Note: Descriptions are shown in the official language in which they were submitted.
CA 02632788 2008-05-30
TITLE
DEGRADATION OF ORGANIC TOXICS BY ELECTRO-OXIDATION
INVENTORS
Patrick Drogui, Lan Huong Tran, Jean-Frangois Blais and Guy Mercier
INVENTION FIELD
This invention relates to an electrochemical process for degradation of toxic
organic
molecules in solution. Particularly, this process includes the treatment of
organic toxics
containing solutions in an electrolytic cell having dimensionally stable
anodes (DSA) with high
oxygen overvoltage. The anodes are made of titanium coated with iridium oxide
(IrO2),
ruthenium oxide (Ru02) or tin oxide (Sn02), with different geometrical forms
(plane, cylindrical
or circular mesh anodes). This process can be used for degradation of
different organic molecules
including one-type or a mixture of polycyclic aromatic hydrocarbons (PAHs),
chlorinated
compounds, pesticides, endocrine disruptors, oils and greases, petroleum
hydrocarbons, PCBs,
PCDD/F or other types of organic compounds.
STATE-OF-THE-ART
In recent years, numerous research works have focused on electro-oxidation
(EO) process
owing to the appearance of emerging pollutants (PCBs, PAHs, EDCs, pesticides,
and others),
which are recalcitrant organic compounds and difficult to oxidize by
traditional biological and
chemical treatments. This type of technology has been widely applied for the
treatment of
different effluents: wastewater (Martinez-Huitle and Ferro 2006), textile
effluents (Wang et al.
2004), landfill leachate (Moraes and Bertazzoli, 2005; Deng and Englehardt
2007), olive oil mill
wastewater (Gotsi et al., 2005), municipal sewage sludge (Zheng et al. 2007),
tannery effluent
(Rao et al. 2001; Panizza and Cerisola 2004) using different electrode
materials.
The interest of using electrochemical oxidation is based on its capability of
reacting on
the pollutants by using both direct and indirect effect of electrical current.
Direct anodic
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oxidation, where the organics can be destroyed at the electrode surface, and
indirect oxidation
where a mediator (HC1O, HBrO, H202, H2S208, and others) is electrochemically
generated to
carry out the oxidation (Grimm et al. 1998; Drogui et al. 2001). Two different
ways can be
followed in anodic oxidation: electrochemical conversion or electrochemical
combustion
(Comminellis and Pulgarin 1993). Electrochemical conversion only transforms
the non-
biodegradable organic pollutants into biodegradable compounds, whereas
electrochemical
combustion yields water and carbon dioxide and no further treatment is then
required.
Direct anodic oxidation
It has been admitted that the direct anodic oxidation is carried out using two
steps
(Comminellis 1994; Gandini et al. 1998). The first reaction (equation 1) is
the anodic oxidation
of water molecule leading to the formation of hydroxyl radicals (HO ) adsorbed
on active sites
on the electrode "M":
H2O + M -> M[HO ] + H+ + e - (1)
Subsequently, the oxidation of organics "R" is mediated by adsorbed hydroxyl
radicals
(equation 2) and may result in fully oxidized reaction product as CO2
(equation 3).
R+M[HO ]-> M+RO+H++e- (2)
R+M[HO ]-* M+mCO2+nHZO+H++e- (3)
Where "RO" represents the oxidized organic molecule, which can be further
oxidized by
hydroxyl radical while it is continuously produced at anode electrodes. The
accumulation of HO
radicals favors the combustion reaction (equation 3). The hydroxyl radicals
are species capable of
oxidizing numerous complex organics, non-chemically oxidizable or difficulty
oxidizable
(Pulgarin et al. 1994). They efficiently react with the double bonds -C=C- and
attack the aromatic
nucleus, which are the major component of refractory organic compounds.
However, during
direct anodic oxidation of organic pollutant, competitive reactions (parasitic
reaction) can take
place and limit hydroxyl radical formation, such as molecular oxygen formation
(equation 4):
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HzO + M[HO ] --> M+ OZ + 3H+ + 3e - (4)
Indirect electro-oxidation effect
The indirect effect of electrolysis is also interesting to destroy
recalcitrant organics. For
instance, in the presence of sulfate and chloride ions, these ions can be
respectively oxidized at
the anode electrodes and formed in peroxodisulfuric acid (H2S208) and
hypochlorous acid
(HC1O) solutions (see equations 5 and 6). Both HC1O and H2S208 are powerful
oxidants capable
of oxidising and modifying the structure of organic molecules and leading to
more oxidized and
less toxic compounds (Canizares et al. 2002, 2005).
2SO4- + 2H+ -> H2S208 + 2e - (5)
Cl -+ 2H20-> HCIO + H3O+ + 2e - (6)
Likewise, during electrolysis, hydroxide peroxide (H202) can be produced from
dissolved
oxygen by cathodic reduction (Bernard and Rumeau 1998; Drogui et al. 2001)
(equation 7):
02(d,ssoUs) + 2H+ + 2e- -> H202 (7)
In such an electrolytic system, the required oxygen was supplied by oxidation
of water
and by transfer from the atmosphere or by pure oxygen injection. Hydroxide
peroxide was
produced by direct current electrolysis using only two electrodes, a carbon
felt cathode and a
Ru02 coated titanium anode. A high peroxide production rate was reached and a
15 mg/L
concentration was maintained. The dissolved organic carbon (DOC) removal in
effluent of
municipal sewage plant corresponded to a breakage of the double bonds (Drogui
et al. 2001). A
remarkable remnant effect was ensured and induced non offensive by products
contrary to
chlorine or hypochlorous acid. It has also been reported that hydroxide
peroxide can be produced
in the electrolytic cell whose cathode is made of porous carbon
polytetrafluorethylene (PTFE)
with oxygen feeding. The degradation of 95% of aniline has been recorded in
the presence of
ultraviolet (UV) irradiation (Brillas et al. 1998).
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Indirect EO can also contribute to generating two mediators such as ozone (03)
and
hydroxide peroxide (H202) by means of a compartmented electrolytic cell using
an ionic
exchange membrane separating anode and cathode chambers (Murphy and Duncan
1999). The
cathode is coated with both a catalytic layer and a diffusion gas layer. The
diffusion gas layer is
comprised of hydrophobic (carbon fiber) and hydrophilic (PTFE) parts. The
oxygen is transferred
into the diffusion gas layer. The anodic oxidation of water induces ozone and
proton (H)
formation. The protons are transferred into cathodic chamber and H202 is
produced from
dissolved oxygen by cathodic reduction. The formation of hydroxyl radical at
the surface of
anode electrode does not take place on all the electrodes. These reactions
depend on the
experimental conditions and, above all, on the nature of the electrode
materials.
Catalytic anodes for organic pollutants destruction
In EO process, two types of insoluble electrode are often used dependently on
the
objectives of the treatment. When the objective is the simple electrolysis of
water (oxygen
formation), an electrode material having a low over-potential of oxygen
evolution is required.
However, when the objective is the degradation of pollutants, a high over-
potential of oxygen
evolution is used. The latter parameter governs the choice of the electrodes
for anodic
combustion or conversion of organic pollutants. Table 1 gives a comparison of
the most
investigated anode materials. Numerous high over-potential electrodes have
been investigated
(Ti/Pt, Ti/Pt-Ir, Ti/IrO2, Ti/Ru02, Ti/Pb02, Ta/PbO2, BDD, Ti/BDD (boron-doped
diamond),
Ti/Sn02, Ti/Sn02-Sb2O5, Gr, and others) to treat effluents containing varied
and relatively high
amount of organic matter (PCBs, PAHs, EDCs, pesticides, phenolic compounds,
surfactants)
which are difficult to oxidize biologically or chemically (Rajeshwar and
Ibanez 1997). Among
these organics, phenolic compounds are the most investigated applications in
EO studies. The
influence of the nature of anodic electrode has been clearly put into evidence
while oxidising
phenol-containing synthetic effluent (Comminellis 1994). Different types of
titanium anode
electrodes coated with a catalytic layer of Pt, Ru02, IrO2, Pb02, and Sn02
have been studied for
electrochemical treatment (i = 50 mA/cm2) of solutions containing phenol (10.6
mM). The best
removal yield was obtained using Ti/Sn02 electrode with an average oxidation
yield of 55% of
current efficiency compared to Ti/Pt, Ti/Ru02, Ti/IrO2 and Ti/Pb02 electrodes
whose values
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ranged between 10 and 15%. The relatively high oxidation yield recorded with
Ti/Sn02 has been
attributed to the highly crystalline nature of the electrode that catalyzed
the electrochemical
oxidation of phenol.
TABLE 1. Summary of the effectiveness of different anode materials used in
electro-
oxidation of water pollutants
Pollutants Anode Pollutant Current References
materials removal efficiency
(%) (%)
Aliphatic alcohol Ti/Ir02 90%, TOC 30-40 Simond and Comninellis (1997)
Phenol Ti/Ir0z - 17 Comninellis (1992)
Phenol Ti/Ru02 - 14 Comninellis (1992)
Phenol Ti/Pt 13 Comninellis (1992)
Carboxylic acids Ti/Pt-Ir 99% 80-100 Bock and MacDougall (2002)
Bock et al. (2002)
Dyes Ti/Pt-Ir 50% 80-100 Szpyrkowicz et al. (2000)
2-chlorophenol Ti/Pb02 80-95%, COD 35-40 Polcaro et al. (1999)
Phenol Ti/Pb02 18 Comninellis (1992)
Phenol Si/BDD 97%, TOC Beck et al. (2000)
Phenol Ta/PbO2 57%, TOC Beck et al. (2000)
Phenol Ti/Sn02 80%, TOC Beck et al. (2000)
Phenol Pt 37%, TOC Beck et al. (2000)
Phenol Ti/Sn0z-Sbz05 71% 58 Comninellis (1992)
Panizza and Cerisola (2004) studied the electrochemical oxidation as a final
treatment of
synthetic tannery wastewater using lead dioxide (Ti/Pb02) and mixed titanium
and ruthenium
oxides as anodes (Ti/TiRuO2). Complete mineralization of the wastewater was
recorded using
either Ti/Pb02 or Ti/TiRuO2 electrode. In particular, the oxidation of
organics took place on the
Ti/Pb02 anode by direct electron transfer and indirect oxidation mediated by
active chlorine,
while it occurred on the Ti/TiRuO2 anode only by indirect oxidation. Likewise,
Ti/Pb02 gave
higher oxidation rate than that observed for the Ti/TiRuO2.
Electrochemical oxidation of PAHs present in sediment has been studied by
Stichnothe et
al. (2005). A total of sixteen PAHs have been measured before and after the
electrochemical
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treatment. A titanium anode electrode coated with iridium oxide (Ti/Ir02)
operated at a current
density of 80 mA/cm2 during 120 min has been used. At the end of the
treatment, the residual
concentration was 0.53 mg PAH/kg, compared to 4.1 mg PAHs/kg recorded in the
initial
sediment, which corresponded to 90% of degradation. IrO2 electrode has been
also studied for
electrochemical elimination of aliphatic alcohols, allowing current efficiency
of 30-40% and 90%
conversion to CO2 (Simond and Comminellis 1997). Overall, low current
efficiencies and high
removal efficiencies are obtained at longer times, resulting in the
competition between the
oxidation of organic and the oxygen evolution.
In particular, the boron-doped diamond (BDD) electrode represents an
interesting and
attractive anode material for the degradation of refractory pollutants such as
phenol,
chiorophenol, carboxylic acids, aniline, various dyes, surfactants and many
others compounds
(Kraft et al. 2003; Martinez-Huitle 2004a, 2004b, 2005). The use of BDD as
anode for organic
pollutant oxidation has been patented by Carey et al. (1995). The
effectiveness of Si/BDD
electrode for the degradation of phenol has been compared to Ti/Sn02, Ta/PbO2
and Pt
electrodes. At a charge loading of 20 A/h, the yield of total organic carbon
(TOC) removal of
97% was recorded using Si/BDD. In comparison, 80, 57 and 37% of TOC removal
were
respectively recorded for Ti/Sn02, Ti/Pb02 and Pt electrodes. Another
peculiarity of BDD
electrode results from its capacity to produce H2S208, a powerful oxidant
capable of participating
in the oxidation of the organic substrates, allowing higher efficiencies.
However, the fragility and
the relatively low conductivity of Si-supported device, constitutes an
obstacle for BDD
application at full scale. Titanium coated with BDD (Ti/BDD) would be more
conducive, stable
and effective for organic compound destruction.
Based on the background of the EO method, several processes have been
developed for
environmental applications. For instance, Kinder et al. (1998) have developed
a procedure for the
treatment of urban wastewater using an oxidative-electrochemical process. This
process is
effective to treat wastewater having high level of colloids and high-molecular
organic residues.
The wastewater was passed multiple times over the stainless steel electrode,
which functions as
the anode, generates oxygen, and has a catalytic effect so that the colloid
and partially dissolved
oxidized dissolved materials are flocculated. In the second stage of the
process, the flocculated
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substances are concentrated using membrane filtration. The concentrate volume
(1/10 of the
volume of wastewater) is then passing over a water-swellable polymer, an
acrylamide-acrylic
acid copolymer to remove the water. The water-swellable polymer can be air-
dried and reused.
An oxidative electrochemical method has also been developed for conditioning
and
stabilizing sewage sludge from municipal and paper mill industries (Drogui et
al. 2005). The
treatment of sludge is carried out using a cylindrical electrolytic cell
having two concentric
electrodes are used. The anode material is made of titanium coated with
ruthenium (Ti/Ru02)
whereas titanium material (Ti) is used as cathode electrode. This process is
characterized by the
following steps: acidification of the sludge (4.0 < pH < 5.0) in such a manner
so as to reach a
sufficiently high pH to avoid corrosion and sufficiently low to significantly
reduce the indicators
of pathogens. Treatment of the acidified sludge in an electrolytic cell able
to generate in situ a
bactericidal oxidant (HC1O or H2S208) in a sufficiently high concentration to
disinfect the sludge
and a sufficiently low concentration to avoid the formation of
organochlorinated compounds in
the sludge; electrolysis of the sludge for a period of time sufficient for
stabilization of the sludge
and to improve their ability to be dewatered. Dryness gain of dewatered-sludge
as high as 10
units are expected when the process is applied. The increasing of the total
solids of treated sludge
allowed reduction from 20 to 30% of the volumes of dewatered-sludge produced.
The process
was also found to be effective in removing indicator microorganisms such as FC
(log-
inactivation of FC was higher than 6 units), while at the same time preserving
its fertilizing
properties by maintaining the concentration of organic matter (chemical oxygen
demand: COD)
and inorganic nutrients (P-P04 and N-NH4) in dewatered-sludge.
Held and Chauhan (2002) describe a process using high electrical voltage
(pulsed-electric
filed, PEF) for dewatering and treating waste-activated sludge (WAS) from
municipal
wastewater treatment plant, from paper mill wastewater treatment plant, and
effluents from
agrofood industries. The method consists in subjecting waste-activated sludge
to electroporation,
which incorporate nonarcing, cyclical high voltages in the range of 15,000 and
100,000 V, which
break down intercellular and intracellular molecular bonds. The release of
intercellular and
intracellular water allows reduction to 50% the amount of dewatered sludge.
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An electrochemical process capable of oxidising and disinfecting water and
wastewater
by electroperoxidation has been developed by Bernard and Rumeau (1998). This
process consists
of oxidisation by direct anodic oxidation dissolved organic compound, viruses
and deactivated
bacteria, while at the cathode electrode, hydrogen peroxide is produced from
dissolved oxygen
reduction. This peroxide has a remarkable remnant effect and induces non
offensive by products
contrary to chlorine. Percolating electrodes of vitreous carbon (cathode) and
platinated titanium
or DSA (anode) are used in the electrolytic cell. Experiments have been
carried out using
synthetic effluent and effluent from municipal wastewater treatment plant. A
10 to 50% of DOC
removal yield can be reached depending on the nature of organic compouds
(aliphatic or
aromatic compounds). Bacterial (FC) removal is as high as 6 log units.
Matsumoto (2005) has developed a process for electrolytic treatment of
photographic
wastewater for COD removal using a conductive diamond electrode as an anode.
The pH of the
effluent is maintained at a pH 4Ø The efficiency of EO on photographic
wastewater is improved
while diluting the gas to be generated /emitted from the waste photographic
processing solution
with air or oxidising gas-mixed air. Ozone may be used as the oxidising gas.
COD components
contained in photographic wastewater can be suppressed to satisfy ambient
environmental quality
standard.
A method and apparatus for treating wastewater containing organic matter and
nitrogen
compounds has been developed by Tabata et al. (2005). The wastewater treatment
apparatus
comprised of an electrolytic cell partitioned with an ion-exchange membrane
into an anode zone
and a cathode zone. Pollutants comprising organic matter and nitrogenous
matter contained in
raw wastewater are oxidatively decomposed by means of hypochlorous acid
generated by
chloride ion oxidation in the anode zone. Then, the treated-effluent is sent
into the cathode zone
and the residual oxidising substance is reduced due to a membrane module
capable of separating
chloride ions provided downstream of the cathode zone.
Coupling electro-oxidation and biological processes
EO process can be advantageously combined with biological process while
treating
effluents containing refractory organic compounds. This system takes advantage
of coupling a
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biodegradation (reduction of operating cost) and a physicochemical process
(shorter retention
time). While the effluent is previously subjected to EO process, the non-
biodegradable organic
pollutants are transformed into biodegradable compounds, which contribute to
increasing the
depurative efficiency of the subsequent biological process. When installed
downstream of
biological process, electrochemical combustion yields water and carbon dioxide
and no further
treatment is then required (Comminellis and Pulgarin 1991, 1993).
The effectiveness of such treatment has been put into evidence by Panizza et
al. (2006)
while treating naphthalenesulfonates in effluent from landfill sites using
biological oxidation
followed by electrochemical oxidation. This indicates that non biodegradable
compounds have
been mineralized by electrochemical oxidation. Likewise, the coupling allowed
reducing energy
consumption from 80 kWh/m3 (in the absence of biological treatment) to 60
kWh/m3 (in the
presence of biological treatment).
Creosote as a source of PAHs
Many industrial processes generate very toxic residual wastes or wastewaters
that are
hardly biodegradable and require a chemical or physicochemical treatment.
Among these organic
pollutants, there are polycyclic aromatic hydrocarbons (PAHs), which are
usually classified as
priority pollutants of water due to their toxic xenobiotics and dangerous
character for humans,
plants, and animals (USEPA 1987; Beltran et al. 1998; Zheng et al. 2007). The
presence of PAHs
in water is due to different sources like pyrolysis of carbon, electrolysis
with graphite electrodes
(waste from aluminium industries), coke plant, creosote rubber or hydrocarbon
synthesis from
natural gas (Deschesne 1995; Beltran et al. 1998). In particular, creosote is
one of the important
sources of PAHs release in the environment.
Creosote is a distillate of coal tar (USEPA 1984) and it is an excellent
fungicide and
insecticide (Betts 1990). Creosote can be toxic to animal, and direct contact
with creosote can
lead to skin irritation and disease (Betts 1990; Becker et al. 2001). The
organic constituent of
creosote include PAHs (up to 85%), phenolic compounds (10%) and N-, S- and 0-
heterocyclic
aromatic compounds (5%) (Mueller et al. 1989; Engwall et al. 1999).
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Creosote is commonly used as wood preservative. Creosote-treated wood is
widely used
for railway construction and poles for the transport of electricity or for
telephone lines
(Gouvernment of Canada 1993; Engwall et al. 1999; Becker et al. 2001; Ikarashi
et al. 2006).
One concern involved in the use of creosote is the long-term release into the
environment. In
natural environment, creosoted wood is in contact with rainwater and moisture
and water
contained in the surrounding soil and may be responsible for severe pollution
of ground water
and surface water. Ikarashi et al. (2006) reported that creosote contaminated
sites have been
identified in Canada, United States, Greenland, Denmark, Sweden and the United
Kingdom.
Creosote contains high quantities of polycyclic aromatic hydrocarbons (PAHs).
The removal of
these compounds from water is a difficult task due to their low solubility and
refractory character
but it can be achieved through some treatment methods, such as chemical
advanced oxidation
(Trapido et al. 1995; Beltran et al. 1998; Goel et al. 2003; Flotron et al.
2005), electrochemical
oxidation (Stichnothe et al. 2004, 2005; Panizza et al. 2006) or biological
oxidation (Romero et
al. 1998; Juhasz and Naidu 2000).
PAHs are usually classified as priority pollutants of water due to their
dangerous or
toxicity character for plants and animals. The United States Environmental
Protection Agency
(USEPA) has specified 16 main PAHs as priority pollutants because of their
known toxicity,
mutagenicity, and carcinogenicity to mammals and aquatic organisms (USEPA
1987; Wilson and
Jones 1993; Mangas et al. 1998) (Figure 5). Main compounds in the creosote
used in this study
were naphthalene (NAP), phenanthrene (PHE), fluorene (FLU), pyrene (PYR) and
fluoranthene
(FLE). The present study focuses on PAHs in creosote solution because of their
potential to
contaminate both surface and ground waters.
The removal of PAHs from water is a difficult task due to their low
concentration and
refractory character but it can be achieved through some treatment methods,
such as chemical
advanced oxidation (Trapido et al. 1995; Beltran et al. 1998; Goel et al.,
2003; Flotron et al.,
2005), electrochemical oxidation (Stichnothe et al. 2005; Panizza et al. 2006)
or biological
oxidation using micro-organisms (Romero et al. 1998; Juhasz and Naidu 2000).
Advanced
oxidation processes (AOPs) are often used for PAHs degradation (Trapido et al.
1995; Goel et al.
2003; Zheng et al. 2007). The aim of AOPs (including, 03/H2O2, UV/03, UV/H202,
H2O2/Fe2+
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etc.) is to produce the hydroxyl radical in water, a very powerful oxidant
capable of oxidising a
wide range of organic compounds with one or many double bonds. In spite of
good oxidation of
refractory organic compounds, the complexity of these methods (AOPs), high
chemical
consumption and relatively higher treatment cost constitutes major barriers in
the field
application (Panizza and Cerisola 2004; Martinez-Huitle and Ferro 2006).
Electrochemical oxidation treatment can be used as an alternate method for
PAHs
degradation. Electro-oxidation process opens new ways and can advantageously
replace or
complete already existing processes. There are two types of anodic oxidations
that are indirect
oxidation process and direct oxidation. The latter may be achieved through
mineralization with
hydroxyl radical (OH ) produced by dimensionally stable anodes (DSA) having
high oxygen
overvoltage, such as Sn02, Pb02 and IrO2 (Comninellis and Pulgarin 1991;
Comninellis 1994;
Panizza et al. 2000).
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SUMMARY OF THE INVENTION
This invention relates to an electrochemical process for the degradation of
toxic organic
molecules in solution. This process includes the treatment of organic toxics
containing solutions
in an electrolytic cell having dimensionally stable anodes (DSA) with high
oxygen overvoltage.
The anodes are made of titanium coated with iridium oxide (IrO2), ruthenium
oxide (Ru02) or tin
oxide (Sn02). The cathodes are made of stainless steel, titanium or another
type of metal. The
electrodes may have different geometrical forms (plane, cylindrical or
circular mesh anodes).
This process can be used for degradation of different organic molecules
including one-type or a
mixture of polycyclic aromatic hydrocarbons (PAHs), chlorinated compounds,
pesticides,
endocrine disruptors, oils and greases, petroleum hydrocarbons, PCBs, PCDD/F
or other types of
organic compounds. The solution is treated with a current density ranging
between 3.0 to
23 mA/cm2 and for a period of time ranging between 10 to 200 min. The inter-
electrode distance
is adjusted between 0.5 to 2 cm. A surfactant can be added to keep the organic
toxic molecules in
solution, whereas a supporting electrolyte, like NaZSO4, can be added with a
concentration
ranging from 0.5 to 4.0 g/L to reduce the energy consumption. The temperature
of the solution is
preferentially maintained between 4 and 35 C. Finally, this process can be
operated in batch,
semi-continuous or continuous mode.
DETAILED DESCRIPTION OF THE INVENTION
In spite of the extensive bibliography which exists on aspects related to the
process
described, no publication is known to date which considers the method of
electrolytic
degradation described herein. The technique to be applied is therefore novel,
and consists in a
simultaneous method of electrochemical destruction of different types of
organic pollutants in a
single-cell process, since the oxidation of polycyclic aromatic hydrocarbons
(PAHs), oils and
grease (O&G), petroleum hydrocarbons (Clo-Cs0), take place at the same time on
the anode
electrode owing to hydroxyl radical generation on the electrode, whereas
others oxidizing species
can be simultaneously generated in solution, such as hypochlorous acid (HCIO),
peroxodisulfuric
acid (H2S208), ozone (03) and hydrogen peroxide (H202) in order to enhance
organic pollutant
degradation.
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According to an embodiment of the invention, this electrochemical process can
be used
for degradation of one-type or a mixture of polycyclic aromatic hydrocarbons
(PAHs) in
synthetic solution or real effluent. The process described herein is effective
in simultaneously
oxidizing several PAHs having 2 to 6 of aromatic rings. More than 85% of PAHs
degradation
can be reached irrespective of the number of aromatic rings.
Another interesting characteristic of the process described herein results
from its capacity
of simultaneously reducing oils and greases (O&G) by direct anodic oxidation
or by
neutralization of charged droplets owing to the electric field induced by the
potential difference,
resulting in oils and grease destabilization.
Moreover, according to an embodiment of the invention, this electrochemical
process is
also effective in simultaneously oxidizing compounds in form of hydrocarbon
chains from 10 to
50 units (Clo-Cso) contained in synthetic or real effluent. More than 80% of
Clo-C5o reduction
can be reached.
On the other hand, reduction in COD is about three times higher than TOC
reduction,
indicating that only a small fraction of PAHs was completely oxidized into
water and carbon
dioxide, the majority of the pollutants being transformed into small molecules
that reduce the
oxygen demand in the treated-effluent.
In another configuration of the invention, this process can be used to degrade
others types
of toxic organic molecules, like chlorinated compounds, pesticides, endocrine
disruptors, BPCs,
PCDD/F or other types of organic compounds.
According to an embodiment of the invention, the surfactant cocamidopropyl
hydroxysultaine (CAS) can be added to keep the organic toxic molecules in
solution. In another
configuration of the invention, other types of surfactants can be used in
replacement of CAS.
In the cell it is advantageous to promote turbulence of the effluent.
Possibly, it should be
provided using a system with recycling flow rate which would allow to
overcoming the
formation of organic substances on the electrode surface. Higher recycling
flow rate decreases
the thickness of the diffusion layer, which may results in a higher removal
rate of organics. It is
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preferable that the raw water to be treated circulate in turbulent regime
either by imposing
conventional and mechanical agitation, or by forced circulation through
turbulence promoters, in
order to favour transportation of the electro-active species to the
electrodes. Preferably, a
recirculation flow rate, between 1.0 and 5.4 L/min is applied.
A direct current of voltage lower than 40 V, preferably between 1 and 20 V, is
applied.
The potential applied must be contributed to increase temperature from about
20 to 25 C during
electrolysis. The increase of the temperature accelerates the electrochemical
decomposition of
organics. However, work can be carried in the entire range of temperature in
which the effluent
to be treated is liquid (over 60 C in pressurized system), although economic
consideration make
it advisable to work at moderate temperature (up to a maximum of about 40 C)
in non
pressurized system.
According to an embodiment of the invention, the temperature of the solution
is
preferentially maintained between 4 and 35 C. Inside the cell, oxygen from the
air or pure oxygen
can be injected in a close loop in order to gradually saturate the liquid in
oxygen and be able to
further generate radical species (OH ) or oxidants (such as ozone, 03) capable
of enhancing
PAHs degradation. It has been already demonstrated by several authors that
ozone could be
formed during electrolysis of water using high oxygen overvoltage anodes
(Wabner and
Grambow, 1985; Tatapudi and Fenton, 1993; Foller and Tobias, 1982). A maximum
for PAHs
degradation efficiency was observed at 5.0 mL 02/min.
Amongst the many metals and alloys which can be used as anode, noble metallic
oxides
fixed on titanium metal are preferably used. According to an embodiment of the
invention, the
process includes the treatment of solution in an electrolytic cell having
dimensionally stable
anodes (DSA) with high oxygen overvoltage. The anodes are preferentially made
of titanium
coated with iridium oxide (Ir02), ruthenium oxide (Ru02) or tin oxide (Sn02).
The cathodes are
made of stainless steel, titanium or another type of metal. The electrodes may
have different
geometrical forms (plane, cylindrical or circular mesh anodes). Mesh electrode
or expanded
electrodes are used in order to favor high transfer coefficient between
electrode and effluent to be
treated. The solution is treated with a current density ranging between 3.0 to
23 mA/cm2 and for a
14
CA 02632788 2008-05-30
period of time ranging between 10 to 200 min. The inter-electrode distance is
adjusted between
0.5 to 2 cm.
According to an embodiment of the invention, an electrolyte can be added to
reduce the
energy consumption. The electrolyte can be one or a mixture of Na2SO4, NaC1,
KCI, MgC12,
CaCl2, HCI, H2SO4, MgSO4, (NH4)2SO4, NH4C1. The concentration of the
electrolyte is usually
ranging between 0.5 to 4.0 g/L.
Alkaline media is not favourable for PAHs oxidation, whereas high performance
of PAHs
degradation can be recorded preferably between pH 4.0 and 7Ø
The method of electrolytic degradation described herein could be used as an
alternative or
complementary method to the conventional biological treatment used today in
many
sewage/wastewater treatment plants (STP). This is because the biological
process suffers from a
number of defects. For instance, the biological purification plant is
essentially a culture of
microorganisms, especially, bacteria, which feed on pollutants, oxidizing
them. Since it is an
ecosystem, it is not easy to maintain in a stationary state. Effectiveness of
biological process
depends to many environmental parameters, such as temperature, nutrients,
oxygen transfer, but
mainly depend of the quantity and type of pollutant contained in the input
water. In order to
avoid unsatisfying results of the conventional biological process in the
presence of refractory
organic pollutants, the method described herein can be advantageously used as
pre-treatment or
as tertiary treatment. While the effluent is previously subjected to the
described process, the non-
biodegradable organic pollutants are transformed into biodegradable compounds,
which
contribute to increasing the depurative efficiency of the subsequent
biological process. When
installed downstream of biological process, electrochemical combustion yields
water and carbon
dioxide and no further treatment is then required. The described process
breaks the double bonds
of PAHs producing smaller molecules. For instance, pyrene molecule having four
aromatic rings
is transform into furanone compounds which are less toxic than the initial
pyrene compound.
Indeed, the described process is able to efficiently reduce more than 90% of
the toxicity of PAH-
containing effluent, based on a biotest battery using Microtox and Daphnia
test.
CA 02632788 2008-05-30
Finally, this electrochemical process can be operated in batch, semi-
continuous or
continuous mode.
16
CA 02632788 2008-05-30
Methodology
Creosote and PAHs solubilization
Commercially-available creosote used in this study was provided by Stella-
Jones Inc.
(Montreal, QC, Canada). It was comprised of 50% (v/v) of creosote and 50% of
petroleum
hydrocarbons. The creosote effluent was prepared in a 100 mL glass-tank
containing 10 to
50 g of oily-creosote in which 10 to 50 g of an amphoteric surfactant, CAS
(Cocamidopropyl
Hydroxysultame, Chemron, Ohio, USA) was added. Conditioning was carried out at
a speed of
750 rpm for a period of time of 24 h. At the end of the conditioning stage,
the suspension was
transferred into a 20 L polypropylene tank containing 10 L of distilled water
(final concentration
= 1.0 to 5.0 g creosote/L). The resulting suspension constituted the synthetic
creosote-oily
solution (COS), which was then subjected to settling for at least 24 h in
order to separate the
insoluble and suspended solids before electrochemical treatment.
Electrochemical treatment using plate electrodes
Electrochemical degradation of PAHs in COS was carried out in a batch square
electrolytic cell made of acrylic material with a dimension of 12 cm (width) x
12 cm (length) x
19 cm (depth) (Figure 1). The electrode sets (anode and cathode) consisted of
ten parallel pieces
metal with an inter-electrode distance of 1 cm. Five anodes and five cathodes
alternated in the
electrode pack. The electrodes were placed in stable position and submerged in
COS. The anodes
are presented in the form of expanded titanium (Ti) covered with ruthenium
oxide (RuO2), each
one having a solid surface area of 65 cm2 and a void area of 45 cm2. The
cathodes are plate
stainless steel (SS, 316L) and having a surface area of 110 cm2 (10 cm width x
11 cm height).
The electrodes were placed 2 cm from the bottom of the cell. Mixing in the
cell was achieved by
a Teflon-covered stirring bar installed between the set of electrodes and the
bottom of the cell.
For all tests, a working volume of 1.5 L of COS was used. The anodes and
cathodes were
connected respectively to the positive and negative outlets of a DC power
supply Xantrex XFR
40-70 (Aca Tmetrix, Mississauga, Canada) with a maximum current rating of 70 A
at an open
circuit potential of 40 V. Current was held constant for each run. Between two
tests, electrolytic
17
CA 02632788 2008-05-30
cell (including the electrodes) were rubbed with a sponge and rinsed with tap
water, and then
soaked with 5% (v/v) nitric acid solution for 15 min.
12cm
12eM
~
19cm
r~ 1
FIG. 1. Configuration of electrolytic cells using plate electrodes: (anodes:
Ti/RuO2;
cathodes: stainless steel)
The first set of electrodegradation experiments consisted to test successively
different
operating parameters such as, current densities (3.08 to 12.3 mA/cm2),
retention times (20 to
180 min), initial pH (2.0 to 9.0), initial PAHs concentration (240 to 540
mg/L), concentration of
electrolyte (500 to 4,000 mg Na2SO4/L) and temperature (4 to 35 C) in view of
determining the
optimal conditions (reduce cost and increase effectiveness) for treating COS.
The pH was
adjusted using sulphuric acid (H2SO4, 5 mol/L) or sodium hydroxide (NaOH, 2
mol/L). Sodium
sulphate, sodium hydroxide and sulphuric acid were analytical grade reagent
and supplied by
Fisher Scientific. During these assays, only the residual PAHs concentrations
were measured to
evaluate the performance of the experimental unit in oxidizing these
refractory organic
18
CA 02632788 2008-05-30
compounds. Once the appropriate values of these parameters were determined,
the optimal
conditions were repeated in triplicate to verify the effectiveness and the
reproducibility of the
electro-oxidation process. In addition to residual PAHs analyzed during the
second set of
experiments, dissolved organic carbon (DOC), total organic carbon (TOC), oil
and grease (O&G)
and petroleum hydrocarbons (Cio-C50) were simultaneously measured. Likewise,
biotests
(Microtox and Daphnia tests) were carried out to have information about the
toxicity of the
initial and treated solution under optimum experimental conditions.
Electrochemical treatment using concentrical and circular electrodes
Three cylindrical electrolysis cells (C1, C2 and C3), each having 2 L of
capacity, were
operated. The cells were made of PVC material with a dimension of 15 cm
(height) x 14 cm
(diameter) and all electrodes were presented in the form of expanded metal.
The electrolytic Cl
was comprised of two concentrical electrodes (Figure 2). The cylindrical anode
electrode (14 cm
height x 10 cm diameter x 0.1 cm thick) was titanium coated with iridium oxide
(Ti/IrO2) having
a solid surface area of 270 em2 and a void area of 170 cm2. The cylindrical
cathode electrode
(14 cm height x 12 cm diameter x 0.1 cm thick) was made of titanium (Ti)
having a solid surface
area of 325 cm2 and a void area of 202 cm2. A perforated cylindrical weir (2
mm diameter of
holes) made of PVC material, was placed in the centre of the C1 cell and
allowed uniformly
distributing the effluent toward the concentrical electrodes. Likewise, the
cylindrical weir
allowed wastewater to remain in the cell for a certain period. The weir had a
diameter of 4.0 cm
and a height of 14 cm. By comparison, inside both electrolytic C2 and C3, the
electrodes were
circular disks (12 cm diameter) and titanium (Ti) was used as cathode with a
solid surface area of
65 cm2 and a void surface area of 45 em2 (Figure 3). Circular Ti/Ir02 and
Ti/Sn02 electrodes
were respectively used as anode electrode in C2 and C3 cells with a solid
surface area of 65 cm2
and a void surface area of 45 cm2. The inter-electrode gap was 10 mm in the
three electrolytic
cells. The circular electrodes were supplied by Electrolytica Inc (Amherst,
NY), whereas the
cylindrical electrodes were provided by Electech (Chardon, Ohio).
The assays were carried out in a closed loop, depicted schematically in Figure
4. A one
litre of PVC tank (4), a centrifugal gear pump (6) and the electrolytic cell
(1) (fully detailed in
Figures 2 and 3) constitute the loop. The first assays were conducted in batch
recirculation mode
19
CA 02632788 2008-05-30
with a flow of wastewater entering the bottom of the cell. The recycle flow
rate (varying from 1.8
to 7.3 L/min) was measured using a flow-meter (13). It worth noting that, the
recycle flow (QR),
induced by the centrifugal pump maintained the liquid phase in sufficient
mixing. A needle valve
(2) installed before a manometer (3) allowed controlling the hydrostatic
pressure inside the cell.
The apparatus included oxygen injection (8) in the closed loop in order to
favour the hydroxyl
radical at anode electrode. The rate of oxygen injected was measured using a
flow-meter (7). An
oxygen probe (9) was connected to an oxymeter and installed in the pipe to
measure dissolved
oxygen concentration (8.0 to 20 mg/L) during electrolysis. The excess of
oxygen was rejected out
of the system by means of a venting pipe (10) fixed on the 1-L PVC reservoir.
At the start of each
assay, the raw effluent was injected in the experimental unit by means of a
funnel (14) installed
in the pipe and connected to a peristaltic pump, which allowed adding a
working volume of
4.5 L. An addition of sulphate sodium (0.5 g/L of Na2SO4) was necessary to
increase the
electrical conductivity. The electrochemical cells were operated under
galvanostatic conditions,
with current densities varying from 4.0 to 23 mA/cm2 imposed during a period
of treatment
ranging from 10 to 180 min. Current densities were imposed by means of a DC
power source,
Xantrex XFR40-70. During the experiments the pH was monitored but not
controlled. While the
experimental unit operated in continue mode, valve (15) was closed, whereas
valves (11) and
(12) were opened. However, in the batch mode, valves (11), (12) and (15) were
closed. Prior to
continue mode operation, the apparatus was initially operated in batch
recirculation mode until
the steady sate for PAHs degradation was reached, followed by feeding the
electrolytic cell with
untreated and freshly creosote effluent by means of peristaltic pump. The
inlet (QE) and outlet
(Qs) flow rates were quite the same and ranged between 50 and 100 mL/min. It
worth noting that
during the continue mode operation, the centrifugal pump was closed and the
recycling flow (QR)
was nil.
CA 02632788 2008-05-30
Treated effluent
Current
connector
2,5 cm Perforated
PVC plate
--~~
anode (8 cm
diameter)
14cm
0.5 m Cathode (12cm
Scm ~ ~
9cm diameter)
1cm Perforated
f cylindrical
weir ` ~'' ~_---
~r----r
2,5 cm Inlet zone
= Effluent
T distributor
Untreated effluent
.F
13 cm
FIG. 2. Configuration of electrolytic cells using cylindrical electrodes: Cell-
1 (anode:
Ti/IrO2; cathode: Ti)
21
CA 02632788 2008-05-30
Treated effluent
= =
~ ~.. -+.
Outlet zone - 6 cm
,.~
=
---
Cathode
1cm . .. . _._... . ..`
Anode = (' ~ ', ~
~ 1cm
=---- __ - 15cm Cathode
---- - ~ -- -
1cm
Anode
Inlet zone - -
6 cm,
~ =
Untreated
effluent
~ -_ - --_ - -
13 cm
U05~
`n!i~~;~tt~i~
$~-
FIG. 3. Configuration of electrolytic cells using circular electrodes: Cell-2
(anode:
Ti/Ir02; cathode: Ti) and Cell-3 (anode: Ti/Sn02; cathode: Ti)
22
CA 02632788 2008-05-30
(14) (3)
(2)
(15)
(9)
outlet ~_ (11)
Inlet _ (12)
(10) I
I
(4)
(~)
O2
~ ----- ----------- (5)
(7)
02
(8) (6)
(13)
(1) : Electro-oxydation cell (11) : Valve (Outlet of water in continuous
operation mode)
(2) : Needle valve (0-5 bars) (12) : Valve (Inlet of water in continuous
operation mode)
(3) : Manometer (13) : Water flowmeter
(4) : PVC tank (1 L) (14) : Funnel for filling in effluent the experimental
unit
(5) : Exhaust pipe (15) : Valve
(6) : Centrifugal Pump
(7) : Air and 0Z flowmeter
(8) : Oxygen bottle
(9) : Oxygen probe
(10) : Venting pipe
FIG. 4. Schematic view of the electro-oxidation cell with a recirculation loop
23
CA 02632788 2008-05-30
Analytical techniques
Operating parameters
The pH was determined using a pH-meter (Fisher Acumet model 915) equipped with
a
double-junction Cole-Palmer electrode with Ag/AgCI reference cell. A
conductivity meter
(Oakton Model 510) was used to determine the ionic conductivity of the
solution. The
temperature of treated-solution was monitored using a thermo-meter (Cole-
Parmer, model
Thermo Scientific Ertco).
Extraction and analysis of PAHs
Analyses of PAHs were carried out after extraction and purification on a solid
phase
using polypropylene-cartridges (Enviro-Clean sorbents, United Chemical
Technology Inc.). The
Enviro-Clean polypropylene-cartridge was successively conditioned by rinsing
with 10 mL of
dichloromethane (99.9% ACS reagent, EMD chemicals Inc., USA), 10 mL of
methanol (99.8%
reagent, Fisher Scientific, Canada) and 10 mL of distilled water.
Subsequently, 500 mL of
sample (creosote-oily solution) containing 5 mL of methanol was loaded onto
the cartridge where
it is entirely filtered drip. PAHs retained on the polypropylene-cartridge
were then eluted with 10
mL of dichloromethane. After elution, the sample was transferred into a filter
containing
anhydrous MgSO4 (EMD chemicals Inc., USA) in order to eliminate all traces of
water, followed
by evaporation of dichloromethane using a rotary evaporator (Buchi Rotavapor-
R, Rico
Instrument Co.). The extraction solution was diluted with dichloromethane, and
a series of
diluted solution (1 x 10 x 100) was prepared and analyzed. PAHs were
quantified using a Perkin
Elmer 500 gas chromatography coupled Mass Spectrometry (GC-MS) on a VF-5MS-FS
column
(0.25 mm diameter, 30 m long and 0.25 m film thickness). A polycyclic
aromatic hydrocarbons
(PAHs) mixture containing 44 PAHs at a concentration of 1,000 mg/L in
dichloromethane-
benzene (3:1) (Supelco, Canada) was used as a standard for PAHs. The PAHs
standard solution
was commercially-available from Sigma Aldrich Canada Ltd (Oakville, ON,
Canada). The 16
major PAHs identified in the creosote solution with the relatively largest
peak area in the
chromatogram are presented in Figure 5. Likewise, Table 2 indicates some
physicochemical
properties of these compounds.
24
CA 02632788 2008-05-30
\ \ \ \ H3
/ / / / \ \ \ \
Naphthalene 2-Methyl Naphtalene Acenaphtylene Acenaphtene
CioHs C12H8 C12HIo
cCc9ccCct Fluorene Phenanthrene Anthracene Fluoranthene
C13H,o C1aHto CiaHio Ci6Hio
\ \ \ \ \ \
\ \ \ \ \ \ \ / \ \ \
Pyrene Benzo(a) anthracene Chrysene Benzo(b)fluoranthene
C16H,o CaH12 C18H12 C2oH12
co
Benzo(j)fluoranthene Benzo(k)fluoranthene Benzo(a) pyrene Indeno (1,2,3-c,d)
pyrene
C2oH12 C2oHl2 C20H12 C22Ht2
\ \ \ \ \
Dibenzo (a,h) anthracene Benzo(ghi)perylene
C22H14 C22HI2
FIG. 5. Molecular structures of PAHs identified in creosote solution
CA 02632788 2008-05-30
Organic measurements
Chemical oxygen demand (COD) determination was made by Hach COD method
(HACH 1995) and a reading spectrophotometer Carry UV 50 (Varian Canada Inc.).
TOC was
measured using a Shimadzu TOC 5000A analyzer (Shimadzu Scientific Instruments
Inc.)
equipped with an autosampler. Samples BOD determinations with required
controls were made
by Standard Methods (APHA 1999). The quality of the treated-solution was also
measured in
terms total oil and grease (O&G) and CI o-C5o petroleum hydrocarbons. O&G were
determined by
gravimetric method which consisted in extracting fat and grease from sample
with hexane at pH
below 2.0 followed by the evaporation of the organic solvent. The
concentration of petroleum
hydrocarbons present in the samples was determined by comparing the total area
of group of
peaks of n-C10 to n-C50 with area of the standard curves obtained under
similar reaction
conditions.
Toxicity tests
The quality of treated-solution (versus untreated solution) has been evaluated
using a
biotest battery to have information about its toxic effect. Microtox and
Daphnia bioassay tests
were applied. Microtox analysis is a standardized toxicity test using the
luminescent marine,
Vibrio fisheri (Software MTX6, version 6.0, Microbics Corporation)
(Environnement Canada
1992; USEPA 1993). This test consisted of one control and six serial dilutions
of each sample
(1.5, 3.0, 6.25, 12.5, 25.0, and 50% v/v). The endpoint of Microtox test is
the measurement of
bioluminescence reduction. The bioluminescence emitted by V. fisheri was first
measured after
min of incubation (without adding any sample, control assay), after which the
creosote-
solution (treated or untreated-solution) was added to the bacterial
suspension. The
bioluminescence reduction was determined after a 5, 15 and 30 min exposure to
the contaminant.
The toxicity effects were monitored as the average percentage of light
emission inhibition
compared to the control assay. By comparison, Daphnia bioassay test consisted
in determining
the lethal concentration for which at least 50% of mortality of crustacean
Daphania magna is
observed after 48 h exposure to the contaminant. This test consisted of one
control and five serial
dilutions of each sample (6.25, 12.5, 25.0, 50.0, and 100% v/v). After 48 h
exposure, the survival
26
CA 02632788 2008-05-30
and death organisms was counted and the toxicity effect was evaluated using a
statistic
calculation software (Computer Basic, Spearman Karber tests, version 2.01,
Microsoft)
(Environnement Canada 2000; Ministere de 1'environnement du Quebec 2000).
Economic aspect
The economic study included chemical and energy consumption. The electric cost
was
estimated about of 0.06 US$/kWh. A unit cost of 0.30 US$/kg was used of
electrolyte (Na2SO4
industrial grade). The acid used to adjust the pH of the solution before and a
long the treatment
was a H2SO4 solution (5 mol/L) which have a cost of 80 US$/t of concentrated
acid (H2SO4
93%). The base pH was adjusted using a NaOH (2 mol/L) solution and it was
about 600 US$/t.
The total cost was evaluated in terms of U.S. dollars spent par cubic meter of
treated solution
(US$/m3).
27
CA 02632788 2008-05-30
a~ p
~x o o
N N N m 7 7 'n ~ r ~ ~ r ~
O C O C C O O O O O O O p p
- ti -
Q" X X X X X X X X X X X X X X ~ X
V l~ 00 O~ v'i N 00 O o0 N M O ~O p p
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> N
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oo 00 00 M ~ ~O ~ 00 O
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Vl kn
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N
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0
Q~ L ~ O O O O O O O ~ N_ ~ N_ ^ ~ ~
++ ~~ ~" x x x x x:T x:~ x x~; x x x x
rN, rN` rN\
.(,~. r00 r, r, V V V V
O ~~ v V ~ V V V V V V V V V
V O
~ ~ a z z d~ w H w r~ d a a z a a
a~ > d w U U a x z~~ d x~ d d z a
~ i: z~" d d w a d w a CG U~ A A'-' A
A d
ct ~ _a c
Y u
N a~ c3 ^
a z
Z % -0 cz 0 p ~A
C 6 C ~ v NN NN N
o
E-~ a z N d d w w¾ u o. aa v cn m Q~ aa
CA 02632788 2008-05-30
Example 1: PAHs solubilization from creosote
The electro-oxidation has been explored at the laboratory pilot scale, to
oxidize refractory
organic compounds from creosote-oily solution (COS). The COS was a synthetic
solution
prepared from a commercial creosote solution in the presence of an amphoteric
surfactant. The
main objective of the present study was to examine the feasibility of electro-
oxidation process in
treating COS and to determine the optimal operating conditions to efficiently
oxidize PAHs.
The first set of experiments consisted to determine the best way of
solubilizing PAHs
from creosote using an amphoteric surfactant (Cocamidopropyl Hydroxysultaine,
CAS).
Different creosote/surfactant mass ratios (1.0, 2.0, 3.0 and 5.0) have been
tested by imposing
either a creosote (CR) concentration of 0.5 g/L or by holding constant the
surfactant
concentration to 1.0 g/L during the assays. The results are summarized in
Table 3. 16 PAHs were
investigated in the creosote and were comprised of different number of
aromatic rings (2-, 3-, 4-,
5- and 6-rings PAHs). The highest total concentration of PAHs in solution were
obtained at a
fixed concentration of surfactant of 1.0 g/L with solubilization of 274, 404,
471 and 538 mg/L
recorded while imposing creosote/surfactant ratios of 1.0, 2.0, 3.0, and 5.0,
respectively. The
total PAHs concentrations increased while increasing CR/CAS for CAS
concentration imposed
of 1.0 g/L, whereas the PAHs concentration decreased with CR/CAS ratio while
imposing a
creosote concentration of 0.5 g/L. It can also be seen that, the total PAHs
measured in solution
were greatly linked to the amount of creosote concentration utilized rather
than surfactant (CAS)
concentration. For instance, for the lowest (1.0 w/w) and the highest (5.0
w/w) CR/CAS ratios
imposed, 123 and 53.3 mg/L of total PAHs were respectively recorded using 0.5
g/L of creosote
concentration. By comparison, while using a fixed concentration of 1.0 g/L of
CAS, 274 and
538 mg/L of PAHs were solubilized for the same ratios of 1.0 and 5.0 imposed,
respectively. The
latter concentrations of PAHs were 2.2 and 10.0 times higher than the first
ones. Indeed, 1.0 and
5.0 g/L of creosote were respectively required to impose the ratios 1.0 and
5.0 in the presence of
1.0 g/L of CAS. Consequently, the best performance of PAHs solubilization
results more
importantly from the amount of creosote concentration in the mixture creosote-
surfactant.
29
CA 02632788 2008-05-30
TABLE 3. PAHs solubilization (mg/L) from creosote
PAHs Creosote (0.5 g/L) Surfactant (1.0 g/L)
Creosote/surfactant ratio (w/w) Creosote/surfactant ratio (w/w)
1.0 2.0 3.0 5.0 5.0 3.0 2.0 1.0
2-ring PAHs
NAP 16.8 12.9 9.27 6.75 69.2 66.4 52.1 35.3
MEN 15.0 11.9 8.63 6.06 61.1 64.2 49.2 33.7
Sum 31.8 24.8 17.9 12.8 130 131 101 69.0
3-ring PAHs
ACN 0.80 0.88 0.70 0.37 3.88 3.50 2.60 1.67
ACA 12.2 10.3 9.78 7.54 72.9 59.1 43.2 40.3
FLU 10.8 8.50 7.43 6.24 61.9 43.5 39.5 33.2
PHE 25.8 18.1 17.6 11.1 127 103 97.5 52.6
ANT 5.67 4.00 2.69 1.16 12.4 10.5 10.4 9.37
Sum 55.3 41.8 38.2 26.4 278 220 193 137
4-ring PAHs
FLE 13.9 12.5 7.36 4.93 53.3 43.9 41.7 24.5
PYR 12.2 10.2 5.72 5.16 44.4 41.5 37.3 25.6
BAA 2.83 3.07 1.93 1.06 10.1 9.71 8.60 5.95
CHR 3.94 2.94 1.88 1.42 11.7 14.1 12.0 6.18
Sum 32.9 28.7 16.9 12.6 120 109 99.6 62.2
5-ring PAHs
BJK 1.82 1.69 0.90 0.91 6.53 6.44 5.93 3.82
BAP 0.74 0.70 0.57 0.38 2.50 3.83 2.85 1.55
DAN 0.13 0.13 0.06 0.07 0.66 0.29 0.53 0.28
Sum 2.69 2.52 1.53 1.36 9.69 10.6 9.31 5.65
6-ring PAHs
INP 0.03 0.03 0.10 0.07 0.26 0.52 0.24 0.16
BPR 0.09 0.04 0.04 0.05 0.51 0.38 0.38 0.18
Sum 0.12 0.07 0.14 0.12 0.77 0.90 0.62 0.34
E PAHs 123 97.9 74.7 53.3 538 471 404 274
CA 02632788 2008-05-30
From the Table 3, it can also be seen that 3-ring PAHs (FLU, PHE, ANT, CAN and
ACA) were present in the highest concentration with the percentage of
solubilization ranging
from 42.7 to 51.7%, followed by 2-rings-PAHs (NAP, MEN) with the yields of
solubilization
varying between 24.0 to 27.7% and 4-rings PAHs (FLE, PYR, BAA and CHR) with
the yields of
solubilization ranging from 22.2 to 29.3%. The lowest yields of PAHs
solubilization from
creosote were recorded for 5-rings PAHs (BJK, BAP and DAN) and for 6-rings
PAHs (INP and
BPR) with the percentage ranging from 0.10 to 2.6%. Despite the maximal PAHs
solubilization
recorded using the ratio 5/1 (creosote/surfactant) (538 mg/L of total PAHs
recorded), the ratio of
3/1 leading to 471 mg/L of PAHs was selected as an optimal ratio to reduce as
much as possible
the concentration of creosote while preparing COS. The COS was then subjected
to
electrochemical oxidation.
Several batch electrolytic tests were performed in order to determine
economical and
optimal conditions for PAHs degradation in COS. Majors operating conditions
investigated
included: (i) current density; (ii) retention time; (iii) initial pH; (iv)
electrolyte concentration; and
(iv) temperature.
Example 2: Effect of current density on electrochemical oxidation of PAHs
Current density is one of the most important parameters that can affect
organic removal.
Table 4 indicates the initial untreated COS and residual PAHs concentrations
after treatment
while imposing different current densities (3.08, 4.62, 6.15, 9.23 and 12.3
mA/cm2) for 180 min.
The control assay consisted only in agitating the COS in the electrolytic cell
without imposing
any current density. The yields of PAHs degradation were obtained by
subtracting the residual
PAHs concentration from the initial value recorded in COS and the resulting
operation was
divided by the same initial concentration of PAHs. A total PAHs concentration
of 476 mg/L was
measured in the initial solution, compared to 418 mg/L recorded in the control
assay, which
corresponded to an abatement of 13.2% of PAHs. The decrease in PAHs
concentration during the
control assay was probably attributed to the volatilization of the fraction of
the molecular organic
while agitating the solution. For instance, some compounds such as, PYR, FLE,
MEN were more
sensitive to the volatilization than CAN, NAP and CHR. While the current
density was imposed
31
CA 02632788 2008-05-30
the degradation of PAHs increased from 72 to 82%. Considering the possible
volatilization of
some organic compounds, the real contribution of electro-oxidation for PAHs
degradation can be
obtained by subtracting the yields of PAHs removal (while imposing current
density) from the
yields recorded without current density. Thus, in our experimental conditions,
the real yields of
PAHs degradation varied from 59 and 69%. The yields of PAHs degradation
increased with
current density unti19.23 mA/cm2 and then remained quite stable at 12.3
mA/cm2. Similar results
have been recorded by Yavuz and Kaporal (2006) while studying electrochemical
oxidation of
phenol by using ruthenium mixed metal oxide electrode. Phenol removal of 47,
67 and 78% were
obtained with current densities of 10, 15 and 20 mA/cm2, respectively for a
charge loading of
269 F/m3. Using a current density of 9.23 mA/cm2, the rates of PAHs
degradation (around 81 to
84%) were quite similar regardless of the number of aromatic rings (2-, 3-, 4-
, 5- and 6-rings
PAHs) of the compounds. Finally, the current density of 9.23 mA/cm2 was
retained for the next
step of the study. The power consumption was 78 kWh/m3 while the current
density of
9.23 mA/cm2 was held constant for a period of treatment of 180 min.
32
CA 02632788 2008-05-30
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CA 02632788 2008-05-30
Example 3: Effect of treatment time on electrochemical oxidation of PAHs
In view of reducing the power consumption and further optimizing the
electrochemical
oxidation of COS, additional experiments were conducted by testing different
retention times.
During these assays, the current density of 9.23 mA/cm2 was imposed. Two sets
of experiments
were carried out: the first one consisted to test relatively short retention
times (0, 10 and 20 min),
whereas the second one allowed testing long retention time periods (30, 60,
90, 150 and
180 min). The results are summarized in Table 5. During the first set of
experiments a total
PAHs concentration of 513 mg/L was measured in the initial solution. By
comparison, 474, 364
and 299 mg/L were recorded while imposing 0, 10 and 20 min, respectively. The
PAHs
degradation yield increased with the retention time. However, it is surprising
to see that, the
initial concentration of PAHs recorded in the untreated solution was different
to that measured at
t= 0 min (i = 0 mA/cm2) in the electrolytic cell. In fact, before each assay,
10 L of COS was
prepared in a 20 L cylindrical tank from which 1.5 L was withdrawn and
transferred into the
electrolytic cell. PAHs concentrations in the initial solution was measured
using a sample
obtained from the 20 L cylindrical tank, whereas the initial values measured
at t = 0 min was
obtained from a sample withdrawn in the electrolytic cell. Thus, this
discrepancy can be
attributed to two main factors. Firstly, the initial solution was not very
homogenous, and
secondly a fraction of PAHs could be deposited on the wall on the tank or on
the electrode
material, so that PAHs concentrations initially measured in both tanks
(cylindrical tank and
electrolytic cell) were different. It was the reason why, at the start of each
set of experiment
(before imposing the current density), a sample of COS (untreated sample) was
withdraw from
the cell and analyzed. During the 2"d set of experiment, a total concentration
of 525 mg PAHs/L
was measured in the untreated solution. The application of electro-oxidation
process allowed
reducing PAHs content and the residual concentration varied from 88.5 to 143
mg/L and
contributed to removal of about 74 to 83% of PAHs depending on the retention
time imposed.
34
CA 02632788 2008-05-30
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CA 02632788 2008-05-30
Figure 6 shows the changes in PAHs degradation yield as a function of charge
loading.
Two different regions could be distinguished. When the charge loading is below
1 A.h/L, the
yield of PAHs degradation increased linearly with charge loading. Beyond 1
A.h/L, the rate of
PAHs degradation remained quite stable. These results are consistent with
those obtained by
Chen and Chen (2006) while oxidizing orange II dye molecular on titanium (Ti)
recovered with
boron-doped diamond (BBD) electrode. The anodic oxidation of pollutant occurs
heterogeneously. First, organic pollutants must be transported toward the
anode electrode
surface, and then be oxidized there. The organic pollutant degradation may be
subjected either to
current control or mass transfer control. In fact, at the start of the
electrolysis, the PAHs
concentration was relatively high, and accordingly the PAHs reduction rate was
subjected to
current control. As the PAHs concentration was lowered to a certain level, the
PAHs reduction
rate was subjected to the mass transfer control (Panizza et al. 2001). In that
case, only a fraction
of current intensity (or charge loading) supplied was used to oxidize
pollutants, while the
remaining charge loading was wasted for generation of oxygen. It was the
reason for which the
yields of PAHs degradation remained stable in spite of high charge loading
applied. Figure 6
presents also the change in energy consumption as a function of charge
loading. The energy
consumption varied in a linear fashion between 0.0 and 6.0 A.h/L, from 0.0 to
78 kWh/m3. Since
the maximum increase in PAHs reduction rate was reached between 1.0 and 3.0
A.h/L, the
energy consumption could be reduced by curtailing the charge loading at 3.0
A.h/L. Indeed, a
charge loading of 3.0 A.h/L was selected (rather than 1.0 A.h/L) to further
oxidize by-products
resulting from PAHs oxidation and, render the treated-solution less toxic. A
charge loaded of
3.0 A.h/L corresponded to a period of treatment time of 90 min and the energy
consumption was
reduced to 41 kWh/m3 (rather than 78 kWh/m3) as expected.
CA 02632788 2008-05-30
90 90
80 80
70 70
60 60
50 50
O m
40 40
a G
30 30
20 -C~ PAH degradation 20
--*-Energy consumption
l - 10
0 0
0 1 2 3 4 5 6
Charge loading (A.h/L)
FIG. 6. Effect of charge loading on the yields of PAHs degradation and on
energy
consumption (current density = 9.23 mA/cm2, without initial pH adjustment
(pHi = 6.0), [Na2SO4] = 0 mg/L, T = 21 C)
Example 4: Effect of initial pH on electrochemical oxidation of PAHs
In order to know if the electrolysis cell could work well in oxidizing PAHs in
a wide pH
range, the removal efficiency at four different initial pH values (2.0, 4.0,
7.0, and 9.0) was
investigated. Initial pH of the solution was adjusted using sulfuric acid
(H2SO4, 5 mol/L) and
sodium hydroxide (NaOH, 2 mol/L). In addition, a control assay was carried out
without pH
adjustment (original pH was around 6.0). During these assays, the current
density was maintained
at 9.23 mA/cm2 and a retention time of 90 min was imposed. The results are
shown in Table 6. It
was found that COS having an initial pH closed to the neutral value (pH 6.0
and 7.0) was more
favorable for PAHs reduction (PAHs removal of 81 and 84% were recorded,
respectively). This
is consistent with the results of Yavuz and Kaporal (2006) while oxidizing
phenol using
37
CA 02632788 2008-05-30
ruthenium mixed metal oxide electrode. They reported that electro-oxidation
without initial pH
adjustment (initial pH around 7.0) was more effective in removing phenol,
compared to pH 3.0
and pH 11Ø However, one study showed that the pH effect is not significant
while oxidizing
orange II dye on Ti/BDD anode at a current density imposed of 200 mA/rn2 (Chen
and Chen
2006). The COD reduction (nearly 100%) at pH 1.2 was comparable with that at
pH 12Ø
Another study showed that the current efficiency increased from 3.0 to 13% as
pH increased
from 2.0 to 11.0 while oxidizing aniline on Pb02 anode (Kirk et al. 1985).
PAHs electro-
oxidation rate variations with pH recorded in the present study are
contradictory with those
obtained by the authors mentioned above. This may be associated with the
differences in
properties of chemical compounds tested and the anodic electrode used.
Finally, as the highest
PAHs removal yield (84%) recorded at pH 7.0 was quite similar to that measured
(81 %) without
pH adjustment (original pH 6.0), it was not necessary to modify the initial pH
before treatment.
38
CA 02632788 2008-05-30
TABLE 6. PAHs concentrations (mg/L) before and after treatment by imposing
different initial pH values*
PAHs Initial Final solution
solution
without pH with pH adjustment
adjustment
pH 6.0 pH 2.0 pH 4.0 pH 7.0 pH 9.0
NAP 72.4 11.7 14.1 13.0 10.9 16.7
MEN 69.0 10.2 11.3 9.70 7.88 13.7
ACN 3.25 0.75 0.78 0.76 0.59 0.83
ACA 75.3 11.2 11.9 11.9 9.19 14.32
FLU 50.1 10.8 11.7 10.5 9.48 12.0
PHE 113 25.8 22.4 24.6 17.3 22.7
ANT 13.3 2.37 2.36 2.25 2.06 2.91
FLE 55.1 10.6 13.4 10.9 8.34 12.5
PYR 35.4 7.15 9.49 7.65 6.50 8.81
BAA 7.94 1.54 1.26 1.29 1.41 2.24
CHR 9.70 2.21 2.35 2.09 2.34 2.92
BJK 4.94 1.44 1.24 1.41 0.58 1.55
BAP 1.56 0.24 0.26 0.26 0.19 0.27
DAN 1.00 0.21 0.25 0.24 0.23 0.34
INP 0.30 0.04 0.06 0.05 0.04 0.07
BPR 0.30 0.06 0.08 0.05 0.06 0.06
E PAHs (mg/L) 513 96.3 103 96.7 77.1 112
Removal (%) - 80.5 78.4 80.2 83.6 75.7
* Operating conditions: current density = 9.23 mA/cmZ, treatment time = 90
min, [NaZSO4] _
O mg/L, T = 21 C.
39
CA 02632788 2008-05-30
Example 5: Effect of supporting electrolyte on electrochemical oxidation of
PAHs
The addition of an electrolyte in solution during electrolysis can influence
the treatment
since it modifies the conductivity of the solution and facilitates the passage
of the electrical
current. Thus, various concentrations of sodium sulfate (NaZSO4 used as
electrolyte) were added
to the system and changes in PAHs reduction rate were noted. The current
density of
9.23 mA/cm 2 was held constant over the retention time of 90 min without
initial pH adjustment.
Table 7 represents the PAHs reduction yields with increasing concentration of
Na2SO4. The
PAHs degradation yields (80 to 83%) were quite similar regardless of
supporting electrolyte
concentration imposed. There was not a significant effect of electrolyte
concentration on the
oxidation efficiency in the investigated range of 500 to 4,000 mg Na2SO4/L.
This is consistent
with the results of Chen and Chen (2006) while oxidizing orange II dye
synthetic solution. The
same trend has also been recorded by Fernandes et al. (2004). However, it can
be interesting to
add a certain quantity of electrolyte in order to reduce the power consumption
and consequently,
to reduce the cost related to the electrochemical treatment. For instance, for
the same oxidation
efficiency of 80% recorded, the treatment cost (including only, energy and
electrolyte cost) was
estimated to 1.35 US$/m3 while adding 500 mg Na2SO4/L in COS, compared to 2.52
US$/m3
recorded without any addition of supporting electrolyte.
CA 02632788 2008-05-30
TABLE 7. PAHs concentrations (mg/L) before and after treatment using different
concentrations of supporting electrolyte (Na2SO4)*
PAHs Initial Na2SO4 concentration (mg/L)
0 500 1,000 2,000 3,000 4,000
NAP 75.0 9.57 9.89 9.88 9.87 10.8 10.7
MEN 72.8 9.21 10.9 10.8 10.1 10.8 9.85
ACN 3.31 0.56 0.64 0.75 0.52 0.60 0.58
ACA 70.2 11.7 11.8 11.1 11.7 11.2 10.9
FLU 59.0 10.1 10.8 10.9 9.7 9.7 8.86
PHE 133 26.3 31.0 29.8 27.1 26.1 25.6
ANT 12.3 2.11 2.28 2.50 2.23 2.04 2.17
FLE 60.2 12.4 12.5 12.4 12.0 11.8 10.3
PYR 41.8 8.17 8.14 8.44 8.08 8.18 7.96
BAA 8.48 1.79 1.59 1.83 1.94 1.82 1.46
CHR 9.42 2.07 2.01 2.11 1.93 1.88 1.52
BJK 5.04 1.15 1.25 1.26 1.16 1.10 0.96
BAP 1.83 0.41 0.36 0.43 0.47 0.43 0.33
DAN 0.20 0.08 0.05 0.04 0.04 0.04 0.05
INP 0.99 0.21 0.29 0.19 0.25 0.28 0.15
BPR 0.61 0.08 0.09 0.08 0.07 0.06 0.07
F. PAHs (mg/L) 554 95.9 104 102 97.1 96.7 91.5
Removal (%) 80.3 80.0 80.3 81.3 81.3 83.1
Cost ($/m3) 2.52 1.32 1.34 1.39 1.66 1.90
* Operating conditions: current density = 9.23 mA/cm2, treatment time = 90
min, without initial pH
adjustment (pHi = 6.0), T = 21 C.
41
CA 02632788 2008-05-30
Example 6: Effect of initial PAHs concentration on electrochemical oxidation
of PAHs
The effect of initial PAHs concentration was investigated while preparing
different
synthetic COS by using creosote/surfactant ratios of 1.0, 2.0, 3.0, and 5.0
(w/w). The surfactant
concentration was held constant at 1.0 g/L, whereas the concentration of
creosote varied from 1.0
to 5.0 g/L (Table 8). Initial PAHs concentrations varied from 274 to 538 mg/L.
At the end of the
treatment, residual PAHs concentrations recorded ranged between 65 to 108
mg/L. Irrespective
of the initial PAHs concentration, the PAHs removal yield was quite similar
with 78 to 80% of
PAHs degradation. There was no effect of initial PAHs concentration on the
oxidation efficiency
in the investigated range 274 to 538 mg PAHs/L.
42
CA 02632788 2008-05-30
TABLE 8. PAHs concentrations (mg/L) before and after treatment at different
initial
PAHs concentrations*
PAHs Initial solution Final solution
Creosote/surfactant ratio (w/w) Creosote/surfactant ratio (w/w)
5.0 3.0 2.0 1.0 5.0 3.0 2.0 1.0
NAP 69.2 66.4 52.1 35.3 12.7 12.0 9.10 7.90
MEN 61.1 64.2 49.2 33.7 11.7 11.0 8.38 6.10
ACN 3.88 3.50 2.60 1.67 0.83 0.76 0.72 0.41
ACA 72.9 59.1 43.2 40.3 13.1 14.1 12.4 5.92
FLU 61.9 43.5 39.5 33.2 14.2 11.0 9.51 7.83
PHE 127 103 97.5 52.6 23.4 20.6 17.5 15.6
ANT 12.4 10.5 10.4 9.37 2.92 2.45 2.04 2.16
FLE 53.3 43.9 41.7 24.5 12.6 11.7 9.69 7.93
PYR 44.4 41.5 37.3 25.6 9.71 8.74 7.54 6.33
BAA 10.1 9.71 8.60 5.95 1.96 1.86 1.73 1.71
CHR 11.7 14.1 12.0 6.18 2.43 2.63 2.08 1.51
BJK 6.53 6.44 5.93 3.82 1.67 1.22 1.55 0.93
BAP 2.50 3.83 2.85 1.55 0.58 0.73 0.66 0.30
DAN 0.26 0.29 0.24 0.16 0.05 0.06 0.04 0.03
INP 0.66 0.52 0.53 0.28 0.07 0.09 0.06 0.03
BPR 0.51 0.38 0.38 0.18 0.06 0.04 0.04 0.03
F, PAHs (mg/L) 538 471 404 274 108 98.7 83.0 64.0
Removal (%) - - - - 80.1 80.1 80.0 78.0
* Operating conditions: current density = 9.23 mA/cm2, treatment time = 90
min, without initial pH
adjustment (pHi = 6.0), [Na2SO4] = 500 mg/L, T = 21 C.
43
CA 02632788 2008-05-30
Example 7: Effect of temperature on electrochemical oxidation of PAHs
The effect of the temperature on PAHs degradation was examined by controlling
the
temperature of the solution in a water bath. Figure 7 shows residual PAHs
concentration of
different number of aromatic rings (2-, 3- and 4-ring PAHs) at different
temperatures (4, 21 and
35 C). These results compare the untreated-solution (initial solution
maintained at the desired
temperature without current imposition) with electro-oxidation of solution
(treated-solution).
Firstly, considering the untreated-solution subjected but maintained only, at
different
temperatures, it can be seen that residual 2-ring PAHs concentrations increase
slightly while
increasing the temperature from 4 to 21 C. The same trend could be observed
for 4-ring and 3-
ring PAHs. The temperature of 21 C enhanced PAHs solubilization. However,
while maintaining
the temperature at 35 C, residual PAHs (2-, 3- and 4-rings) concentrations
decreased compared
to that recorded at 4 C or at 21 C. For instance, at 21 C, the 2-rings PAHs
concentration
measured was 26.0 mg/L. When the temperature increased to 35 C, the residual 2-
rings PAHs
concentration was lowered to 14.9 (42.9% 2-rings PAHs reduction). It is
believed that, from a
certain level of temperature, the heat induced a loss of a fraction of PAHs
either by volatilization
or by PAHs deposition on the wall of the electrolytic cell so that PAHs
concentrations in solution
were reduced. Considering now the effectiveness of electro-oxidation process
at different
temperatures imposed, it can be seen that about 50% of PAHs was oxidized at 4
C. However, the
yields of PAHs removal increased to around 80% while increasing the
temperature either at 21 or
35 C. The increase of the temperature accelerates the electrochemical
decomposition of PAHs.
These results were consistent with several works mentioned in the literature
(Sharifian
and Kirk 1986; Tahar and Savall 1998; Chen and Chen 2006; Yavuz and Kaporal
2006). Since
the temperature of the solution naturally (without temperature control)
increased from about 20
to 25 C during electrolysis, it was not necessary to adjust the temperature to
have its beneficial
effect on PAHs degradation.
44
CA 02632788 2008-05-30
250
^ 4oC (initial solution)
^ 21 oC (initial solution)
200
35oC (initial solution)
4oC (final solution)
B 21oC (final solution)
150 o islut'on
35 C(f a o~ )
L
=+
O
d
V
100 -
0
2-ring PAHs 3-ring PAHs 4-ring PAHs
FIG. 7. Effect of temperature on the residual PAHs concentrations (current
density =
9.23 mA/cm2, treatment time = 90 min, without initial pH adjustment (pHi =
6.0), (Na2SO4] = 500 mg/L)
CA 02632788 2008-05-30
Example 8: Effectiveness and reproducibility of electro-oxidation
performance in treating COS
According to the results mentioned above, the electrolytic cell operated at
current density
of 9.23 mA/cm2 through 90 min of treatment in the presence of 500 mg/L but
without pH and
temperature adjustment gave the best performance of electro-oxidation of COS.
It was now
important to determine whether the results of these tests are reproducible or
not. For that, the
optimal assay (determined in terms of effectiveness and cost) was repeated in
triplicate to verify
the effectiveness and reproducibility of electro-oxidation performance in
treating COS.
Degradation of PAHs
Table 9 compares the untreated and treated-solutions by electro-oxidation. An
average
value of total PAHs concentration of 462 5 mg/L were measured in the initial
solution. It was
found that PHE (77.7 0.5 mg/L), ACA (66.5 0.1 mg/L), NAP (65.3 0.3 mg/L)
and MEN
(62.2 0.7 mg/L) were present in the highest concentrations (2 to 3-rings
PAHs). In contrast, the
compounds having 5 and 6-rings PAHs were represented in the lowest
concentrations: INP
(0.79 0.00 mg/L), DAN (0.15 0.04 mg/L) and BPR (0.48 0.01 mg/L). By
comparison, the
application of electrochemical treatment reduced the total concentration of
PAHs to an average
value of 105 2 mg/L. The PAHs removal yield had a mean value of 80.1 % with
a standard
deviation of only 0.2, which means that it can be considered as constant with
0.3% accuracy. The
compounds initially represented in the highest concentrations in untreated-
solution were
effectively oxidized. The residual concentrations of these PAHs were as
follows: PHE (17.4
0.4 mg/L), ACA (16.9 0.5 mg/L), NAP (14.4 0.2 mg/L) and MEN (11.5 0.4
mg/L). It worth
noting that these residual concentrations were obtained with a percentage of
accuracy inferior to
4.0%, consequently, they can be considered as constant. It corresponded to
PAHs degradation
rates of 78, 75, 78 and 81 %, respectively.
46
CA 02632788 2008-05-30
TABLE 9. PAHs concentrations (mg/L) before and after treatment in optimal
conditions*
PAHs Solution Removal
Initial Final (%)
NAP 65.3 0.3 14.4 0.2 77.9
MEN 62.2 0.7 11.5 0.4 81.4
ACN 3.18 0.02 0.81 0.04 74.6
ACA 66.5 0.1 16.9 0.5 74.6
FLU 49.9 1.0 12.8 0.3 74.4
PHE 77.7 0.5 17.4 0.4 77.6
ANT 16.5 0.1 3.28 0.05 80.1
FLE 50.8 1.0 11.4 0.1 77.5
PYR 39.4 0.6 10.4 0.2 73.5
BAA 11.4 0.4 2.20 0.07 80.8
CHR 10.2 0.4 2.38 0.05 76.6
BJK 5.12 0.13 1.01 0.00 80.2
BAP 1.94 0.05 0.40 0.01 79.2
DAN 0.15 0.04 0.01 0.00 93.3
INP 0.79 0.00 0.07 0.00 91.1
BPR 0.48 0.01 0.05 0.00 90.2
E PAHs 462 5 105 2 80.1 0.2
* Operating conditions: current density = 9.23 mA/cm2, treatment time = 90
min, without initial pH
adjustment (pHi = 6.0), [Na2SO4] = 500 mg/L, T = 21 C.
47
CA 02632788 2008-05-30
Organics removal
In addition to PAHs measurements, other parameters such as O&G, Clo-C50, COD
and
TOC related to the organics were also measured in the initial and treated-
solution. The results are
summarized in Table 10. The residual O&G and CIo-Cso concentrations recorded
at the end of
the treatment were 290 mg/L and 27 mg/L, respectively, compared to 940 mg
O&G/L and
170 mg O&G/L measured in the initial solution. A yield of 69% of O&G removal
was recorded,
whereas 84% of C10-C50 was removed.
On the other hand, reduction in COD and TOC were 62% and 27%, respectively.
The
residual concentration COD and TOC recorded at the end of electro-oxidation
were 809 mg
DCO/L and 174 mg TOC/L. By comparison, 2,102 mg/L and 237 mg/L were measured
respectively in the initial solution. The relatively low yield of TOC removal
(27%) compared to
62% of COD removal, indicated that only a small fraction of PAHs was
completely oxidized into
water and carbon dioxide, the majority of the pollutants being transformed
into small molecules
that reduce the oxygen demand in the treated-solution. In fact, the
electrolytic cell broke the
double bonds producing smaller molecules. It is worth noting that, during
electrolysis, organic
pollutants can be subjected to two different paths in anodic oxidation:
electrochemical
conversion or electrochemical combustion (Comninellis 1992; Grimm et al. 1998;
Drogui et al.
2001; Drogui et al. 2007). Electrochemical conversion only transforms the non-
biodegradable
organic pollutants into biodegradable compounds, whereas electrochemical
combustion yields
water and carbon dioxide and no further treatment is then required. In the
present study,
electrochemical conversion was the predominant reaction.
48
CA 02632788 2008-05-30
TABLE 10. Concentrations of parameters related to the organics and toxicity
measurements before and after treatment in optimal conditions*
Parameters Solution Removal
Initial Final (%)
Organics
O&G (mg/L) 940 290 69.2
(Cio--Cso) (mg/L) 170 27 84.1
COD (mg/L) 2,102 809 61.5
TOC (mg/L) 237 174 26.6
Toxicity
Daphnia magna test (TU) 4,762 453 90.5
Vibriofischeri test (Microtox) (TU) 1,000 200 80.0
* Operating conditions: current density = 9.23 mA/cm2, treatment time = 90
min, without initial pH
adjustment (pH; = 6.0), [NazSO4] = 500 mg/L, T= 21 C.
Toxicity reduction
Microtox and Daphnia bioassay tests were carried out to estimate the toxic
effect of the
initial and treated solutions under optimum experimental conditions. The
Microtox test used the
luminescent marine bacterium (Vibrio fisheri) and the toxicity results effects
were monitored as
the average percentage of light emission inhibition. The Daphnia test
consisted in determining
the lethal concentration for which at least 50% of mortality of crustacean
Daphnia magna is
observed after 48 h exposure to the contaminant. The results are given in
toxicity unit (TU) and
are summarized in Table 10. The comparison of the results shows a reduction of
the toxicity
while applying electro-oxidation treatment. Thus, relatively high toxicity of
4,762 TU was
measured for crustacean Daphnia and 1,000 TU was recorded for luminescent
bacterium Vibrio
fisheri in the initial solution. By comparison, only 453 TU and 200 TU were
recorded after
treatment, respectively. It corresponded to 91 % of toxicity reduction on
crustacean Daphnia,
whereas 80% of toxicity reduction was accomplished on luminescent bacterium V.
fisheri. In
fact, the electro-oxidation process breaks the double bonds of PAHs producing
smaller molecules
which are less toxics. For instance, the electrolysis of pyrene-containing
synthetic solution is
49
CA 02632788 2008-05-30
transform into furanone compounds which are probably less toxic than the
initial pyrene
compound.
CA 02632788 2008-05-30
Example 9: Selection of electrolytic cell configuration and anode material
Initial characteristics of the COE are given in Table 11. 16 PAHs were
investigated in the
COE and were comprised of different number of aromatic rings (2-, 3-, 4-, 5-
and 6-rings PAHs).
From the Table 11, it can be seen that 3-ring PAHs (ACA, PHE, FLU, ANT, and
ACN) were
present in the highest concentration with a sum of 104 mg/L, followed by 4-
rings PAHs (FLE,
PYR, BAA and CHR) with a total concentration of 91 mg/L, and 2-rings-PAHs (NAP
and MEN)
with a sum of 41 mg/L. The lowest concentration of PAHs in COE were recorded
for 5-rings
PAHs (BJK, BAP and DAN) and for 6-rings PAHs (INP and BPR) with total
concentrations of
10.1 and 0.9 mg/L, respectively. Effectiveness of electro-oxidation process in
treating COE was
evaluated by measuring the residual 16 PAHs concentrations.
The primary objective of the preliminary screening tests was to verify the
efficacy of
PAHs degradation in COE. The assays were carried out using electrolytic cells
made up of either
Ti/Ir02 or Ti/Sn02 anode electrodes at current densities of 9.0 mA/cm2 and 12
mA/cm2 for 90
min. Table 12 presents initial and final conditions of each test as well as
PAHs degradation rates
obtained during treatment using different electrolytic reactors, C1, C2 and
C3. The initial pH was
around 6.0, whereas at the end of the treatment the values varied from 6.9 to
7.8. The power
consumption has been evaluated between 3.09 and 9.50 kWh/m3, and the highest
consumption
was obtained for CI (6.14 and 9.50 kWh/m3) comprising of cylindrical
electrodes. This was
mainly due to higher current intensities imposed to reach the desired current
densities with regard
to high surface area of cylindrical anode in the C1. For instance, for the
same current density of
9.0 mA/cm2 and the same nature of electrode of Ti/Ir02 imposed (comparison
between C1 and
C2), the current intensities required were 2.4 A and 1.2 A, respectively,
whereas the average
voltage was around 7.1 or 7.4 using either the C, or the C2. However,
considering the energy
consumption, it can be seen that the electrical energy (6.14 kWh/m3) using C,
was approximately
two times higher than that (3.09 kWh/m3) recorded with C2. This confirms that
the parameter that
influenced the energy consumption during assays using C1 and C2 is the current
intensity.
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TABLE 11. Characterization of the creosote oily effluent
Parameters Means values and
standard deviation
pH 6.0 0.1
Conductivity ( S/cm) 322 8
POR (mV) 213 8
Temperature 20 I
PAHs (mg/L) Aromatic rings
Naphthalene (NAP) 2 21.8 3.8
2-Methyl-naphtalene (MEN) 2 18.8 3.4
Acenaphtylene (ACN) 3 2.10 0.5
Acenaphtene (ACA) 3 43.6 17.5
Fluorene (FLU) 3 18.4 2.0
Phenanthrene (PHE) 3 28.3 5.8
Anthracene (ANT) 3 11.4 3.9
Fluoranthene (FLE) 4 42.7 19.8
Pyrene (PYR) 4 30.8 10.9
Benzo(a)anthracene (BAA) 4 9.1 3.8
Chrysene (CHR) 4 8.7 3.2
Benzo(b,j,k)fluoranthene (BJK) 5 5.8 1.9
Benzo(a)pyrene (BAP) 5 3.5 1.5
Dibenzo(a,h)anthracene (DAN) 5 0.8 0.3
Indeno(1,2,3-c,d)pyrene (INP) 6 0.3 0.2
Benzo(ghi)perylene (BPR) 6 0.6 0.3
E PAHs 247 65
52
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TABLE 12. Treatment of creosote oily effluent using different electrolytic
cells
Parameters Electrolytic cells
Ci C2 C3
Anodic current density (mA/cm2) 9 12 9 12 9 12
Current intensity (A) 2.4 3.2 1.2 1.6 1.2 1.6
Anode electrode Ti/IrOz Ti/IrOz Ti/IrOz Ti/IrO2 Ti/Sn02 Ti/Sn02
Cathode electrode Ti Ti Ti Ti Ti Ti
Geometric form concentric concentric circular circular circular circular
Recycling rate (L/min) 3.6 3.6 3.6 3.6 3.6 3.6
Treatment time (min) 90 90 90 90 90 90
Average voltage (V) 7.4 9.5 7.1 9.7 9.8 10.5
Initial pH 6.0 6.0 6.0 6.0 6.0 6.0
Final pH 6.9 7.1 7.8 7.3 7.3 7.5
Energy cons. (kWh/m3) 6.14 9.50 3.09 5.54 4.33 6.00
Energy cost ($/m3) 0.37 0.57 0.19 0.33 0.26 0.36
Electrolyte cost ($/m3) 0.15 0.15 0.14 0.14 0.14 0.14
Total cost ($/m3) 0.52 0.72 0.33 0.48 0.40 0.50
E PAHs (before treatment) 146 140 146 146 155 155
E PAHs (after treatment) 43.0 33.4 27.9 28.0 44.5 32.2
Removal (%) 67.3 73.5 79.8 78.0 74.8 82.4
53
CA 02632788 2008-05-30
The efficacy of the electro-oxidation process in terms of PAHs removal from
COE using
different electrolytic cells was in the following order: C3 (75 to 82%) > C2
(78 and 80%) > C1 (67
to 74%). In fact, the electrolytic cells (C2 and C3) including circular
electrodes were more
effective than the other one comprised of cylindrical electrodes. Considering
both electrolytic
cells (C1 and C2) for which the same material anode electrode (i.e., Ti/IrO2)
was used, it can be
seen that the PAHs removal yields (80 and 78%, respectively) using the C2 were
better than those
recorded (67 and 74%, respectively) using the C, while imposing respectively
9.0 and
12 mA/cm2 of current densities. This can be attributed to the different
hydrodynamic conditions
(or mass transfer) imposed inside the cells. It is well-known that,
hydrodynamic conditions
insides the reactors are greatly linked to the cell configuration or cell
design. Indeed, in the direct
anodic oxidation, the oxidation of pollutants occurs heterogeneously.
Pollutant must be
transported to the electrode surface first, and then be oxidized there owing
to hydroxyl radical
formation (OH ) (Grimm et al. 1998; Drogui et al. 2001; Martinez-Huitle and
Ferro 2006). In the
C2, the liquid arrived rapidly and directly on the anode material and passed
through the cathode
material, followed by the circulation through a second anode and cathode
electrodes. By
comparison, in the C1 comprising of cylindrical electrodes, the liquid firstly
arrived in the centre
of the cell inside a perforated cylindrical weir, before being distributing
gradually and
successively toward the anode and cathode electrodes. From the hydrodynamic
descriptions
(mentioned above), it believed that, the mass transfer between electrode and
electrolyte was
better inside the C2, resulting in an increase in PAHs oxidation rates by
comparison to the CI. On
the other hand, in view of putting into evidence the influence of anode
electrode material on
PAHs removal from COE, additional experiments were conducted by using Ti/SnO2
circular
electrodes (C3). The hydrodynamic conditions and the configuration of the C2
and C3 were the
same; all parameters were kept constant with the exception of the anode
material. For the
relatively high current density of 12 mA/cm2 imposed, the highest yield of
PAHs degradation
(82.4%) was recorded using Ti/Sn02 anode electrode installed in the C3 in
comparison to 78%
PAHs removal obtained with Ti/IrO2 anode using the C2 for the same current
density imposed.
As reported by Comninellis and Nerini (1995), Comninellis (1992) and Feng et
al. (2003), tin
oxide is one of the noble metal oxides having a better performance for organic
compounds
degradation in comparison to traditional electrodes (Pt, IrOZ and Ru02). This
is attributed to the
54
CA 02632788 2008-05-30
highly crystalline nature of tin oxide, which catalyzes the reaction of
electrochemical oxidation
(Comninellis 1992). Finally, the C3 including circular Ti/Sn02 anode was
selected for the next
experiments.
CA 02632788 2008-05-30
Example 10: Influence of applied current density on PAHs degradation using
Ti/Sn02 circular mesh anode
In order to determine economical and better conditions for PAHs degradation in
COE,
several batch electro-oxidation assays were performed using the C3 containing
circular Ti/SnO2
anode electrode. Majors operating conditions such as current density,
retention time, recycling
flow rate and oxygen flow rate in the close loop were investigated.
One of the main factors affecting the electrochemical oxidation efficiency is
the current
density. Current densities were obtained by dividing each current by the
corresponding total
anode area. The effect of current density on PAHs degradation is shown in
Table 13. This table
indicates the initial untreated COE and residual PAHs concentrations after
treatment while
imposing different current densities (4.0, 9.0, 12, 15 and 23 mA/cm2) for 90
min at a recycling
flow rate of 3.6 L/min. The residual PAHs concentrations recorded at the end
of the treatment
varied from 52 to 26 mg/L compared to 155 mg/L measured in untreated COE. PAHs
degradation increased with current density in the range of 4.0 to 15 mA/cm2.
The largest PAHs
oxidation was observed at 15.0 mA/cm2. However, when a current intensity of 23
mA/cm2 was
imposed, the PAHs removal slightly decreased. Indeed, the increase of current
intensity above
15.0 mA/cm2 further induced parasitic reactions such as water reduction,
leading to high amount
of oxygen bubbles (02) formation, which disturbed PAHs oxidation on anode
electrodes.
56
CA 02632788 2008-05-30
TABLE 13. PAHs concentration (mg/L) before and after treatment using
experimental
C3 (Ti/SnO2) operated at different current densities*
PAHs Control Current density (mA/cmz)
4.0 9.0 12 15 23
NAP 17.7 3.65 3.99 2.35 1.73 2.34
MEN 14.3 3.18 2.05 2.02 1.73 1.62
CAN 1.46 0.38 0.25 0.21 0.17 0.17
ACA 19.8 7.72 5.92 4.91 3.91 4.12
FLU 16.6 6.34 4.80 4.14 3.34 3.35
PHE 35.6 13.5 10.1 8.28 6.74 7.63
ANT 6.76 1.71 1.36 1.05 0.78 0.78
FLE 14.9 6.13 5.48 3.85 3.59 3.33
PYR 15.8 5.02 4.53 3.01 2.43 2.43
BAA 4.16 1.33 1.25 0.80 0.46 0.60
CHR 4.35 1.31 1.24 0.77 0.43 0.59
BJK 3.27 0.88 0.92 0.48 0.28 0.35
BAP 1.78 0.36 0.33 0.22 0.11 0.14
DAN 0.13 0.03 0.03 0.02 0.01 0.01
INP 0.50 0.14 0.12 0.07 0.06 0.05
BPR 0.31 0.09 0.08 0.05 0.04 0.03
E PAHs (mg/L) 155 51.7 42.4 32.2 25.8 27.5
Removal (%) - 70.5 74.8 82.4 86.9 86.2
* Operating conditions: treatment time = 90 min, recycling rate = 3.6 L/min.
57
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Example 11: Influence of reaction time on PAHs degradation using Ti/SnOZ
circular mesh anode
Figures 8, 9 and 10 show the results of electrolysis of COE for various
retention times (10
to 180 min). It can be observed that the pH of COE first increased and then
remained quite stable
around pH 6.8 (compared to the original value of 5.8) until the end of
experiment. These changes
can be justified in terms of anodic and cathodic processes that develop in the
cell. On the cathode
electrode, the main reaction is the water reduction which generates hydroxyl
ions and induces an
increase of the pH. On the anode electrode several reactions take place
simultaneously. The main
reaction is the oxidation of organic matter. Generally, the first stages in
electro-oxidation
processes are the formation of carboxylic acid in addition to proton formation
owing to water
oxidation. These acidic compounds compensate the cathodic hydroxyl ion
generation rate. The
cell potential decreases slightly during the electrolysis and then remains
constant (around
10.5 V). This fact could be explained in terms of the increase of the ionic
conductivity due to
water oxidation and reduction reactions that generate ions in solution. The
behavior of
electrochemical oxidation of PAHs is presented in Figure 9. PAHs removal
increase to 92% with
the reaction time elapsed 180 min. From Figure 10, it can be seen that the
decomposition of
PAHs followed first order kinetics. Therefore, the reaction rate constant "k"
could be calculated
from the slope value of the plot (t) versus -Ln(C/Co) of equation (8).
- Ln ~ = k.t (8)
0
Where "Co" is the initial concentration of PAHs, "C" the concentration of PAHs
at time t,
"t" the reaction time, and "k" is the first order reaction rate constants
(f'). As shows in Figure 10,
the first order decomposition reaction rate constant of PAHs by the
electrochemical oxidation
was 0.015 miri l. It is interesting to compare the constant rate of PAHs
degradation in COE with
those obtained under different experimental conditions. The constant rate of
organic degradation
has been determined by Kim et al (2003) while studying electrochemical
oxidation of polyvinyl
alcohol (PVA) using titanium coated with ruthenium oxide (Ti/Ru02). The
constant rate
decreased from 0.0162 miri I to 0.0021 miri 1 while increasing initial PVA
concentration from 33
58
CA 02632788 2008-05-30
to 2,400 mg/L. The smaller the initial PVA concentration, the faster it could
be removed from
solution by anodic oxidation. It can be seen that, the kinetic rate constant
determined in the
present study (0.015 miri ) was quite similar to that measured (0.0162 min")
by Kim et al.
(2003) while imposing the lowest concentration of 33 mg/L of PVA, although the
experimental
conditions were different. For instance, in the present study a current
density of 15 mA/cm2 was
imposed, whereas Kim et al. (2003) imposed a current density of 1.34 mA/cm2,
which was
times lower. For the same kinetic constant rate, high current density was
required in treating
CEO probably due to the fact that PAHs in COE was more difficult to oxidize
than PVA.
12 8
11 7
i.+
10 6
9 - 5
"C'-Cetl potential -4HpH
8 4
0 20 40 60 80 100 120 140 160 180 200
Time (min)
FIG. 8. Variation of cell potential and pH with the reaction time using the
electrochemical C3 during the recycling batch tests (operating conditions:
current density: 15 mA/cm2, recycling rate: 3.6 L/min)
59
CA 02632788 2008-05-30
1.0 ._........._..____..... _.... _...... ...._.... 300
250
0.8
o ..
~ 200
0.6
u ~ C
0 Normalized concentration 150
-O-Residual PAHs concentration
d 0.4 I - w
100
L y
z
0.2
0.0 0
0 20 40 60 80 100 120 140 160 180 200
Time (min)
FIG. 9. Variation of residual PAHs and yields of PAHs degradation with the
reaction
time using the electrochemical C3 during the recycling batch tests (operating
conditions: current density: 15 mA/cm2, recycling rate: 3.6 L/min)
CA 02632788 2008-05-30
3.0 ~ ~ _.. _._....._._..____.
2.5 = 3
2.0
y = 0.0151x V = R2 = 0.9614
V 1.5
=
1.0
0.5
.=0.0
0 20 40 60 80 100 120 140 160 180 200
Time (min)
FIG. 10. First-order relationship of PAHs degradation by electrochemical
oxidation
using the C3 during the recycling batch tests (operating conditions: current
density: 15 mA/cm2; recycling rate: 3.6 L/min)
61
CA 02632788 2008-05-30
Example 12: Influence of recycling flow rate on PAHs degradation using
Ti/Sn02 circular mesh anode
Due to Joule effect, the temperature of the liquid can increase dramatically
due to low
flow rates in the cell and excessive electricity consumption. Recirculating
waste could be
absolutely necessary for efficient treatment. Experiments were conducted at
constant current
density (15 mA/cm2) for different recycling flow rates (1.8, 2.7, 3.6, 5.4 and
7.3 L/min) during a
period of treatment of 90 min. Degradation efficiency increased slightly (from
81 to 85%) as
recycling flow rate increased from 1.8 to 5.4 L/min, as shown in Table 14. A
maximum for PAHs
degradation of 85% was observed at 5.4 L/min. Higher recycling flow rate
decreases the
thickness of the diffusion layer, which may results in a higher removal rate.
These results can be
compared to those obtained by Nagata et al. (2006) while treating different
effluents containing
endocrine disrupting chemicals (17(3-estradiol, biphenol, pentachlorophenol,
dichlorophenol,
etc.) using electro-oxidation process with a three-dimensional electrode
system. They observed
that removal efficiency increased from 60 to 90% as the recycling flow rate
increased from 0.1 to
0.8 L/min. However, in our experiment conditions, while increasing the
recirculation rate to
7.3 L/min, degradation efficiency decreased to 81%. It is worth noting that an
increase in the
recirculation rate is accompanied by higher velocity in the cell and shorter
residence times. For
instance, a linear velocity of 0.71 cm/s was imposed for 7.4 L/min compared to
0.55 cm/s
measured for 5.4 L/min. It is believed that from a certain level of the linear
velocity imposed, the
fluid did not sufficiently remain inside the cell, so that the degradation
efficiently decreased.
Thus, a recycling flow rate of 3.6 L/min was selected for the next step of the
experiments, as
PAHs degradation efficiency was quite similar to that at 5.4 L/min.
62
CA 02632788 2008-05-30
TABLE 14. PAHs concentration (mg/L) before and after treatment using
experimental
C3 (Ti/SnO2) operated at different recycling flow rates*
PAHs Control Recycling rates (L/min)
1.8 2.7 3.6 5.4 7.3
NAP 26.7 5.36 5.27 4.28 4.02 3.89
MEN 22.8 4.36 3.82 3.65 3.54 4.43
CAN 2.30 0.36 0.34 0.32 0.30 0.39
ACA 63.6 14.3 12.9 12.3 11.5 14.9
FLU 18.7 4.21 4.17 4.02 3.73 3.75
PHE 20.4 2.30 1.88 1.75 1.65 2.18
ANT 10.4 2.30 1.88 1.87 1.65 2.18
FLE 59.5 10.7 8.63 8.29 8.03 10.0
PYR 35.9 8.25 6.75 6.33 6.25 7.82
BAA 9.69 1.73 1.41 1.38 1.36 1.64
CHR 9.59 1.65 1.36 1.31 1.29 1.57
BJK 5.89 1.02 0.87 0.84 0.80 1.00
BAP 3.10 0.50 0.44 0.42 0.41 0.51
DAN 0.25 0.03 0.03 0.03 0.02 0.03
INP 0.70 0.15 0.14 0.14 0.15 0.23
BPR 0.50 0.12 0.11 0.10 0.09 0.12
E PAHs (mg/L) 290 57.3 50.0 47.0 44.8 54.7
Removal (%) 81.2 83.5 84.3 85.0 81.2
* Operating conditions: current density = 15 mA/cmZ, treatment time = 90 min.
63
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Example 13: Influence of injection of oxygen in a close loop on PAHs
degradation using Ti/Sn02 circular mesh anode
It is worth underling that, the results discussed above were obtained without
any oxygen
injection in the close loop. Then, some experiments have been carried out for
different oxygen
flow rates (5, 10 and 20 mL 02/min) injected in the close loop and compared
with a control assay
without 02 injection. The interest of continuously injecting oxygen in the
system was to
gradually saturate the liquid in oxygen and be able to further generate
radical species (OH ) or
oxidants (such as ozone, 03) capable of enhancing PAHs degradation. It has
been already
demonstrated by several authors that ozone could be formed during electrolysis
of water using
high oxygen overvoltage anodes (Foller and Tobias 1982; Wabner and Grambow
1985; Tatapudi
and Fenton 1993). The results are presented in Table 15. The initial PAHs
concentration
measured in the untreated COE was 264 mg/L. While injecting oxygen in the
close loop, residual
PAHs concentrations varied from 31.2 to 52.9 mg/L. By comparison, a residual
PAHs
concentration of 40.5 mg/L was recorded during the assay without Oz injection
(control assay). A
maximum for PAHs degradation efficiency (88%) was observed at 5 mL 02/min.
While the
oxygen flow rate increased to 10 mL/min, no significant effect was observed by
comparison with
the assay carried out without oxygen injection (83% of PAHs was removed).
However, for 20
mL 02/min imposed, a negative effect was recorded, PAHs degradation efficiency
decreased to
79%. This can be due to the fact that, high oxygen flow rates may favor
hydrophobic conditions
inside the cell, so that the reaction at the electrodes were hampered or
disturbed. As this
operating parameter had moderately significant effect, oxygen injection in the
close loop was not
pursued.
Finally, the best operating conditions retained for PAHs degradation in COE
were as
followed: the utilization of the C3 containing circular electrode comprised of
Ti/Sn02 anode
operated at a current density of 15 mA/cm2 through 90 min of treatment with a
recycling rate of
3.6 L/min in the presence of 500 mg Na2SO4/L (used as electrolyte support) but
without 02
injection the close loop.
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TABLE 15. PAHs concentration (mg/L) before and after treatment using
experimental
C3 (Ti/SnO2) operated at different oxygen flow rates*
PAHs Raw Control Oxygen flow rates (mL/min)
effluent (without 02 5 10 20
injection)
NAP 18.9 4.59 5.87 7.57 8.53
MEN 18.0 2.41 2.35 3.14 3.83
CAN 2.41 0.36 0.36 0.45 0.54
ACA 48.8 6.39 5.07 7.43 8.71
FLU 16.7 3.34 3.24 3.01 3.79
PHE 23.8 2.14 1.87 2.12 2.67
ANT 13.8 2.14 1.17 2.12 2.67
FLE 53.0 7.65 4.77 7.11 8.47
PYR 35.4 5.37 3.38 5.09 6.20
BAA 10.3 1.88 1.00 1.75 2.25
CHR 10.2 1.82 0.96 1.67 2.14
BJK 6.78 1.36 0.63 1.24 1.88
BAP 3.45 0.71 0.32 0.65 0.90
DAN 0.60 0.06 0.06 0.02 0.06
INP 0.84 0.17 0.07 0.18 0.21
BPR 0.63 0.11 0.05 0.12 0.10
E PAHs (mg/L) 264 40.5 31.2 43.7 52.9
Removal (%) 83.5 88.2 82.8 78.7
* Operating conditions: current density = 15 mA/cm2, treatment time = 90 min,
recycling rate =
3.6 L/min.
CA 02632788 2008-05-30
Example 14: Efficacy and reproducibility of batch electro-oxidation
treatment for PAHs degradation using Ti/Sn02 circular mesh anode
It was now important to determine whether the results of these tests are
reproducible or
not. For that, the optimal assay (determined in terms of effectiveness and
cost) was repeated in
triplicate to verify the effectiveness and reproducibility of electro-
oxidation performance in
treating COE.
The Table 16 compares the untreated and treated-effluents by electro-
oxidation. An
average value of total PAHs concentration of 292 24 mg/L was measured in
untreated effluent.
It was found that ACA (59.5 5.1 mg/L), FLE (55.0 3.1 mg/L), PYR (38.3
2.2 mg/L) and
PHE (24.5 3.6 mg/L) were present in the highest concentrations (3 to 4-rings
PAHs). In
contrast, the compounds having 5 and 6-rings PAHs were represented in the
lowest
concentrations: INP (0.42 0.21 mg/L), DAN (0.96 0.28 mg/L) and BPR (0.71
0.22 mg/L).
By comparison, the application of electrochemical oxidation reduced the total
concentration of
PAHs to an average value of 50.5 4.3 mg/L. The yield of PAHs removal had a
mean value of
81.6% with a standard deviation of 2.2, which means that it can be considered
as constant with
4.3% accuracy. The compounds initially represented in the highest
concentrations in untreated-
effluent were effectively oxidized. The residual concentrations of these PAHs
were as following:
ACA (9.10 0.39 mg/L), FLE (8.94 0.70 mg/L), PYR (6.78 0.52 mg/L) and PHE
(3.78 0.42 mg/L). It worth noting that these residual concentrations were
obtained with a
percentage of accuracy inferior to 4.0%, consequently, they can be considered
as constant. It
corresponded to PAHs degradation rates of 85, 84, 82 and 84%, respectively.
66
CA 02632788 2008-05-30
TABLE 16. PAHs concentration before and after treatment using experimental C3
(Ti/SnO2) and the optimal conditions*
PAHs Effluent Degradation (%)
Untreated Treated
NAP 23.5 0.8 4.86 0.20 79.3 0.4
MEN 20.5:L 0.5 3.26t0.12 84.0f0.8
CAN 2.46 f 0.18 0.39 t 0.03 84.3f 0.5
ACA 59.5 5.1 9.10f0.39 84.7t0.9
FLU 19.1 t 1.1 3.90 0.51 79.6 3.4
PHE 24.5 f 3.6 3.78 0.42 84.5 0.6
ANT 14.3t 1.7 2.89t0.74 79.8f4.5
FLE 55.0 f 3.1 8.94 0.70 83.7 t 1.4
PYR 38.3t2.2 6.78f0.52 82.2 2.3
BAA 11.4f 1.7 2.12 0.14 81.1t3.3
CHR 10.8f1.1 2.11f0.31 80.5 1.9
BJK 6.99 0.95 1.28f0.10 81.4 3.8
BAP 4.13 f 1.23 0.83 t 0.11 79.2 3.2
DAN 0.96f0.28 0.18t0.03 80.5f2.7
INP 0.42f0.21 0.07t0.02 83.0t2.9
BPR 0.71f0.22 0.16t0.05 77.7f0.7
E PAHs (mg/L) 293 t 24 50.5 f 4.3 -
Removal (%) - 81.6 2.2 -
* Operating conditions: current density = 15 mA/cm2, treatment time = 90 min,
recycling rate =
3.6 L/min, without oxygen injection.
67
CA 02632788 2008-05-30
Example 15: Combining successively batch and continuous electro-oxidation
treatment for PAHs degradation using Ti/SnO2 circular mesh anode
Three sets of experiments were performed to evaluate the performance of the
electro-
oxidation process while combining successively batch and continuous mode
operations. During
these assays, a constant current density of 15 mA/cm2 was imposed for various
inlet flow rates
(50, 75 and 100 mL/min). The experimental conditions are summarized in Table
17. For the first
set of experiments, the electrochemical system was previously maintained in
the recirculating
batch test (run A, 3.6 L/min of recycling flow rate) for 90 min, followed by
the continuous mode
operation (runs B to F) by imposing a constant inlet flow rate at 50 mL/min,
which corresponded
to 90 min of HRT. By comparison, during the second set of experiment (runs H
to K) 60 min of
HRT was imposed in continuous mode operation by imposing a constant inlet flow
of 75
mL/min, whereas the system was previously maintained in the recirculating
batch test (run G,
3.6 L/min of recycling flow rate) for 90 min. Similarly to the 1 St and 2nd
set of experiments, a
recirculating batch test (run L) was carried out prior to continuous mode
operation (runs M to 0)
during the third set of experiment where 45 min of retention time (100 mL/min
of inlet flow
rate). The interest of imposing recirculating batch tests (Runs A, G and L)
was to maintain
initially a steady state inside the cell prior to start the continuous run
tests. The results are
presented in Table 17. This table compares sum of PAHs concentration measured
in the inlet
solution versus those recorded in the outlet solution. As expected, the best
performance of the
electrolytic C3 operated in continuous mode was obtained while a HRT of 90 min
was imposed.
Residual PAHs concentration varied from 19.1 to 34.4 mg/L compared to 150 mg/L
of PAHs
continuously injected inside the electrochemical system. By comparison, while
decreasing HRT
(60 or 45 min), residual PAHs concentration increased rapidly and residual
concentrations up to
80 and 90 mg/L could be recorded in the outlet solution (compared to 176 mg/L
injected in the
system). Figure 11 represents the change in PAHs degradation with reaction
time for various
HRT. The values reported correspond to the values obtained after a period of
time equal at least
to three HRT (i.e. when the initial effluent electrolyzed in the recirculating
batch test was
completely replace by freshly effluent). The percentage of PAHs oxidized
remained in a steady
state (around 85%) for a long period of time (from 300 to 1,200 min), then
slightly decreased to
68
CA 02632788 2008-05-30
79% of total PAHs removal. The slight decrease of degradation efficiency cans
probably due the
formation of organic substances on the electrode surface that reduce its
electrode active surface.
Nagata et al. (2006) analyzed the electrode surface (Ti/Pt anode electrode)
before and after the
continuous electrochemical by using X-ray photoelectron spectrometry (XPS).
Before treatment,
a big Pt peak, a small oxygen d and carbon were observed, translating to the
fact that electrode
surface was comprised a clean Pt. However, after treatment a big carbon peak
was observed
instead of the Pt peak, meaning that the electrode surface was covered with
organic substances
that were formed during the treatment of organic-containing effluent. From
Figure 11, it can be
seen that PAHs degradation efficiency decrease rapidly using 60 min of HRT
(with a relatively
high slope). Degradation efficiency passed from 77% to 54% between 360 min and
1,080 min of
treatment period. In fact, the formation of organic substances on the
electrode surface increased
while HRT decreased to 60 min. Otherwise, while further decreasing HRT, the
percentage of
PAHs degradation was low, but it remained quite stable around 50%, meaning
that the process of
the formation of organic substances on the electrode surface decreased owing
to a relatively high
linear velocity of liquid. In all case, during continuous treatment, the
electrode surface can be
easily recovered with organics dependently on HRT imposed. This situation may
affect the
treatment performance in a long term experiment. To overcome this process, the
polarity
inversion during the treatment could be one of the easier and feasible
regeneration methods of the
electrode surface.
69
CA 02632788 2008-05-30
~
~.+ u
G ~ M ~t l~ 00 O o G 10 Vt V' O
=~ a ~ ~ N ~ N N M N M 00 N ~O 00 O~
bQ
a a - - - - - - - - - - r -
W C v
..,
E o O o p 00 oV O o O O 00 o O C) O
~ M N
ZC ~ M O 'Y O~ M
Cd O O~ ~ r
O + E .- .-~
y ..i
~ F
ei
O
0
~ i : o p p p o o p o p o O o ~., n n
o, o, c, rn c, ~o ~o ~ ~c o, v rr ~
4u õ ~.
~ O O a p i/'1 i!1 i!1 vl ~ p l~ l~ [~ [~ o
~i
O
~
i, ++ G
p o O O
O O
O
~r
++ ^
L ~O O O O O O ~O O O O O ~O O O O
3 M O O O O O M O O O O M O O O
O
L H ` ~/'~ ~fl Ul ~1 V~ V1 V'1 v"~ V1 Vl ~1 v~ ~!1 Vl ~
.--.--~ ,~
- - - - - - - - .-r - r.,
~ O ~ ~ tn rn N V~ V~ N V) tn v1 V1 V1
~ ~ ~ ~ ~ ~ =~ '~' ~ ~ ~ ~
=0000
~~.y F ~ ~ o ~ c .~ ~ ~ .~ ~ .~ ~ ~ ^ ro ~ ~ ~
~.y `~ ~ ~+ Y Y Y Y Y Y Y ~-+ Y ~+ Y
h=l
d 0 0 0 0 0 0 0 0 0 0 0 0
u U
O D U U U U U U U U U U U
0 ro)
k a
F~ W s . Q m U ~1 W w C7 x .~ ti ~C . a ~ Z O
CA 02632788 2008-05-30
1.00
0.80
U
U
0.60
a
'a 0.40 -
~
0
-4- HRT= 90 min
0.20 fHRT=60 min
j f HRT= 45 min
0.00
0 200 400 600 800 1000 1200 1400 1600
Time (min)
FIG. 11. Variation of normalized concentration with the reaction time using
the
electrochemical C3 during continuous mode operation at different HRT
(operating conditions: current density: 15 mA/cm2). Values reported after a
period of time equal to three HRT
71
CA 02632788 2008-05-30
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