Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IN SITU TREATMENT PROCESS TO REMOVE METAL
CONTAMINATION FROM GROUNDWATER
BACKGROUND OF THE INVENTION
[0001] Groundwater is an important water source.
In consideration of the purity of the water, there
are thresholds of acceptable tolerance for various
metal ions. Amounts of metal ions dissolved in water
that are above desirable or acceptable limits can be
considered contamination. Heavy metal ions are
particularly undesirable contaminants in many cases.
The present invention provides methods for creating
an unsatisfied demand for metal sorption in water-
bearing formations in such a way as to effectively
remove metal contaminants from groundwater passing
through the water-bearing formation.
[0002] A water-bearing formation, in many cases
referred to as an aquifer, typically is composed of
areas through which groundwater flows rapidly, as
well as bypassed areas through which water passes
more slowly. Water-bearing formations (aquifers) are
commonly bounded by relatively impermeable
formations, referred to as aquitards. Metal
contamination typically enters an aquifer and flows
through the most conductive portions, bypassing less
conductive areas and aquitards. Metal ions commonly
diffuse into the bypassed areas and aquitards, and
may sorb there. In any attempt to flush metal
contamination from an aquifer, metal is gradually
released into the main groundwater flow from bypassed
areas and adjoining aquitards. Clean water passing
through such a water-bearing formation can become
contaminated in this way over an indefinite period.
The resulting concentration of contaminants in the
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water-bearing formation may be small, but nonetheless
significant relative to health standards.
[0003] There are several sources of heavy metal
contamination. These include, but are not limited
to, drainage from mining districts, electrical and
electronics manufacturing processes, munitions
production and weapons laboratories, metal plating
processes, battery recycling, coal combustion and fly
ash disposal, petroleum refining, chemical production
and storage and the nuclear industry. The metal-
bearing fluids that carry contamination into a water-
bearing formation are commonly acidic.
[0004] It has been reported that in the United
States, acidic mine drainage affects more than 19,000
kilometers of rivers and streams. Some scientists
rate toxic mine drainage as the greatest water-
quality problem facing the western United States. It
is said that in Colorado alone, the effluent from
more than 7,000 abandoned mines contaminates more
than 2,500 km of streams. A significant, but
unknown, amount of acidic drainage has infiltrated
the subsurface, acidying drinking water aquifers
there, and contaminating them with metals.
[0005] Known methods for remediation of water
contaminated with metals include active remediation
methods, direct precipitation methods, reactive
barriers, and monitored natural attenuation methods.
[0006] Active remediation methods for groundwater
treatment involve pumping out contaminated water from
an aquifer and treatment of the contaminated water to
remove the metal contaminants (for example through
precipitation processes, sorbent processes, or
electrochemical processes). The treated water is, in
some cases, returned to the aquifer where it is drawn
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back toward the water production well. Such pumping
facilities require a long-term commitment and the
facilities and processes tend to be expensive. A
further problem is the disposal of the metal
contaminant that had been removed, because the
treatment process typically generates large
quantities of metal-contaminated waste.
[0007] One of the difficulties encountered in the
art with active remediation methods of metal
attenuation is that the methods do not effectively
address the problem of gradual metal release from the
water-bearing formation itself. For example, metal
contaminants may diffuse from bypassed low
conductivity sediment lenses into high conductivity
areas.
[0008] Water treated by active remediation is, in
some cases, pumped back into the aquifer prior to
pumping for use. If the problem of metal release
from the water-bearing formation itself is not
addressed, then metals may diffuse into the
previously-treated water, and may be thus rendered
again unsuitable for immediate use without further
treatment.
[0009] Another persistent problem with active
remediation is that the difficulty in removing a
certain amount of metal contaminant increases
significantly with the decrease in metal ion
concentration. As a result, it is significantly more
costly, in time and money, to treat a large volume of
slightly contaminated water than it is to treat a
small volume of highly contaminated water.
[0010] Direct precipitation methods for
groundwater treatment involve precipitation of the
metal contaminants within the water source (e.g.
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aquifer) to keep the metals out of the moving
groundwater. Such methods in many cases involve
converting the metals to sulfide or other insoluble
forms, and both biotic and abiotic approaches have
been utilized. Biotic approaches use de-toxifying
microorganisms to convert the metals to insoluble
granules; such approaches may involve introducing
sulfur compounds and stimulating sulfate-reducing
bacteria. Abiotic approaches use solution methods to
provide ligands and reaction conditions suitable for
forming insoluble precipitates. A drawback of this
type of approach is that they are able to precipitate
only the metal ions present in the aquifer at the
time of treatment. An additional difficulty of this
method is that the precipitated metals are subject to
re-dissolution (for example, by oxidation of sulfide
granules), allowing metal ions to once again
contaminate water in the aquifer.
[0011] Reactive barriers are constructed within a
trench dug across an aquifer. The barriers, through
which groundwater is allowed to pass, are designed to
create a zone of chemical or biological reaction
where metal contaminants are immobilized. Drawbacks
to reactive barriers include the expense of
constructing them, the difficulty in applying them to
areas where the contamination is widespread or not
limited to shallow depth, the possibility that water
flow will bypass them, the possibility that metal
concentrations will not be reduced to acceptable
levels, and uncertain long-term performance.
[0012] Monitored Natural Attenuation methods for
groundwater treatment use the naturally-present
composition, structure, and microbial content of the
aquifer and sediments to immobilize undesirable
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compounds, such as metal ions. Such methods may be
employed following periods of active remediation.
Such methods have the potential, where successful, to
significantly reduce costs. Such methods for
processing groundwater sources run into
administrative barriers such as extensive
environmental monitoring required for process
approval, coupled with a long and expensive
application approval process in the United States.
In the U.S., there is also political opposition, and
the long-term effectiveness of the methods is
uncertain.
[0013] In natural attenuation methods, the removal
of metal contaminants may be dependent upon the
sorption capacity of the ground structures that the
ground water moves through. When groundwater is
infiltrated with acidic fluids, the water-bearing
formation becomes acidified, markedly decreasing the
effectiveness of natural attenuation methods. The
groundwater itself is commonly unable to rapidly
affect the pH of the acidified regions in the ground,
where metal sorption to the sediments is hindered.
Most groundwaters have insufficient alkalinity to
rapidly neutralize the sediment surfaces. As a
result, the surface acidity remains high during
remediation, thus decreasing the effectiveness of the
formation in attenuating metal ion concentration.
Furthermore, the high surface acidity may permit
previously sorbed metals to desorb and reenter the
groundwater flow.
[0014] Source control refers to processes for
control of contaminants from a wastewater source.
For example, such treatments may involve
precipitation of heavy metals using alkali, and may
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further include addition of a precipitation agent
such as silica, see, e.g. U.S. Patent No. 5,370,827
and 3,579,443. Such treatments do not occur in a
groundwater-bearing formation, but are carried out
externally, above the land surface.
[0015] Flushing techniques are known in the art
for flushing undesirable metals from out of land
formations that are responsible for contaminating
groundwater. U.S. Patent No. 5,324,433 and
concurrent 5,275,739, describe in situ methods of
removing and stabilizing soluble heavy metal
contaminants in soil and groundwater. They disclose
an ion displacement method of introducing an aqueous
remediation solution into a land formation to
solubilize, mobilize and remove heavy metal ions from
the soil, counteracting the retention of the ions by
the charged clays, displacing the heavy metal or
radioactive ions with harmless, naturally-occurring
ions. The disclosed remediation solution contains at
least one remediation ion selected from the group
consisting of aluminum, magnesium, calcium,
potassium, sodium, hydrogen, chloride, sulfate,
carbonate, bicarbonate, hydroxide, or any mixture
thereof. After the land formation was sufficiently
flushed with remediation solution to decrease
effluent undesirable metal ion content, the land
formation was treated with a stabilization solution
consisting essentially of sodium silicate, potassium
silicate, or a mixture thereof to co-precipitate
remaining metal contaminants and inhibit their
remobilization.
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[0017] In summary, there are no existing methods
for treating a water-bearing formation to improve or
regain its ability to sorb metals. There are no
methods that effectively address the problem of
metals diffusing out of bypassed regions into the
main flow of a water-bearing formation. There is a
need for an alternative or addition to existing
groundwater treatment processes for attenuating the
metal contaminant content. There is a great demand
for such a process that does not require costly
sorbent resins or off-site treatment. There is need
for improvement in natural attenuation methods of
groundwater treatment.
BRIEF SUMMARY OF THE INVENTION
[0018] The present invention provides a process
for enhancing or regaining the metal contaminant
sorption capacity of mineral compounds within a
groundwater-bearing formation by manipulating the pH
and the surface acidity of the mineral compounds.
The process may be useful (i) for providing a
groundwater-bearing formation capable of more
effectively attenuating the metal content of
groundwater currently within the formation as well as
groundwater that passes through the formation in the
future, (ii) for providing a backstop treatment for
groundwater after previous water treatment, and (iii)
for protection of water sources, for example well-
head protection.
[0019] In a contemplated process, a water-bearing
formation is exposed to an alkaline flush solution.
With this process, metals sorb in place in the water-
bearing formation instead of contaminating the
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groundwater. The alkaline flush causes mobile metals
to sorb or precipitate, strengthens metal sorption
and works against desorption. Most importantly, the
alkaline flush creates the unsatisfied demand for
metal sorption that will continue to remove metal
contaminants from groundwater passing through the
formation.
[0020] A preferred process for the attenuation of
the metal content of groundwater from a groundwater-
bearing formation is disclosed herein. A
groundwater-bearing formation is typically composed
of at least one mineral compound, and often several.
Of course, all mineral compounds don't sorb metals to
a significant extent. For purposes of discussing the
present invention, a contemplated groundwater-bearing
formation has at least one sorbent mineral compound
that is capable of sorbing metals. This required
mineral compound is referred to in the claims as "a
mineral compound" for ease of reference. It is to be
understood that the use of "a" in the claims is open
to having more than one, but at least one, unless
more is specifically recited.
[0021] A groundwater-bearing formation is provided
that is made up of groundwater and a mineral
compound. The mineral compound has a mineral surface
with sorbing sites. The groundwater may contain
mobile metal contaminants, the immobilization of
which is desired. In this preferred process, some or
effectively all of the mobile metal contaminants are
removed from the groundwater through the following
sorption process. An aqueous alkaline solution is
applied to the groundwater-bearing formation to
neutralize groundwater acidity and the surface
acidity of the mineral compound. Decreasing the
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surface acidity of the mineral compound has the
effect of enhancing the ability of the sorbing sites
provided by the mineral compound to sorb metal ions.
In preferred processes, the mineral compounds include
ferric oxide, manganese oxide, alumina or silica;
each of these compounds may also be composed of
hydroxide and/or water, as well minor amounts of
various chemical constituents. The equilibrium of the
sorbing sites on the surface of the compound is
shifted by raising the surface pH in such as way as
to enhance sorption strength and capacity for metal
ion contaminants. The mineral compound in the
groundwater-bearing formation that has been in
contact with the alkaline solution is referred to
hereinbelow as a "treated mineral compound". When
the metal-contaminated groundwater contacts the
treated mineral compound, the sorbing sites sorb the
mobile metal contaminants. A treated mineral
compound has an increased sorption capacity as a
result of the treatment. As a result, the
groundwater in the treated water-bearing formation
has an attenuated metal content as compared to the
groundwater in the water-bearing formation prior to
treatment.
[0022] In an embodiment of the preferred process,
the application of the aqueous alkaline solution is
accomplished through injection of the aqueous
alkaline solution into the groundwater-bearing
formation.
[0023] In an embodiment of the preferred process,
the aqueous alkaline solution includes one or more of
hydroxide, carbonate, phosphate, phosphite, or
silicate, preferably hydroxide, carbonate or
silicate. The cationic composition of the aqueous
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alkaline solution is discussed in the detailed
description. The type of mineral(s) in the water-
bearing formation affects the preference of aqueous
alkaline solution, as discussed in more detail below.
[0024] in an embodiment of the preferred process,
the mineral compounds included in the groundwater-
bearing formation is one or more of ferric oxide,
alumina, silica, or manganese (III) oxide, which
provide sorbing sites, including their hydrous and
anhydrous oxide and hydroxide forms.
[0025] A contemplated process creates an
unsatisfied demand for metal sorption. In addition,
in some embodiments of the preferred process, the
application of aqueous alkaline solution to
groundwater-bearing formation inhibits desorption of
metal contaminants from the mineral compounds in the
groundwater-bearing formation. In some embodiments,
leaching of metals from the low-conductivity lenses
to form new mobile metal contaminant is inhibited.
[0026] The present invention has many benefits and
advantages, several of which are listed below.
[0027] One benefit of the invention is that the
process can be carried out in the aquifer and does
not require removal of the aquifer sediment.
[0028] One advantage of the invention is that the
process also prevents desorbing of metals from
aquifer sediment into the groundwater.
[0029] Another benefit of the process is that it
does not add harmful chemicals to the environment.
[0030] An advantage of an embodiment of the
invention is that it is useful for treating a
groundwater-bearing formation whose metal sorption
capacity has been diminished by acidic water drainage
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(for example, from mines) that has contaminated subsurface water.
[0031] A benefit of an embodiment of the invention is that it is useful as
a backstop treatment for water effluent from a contaminant plume after primary
treatment, such as a permeable reactive barrier or source control.
[0032] An advantage of an embodiment of the invention is that it can
provide protection from a contaminant plume for well water supply.
[0032a] In one aspect, the invention relates to a process for enhancing
the sorption capacity of a groundwater-bearing formation comprising the steps
of: a.
applying an aqueous alkaline solution to the groundwater-bearing formation
comprising groundwater containing mobile metal contaminants and a mineral
compound having a mineral surface with sorbing sites having an initial metal
contaminant sorption capacity, said mineral compound comprising one or more of
ferric oxide, manganese oxide, alumina, silica or their respective hydrous,
anhydrous
hydroxy, or oxyhydroxy forms; and b. contacting said mineral compound with
said
aqueous alkaline solution to convert the mineral compound to a form having a
higher
metal contaminant sorption capacity than the initial metal contaminant
sorption
capacity thereby forming a treated mineral compound within said groundwater-
bearing formation, wherein said contact inhibits desorption of metals from the
mineral
surface.
[0032b] In a further aspect, the invention relates to a process for the
remediation of groundwater contaminated by metal ions through attenuation of
the
metal content therein, wherein said groundwater is in a groundwater-bearing
formation, comprising the steps of: a. applying an aqueous alkaline solution
to the
groundwater-bearing formation comprising groundwater containing mobile metal
contaminants and a mineral compound having a mineral surface with sorbing
sites
having an initial metal contaminant sorption capacity, said mineral compound
comprising one or more of ferric oxide, manganese oxide, alumina, silica or
their
respective hydrous, anhydrous hydroxy, or oxyhydroxy forms, in one or more
positions near the mobile metal contaminants; b. contacting said mineral
compound
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with said aqueous alkaline solution to convert the mineral compound to a form
having
a higher metal contaminant sorption capacity than the initial metal
containment
sorption capacity to form a treated mineral compound within said groundwater-
bearing formation wherein said contact inhibits desorption of metals from the
mineral
surface; and c. contacting said treated mineral compound with the groundwater
that
contains the mobile metal containments to permit the treated mineral compound
to
sorb the mobile metal contaminants to form the groundwater having an
attenuated
metal content.
[0032c] In a still further aspect, the invention relates to a process for
the protection of a well-head from metal contaminants, wherein said well-head
is in a
groundwater-bearing formation, comprising the steps of: a. applying an aqueous
alkaline solution to the groundwater-bearing formation, wherein said
groundwater-
bearing formation comprises groundwater and a mineral compound, and wherein
said
mineral compound has a mineral surface comprising sorbing sites having an
initial
metal contaminant sorption capacity, and wherein said groundwater contains
mobile
metal contaminants, in one or more positions near the metal contaminants such
that
at least some of the groundwater reaching the well-head has had to pass
through a
portion of the groundwater-bearing formation that has a treated mineral
compound
formed in step b; b. contacting said mineral compound with said aqueous
alkaline
solution to convert the mineral compound to a form having a higher metal
contaminant sorption capacity than the initial metal contaminant sorption
capacity to
form the treated mineral compound within said groundwater-bearing formation
wherein said contact inhibits desorption of metals from the mineral surface;
and c.
contacting said treated mineral compound with the groundwater that contains
the
mobile metal contaminants to permit the treated mineral compound to sorb the
mobile
metal contaminants to form the groundwater having an attenuated metal content
thereby protecting the well-head from metal contaminants.
[0032d] In a yet further aspect, the invention relates to a process for
providing a backstop to a permeable reactive barrier water treatment method or
a
water source control method to further remove metal contaminants, wherein
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groundwater in a groundwater-bearing formation has been previously treated by
a
permeable reactive barrier water treatment method or a water source control
method,
said backstop comprising the steps of: a. applying an aqueous alkaline
solution to
the groundwater-bearing formation wherein said groundwater-bearing formation
comprises a mineral compound and the groundwater that has been previously
treated
by the permeable reactive barrier water treatment method or the water source
control
method, and wherein said mineral compound has a mineral surface comprising
sorbing sites having an initial metal contaminant sorption capacity, and
wherein said
groundwater contains mobile metal contaminants, in one or more positions near
the
10, mobile metal contaminants such that at least some of the groundwater
reaching a
portion of the groundwater-bearing formation that has a treated mineral
compound
formed in step b has been previously treated by the permeable reactive barrier
water
treatment method or the water source control method; b. contacting said
mineral
compound with said aqueous alkaline solution to convert the mineral compound
to a
form having a higher metal contaminant sorption capacity than the initial
metal
contaminant sorption capacity to form the treated mineral compound within said
groundwater-bearing formation wherein said contact inhibits desorption of
metals
from the mineral surface; and c. contacting said treated mineral compound with
the
groundwater that contains the mobile metal contaminants to permit the treated
mineral compound to sorb the mobile metal contaminants to form the groundwater
having an attenuated metal content thereby providing the backstop to the
permeable
reactive barrier water treatment method or the water source control method to
further
remove the mobile metal contaminants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In the drawings forming a portion of this discloser,
[0034] Fig. 1 illustrates the form of iron (y-axis "Number of Surface
Sites") in surface structures that contain ferric oxide minerals, depicted as
a function
of acidity (x-axis "pH"). The sorption of metal ions (preferably cationic)
increases as
the pH increases, because the sorption of metal cations onto the iron (111)
surface
liberates hydrogen ions (Hf'). As such, an increase in pH serves to favor the
sorption
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of metal cations, and to cause metal cations to sorb more tightly. Acidic
environmental factors, conversely, inhibit the sorption capacity of the water-
bearing
natural structures containing ferric minerals.
[0035] Fig. 2 illustrates sorption profiles for the metal ions lead (Pb++)
copper (Cu++), zinc (Zn++), and mercury (Hg++). The graph shows the fraction
of the
metal sorbed as a function of pH. Under non-saturating metal contaminant
conditions, when the pH is about 6.5, the ferric mineral is able to sorb
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essentially all of the lead (II), copper (II) and
zinc (II), while the mercury is not sorbed.
Providing conditions such that the pH is about 9
ensures sorption of essentially all of the mercury
(II) as well as the other metal ions.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention provides a process
for enhancing the capacity of mineral compounds
within a groundwater-bearing formation to sorb metal
contaminants. The parameters of the contemplated
processes are discussed in more detail below. The
processes center upon manipulation of the pH in the
aqueous local environments in a groundwater-bearing
formation and the surface acidity of the mineral
compounds. The processes have useful applications in
groundwater purification, such as in providing a
groundwater-bearing formation that has enhanced
sorption capacity and is therefore effective in
attenuating the metal content of groundwater,
providing a backstop treatment for groundwater after
previous water treatment, and for the protection of
water sources, for example well-head protection.
[0037] For groundwater treatment processes of
attenuating metal contaminant content, the present
inventor recognized that the surface acidity of the
mineral compounds in the groundwater-bearing
formation plays an important role. The infiltration
of acidic fluids into groundwater-bearing formations
increases the surface acidity of the mineral
compounds, and this decreases the effectiveness of
natural attenuation methods and can even exacerbate
heavy metal ion contamination.
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[0038] Certain negative effects, such as acidic
runoff, decrease the local pH of the mineral
compounds responsible for providing natural sorption
sites within the groundwater-bearing formation. Under
acidic conditions, the sorption affinity of the
surface sites for contaminant metal ions is decreased
considerably. The surface acidity of the mineral
compounds also creates a pH-buffered region that
serves to maintain acidic conditions in the
subsurface.
[0039] These negative effects are not countered
naturally. Most groundwaters lack sufficient
neutralizing capacity to neutralize surface acidity
in a reasonable time period for remediation. As a
result, the mineral surface acidity remains high
during remediation, thus decreasing the effectiveness
of attenuating metal ion content.
[0040] Another negative effect of high surface
acidity is that the acidic conditions permit
previously sorbed metals to desorb and diffuse into
the groundwater flow. The high acidity also
encourages mobilization of metal ions from low
conductivity lenses within the groundwater-bearing
formation into the main groundwater flow.
[0041] The processes of the invention provide a
means for enhancing the metal contaminant sorption
capacity of groundwater-bearing formations through
manipulation of the pH using aqueous alkaline
solutions. The goal is to immobilize metal
contaminants from the groundwater solution. This is
in contrast to U.S. Patent No. 5,324,433, where the
goal was to mobilize and thus remove metal
contaminants from land formations.
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[0042] The processes of the invention are
particularly useful for remediation of an acid-
contaminated groundwater-bearing formation. An
aqueous alkaline solution is introduced into an acid-
contaminated groundwater-bearing formation to
neutralize the acidity, and convert the mineral
compounds within the groundwater-bearing formation
back to a chemical form that has the desired sorption
capacity for the mineral compounds. Although the net
effect of a process of the invention is a process for
attenuating the metal content of groundwater, a
contemplated process is in many cases a remediation
process for the groundwater-bearing formation itself,
restoring the innate ability of the mineral compounds
therein to sorb metal contaminants from the
groundwater.
[0043] The introduction of an aqueous alkaline
solution into a groundwater-bearing formation results
in the entrainment of the aqueous alkaline solution
into the groundwater flow, which is naturally often
quite slow. The entrainment typically results in
dilution of the introduced aqueous alkaline solution
into the groundwater. The mineral compounds in the
groundwater-bearing formation are treated at the
solution front of the aqueous alkaline formation,
consuming units of basicity while neutralizing
acidity within an acid-contamination groundwater-
bearing formation. Groundwater flowing behind the
alkaline front enjoys the benefit of the higher
sorption capacity of the treated metal compounds
within the formation.
[0044] In some cases, it may be advantageous, when
introducing aqueous alkaline solution into a
groundwater-bearing formation, to also remove water
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from the formation. Such removal can make
introduction of the aqueous alkaline solution more
efficient and/or faster. Preferably, the water is
removed down-gradient (relative to groundwater flow)
of the point(s) of introduction of the alkaline
solution.
[0045] A process of the present invention is
exemplified by the following embodiment. An aqueous
alkaline solution is pumped into an aquifer and
begins to migrate along the direction of flow (i.e.
toward a production well). The transit time for the
injected solution is likely weeks, months, or years,
due to the slow speed that groundwater typically
migrates through a groundwater-bearing formation. As
the solution migrates through the aquifer, its
alkalinity reacts with sediments, leaving the water
more acidic and the sediment surfaces neutralized.
[0046] The reaction occurs along a "front" that
migrates in the direction of groundwater flow, but at
a rate slower than the flow rate of the groundwater,
because the alkalinity is stripped from the moving
front as it passes through the groundwater-bearing
formation. Upstream of the alkaline front, the
solution is alkaline; downstream, its alkalinity has
been consumed.
[0047] If one were to consider the process in
terms of pore volumes of alkaline solution required,
the number of pore volumes required depends on the
number of moles of alkalinity that the specific
aquifer consumes as it is neutralized, and the moles
of alkalinity contained in each pore volume of
alkaline solution applied (concentration-dependent).
The optimal treatment conditions will vary from one
groundwater-bearing formation to another.
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Preferably, a sufficient quantity of alkaline
solution of sufficient strength is injected to drive
the reaction front through the affected zone of the
aquifer (i.e. the contaminated area or the zone to
act as a backstop).
[0048] Groundwater-bearing formations.
Contemplated groundwater-bearing formations have
mineral compounds that provide metal contaminant
sorbing sites. Such formations include aquifers,
which include water-bearing sediment and water-
bearing rock. Water-bearing rocks and other
groundwater-bearing formations include sandstone,
which is commonly made up of silica-based mineral
compounds. Contemplated groundwater-bearing
formations can include, inter alia, sediment, soil,
and rock.
[0049] Groundwater-bearing formations also include
those formations from which groundwater can be
obtained, although the formation may arguably not
rise to the level of being called an "aquifer". The
term "aquifer" has a functional definition involving
the ability to obtain "useful quantities" of water.
Such a limitation does not apply to the contemplated
groundwater-bearing formations of the present
inventive process.
[0050] Mineral compounds. Contemplated mineral
compounds that provide metal contaminant sorbing
sites include but are not limited to metal oxides,
hydroxides, and oxyhydroxides with surface sites that
convert between the protonated hydroxy, hydroxy and
oxy forms. Figure 1 shows these forms for ferric
oxide, including the number of metal contaminant
sorption sites as a function of surface acidity.
Preferred mineral compound examples are composed of
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the metals ferric iron, manganese, silicon, and
aluminum. It is to be understood that the surface
sites of these mineral compounds exist in
equilibrium, depending their respective pH profile
and other environmental factors, in oxide form and
hydrous oxide (oxy-hydroxide) form.
[0051] Metal contaminants. When metal
contaminants are immobilized, they are thus taken out
of the groundwater moving through a groundwater-
bearing formation. The metal contaminants of concern
are dissolved or dispersed in the groundwater, and
are referred to herein as mobile metal contaminants.
Metal contaminants in groundwater that are of primary
concern include heavy metals, such as lead and
mercury. Copper and zinc also rise to contaminating
levels in certain groundwaters. Further significant
divalent metal ions that commonly require remediation
are Ni2+ and Co2+ (the latter may be radioactive). For
the purposes of the present invention, contemplated
metal contaminants are those whose levels are above
the desired concentration in groundwater. The metal
contaminants that tend to be sorbed by the
contemplated mineral compounds are commonly cations.
[0052] Sorbing sites. For example, a metal oxide
surface sorbing site, represented in the equations
below as =MOH, showing the metal in the mineral
compound, M, bonded to the bulk mineral by three
(representing multiple) bonds to oxides to the
extended mineral structure, and having an -OH at the
sorbing site. The mineral compound sorbs a metal
contaminant as shown in the exemplary equations
below.
Pb2+ + =MOH --> =MOPb+ + H+
Hg 2+ + =MOH --> MOHg+ + H+
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Cu 2+ + =MOH --> =MOCu+ + H+
Zn2+ + =MOH --> =MOZn+ + H+
[0053] In the typical mineral compounds that
provide sorbing sites, ferric oxides, manganese (III)
oxide, silica and alumina, the M in the above
equations represents iron, manganese, silicon and
aluminum, respectively.
[0054] Aqueous alkaline solution. A contemplated
aqueous alkaline solution is a high pH solution, pH 8
or greater, capable of raising the local pH at the
surface of the mineral compounds within the
groundwater-bearing formation. Preferably, such a
solution is an aqueous solution of a strong base or a
mixture of strong bases. Preferred basic anions
include but are not limited to hydroxide, carbonate,
phosphate, phosphite, silicate, with hydroxide,
carbonate or silicate being particularly preferred.
[0055] Two factors must be considered in
determining the concentration of base to be included
in the alkaline solution. First, the pH of the
aqueous alkaline solution needs to be high enough to
permit, once the solution mixes with the ambient
groundwater flow, the sorbing mineral compounds in
the groundwater-bearing formation to sorb the metal
contaminants. Please refer, for example, to the pH
profile for iron oxide in Fig. 2. Second, the
alkaline flood should be designed so that as much of
the alkalinity as possible will be used for
neutralizing surface acidity. For this second
factor, it is important to avoid mineral
precipitation of the base, which could then consume
the majority of the aqueous alkaline solution's
alkalinity (and thus, neutralizing power). The
latter criterion can also bear upon the selection of
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the cationic composition of the aqueous alkaline
solution to be used for the alkaline flood. For
example, a calcium solution should not be used in a'
groundwater-bearing formation where precipitation of
carbonate minerals (e.g CaCO3) is possible.
[0056] Preferably, upon introduction of the
aqueous alkaline solution and mixing with the ambient
groundwater flow, the pH of the mixture is not higher
than about 12, in order to prevent precipitation of
the pH-controlling base from the solution or the
dissolution of the sorbent mineral compounds. For
example, alumina will dissolve at very high pH, such
as above pH 12. Examples of a contemplated strong
base include, but are not limited to a metal
hydroxide, metal carbonate, or metal silicate; where
the metal is a highly prevalent low-valence metal
ion, such as sodium or potassium, although divalent
cations magnesium or calcium are also useful, and
preferably the metal ion is abundant in naturally
occurring uncontaminated groundwater; and where the
anion (basic) moiety is a naturally-occurring mineral
anion.
[0057] The concentration of the pH-controlling
base in the aqueous alkaline solution should be
sufficiently high that the alkalinity is not consumed
by reactions within the water-bearing formation
immediately. Preferably, the concentration before
mixing with ambient groundwater is at least 0.1 N
base, more preferably 1 to 5 N.
[0058] The cationic composition of the aqueous
alkaline solution may be any cation, preferably
inorganic in nature. The cationic composition of the
aqueous alkaline solution is not a metal ion whose
removal would be desired, but preferably is a
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standard cation such as an alkali metal or alkaline
earth metal from group IA or group IIA of the
periodic table, preferably rows 2 through 4, (e.g. K+,
Na+, Ca++, Mg++), or a typical inorganic cation such as
ammonium ion (NH4+), among others. Particularly
preferred examples are alkali metal ions such as K+,
Na+, Ca++, and Mg++. Also particularly preferred is
the ammonium ion, NH4+, which also serves as a
nitrogen source for some bacteria that can also
enhance water-purifying properties of aquifers. The
type of mineral(s) in the water-bearing formation
affects the preference of aqueous alkaline solution,
as discussed in more detail below.
[0059] The anionic composition of the aqueous
alkaline solution is a strong base, preferably an
inorganic base. Such bases include but are not
limited to hydroxide ion, silicate ion, aluminate,
phosphate, and carbonate.
[0060] In an embodiment of the preferred process,
the aqueous alkaline solution comprises one or more
of NaOH, Na2CO3, or Na2SiO3.
[0061] The nature of the mineral makeup of the
water-bearing formation should be considered when
selecting a base and counter-ion for preparing the
aqueous alkaline solution. For example, if a water-
bearing formation is contains a high concentration of
calcium-laden minerals, the use of carbonate ions in
the aqueous alkaline solution will cause the
precipition of calcium carbonate within the water-
bearing formation, consuming alkalinity and possibly
impeding the flow of groundwater. As another
example, if a water-bearing formation contains a high
concentration of alumina-based minerals, such as
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kaolinite, the mineral compounds will react with a
KOH solution to make an undesired potassium clay.
[0062] In order to evaluate the risk of mineral
precipitation, calculations (on the basis of mineral
solubility) and/or experiments are preferably used.
The type of experiment commonly used for evaluating
the risk of mineral precipitation is a column
experiment where a large tube is filled with sediment
from the subject groundwater-bearing formation,
solution is slowly pumped through the column, and the
chemistry of the effluent is observed and analyzed.
[0063] Application of the alkaline solution. The
alkaline solution can be introduced into the
groundwater-bearing formation by a suitable means.
For example, it may be introduced by injection wells.
In order to encourage flow into the groundwater-
bearing formation, it may be helpful to remove
groundwater at another location at the same time, for
example by pumping downstream.
[0064] For example, injection systems as defined
by any suitable arrangement of wells for the
particular application (groundwater treatment, well-
head protection or backstop) may be used. The
injection wells can be arranged in any convenient
pattern, for example a conventional five spot pattern
wherein a central well is surrounded by four somewhat
symmetrically located injection wells.
Alternatively, other suitable patterns include inter
alia, line drive, staggered line drive, four spot,
and seven spot.
[0065] The term "pore volume" is generally
understood in the geologic arts. ASTM International
D44404-84(1998)el describes a standard test method
for determination of pore volume and pore volume
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distribution of soil and rock by mercury intrusion
porosimetry. Pore volume is used herein as it may
apply to a groundwater-bearing formation as the
volume of water required to replace or flush out the
water in a certain volume of the formation. Although
there may be substantial heterogeneity in a
groundwater-bearing formation, the amount applied
need not be precise to successfully carry out the
present invention.
[0066] Furthermore, as the aqueous alkaline
solution passes through an acidic groundwater-bearing
formation, basic equivalents are lost at the front as
neutralization occurs, so the alkaline front lags
farther and farther behind the spreading front of
introduced solution as it progresses. As a result,
the concept of pore volumes introduced does not
directly reflect the number of pore volumes of
alkaline solution actually in the groundwater-bearing
formation at any given time, depth or area. As a
result, the "pore volumes" referred to herein are
used in a very general manner. In a preferred
embodiment, at least a few pore volumes of aqueous
alkaline solution are applied to the portion of the
groundwater-bearing formation to be treated.
[0067] The aqueous alkaline solution equilibrates
rapidly with the sorption surface in comparison to
the time that it typically takes for water to migrate
through the subsurface of a groundwater-bearing
formation. Acid-base reactions tend to be very rapid
(e.g. diffusion-limited). Typically, the sorbing
sites equilibrate right away with the pH of the
aqueous alkaline solution.
[0068] However, due in part to the volume of a
groundwater-bearing formation and the rate of flow, a
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typical process carried out according to the present
invention uses a large volume of aqueous alkaline
solution (for example, 50 to 100,000 gallons) applied
over a long period of time (for example, several
hours to several months). A worker of ordinary skill
in the field is immediately able to appreciate the
range of variation in both volume and time that will
be helpful for various applications, depending on the
particular groundwater-bearing formation to be
treated. The rough pore-volume suggestions provide
sufficient guidance to a worker of ordinary skill,
who is also easily familiar with flow rates
acceptable through the various groundwater-bearing
formations and thus the application time needed to
introduce the desired volume.
[0069] Treated mineral compound. Exposure of a
mineral compound present in a groundwater-bearing
formation to an aqueous alkaline solution returns the
sorbing sites therein to a basic form that has a
higher affinity for sorbing metal contaminants than
the mineral compound sorbing sites at low pH. See,
for example, Fig. 2. In a contemplated process, the
metal contaminant sorption capacity is enhanced or
regained through application of the alkaline solution
to the groundwater-bearing formation. After
treatment, the mineral compound is referred to herein
as the "treated mineral compound".
[0070] Attenuated metal content. The goal of the
present invention is to reduce the level of
contaminants in groundwater that passes through a
groundwater-bearing formation. Relative to the
initial metal content of groundwater from an
untreated groundwater-bearing formation, the methods
of the present invention result in a lower
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concentration of contaminants in the groundwater
collected from the treated groundwater-bearing
formation. This is referred to as attenuated metal
content.
[0071] An alternative usage in the art refers to
metal attenuation through passing contaminated
groundwater through a groundwater-bearing land
formation that contains adsorbent clay and/or other
minerals. This is the natural process of contaminant
attenuation by the earth. For the natural process,
the attenuation of metal content refers to the metal
concentration before and after the water passes
through the ground. For a process of the invention,
the attenuation refers to the metal concentration in
groundwater passing through the ground after
treatment according to the present invention relative
to without (or before) treatment. As it applies to
the natural process, the invention has been termed
"accelerated attenuation" of metal content, because
treatment according to the invention permits the
groundwater-bearing formation to more effectively
reduce the metal content of the groundwater passing
through the formation.
[0072] Desorbing and leaching. When metal
contaminants that were already adsorbed to mineral
compounds within a groundwater-bearing formation
release those metal contaminants into the
groundwater, that is termed "desorption". At a
higher pH, the binding constant (adsorption affinity)
is higher for metal cations. Acid contamination of
the groundwater causes the pH, and thus the
adsorption affinity, to drop, so metal contaminants
desorb. Leaching refers to metal contaminants moving
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into the groundwater from within the minerals of the
groundwater-bearing formation.
[0073] Dissolved metal species. In some
embodiments the groundwater contains other metal
species that are capable of precipitating and
providing additional sorbing sites. In preferred
embodiments, the metal species is iron (III),
aluminum (III) or silicon (IV) that can precipitate
to form iron oxide, alumina or silica that have the
preferred sorbing sites. In addition, iron (II) and
manganese (II) can oxidize to (III) in the presence
of high pH fluids, and then precipitate as additional
sorbing mineral.
[0074] Applications of the process. The processes
of the invention are useful for treating groundwater-
bearing formations in situ. Due to the pH-altering
nature of the treatment, its application is most
useful in formations that have been exposed to acidic
conditions that have altered the contaminant-
sequestering nature of the formation.
[0075] Processes according to the invention are
also useful as a local treatment to attenuate metal
concentrations, for example to decrease contaminants
in groundwater that departs from a contamination
source or that approaches a water source to be
protected.
[0076] The following examples of the invention are
provided to explain in detail applications of
selected embodiments of the invention, and are not
intended to be limiting.
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[0077] Example 1. Remediation of Acid
Contamination
[0078] An aquifer is contaminated by metal-rich
acid drainage from a mining operation. The mine is
situated upgradient of a groundwater-bearing
formation that has a slow but general flow direction
downgradient. The acid contamination of the
groundwater-bearing formation decreases the metal
sorption capacity of the mineral compounds within the
groundwater-bearing formation. The sporadic, ongoing
release of acid contamination results in several
plumes of heavy metal contamination that extend up to
1000 yards downgradient of the mining operation.
[0079] The groundwater-bearing formation contains
significant portion of kaolinite mineral compounds
(alumina-based), but little calcium.
[0080] Drainage from the mining operation is
modified so that metal-contaminated water no longer
enters the aquifer.
[0081] An extraction well is drilled about 1000
yards downgradient of the mining operation to assist
in flushing water through the contaminated portion of
the aquifer, and to serve as a monitoring well. The
extraction well initially produces groundwater
samples that have a high metal content.
[0082] An injection well is drilled at the mining
operation, near the original source of contaminated
water. For an initial period, 3,000 gallons per day
of clean water is injected in this well, and an
equivalent amount produced from the extraction well.
Water drawn from the extraction well is treated to
remove metal contaminants and neutralize acidity, and
reinjected at the injection well.
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[0083] After a period of three months, the
injected water has displaced the original
contaminated water from the highly conductive portion
of the aquifer. At this point, the metal
concentration observed at the extraction well drops
sharply, although it remains in excess of
environmental standards and continues to require
treatment.
[0084] At this point, 200 gallons per day of a 1 N
solution of sodium carbonate is injected into the
ground below the surface into each well of a linear
array of five wells penetrating the aquifer, evenly
spaced along the contaminant plume, from the mining
operation to 500 yards upgradient of the extraction
well.
[0085] After six months of injection of the
alkaline solution, the metal concentration in water
produced from the extraction well is observed to fall
within environmental standards, and pH is observed to
have risen to above 8. At this point, injection of
the alkaline solution and pumping of the extraction
well cease. Water samples are drawn from the
extraction well weekly for a period of three years
and monitored to assure that metal concentration
there meets or exceeds environmental standards.
[0086] Example 2. Backstop for Source Control
[0087] A mine operator carries out several
processes for recovery of precious metals from ore-
laden earth using acidic solutions. Some of the
acidic solutions percolate into an aquifer, creating
a plume of metal contamination gradually migrating
downgradient. The mine operators terminate the
recovery processes and carry out treatments, such as
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sulfide precipitation of the acid washes, to remove
dangerous heavy metals from the contaminated
groundwater. The treatments are largely but not
completely effective and contaminant metals continue
to migrate downgradient through the aquifer in small
but significant concentrations. There is furthermore
concern that some of the previously immobilized
metals will be remobilized at a future date.
[0088] The invention is applied as a source
control to minimize continued movement of
contaminants along the aquifer. A 2N aqueous solution
of sodium hydroxide is pumped into the ground in a
linear arrays of ten injection wells that spans an
interval of the aquifer downgradient of the
contaminant plume. The wells are built about 50
yards downgradient from the current position of the
toe of the plume, spaced 20 yards apart, and
positioned to intercept the plume as it advances.
[0089] Aqueous alkaline solution is pumped into
the injection sites until the high-pH front breaks
through to an observation well 100 yards downgradient
from the injection wells. The mineral compounds of
the groundwater-bearing formation between the
injection sites and the observation well are thus
treated according the invention. The result is an
effective sorption zone in situ protecting the
aquifer downgradient of the mining operation from the
contamination source.
[0090] Monitoring of the metal content in
groundwater downgradient of the treatment zone shows
that the contaminant metals are present at
concentrations that meet or exceed environmental
standards.
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[0091] Example 3. Backstop for permeable
reactive barriers
[0092] A chemical waste site provides a metal
contamination source. Industrial waste buried in
containers underground is subject to leaking as the
old, buried containers deteriorate. The waste site
overlies an aquifer through which water is migrating
in a certain direction. The metal contamination
enters the aquifer and forms a plume that migrates
downgradient. A reactive barrier is set up that
approximately, spans the width of the metal
contamination plume (as determined from sample
testing) and is located downgradient of the metal
contamination source. The permeable reactive barrier
serves as a first protective stage that prevents
contamination from migrating off the site.
[0093] To enhance the effectiveness of the
cleansing properties of the groundwater-bearing
formation, injection points are instituted in a
linear pattern downgradient of the chemical waste
site and reactive barrier. A volume of A 5N solution
of sodium hydroxide is pumped into the injection
points that is sufficient to displace several times
the pore volume for about 100 yards surrounding the
waste site about 50 feet in depth, or a volume of
15,000 square feet per linear foot around the
injected perimeter of the site.
[0094] The treated mineral compounds are thus
formed for about 100 yards of the region downgradient
of the site. The groundwater further downstream is
not removed to make space for the aqueous alkaline
solution that had been applied. Rather, the solution
is permitted to diffuse further into the groundwater-
bearing formation to create an expanded treated zone,
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although the concentration of alkali is lessened due
to the previous passage through the formation. The
treated zone serves to immobilize metal ions that
pass through or bypass the reactive barrier.
[00951 Groundwater monitoring is carried out in
various locations near the chemical waste site.
[0096] Example 4. Well-Head Protection
[0097] A well is situated in a groundwater-bearing
formation. As a result of the pumping from the well,
the groundwater flows from the groundwater-bearing
formation generally toward the well. in a circular
pattern surrounding the well, about 30 yards from the
well site, a 2N aqueous solution of sodium carbonate
is injected into the groundwater-bearing formation.
The well is pumped to draw the alkaline solution
through the groundwater-bearing formation until a
sharp rise in pH is noted. Reaction of the alkaline
solution with the mineral compounds creates a 30-yard
wide swath of groundwater-bearing formation
surrounding the well-head that contains treated
mineral compounds. The well-head is thereby
protected from contamination by metal ions.
