Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W O 94/292~8 21 6 3 8 5 7 PCT~US94/05849
WATER PURIFICATION SYSTEM AND REMOVAL OF HALIDES
Fiel~ ~f the Invention
Methods for impregnating activated carbon with silver to
yield silver loadings much higher than that available
commercially are provided, which provide products useful for
purifying water, especially from iodides and bromides which
may be eluted from iodinated and halogenated disinfecting
resins, lowering the concentrations.of the cont~nAting
halides to levels acceptable for continuous drinking
10 applications and enabling regeneration of the silver and
possibly also of iodine from the precipitated silver halides.
Backqr~un~ of the Invention
15 Silver impregnated activated carbon, (Ag-GAC), is a well
known product, used widely for purifying process-water and
drinking-water. The function of the silver bound to the
activated carbon is to avoid bacterial growth on the carbon
bed an~ in the water in contact with it. Filter cartridges
20 contA;nl; ng Ag-GAC do not kill bacteria during normal
filtration rates. For this reason Ag-GAC is described as a
Bacteriostatic medium and not a Bactericide.
The ~;~um admissible concentration of silver in drinking
25 water is : 0.05 mg/L ( 50ppb) in the USA and Canada and 0.01
mg/L (10 ppb) in the EEC. Therefore, in order to enable
application of Ag-GAC for purifying drinking water, the
loading of silver on the carbon is low, usually lower than
one percent. The highest silver loading offered commercially
30 is 1.0~%.
W094/29~8 2 ~ 6 ~ 8 5 ~ ` PCT~S94/05849
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Recently iodinated and halogenated resins have been
introduced as water purifiers capable of "instantaneously"
killing bacteria and viruses upon contact between the treated
water and the disinfecting resin. The use of equipment
5 contA;~;ng such disinfecting resins for purifying drinking
water has been approved by the US-EPA for emergency and
intermittent applications, not for continuous consumption,
because disinfecting resins elute minute amounts of iodine
and iodides into the treated water, and excessive consumption
10 of iodine/iodide may cause hyperthyroidism. The "ad~usted
acceptable daily intake" (AADI), of iodine/iodide has been
determined to be 1.19 mg/L (Handbook of Water Quality,
Standards and Controls by John De Zuane, P.E., Van Nostrand
Reinhold, New York 1990). The concentrations of iodine/iodide
15 eluted to the treated water by commercial disinecting resins
vary between 2 to 15 ppm (mg/L), depending on the type of the
resin and on the salinity and temperature of the treated
water. Therefore, application of equipment containing
iodinated resins for purifying drinking water for continuous
20 consumption can be possible only if the drinking water
purifier contains means which reduce the concentration of
iodine/iodide in the product water to 1.19 ppm and preferably
considerably lower.
25 Iodides can be removed almost absolutely from a~ueous
solutions by precipitating them with silver ions. The
solubility product of AgI at 25C is 1.5x10-16, a million
times lower than that of AgCl (1.56x10-1) at the same
temperature. Therefore, iodides can effectively be eliminated
wo 94,2g~8 2 1 6 3 8 S 7 PCT~S94/05849
--3--
from drinking water via precipitation as AgI, even in the
presence of a large excess of chloride ions such as those
existi~g in drinking water. Metallic silver or silver
chloride can generate sufficient concentrations of silver
ions for precipitating iodides. However, in order to be
effective, the surface area of the silver-ion generating
solid must be very large. Otherwise it will very quickly be
coated with AgI precipitate and become "blind" to the aqueous
solution. Granular Activated Carbon (GAC), has a large
10 surface area and can be an efficient "carrier" for silver.
Indeed, Ag-GAC has proven to be effective in removing iodide
ions from drinking water.
Nevertheless, commercially available Ag-GACs are not
15 practical for removing iodides from water disinfected by
iodinated resins because their silver loading is very low.
Thus, even if we take GAC impregnated with 1.05% silver (the
highest silver loading available commercially), than 300 ml
of such Ag-GAC (the amount which can be included in a 10"
20 CounterTop Drinking Water Purifier) weigh ~150 grams and
contain only -1.50 gr. (1,500 mg) of silver. This amount of
silver can, theoretically, precipitate only 1.5x(126.9/107.9)
= 1.76 gr. of iodides. Practically, the precipitation
efficiency is less than 80%. If we assume that the
25 concentration of iodine/iodide in the water disinfected by an
iodinated resin is only 3 ppm, then 300 ml. of 1.05% Ag-GAC
would be sufficient for treating less than 470 liters ~f
disinfected water, which is too low a capacity. The minimum
reasonable capacity for a 10" CounterTop Drinking Water
-
W094/29~8 2 ~ ~ ~ 8 ~ ~ PCT~S94/05849
Purifier (which contains a 5~ PreFilter, 300 ml iodinated
resin and 300 ml of Ag-GAC) is 1,500 liters - the annual
drinking water consumption of one person. With 1.05% Ag-GAC
we can attain less than one third that value.
SummarY of the Invention
The present invention relates to a purification device for
10 producing potable water for drinking by humans, comprising a
sequence of a bed of a halogenated resin, followed by a bed
or separate container containing silver-impregnated activated
carbon, loaded with more than two weight per cent of silver.
The carbon is preferably loaded with from 4 to 14 weight-% of
15 silver and a filter is provided for filtering out particulate
material.
The invention further relates to a process for the
purification of water and for the purpose of converting it to
20 water fit for human consumption comprising passing feed water
through a sequence of a halogenated resin, followed by a bed
of silver-impregnated activated carbon containing at least
two weight per cent of silver. Preferably the resin used is
an iodinated one, and preferably the impregnated carbon
25 contains between 4 and 14 weight per cent silver.
The invention also relates to the novel silver impregnated
activated carbon containing at least two percent by weight of
silver. The system is useful for the purification or
30 treatment of water for reducing the concentration of
iodide/iodine in water and also for reducing the
~ 5 - PCT~S 94/ 05 ~ 4~ ~ PEA/Us 2 7 D~C 1~9~
concentration of bromide/bromine, or of mixed iodide/bromide in
water. It can be used as a post treatment for water disinfected by
passage through an iodinated or halogenated resin. It can be used
in conjl~nction with iodinated resins, utilizing the precipitated
silver iodide for regenerating silver and iodine.
Ag--GAC loaded with at least 3% silver is therefore desired for
practical elimination of iodine/iodide from water disinfected by
iodinated resins. Such a product also enables a considerable
reduction of the cost of water purification by disinfecting resins,
because both the eYrencive silver and the ~Yp~ive iodine when
precipit:ated as AgI can easily be trapped (filtered out) by a
coarse post filter and regenerated. Furthermore, due to the
extremely low solubility of AgI, p}acement of Ag-GAC treatment
down-stream the iodinated-resin-filter guarantees that the
concentration of both silver and iodine will be lower than the
maximum permissible l~vels for continuous dri nki nq applications.
A device for purifying water cont~i~ing iodine, anion ~Ych~ge
resin and bacteriostatic carbon has been described in U.S. Patent
4,769,143. This device contains a commercial silver impregnated
activated carbon in which the silver lo~n~ i8 1.05% or less. The
p~L~- q of 1naoL~oL~ting thi~ Ag-GAC in thQ device is to prevent
bacteria, growth in the activated carbon chamber at the exit port.
~he inv~ntors apparently were ~naware that the pre~nc~ of
relatively high conc~tration ~2-15 ppm) of iodide ions in the
treatéd wate~ would qu$ckly convert the small amount (~ 1.05%) of
- silver present to silver iodide, which i8 inactive as a
bacteriostat. ~o mention is made of the need to lower the level of
- ~E~E~ S~E~
~16~8S7
_ 5/1 - P ~ 9 4 / 0 5 ~ ~ 9
IPEAIUS 2, DE~ 1994
iodine in the treated water for making it suitable for continuous
human consumption, nor is such a function claimed or hinted at.
Furthe~ore, the device contains so little silver that it would be
useless for rendering the treated water relatively free of iodide
S ions for more than a very limited use. Therefore, although the
prior alt does reveal the existence of highly silver impregnated
carbon CU.S. Patent 3,294, 572), there is no mention of the use of
silver lmpregnated carbon, of any silver loading, for rendering
wàter treated with iodinated resins suitable for long term human
consumpt:ion. As stated above, the reduction of iodine levels is a
requirement for the ~Y~n~e~ utilization of iodinated resin
te~-h~ology for treating un~afe dri~k~ ng water, a ma~or health
menace worldwide.
BRIFF D~C~TPTION OF T~ FIGr~:
The invention i~ illustrated with reference to the enclosed
schematical graphical ~ s~ntations in which:
Fig. l i~lustrates total iodine elimination of AG/GAC against
quantity of water p~ through the filer, according to Example 1.
Fig. 2 i~ a similar graph, relating to Example 2.
Fig. 3 i8 a similar graph, relating to Example 3.
Fig. 4 is a similar graph, relating to Example 4
Fiq. 5 illustrat-s the effectivity of activates carbon with
different.silver content i~ reducing iodin~ content.
Fig. 6 illu~trates the use of 10 percent ~ilver im~e~.. ated
active c:arbon with prefiltration in a water purifier.
~l~h~cl~ SHE~ '
W094/2g~8 PCT~S94/05849
2~G3~S~ -6-
E x a m P l e s .
A. Preparation of HiqhlY T.o~e~ Aq-GAC.
Example 1: 25.8 g Silver nitrate, 70 ml water, and 31 ml
of a 25% Ammonia (as NH3) solution and were combined and
stirred until dissolved. The solution was added slowly in 10
ml portions to 165 g 20-50 mesh granulated activated carbon,
10 which had been previously washed with nitric acid and dried.
Agitation between additions was necessary to assure an even
mixture. After standing 15 minutes a solution of 15 g
fructose in 80 ml o~ water was added. The mixture was placed
in an 80C oven for 68 hrs. The small amount of supernatant
15 liquid showed no precipitate with HCl. The product was
rinsed well with water and dried at 250C. The yield was
181.2 g, corresponding to 9% silver.
Example 2: 81 g of nitric-acid-washed, dried GAC and 25
20 ml of a cold 1.65 M Hydrazine solution were combined and
mixed. A solution chilled to - 8C containing 12.6 g silver
nitrate, 5 g EDTA, 45 ml water and 20 ml of a 2S% ammonia
solution was added. The mixture was left at room temperature
for one hour. The supernatant liquid was tested with 0.1 N
25 HCl and no precipitate was observed, indicating complete
reaction. The product GAC was washed and dried at 250C,
yielding 97.6 g, corresponding to 8.2% silver.
Example 3: To 90 g of nitric-acid-washed, dried GAC was
30 added 18 g silver nitrate dissolved in 50 ml water with
gentle stirring. The mixture was let stand for 15 min. A
~ WO 94/~ 28 216 3 8 5 7 PCT/US94105849
solution ContA~ ni ng 100 ml water, 8.5 g sodium hydroxide and
26 g sodium dithionite (sodium hydrosulfite, 85% min assay)
was added rapidly and mixed gently. After 1 hr the GAC is
rinsed with water and dried at 250C. The yield was 103 g,
corresponding to 11.1% silver; however, it appeared that some
silver was lost during the rinse.
Example 4: To 170 ml of nitric-acid-washed, dried GAC
(91.4 g) was added a solution a solution of 10.8 g silver
10 nitrate in 100 ml water giving a moist mixture with no free
solution. This mixture is added slowly with stirring to a
solution of 50 g sodium chloride in 500 ml deionized water.
The supernatant solution was filtered, revealing oniy 0.7 g
of residue (mostly carbon fines). After rinsing and drying
15 the weiyht was 100.8 g corresponding to 6.8% silver as silver
chlorid~e.
B. ~-; in~tion of Iodine/Iodides from Water treated by an
20 Iodinatea Resin .
Figure I is a graph of total iodine content in feed water and
treated water as a function of the quantity of water passed
through a sample of example 1. Figure 2 is a similar graph
25 relating to example 2. Figure 3 is a similar graph relating
to example 3. Figure 4 is a similar graph relating to
example 4. Figure 5 is a graph of the same parameters,
relating to four samples of commercially prepared
compositions of the invention. Figure 6 is a graph of eluted
wo 94,2gZ~ 7 ~ PCT~S94/05849 ~
iodine and iodide versus the quantity of water passed through
a Counter-Top water purifier conta;n;ng disinfecting resin
(PDR) and an EC-10 Ag/GAC sample of figure 5.
Figures 1 to 4 demonstrate the effectiveness of the samples
of Ag-GAC prepared according to Examples 1 to 4, above in
reducing the concentrations o~ iodine/iodide contained in
water treated by PuroTech Disinfecting Resin (PDR - a
commercial iodinated resin) to levels permissible for
10 continuous human drinking applications. The water treated is
Tel-Aviv City tap water, having a conductivity of 950-1,600
~S/cm, and at a temperature of 21-25C. The water was passed
through a chamber containing PDR and then through 150-160 ml
of the Ag-GAC sample at a rate of 2 liters/min. Samples were
15 taken of water entering and leaving the Ag-GAC chamber, as
shown in the individual graphs.
Figure 5 demonstrates the effectiveness of various types of
5-10~ Ag-GAC, produced specially for these tests by
20 manufacturers of activated carbon and of Ag-GAC, in reducing
the concentrations of iodine/iodide contained in Tel-Aviv
City tap water treated by PDR, as described above, to levels
permissible for continuous human drinking applications. The
relevant properties of the Ag-GAC samples tested in this
25 experiment, and their actual performance, are summarized in
the Table below. In these tests, 120 ml samples were
treated with the same feed stream at a flow rate of 0.72
liters/min each.
~ WOg4/2g~8 21 6 3 ~ ~ ? PCT~S94/05849
_9_
Code EC-10 PC-7 EC-5 SS-5
Density , gr/ml 0.606 0.537 0.573 0.560
Silver Content, % w/w 10 7 5 5
5 gr/L 60.6 37.6 28.6 28.0
Theoretical Capacity for
2.5 ppm I- Feed, liter 28,500 17,700 13,450 13,200
Practical Capacity for
10 < 1 ppm I- Product, lit 21,000 11,000 10,050 9,500
Practical Efficiency, % 74 62 75 72
Figure ~ illustrates the practical use of 10% silver
impregnated granular activated carbon together with 5~
prefiltration and iodinated resin disinfection in a single
10" standard CounTertop drinking water purifier. The amount
20 of Ag-GAC used in this example was 280 ml. The capacity of
the filter is limited by the permissible maximum or average
level of iodide in the product water. Using the "ad~usted
acceptable daily intake" tAADI), of iodine/iodide, determined
to be 1.19 mg/L, as mentioned previously, the capacity of the
25 filter is approximately 5,000 liters. At the average daily
use of 4 liters per person, this capacity is sufficient for a
family of four for over nine months or a family of seven for
six months, both practical figures, common to many popular
water filtration devices.
