IL105905A - Device and process for purification of water - Google Patents

Description

DEVICE AND PROCESS FOR PURIFICATION OF WATER Pearl Cohen Zedek Latzer P-62591-IL Summary of 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 contaminating halides to levels acceptable for continuous drinking applications and enabling regeneration of the silver and possibly also of iodine from the precipitated silver halides- Background of the Invention 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 and in the water in contact with it. Filter cartridges containing Ag-GAC do not kill bacteria during normal filtration rates. For this reason Ag-GAC is described as a Bacteriostatic medium and not a Bacteriocide.

The maximum admissible concentration of silver in drinking 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 is 1.05 % .

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 containing 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 of iodine/iodide may cause hyperthyroidism. The "adjusted acceptable daily intake" (AADI), of iodine/iodide has been estimated as 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 eluted to the treated water by commercial disinfecting 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 consumption can be possible only if the drinking water purifier contains means which reduce the concentration of iodine/ odide in the product water to 1.19 ppm and preferably considerably lower.

Iodides can be removed almost absolutely from aqueous solutions by precipitating them with silver ions. The solubility product of Agl at 25°C is 1.5xl0~16, a million times lower than that of AgCl (1.56x10"1^) at the same temperature. Therefore, iodides can effectively be eliminated from drinking water via precipitation as Agl, even in the presence of a large excess of chloride ions such as those existing 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 Agl precipitate and become "blind" to the aqueous solution. Granular Activated Carbon (GAC), has a large 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 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" 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 1.5x026.9/107.9) - 1.76 gr. of iodides. Practically, the precipitation efficiency is less than 80%. If we assume that the concentration of iodine/iodide in the water disinfected by an iodinated resin is only 3 ppm than 300 ml. of 1.05% Ag-GAC would be sufficient for treating less than 470 liters of disinfected water, which is too low a capacity. The minimum reasonable capacity for a 10" CounterTop Drinking water Purifier (which contains a 5μ 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.

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 expensive silver and the expensive iodine when precipitated as Agi can easily be trapped (filtered out) by a coarse post filter and regenerated. Furthermore, due to the extremely low solubility of Agi, placement of Ag-GAC treatment down-stream the iodinated-resin-filter guarantees that the concentrations of both silver and iodine will be lower than the maximum permissible levels for continuous drinking applications.

E x a m p l e s .

A. preparation of Highly Loaded Ag-GAC .

Example 1: 25.8g Silver nitrate, 70ml water, and 31ml of a 25% Ammonia (as NH3 ) solution and were combined and stirred until dissolved. The solution was added slowly in 10 ml portions to 165g 20-50 mesh granulated activated carbon, 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 15g fructose in 80ml of water was added. The mixture was placed in an 80°C oven for 68 hrs. The small amount of supernatant liquid showed no precipitate with HC1. The product was rinsed well with water and dried at 250°C. The yield was 181.2g, corresponding to 9% silver.

Example 2: 81 g of nitric-acid-washed, dried GAC and 25ml of a cold 1.65M Hydrazine solution were combined and mixed.

A solution chilled to - 8°C containing 12.6g silver nitrate, 5g EDTA, 45ml water and 20ml of a 25% ammonia solution was added. The mixture was left at room temperature for one hour. The supernatant liquid was tested with 0.1N HCl and no precipitate was observed, indicating complete reaction. The product 3AC was washed and dried at 250°C, yielding 97.6g, corresponding to 8.2% silver.

Example 3: To 90g of nitric-acid-washed, dried GAC was added 18g silver nitrate dissolved in 50ml water with gentle stirring. The mixture was let stand for 15 min. A solution containing 100ml water, 8.5 g sodium hydroxide and 26g 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 250°C. The yield was I03g, corresponding to 11.1% silver; however, it appeared that some silver was lost during the rinse.

Example 4: To 170ml of nitric-acid-washed, dried GAC (91. g) was added a solution a solution of 10, 8g silver nitrate in 100ml 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 500ml deionized water. The supernatant solution was filtered, revealing only 0.7g of residue (mostly carbon fines). After rinsing and drying the weight was 100.8g corresponding to 6.8% silver as silver chloride.

B. Elimination of Iodine/Iodides front Water treated bv an Iodinated Resin .

Figures 1 to 4 demonstrate the effectivity of the samples of A -GAC prepared according to Examples 1 to 4, above in reducing the concentrations of iodine/iodide contained in water treated by PuroTech Disinfecting Resin (PDR - a commercial iodinated resin) to levels permissible for continuous human drinking applications. The water treated is Tel-Aviv City tap water, having a conductivity of 950-1,600 s/cai, and at a temperature of 21-25 °C. The water was passed through a chamber containing PDR and then through 150- 60 ml of the Ag-GAC sample at a rate of 2 liters/min. Samples were taken of water entering and leaving the Ag-GAC chamber, as shown in the individual graphs.

Figure 5 demonstrates the effectivity of various types of 5-10% Ag-GAC, produced specially for these tests by 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 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.

Code A-10 B-7 A-5 C-5 Density , gr/ml 0.606 0.537 0.573 0.560 Silver Content, % / 10 7 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 < 1 ppin I" Product, lit 21,000 11,000 10,050 9/500 Practical Efficiency, % 74 62 75 72

Claims (7)

9 105905/2 Claims:

1. A process for purification of drinking water for use over prolonged periods of time without adverse effects due to iodine ingestion comprising the steps of: contacting the water to be purified with an iodinated resin so as to eliminate bacteria and viruses and produce disinfected water, and passing the disinfected water, containing iodide and iodine eluted from the iodinated resin, through a bed of activated carbon which is loaded with at least 2 weight percent of silver so as to reduce the concentration of iodine/iodide in the water and produce water which is potable over prolonged periods of time.

2. A process according to claim 1, wherein a final iodide/iodine concentration is less than 1.19 ppm.

3. A process according to claim 1, wherein the activated carbon is granulated and contains from 4 to 14 weight-% of silver.

4. A process according to claim 1, wherein the silver and iodine are recovered by working up precipitated silver iodide.

5. A purification device for producing potable water which can be drunk by humans over prolonged periods of time without any adverse effect due to intake of too much iodine/iodide comprising: a bed of an iodinated resin, and a bed of granulated activated carbon following said bed of iodinated resin and carrying at least 2 weight-% silver.

6. A device according to claim 5, and further comprising a filter for filtering out particulate material. 10 105905/2

7. A device according to claim 5, wherein said granular activated carbon is loaded with from 4 to 14 weight-% silver. For the Applicant Peaj/i Co ere Advocates, Notaries & Pate P-62591-IL