Silver in Coal
One of the main applications of granular activated carbon is water purification. Its function is the retention of organic contaminants and the elimination, by chemical reaction, of the residual free chlorine that remains in the water after the disinfection stage. This is why silver is applied to the carbon.
The elimination of free chlorine takes place in the first few centimeters of the carbon bed, so that the rest of the bed is no longer protected against the development of free chlorine. development bacterial growth. Tarde o temprano, pueden incidir bacterias que provienen de un influente mal desinfectado, o de algún punto de la tubería del efluente. Esto sucede, principalmente cuando no circula el agua, mientras el equipo está fuera de servicio. Las bacteria reproduce, and turn the charcoal bed into a source of contamination.
In order to inhibit bacterial growth, the surface of the charcoal is impregnated with metallic silver. The resulting charcoal is called bacteriostatic.
But...
What about its effects on health?
Silver has negative effects on humans only in very high doses. In cases of chronic ingestion of this metal (average daily ingestion doses of 0.14 μg per kg body weight for 70 years) it can cause argyria, which consists of an irreversible blue-gray coloration of the skin. This effect does not alter any functional organ, and is therefore considered as a cosmetic defect.
No mutagenic or carcinogenic effects have been found and, therefore, silver has not been classified as a carcinogen.
Originally, the World Health Organization recommended that drinking water should contain no more than 0.05 parts per million of this element. Legislation in several countries, in
The Mexican Health Ministry set this value as the maximum permissible value for drinking water. Currently, due to the evidence of its relative harmlessness, the maximum permissible limits have become more lax, and for example, in the United States and Mexico, this value is 0.1 mg/liter.
In order to comply with drinking water standards, it is important that the silver impregnated in activated carbon is sufficiently well bound to the carbon to prevent its release into the water. Such release, in addition to causing non-compliance with the standard, means that the carbon will lose its bacteriostatic protection in a shorter period of time.
METHODS OF SILVER IMPREGNATION IN ACTIVATED CARBON
Not all methods of silver impregnation in activated carbon achieve a good fixation of this metal. Basically three methods are known: colloidal silver, chemical and electrochemical.
Colloidal Silver
The colloidal silver method consists of preparing a solution in which silver is found as a colloid. A colloid is a physical state at the boundary between a suspended and a dissolved solid. The colloidal silver solution has the appearance of a silver paint. Activated carbon is bathed with it, so that the silver is applied as a coat of paint.
Unfortunately, the silver does not remain sufficiently fixed, and therefore it is easily released into the water.
*Because of the above, the colloidal silver impregnation method has been declared unacceptable in the United States.
Chemical impregnation
The chemical impregnation method consists of a reaction between ionic silver dissolved in an aqueous solution and carbon subjected to pre-oxidation. It is a reduction reaction at high temperature, in which the silver is chemically bonded to the carbon.
The chemical impregnation method achieves good fixation and is therefore permitted in the United States as well as in Europe and Japan. However, there is always a small proportion of silver that is released into the treated water. Therefore, the carbon remains bacteriostatic for a relatively short period of time.
Electrochemical
Finally, the electrochemical method consists of the deposition of silver on the carbon surface by means of an electric current that causes the reduction of the silver. This method achieves a much better metal-carbon fixation, so it is considered the most acceptable method.
DIFFERENCE BETWEEN A BACTERICIDAL AGENT AND A BACTERIOSTATIC AGENT
A bactericidal agent is one that is applied to a fluid in order to kill the bacteria contained in it. Examples of bactericidal agents for water purification are chlorine, iodine, ozone, chlorine dioxide, chloramines and silver ions. The latter are dosed by electrolysis, starting from a silver anode, in which an electric current causes the oxidation of the metal, which is released into the water in its ionic state (Ag +). On the contrary, a bacteriostatic agent is not dosed into the fluid, but remains fixed in the solid. Therefore, charcoal impregnated with silver is bacteriostatic, but not bactericidal. That is to say, it fulfills the function of inhibiting bacterial growth on its surface, but does not guarantee the annihilation of microorganisms in case they are carried by the water in relatively high concentrations.
METHOD FOR QUANTITATIVE ANALYSIS OF SILVER CONTENT IMPREGNATED ON ACTIVATED CARBON
The fixation of silver by chemical and electrochemical methods is not simple because of the difficulty with which it can be detached from the carbon surface to be extracted in a liquid in which the concentration of silver can be analyzed. To achieve a good analysis, a double extraction is necessary: firstly, with a nitric acid solution, at reflux. Secondly, with ammonium hydroxide solution, also at reflux.
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BIBLIOGRAPHIC REFERENCES
- De Zuane, J.: HANDBOOK OF WATER QUALITY, 2ª. Ed., Wiley, N.Y., 1997.
- Budavari, S. (Ed.): THE MERCK INDEX, 12ª. Ed., Merck & Co., Inc., N.J., 1996.
- Groso, G.: GRANULAR ACTIVATED CARBON IN WATER TREATMENT.Aconcagua, Mexico, 1997
- Nalco: WATER MANUALVolume III, Mc Graw Hill, Mexico, 1989.
- U.S.E.P.A., Office of Water: "SILVER. Drinking Water Health Advisory", April 1991.