How does bacteriostatic activated carbon work?
One of the main applications of granular activated carbon is the purification of water. Its function is the retention of organic contaminants and the elimination, by chemical reaction, of the free residual chlorine that remains in the water after the disinfection stage.
The removal of free chlorine is carried out in the first few centimeters of the carbon bed, so there is no longer any protection against bacterial growth in the rest of the bed. Sooner or later, bacteria can enter from a poorly disinfected influent, or from some point in the effluent pipe.
This happens mainly when there is no water circulation while the equipment is out of service. Bacteria reproduce, and turn the coal bed into a source of contamination.
In order to inhibit bacterial growth, the surface of the carbon is impregnated with metallic silver. The resulting carbon is called bacteriostatic.
Effects of silver on human health.
Silver has negative effects on man, only in very high doses. In cases of chronic ingestion of this metal (average daily dose of 0.14 g per kg of body weight, during 70 years) it can cause argyria, which consists of coloration, irreversible, grayish blue of the skin. This effect does not alter any functional organ, and is therefore considered 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 not contain
more than 0.05 parts per million of this element. The health legislation of several countries set this value as the maximum permissible for drinking water. Currently, due to evidence of its relative safety, the maximum permissible limits have become laxer, and for example, in the USA and Mexico, this value is 0.1 mg / l.
In order to comply with drinking water standards, it is important that silver impregnated with activated carbon is sufficiently well bound to it to prevent it from being released into the water. Such release, in addition to causing non-compliance with the standard, means that the carbon will lose its bacteriostatic protection in less time.
Methods of impregnation with silver in the activated carbon.
Not all methods of impregnation of silver into activated carbon achieve good fixation of this metal. There are basically three know methods: colloidal silver, chemical and electrochemical.
The colloidal silver method consists of preparing a solution in which the silver is found as a colloid. A colloid is a physical state that lies on the boundary between a suspended and a dissolved solid. The colloidal silver solution has the appearance of a silvery paint. It is bathed the activated carbon, so that the silver is applied as a layer of paint. Unfortunately, the silver is not sufficiently fixed, and therefore easily detached into the water.
Therefore, the colloidal silver impregnation method has been declared unacceptable in
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 high-temperature reduction reaction in which silver is chemically bonded to carbon.
The chemical impregnation method achieves good fixation and is therefore permitted in the USA as well as in Europe and Japan. However, there is always a small proportion of silver released into the treated water. Therefore, the carbon remains bacteriostatic for a relatively short period of time. Finally, the electrochemical method, consists in the deposition of silver on the surface of the carbon by means of an electric current that causes the reduction of silver. This method achieves a
much better metal-carbon fix, so it is considered the most acceptable method.
- Reverse osmosis water was used.
- The test was performed on a 5 cm diameter column. The carbon bed was 20 cm high. The volume of the bed was 0.393 dm3, and the amount of carbon was 216 grams. The water flow was 1.57 liters / min.
- The carbon used was BIOSTAT 20x50, impregnated with 0.1 % metallic silver.
Difference between a bactericidal agent and a bacteriostatic one.
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 treatment are chlorine, iodine, ozone, chlorine dioxide, chloramines and silver ions. The latter are dosed by electrolysis, 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+).
In contrast, a bacteriostatic agent is not dosed into the fluid, but remains fixed in the solid. Therefore, silver-impregnated carbon is bacteriostatic but not bactericidal. In other words, it serves to inhibit bacterial growth on its surface, but does not guarantee the annihilation of microorganisms if they are carried by water in relatively high concentrations.
Method for the quantitative analysis of the impregnated silver content in activated carbon.
To achieve a good analysis, a double extraction is necessary: first, with a solution of nitric acid, at reflux. Secondly, with ammonium hydroxide solution, also at reflux.
1. De Zuane, J.: HANDBOOK OF WATER QUALITY, 2nd. Ed., Wiley, N.Y., 1997
2. Budavari, S. (Ed.): THE MERCK INDEX, 12th. Ed., Merck & Co., Inc., N.J., 1996
3. Groso, G.: EL CARBON ACTIVADO GRANULAR EN EL TRATAMIENTO DEL AGUA, Aconcagua,
4. Nalco: MANUAL DEL AGUA, Tomo III, Mc Graw Hill, Mexico, 1989
5. U.S.E.P.A., Office of Water: “SILVER. Drinking Water Health Advisory”, Abril 1991