This work examined the efficacy of silver as a bactericidal agent against Escherichia coli. The minimum inhibitory concentration (MIC) for Ag(I) was observed to be between 0.1 mg Ag/l and 0.05 mg Ag/l following a 24-h incubation period at 25°C. Additional forms of silver that were effective included Ag2O, a protein-based silver and AgCl(s). All of these forms of silver resulted in MICs that were comparable to Ag(I). Predictions from a chemical equilibrium modelling system indicated that a silver-histidine complex may have contributed to the observed bactericidal activity. A MIC for colloidal metallic silver (Ag0) was not observed up to a total silver concentration of 82 mg Ag/l—the highest concentration evaluated. Moreover, aqueous silver was not detected at this total silver concentration. It was concluded from these findings that cationic Ag(I) or Ag(I)-complexes were responsible for the bactericidal activity of silver. In the batch systems evaluated, the MICs increased with time, over a 72-h incubation period, to values above the US Environmental Protection Agency Secondary Standard for silver, for all forms of silver tested. A desorption kinetics study indicated that less than 10% of silver was readily leachable from a granular activated carbon surface that was first saturated with silver.

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