BIOTRANSFORMATION OF CADMIUM SPECIES BY MICROORGANISMS

Experiments to study the biotransformation of chemical forms of cadmiu m by Pseudomonas sp. bacteria were carried out in a nutrient medium so lution having a cadmium concentration from 0.7 to 14 mg l(-1). Culture s of these bacteria were isolated from a polluted reservoir, a special ized burial site of industrial waste which has been operational for ab out 25 years. During this period, as a result of adaptation, the local microflora have become tolerant of high concentrations of various pol lutants, in particular, to high concentrations of heavy metals. The ba cteria exhibit high tolerance of such elements as cadmium and zinc. Th e kinetics of biotransformation of cadmium in aqueous solutions by the action of these resistant strains of microorganisms, were investigate d by means of measurement of the cadmium concentration in the solution s with the bacteria and in the gas phase over the solutions. The chang e in the cadmium concentration with time, after the bacterial cultures are added, is most pronounced in the first few days. During this peri od the cadmium concentration decreases to 20-30% of the initial value. Variations in cadmium concentrations in the following days can be exp lained by the release of cadmium to solution because of lysing of dead cells and its accumulation by new generations of microorganisms. In t he study of biosorption processes, a relatively uniform decrease in th e total cadmium content was detected, which was explained by the forma tion of volatile cadmium species by these bacteria. By collecting vola tile spices of cadmium in a graphite furnace (GF) by electrostatic dep osition and by ETA-AAS determination of cadmium the presence of signif icant quantities of cadmium in the gas phase above the bacterial cultu res was observed. Transformation of a part of the total cadmium into v olatile species may be explained by interaction of cadmium ions with d imethyl disulphide, which formed as a byproduct of bacterial action an d was detected by GC measurements; the volatile dimethyl cadmium forme d as the final product.