Recovery of rhodium(III) from solutions and industrial wastewaters by a sulfate-reducing bacteria consortium

A quantitative analysis of the rate of removal of rhodium(III) by a resting sulfate-reducing bacteria (SRB) consortium under different initial rhodium and biomass concentrations, pH, temperature, and electron donor was studied. Rhodium speciation was found to be the main factor controlling the rate of its removal from solution. SRB cells were found to have a higher affinity for anionic rhodium species, as compared to both cationic and neutral species, which become abundant when speciation equilibrium was reached. Consequently, a pH-dependent rate of rhodium removal from solution was observed. The maximum SRB uptake capacity for rhodium was found to be 66 mg of rhodium per gram of resting SRB biomass. Electron microscopy studies revealed a time-dependent localization and distribution of rhodium precipitates, initially intracellularly and then extracellularly, suggesting the involvement of an enzymatic reductive precipitation process. When a purified hydrogenase enzyme was incubated with rhodium chloride solution under hydrogen, 88% of the rhodium was removed within 1 h, whereas with a soluble extract from SRB 77% was removed within 10 min. Due to the low pH of the industrial effluent (1.31), the enzymatic reduction of rhodium by the purified hydrogenase was greatly limited, and it was apparent that an industrial effluent pretreatment was necessary before the application of an enzymatic treatment. In the present study, however, it was established that SRB are good candidates for the enzymatic recovery of rhodium from both aqueous solution and industrial effluent.