Zinc precipitation by heavy-metal tolerant sulfate-reducing bacteria enriched on phosphogypsum as a sulfate source

In the present study, heavy-metal tolerance and precipitation by a mixed culture of sulfate-reducing bacteria (SRB) were evaluated. These bacteria have been enriched during a previous study from a sewage sludge using phosphogypsum as sulfate source. Taking into account that both sulfate and zinc are naturally occurring in phosphogypsum, zinc tolerance of SRB was tested in synthetic media containing 20 mM sulfate and zinc chloride at concentrations ranging from 0 to 200 mg L−1. Zinc tolerance was determined by bacterial growth susceptibility and zinc removal monitoring. Bacterial growth and sulfate reduction were possible between 10 and 150 mg L−1 of initial zinc concentration. Zinc concentrations more than 150 mg L−1 were lethal to SRB. Zinc was removed effectively by SRB to less than 5% from medium containing 150 mg L−1 initial zinc concentrations or less. Energy-dispersive X-ray analysis showed that precipitation of zinc occurred in the form of sulfide. The results presented in this paper have shown that this mixed culture might be of use for bioremediation of sulfate and heavy-metals containing wastewaters.