Use of biogenic sulfide for ZnS precipitation

A 600 ml continuously stirred tank reactor was used to assess the performance of a zinc sulfide precipitation process using a biogenic sulfide solution (the effluent of a sulfate-reducing bioreactor) as sulfide source. In all experiments, a proportional-integral (PI) control algorithm was used to control the pH and the sulfide (S2−) concentration at the desired level in the precipitator. The pS (defined as: −log [S2−]) and pH were optimised using a chemical Na2S solution as sulfide source. A S2− concentration of 10−15 M (i.e. pS 15) was found to be optimal for zinc sulfide precipitation, resulting in a residual zinc concentration of 0.07 mg/l from a 3 g/l Zn2+ influent, for both chemical Na2S and biogenic sulfide solutions. The mean particle size of the ZnS precipitates at pS 15 and pH 6.3 was 7.5 and 10.2 μm when using biogenic sulfide and chemical Na2S, respectively, indicating that both sulfide sources are adequate for solid–liquid separation by sedimentation. When biogenic sulfide instead of chemical Na2S was used, the efficiency of the ZnS precipitation process slightly decreased both in terms of zinc effluent concentration (at pS 10 and 20) and particle size of the precipitate (at pS 10, 15 and 20). This was shown to be attributed to the presence of various substances (phosphate, micro-nutrients, acetate, EDTA) present in the sulfate-reducing reactor effluent.