Influence of sulfide concentration and macronutrients on the characteristics of metal precipitates relevant to metal recovery in bioreactors

Purity and settling properties determine metal sulfide recovery from bioreactors. The influence of macronutrients commonly present in mineral media and wastewaters on Cu, Pb, Cd and Zn depletion kinetics and characteristics was evaluated in batch experiments with chemically produced sulfide at different concentrations. The metal depletion kinetics showed that metals with slower depletion rates (Zn and Cd) are susceptible to other removal mechanisms such as biosorption onto the sulfate reducing biofilm and precipitation with macronutrients when sulfide is below the stoichiometric metal to sulfide ratio. For Zn, the main mechanism of removal is its sorption onto apatite (Ca-5(PO4)(3)(+)(OH-), a compound formed due to the presence of CaCl2 center dot 2H(2)O and KH2PO4 in the mineral medium. All precipitates were 8.1-10.0 mu m regardless the sulfide concentration demonstrating that this parameter is less relevant for particle growth and settling, compared to the agglomeration of the precipitates. (C) 2012 Elsevier Ltd. All rights reserved.