Novel biosulfidogenic system for selective recovery of metals from acidic leach liquors and waste streams

Many heavy metals, such as cadmium, copper and zinc, form highly insoluble sulphides and are readily removed from aqueous solution in contact with trace amounts of hydrogen sulphide. Since metal sulphides have differing solubility products, it is possible to separate these metals by controlling concentrations of the reactant S2-. Hydrogen sulphide is a weak acid with two dissociation constants ( pK(1)=6.9, pK(2) approximate to 14), and solution pH may therefore be used to control the relative concentrations of the anionic dissociation products (HS2 and S2-). Some microorganisms ( the sulphate reducing bacteria or SRB) can reduce sulphate to hydrogen sulphide, using either simple carbon compounds or hydrogen as energy sources. Biogenic sulphidogenesis has the advantage in commercial operations, in that it can be established at the site of use, thereby avoiding the transport of hydrogen sulphide, which is highly toxic. However, a major factor that constrains the design and application of sulphidogenic remediation systems is the sensitivity of known SRB to even mild acidity (pH < 5). Systems are usually engineered so that the SRB reactors are `offline' and direct contact of the SRB with acidic wastewaters is thus avoided. This paper describes the development of a syntrophic sulphidogenic consortium that can operate at low pH and could therefore be utilised in simple `inline' reactor systems. Sulphidogenesis has been demonstrated in acidic (pH 3-4) liquors using glycerol, acetic acid and hydrogen as energy sources. In contrast to some other biosulphidogenic systems, this microbial consortium completely oxidises organic substrates to CO2, thus optimising the efficiency of the process. By controlling the pH of test solutions containing both soluble iron and zinc, it has been shown that this consortium leads to the selective precipitation of zinc as ZnS. The feasibility of using this microbiological system to recover (and reuse) metals from pregnant liquors and acidic waste streams, such as acid mine drainage, is discussed.