Pretreatment of e-waste and mutation of alkali-tolerant cyanogenic bacteria promote gold biorecovery

Gold was recovered from electronic scrap material (ESM) as gold–cyanide complex by Chromobacterium violaceum which produces cyanide as a secondary metabolite. The effect of pretreatment and mutation of alkali-tolerant bacteria was examined. Pretreatment dissolved most of the base metals, thereby reducing competition for the cyanide ion from other metals. As the pKa of HCN is 9.3, alkaline pH increases the cyanide ion concentration available for bioleaching, and the bacteria were mutated to grow at pH 9, 9.5 and 10. Results showed that at 0.5% pulp density of pretreated ESM, mutated bacteria attained gold biorecovery of 18% at pH 9, 22.5% at pH 9.5 and 19% at pH 10 while that of unadapted bacteria (at pH 7) yielded only 11% recovery. Results showed that gold bioleaching efficiency from electronic scrap was enhanced under alkaline conditions with mutated bacteria compared to bioleaching at physiological pH (around 7) of C. violaceum.