Two-Step Bioleaching and Spent Medium Leaching of Gold from Electronic Scrap Material Using Chromobacterium violaceum

Rapid technological advancement and the relatively short life time of electronic goods have resulted in an alarming growth rate of electronic waste which often contains significant quantities of toxic and precious metals. Compared to conventional chemical recovery methods, bioleaching has been shown to be an environmentally friendly process for metal extraction. In this work, gold bioleaching from electronic scrap material (ESM) was examined using batch cultures of the bacterium Chromobacterium violaceum which produces cyanide as a secondary metabolite. Gold was bioleached via gold cyanide complexation. The ESM was pretreated using nitric acid to dissolve the base metals (mainly copper) in order to reduce competition for the cyanide ion from other metals present in ESM. ESM was added to the bacterial culture after it reached maximum cyanide production during early stationary phase. Spent medium bioleaching using bacterial cell- free metabolites showed a higher gold recovery of 18%, compared to that of two-step bioleaching of 11% at 0.5% w/v pulp density of ESM. Gold bioleaching was further enhanced to 30% when the pH of the spent medium was increased to shift the equilibrium in favor of cyanide ions production. Spent medium bioleaching of pretreated ESM yield a higher gold recovery compared to two-step bioleaching at a pulp density of 0.5% w/v.