Microbiological potential for gold leaching and recovery from E-waste

It is believed that production of electronic waste (e-waste) is one of the most rapidly growing problems in the world, about 5-8% of which being exported to developing countries, especially in Asia and Africa. From the viewpoint of its metal composition, e-waste can be a promising source for recycling of a number of valuable metals. Among them, this study focused on recycling of gold (Au). A two-stage biohydrometallurgical process; (i) bio-thiourea (CS(NH₂)₂; TU) leaching, followed by (ii) precipitation of bio-Au nanoparticles (bio-AuNPs) was set as the final objective of study. Chemical-TU leaching has been performed by a number of studies for Au recovery, yet it is generally considered that its implementation needs further cost-feasibility. Modification of this method was attempted by using the extremely acidophilic Fe-oxidizing archaeon, Acidiplasma sp. strain Fv-Ap, as the tool for microbiological redox potential control during the TU leaching. Several factors (such as TU/Fe(III) ratio and pulp density) were shown to affect Au recovery. Under the optimal condition at 3% pulp density, about 9% of Au was recovered by addition of strain Fv-Ap, while the recovery remained 7% in sterile control. Before attempting microbial Au() recovery from the TU-leached Au(I), fundamental studies were carried out using Au(III) reagent to optimize bio-AuNPs production by using the acidophilic Fe-reducing bacterium, Acidocella aromatica strain PFBC. Different concentrations of formate (as e^--donor; 1, 5, 1, and 2 mM) were tested for optimization. Use of higher formate concentrations allowed production of finer particles: 2 mM formate enabled production of the finest AuNPs (12 nm) with the highest density (97 particles/ cell). The possibility to recover Au from the resultant TU leachate was also tested. Au(I) reduction occurred slower compare to synthetic Au(III) reagent. Approximately 9% of Au(I) was reduced from the leachate. The results indicated that Bio-AuNPs method might be an alternative way to recover Au from solutions, yet further reaction optimization is necessary.