Simultaneous gold and silver recovery in microbial fuel cells operating in a short-circuited mode

We suggest a novel bioelectrochemical approach for simultaneous precious metals recovery from Au(I)- and Ag(I)-dithiosulfate complexes with equivalent concentrations of Ag⁺ and Au⁺ in a two-chamber microbial fuel cell. The graphitized paper, used as a cathode, was short-circuited with the bioanode of a sediment microbial fuel cell thus operating the system as a microbial electrochemical snorkel without additional energy input. The generated current, the cathodic, and the anodic potentials were continuously recorded. Cyclic voltammetry was carried out as a fast method for the determination of the ion concentration depletion in the catholyte. The bimetallic deposits on the cathode surface have been characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, etc. It was established that the silver and gold particles are deposited at zero valence state only under polarization in proportion 0.89:1.00 on the cathode. The Au-Ag recovery of 96.7 ± 3.3 % was achieved for 24h of operation in short-circuited mode, and the cathodic efficiency approached 100 %. The obtained bimetallic deposits reveal better performance as electrocatalysts for hydrogen evolution reactions compared to deposits obtained from the individual thiosulfate complexes of both metals suggesting their combined use as modified cathodes.