Spontaneous reduction of low-potential silver(I) dithiosulfate complex in bioelectrochemical systems for recovery of silver and simultaneous electricity production

Abstract

ABSTRACT Waste fixer solutions generated from photographic processing are silver-rich effluents, in which silver exists in the form of a dithiosulfate complex ([Ag(S2O3)2]3−), a stable and water-soluble chemical compound. This study investigated the electrochemical reduction of [Ag(S2O3)2]3− at different initial concentrations for silver recovery, combined with electricity production in a two-chamber bio-electrochemical system using a cation exchange membrane as the separator. During the biological oxidation of acetate to produce electrons in the anode chamber, [Ag(S2O3)2]3− was reduced spontaneously by acting as an electron acceptor in the cathode chamber, despite its low standard redox potential ([Ag(S2O3)2]3−/Ag0, E 0 = 0.016 V). After 48 h in each batch of operation, a Ag recovery efficiency of 81.7–95.2%, with a columbic efficiency of 12.9–21.4% and a maximum power density of 1500–2647 mW/m3, were obtained with an initial [Ag(S2O3)2]3− concentration of 10–20 mM, respectively. When the initial [Ag(S2O3)2]3− concentration increased to 30 mM, cell voltage production did not improve significantly, and a small decrease in Ag recovery efficiency to 93.2% was found. After 61 days of operation, the cathode surface was covered by different-sized silver clusters under SEM observation. The results were confirmed by EDX and XRD characterization, in which metallic silver with high purity was detected. SEM-EDX-XRD characterization of the membrane and the measurements in the control reactor confirmed that there was no diffusion of negatively charged [Ag(S2O3)2]3− complex through the membrane. Thus, this study showed a successful recovery of Ag from the low-potential [Ag(S2O3)2]3− complex without energy consumption, secondary waste generation, and loss of Ag.