Electrochemical reduction of different Ag(i)-containing solutions in bioelectrochemical systems for recovery of silver and simultaneous power generation

Abstract

In this study, dual-chamber bioelectrochemical reactors (i.e., R1, R2, R3, and R4) were employed to investigate the Ag recovery and electricity production from different Ag(I)-containing artificial wastewaters (i.e., Ag⁺ solution, [Ag(NH₃)₂]⁺ and [Ag(S₂O₃)₂]³⁻ complexes, and mixed metal solution). Results showed that the electrochemical reductions of Ag(I) ions in all reactors were rapid reactions. The reaction rate in R1 was the fastest. At the same initial conditions (i.e. Ag(I) concentration of 1000–1080 mg L⁻¹), the Ag recovery efficiency was 81.8% for R3 operated with the [Ag(S₂O₃)₂]³⁻ complex. Although high Ag removal efficiency (i.e., >99%) was found in other reactors, some diffusion of positively charged Ag(I) ions through the membrane was also observed along with the electrochemical reduction. In all cases, pure silver electrodeposits, mainly as dendrites and crystals in different morphologies, were observed at the cathode surfaces when characterized by SEM, EDX, and XRD. The performance of electricity production was evaluated by the open circuit voltage (OCV) and maximum power density (P_max) obtained during the BES operation. Reactor R1 showed better performance (i.e., OCV of 828 mV, P_max of 8258 mW m⁻³), due to its high standard reduction potential. The lower performance in other reactors was due to the complexity of solutions, other co-existing metals (mixed metal solution), and lower standard reduction potential. In general, the existing forms of Ag(I) in solutions affect the Ag(I) reduction rate. This further influences the Ag removal efficiency, morphology of electrodeposits, and power generation.