DOI: 10.14457/tu.the.2017.332
title: Recovery of silver from aqueous solution by using bio-electrochemical system (BES) technology

description: A large amount of silver (Ag) from global supply has been used in many industrial applications. Accordingly, the Ag(I)-laden wastewater, either in acidic or alkaline conditions, are normally discharged from these processes. Generally, Ag(I) exists in different forms such as free silver ions (Ag+), silver complexes, and multi-metal solutions. Therefore, for both economic and environmental aspects, recovery of silver from effluents, is essential.In this study, dual-chamber BES reactors are used to investigate the recovery of silver from different Ag(I)-containing solutions, accompanied with power generation and wastewater treatment. In each BES reactor, carbon brush serves as the anode inoculated by anaerobic sludge, while graphite plate works as the cathode. Depending on the target of experiments, either anion exchange membrane (i.e. AMI-7001S) or cation exchange membrane (i.e. CMI-7000S) is used as a separator. The research objectives are (i) to investigate the electrochemical reduction of Ag+ ions introduced as silver nitrate AgNO3; (ii) to examine the migration and diffusion of electrolytes through AMI-7001S and CMI-7000S membrane; (iii) to characterize the anodic microbial communities and identify the dominant species along with exoelectrogen; (iv) and to investigate the reduction of Ag(I) complexes (i.e. diammine complex ([Ag(NH3)2]+), dithiosulfate complex ([Ag(S2O3)2]3-), and mixed multi-metal solution at different initial Ag(I) concentration, to understand the difference in BES performance between them.Results and findings obtained in this study are summarized as follows:(i)The reduction of Ag+ ions from AgNO3 solution in the cathode chamber was a fast reaction, which almost completed within 24 hours of operation. A change in initial catholyte pH (2-7) and Ag+ concentration (100-1,000 mg/L) had little effect on Ag removal but significantly affected the power generation. After 24 hours, a removal efficiency (RE) of 96.54-99.28% and a maximum power density (Pmax) of 607-8,258 mW/m3 along with a Columbic efficiency (CE) of 4.36-21.61% were achieved when the initial Ag+ concentration was varied from 100 to 1,000 mg/L. These results did not take into account the diffusion of Ag+ ions through CMI-7000S membrane.(ii)Effects of AMI-7001S and CMI-7000S membranes on the overall performance in BES reactors operated with AgNO3 solutions was demonstrated. A RE of 83.73-92.50%, and CE of 11.50-19.89% were obtained in CMI-based reactor after 24 hours with the initial Ag+ concentration of 100-1,000 mg/L, although some diffusion of Ag+ ions was observed. In contrast, the substrate loss through AMI-7001S caused low cell voltage production, and led to lower silver removal and power density in AMI-based reactor. This finding suggests that CMI-7000S showed better performance when compared to AEM-7001S at the same operational conditions. (iii)After running BES reactors for 73 days with AgNO3 solutions as the catholytes, the gene sequence results showed a considerable phylogenetic diversity in the anodic microbial communities, in which Firmulates, Proteobacteria, and Bateroidetes were dominant phyla. Geobacter sp. and other reported exoelectrogenic bacteria could be detected while no Shewanella-like sequences were retrieved. (iv)The reduction of silver complexes and mixed-metal solutions in BES reactors was also found in similar trends. However, as compared to free Ag+ ions which has high standard reduction potential ((Ag+/Ag0, E0 = 0.799 V versus the Standard Hydrogen Electrode (vs. SHE)) to facilitate the spontaneously electrochemical reduction in the cathode chamber, [Ag(NH3)2]+ and [Ag(S2O3)2]3- complexes showed a slower reduction rate due to their lower standard E0 (i.e. [Ag(NH3)2]+/Ag0, E0 = 0.373 V and [Ag(S2O3)2]3-/Ag0, E0 = 0.016 V vs. SHE).•The reduction of [Ag(NH3)2]+ complex (1000-3000 mg/L) could be completed with RE &gt; 96% when the duration was extended to 48 hours. Similarly to Ag+ ions, [Ag(NH3)2]+ complex also transported through CMI-7000S membrane, which led to the formation of silver deposits at the membrane surface and Ag adsorption in the anodic biomass. This diffusion affected the internal resistance and decreased the electricity production performance (i.e. Pmax) when the system was operated at high initial Ag(I) concentration (i.e. 3000 mg/L). The accumulated amount of silver in the biomass (i.e. mixture of suspended solid and the biofilm attached to the carbon brush anode) after operating BES reactor with [Ag(NH3)2]+ solutions at the initial concentration of 1000 – 3000 mg Ag/L was  0.853 mg Ag/g biomass.•In case of [Ag(S2O3)2]3- solutions (10, 20, and 30 mM), although lower RE was found (i.e. 81.87, 95.17, and 93.22%, respectively) at the same operation time (48 hours), there was no loss of Ag as the diffusion of negatively charged [Ag(S2O3)2]3- complex through CMI-7000S was avoidable. Electricity (i.e. a CE of 12.85 – 25.73%, a Pmax of 1500 – 2893 mW/m3) was produced simultaneously without any supplying of external energy, indicating the reactions could occur spontaneously in spite of low cathodic reduction potential (i.e. [Ag(S2O3)2]3-/Ag0 = 0.016 V vs SHE).•When synthetic mixed metal solution containing 10 mM Ag+, 10-20 mM Fe3+, and 1 mM Cu2+ was investigated, the cathodic reduction occurred differently, due to the difference in the standard reduction potential among these metallic ions (i.e.  Ag+/Ag0 = 0.799 V, Cu2+/Cu0 = 0.337 V, and Fe3+/Fe0 = -0.44 V vs. SHE). During the first 24 hours, Ag+ concentration (10 mM) reduced rapidly, while Fe3+ concentration (10 mM) slightly decreased, and Cu2+ concentration (1 mM) was almost constant. A high Ag removal efficiency of 99.71% was obtained in this case. However, the diffusion of Ag+, Fe3+, and Cu2+ through the membrane was observed, which is believed as the main reason for high removal efficiency of Cu (86.84%) and Fe (33.89%) after 48 hours. For electricity production, the increase of initial Fe3+ concentration, from 10 to 20 mM, helped to improve OCV and Pmax value (i.e. OCV increased from 514 to 645 mV, Pmax increased from 1542 to 2472 mW/m3).(v)After BES operation with different Ag(I)-containing solutions, different morphologies of silver deposits formed at the cathode surfaces were found under SEM, in which dendrite and crystal were the main structures. High purity of silver in the cathodic deposits was confirmed by EDS and XRD analysis, indicating one-step Ag recovery by BES technology, as compared to traditional methods for silver recovery where further steps for refinement are required.In conclusion, this study successfully demonstrated that BES technology can be employed for recovery of silver from different Ag(I)-containing solutions, coupled with wastewater treatment and electricity production without energy consumption. The vital roles of bacteria, especially exoelectrogen, in BES reactors was pointed out, indicating BES as a cost-effective method for the recovery of silver in particular and other metals in general.