Recovery of silver from silver(I)-containing solutions in bioelectrochemical reactors Hu-Chun Tao a,⇑ , Zhu-You Gao a , Hui Ding a , Nan Xu a,⇑ , Wei-Min Wu b a Key Laboratory for Urban Habitat Environmental Science and Technology, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China b Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305-4020, USA article info Article history: Received 23 December 2011 Received in revised form 2 February 2012 Accepted 4 February 2012 Available online 13 February 2012 Keywords: Bioelectrochemical system Silver recovery Photographic waste abstract A novel approach was tested for metallic silver recovery and power generation by using cathodic reduc- tion in bioelectrochemical systems (BESs). In dual-chamber BESs (130 mL volume) with acetate as elec- tron donor on anode, both Ag + ions and Ag(I) thiosulfate complex in catholyte were reduced on cathode. The reduction rate of Ag + was more rapid than the Ag(I) complex as expected by energetic analysis. X-ray diffraction (XRD) analysis indicated that electrodeposits on cathodes from both catholyte were metallic silver with >91% purity. The feasibility of metallic silver recovery with the BESs was confirmed using sim- ulated photographic wastewater and up to 95% of Ag(I) removal was achieved. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Demand for precious metal silver is built on three main pillars: industrial and decorative uses, photography, and jewelry and sil- verware. In 2010, silver used was 13,818 tons for industrial appli- cations, 4734 tons in the jewelry market, 1425 tons in the silverware market and over 2863 tons in coins and medals accord- ing to the Silver Institute (www.silverinstitute.org). Photography has been a major silver consumption and for waste silver recovery although because of the growth of digital photography, the use of silver-based imaging by consumers has been steadily dropping for almost a decade. Various types of technologies to recover silver from photographic waste materials and industrial wastewater, such as replacement (Ye et al., 2009) and, ion exchange and chem- ical reduction (Blondeau and Veron, 2010) were developed. Electrolysis is the most common, effective method for sliver recov- ery from solution by electroplating it on a cathode (Chatelut et al., 2000). Bioelectrochemical systems (BESs) use microorganisms to cata- lyze an oxidation and reduction reaction at an anodic and cathodic electrode, respectively (Rabaey et al., 2009). When electrical power is harvested from this circuit, the system is called a microbial fuel cell (MFC). Using BESs for metal reduction have been tested for chromium(VI) reduction (Li et al., 2008), ferric iron reduction and ferrous iron oxidation (Heijne et al., 2006, 2007), permanganate reduction (You et al., 2006), Hg 2+ removal (Wang et al., 2011), and copper removal and recovery (Heijne et al., 2010; Tao et al., 2011a,b,c). In this study, a novel approach was proposed to remove and recover silver from Ag(I)-containing solutions via cathodic reduc- tion in BESs by incorporating BESs concept and electrolysis to- gether (Fig. 1). At the anode, organic electron donor (such as acetate) is oxidized to CO 2 and protons, and electrons are gener- ated. At the cathode, Ag(I) is reduced to metallic silver and depos- ited on the cathode. The source of Ag(I) can be either Ag(I) ion (Ag + ) or Ag(I) complex (such as[AgS 2 O 3 ]  ). Specially, in photographic wastewater, Ag(I) is present as silver halides, e.g. AgBr, a typical components of photographic emulsions and is dissolved upon treatment with thiosulfate as Ag(I) complex: S 2 O 3 2 þ AgBr !½AgS 2 O 3   þ Br  ð1Þ Therefore, the reduction of Ag(I) by using Ag + versus [AgS 2 O 3 ]  solution was investigated. The pH of photographic wastes was low, ranging between pH 2.0 and 4.0 in the presence of inorganic or/and organic acid based on the measurement of waste solution of black–white fixer formula No. 37 F-5, which were widely used in China (HGT3680-2000, China). At cathode, the electrochemical reaction could be: Ag þ ðaqÞþ e  ¼ Ag ðsÞ; E 0 ¼ 0:799 V ð2Þ or ½AgS 2 O 3   ðaqÞþ e ¼ ¼ Ag ðsÞþ S 2 O 3 2 ðaqÞ; E 0 ¼ 0:250 V ð3Þ When acetate is used as electron donor, at anode acetate is oxidized as CH 3 COO  þ 4H 2 O ¼ 2HCO 3  þ 9H þ þ 8e  ; E 0 ¼ 0:293 V ð4Þ 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.02.029 ⇑ Corresponding authors. Tel.: +86 755 26032007 (H.-C. Tao), tel.: +86 755 26035347 (N. Xu). E-mail addresses: taohc@pkusz.edu.cn (H.-C. Tao), xunan@pkusz.edu.cn (N. Xu). Bioresource Technology 111 (2012) 92–97 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech