Activated Carbon Cloth as Anode for Sulfate Removal in a Microbial Fuel Cell FENG ZHAO, † NELLI RAHUNEN, ‡ JOHN R. VARCOE, † AMREESH CHANDRA, † CLAUDIO AVIGNONE-ROSSA, ‡ ALFRED E. THUMSER, ‡ AND ROBERT C. T. SLADE* ,† Chemical Sciences, Biological Sciences, University of Surrey, Guildford, GU2 7XH, United Kingdom Received February 6, 2008. Revised manuscript received April 2, 2008. Accepted April 9, 2008. By employing the sulfate-reducing bacterium Desulfovibrio desulfuricans we demonstrate the possibility of electricity generation in a microbial fuel cell (MFC) with concomitant sulfate removal. This approach is based on an in situ anodic oxidative depletion of sulfide produced by D. desulfuricans. Three different electrode materials, graphite foil (GF), carbon fiber veil (CFV), and high surface area activated carbon cloth (ACC), were evaluated for sulfide electrochemical oxidation. In comparison to CFV and GF electrodes, ACC was a superior material for sulfide adsorption and oxidation and showed significant potential for harvesting energy from sulfate-rich solutions in the form of electricity. Sulfate (3.03 g dm -3 ) was removed from a bacterial suspension, which represented 99% removal. A maximum power density of 0.51 mW cm -2 (normalized to geometric electrode area) was obtained with a one-chamber, air- breathing cathode and continuous flow MFC operated in batch mode at 22 °C. Introduction Sulfate-rich wastewaters are generated by many processes: they are present in waste streams from animal husbandry, mining, food processing, the pharmaceutical industry, pulp and paper wastewater, etc (1). Numerous adverse effects from pollution by sulfur compounds are already known: (a) they affect the aquatic ecosystem by increased acidity; (b) the odors of wastewaters are commonly from generation of sulfur compounds when sulfate-reducing bacteria use sulfate as a terminal electron acceptor for respirations (2, 3); and (c) such gaseous sulfur-based compounds raise serious health risks and can be corrosive to metals and concrete. For these reasons, a large amount of effort and expense has been undertaken to treat sulfate-rich wastewater. Biological sulfate reduction has been recognized as an efficient method (4); however, the main problem related to this process is due to generation of sulfide that inhibits bacterial growth, decreases the rate of sulfate reduction, and causes physical or biological constraints that may lead to process failure (5). The lack of development of a low-cost, high-efficiency desulfurization process remains the principal barrier for treatment of sulfate- rich wastewaters. Utilizing microbial metabolism to produce an electrical current from the degradation of organic/inorganic matter provides an elegant solution for simultaneous wastewater treatment and electricity generation; these systems are termed microbial fuel cells (MFCs) and represent a clean and renewable energy resource. To date little research effort has been reported regarding development of MFCs for sulfate removal from wastewater treatment: Habermann et al. (6) reported a MFC where the sulfate was biologically reduced to sulfide, which was then catalytically oxidized to sulfate on a metal hydroxide-modified graphite anode; Cooney et al. (7) utilized the same anodic reaction mechanism (see following) for electricity generation in a MFC where charcoal was used as the anode sulfate 9 8 D . desulfuricans... sulfide 9 8 anode sulfate (1) Rabaey et al. (8, 9) developed a MFC that used graphitic granule anode and ferricyanide for the cathodic reaction, where a different anodic oxidation mechanism was reported compared with earlier work (6, 7), and involved removal of sulfate (8) and sulfide (9) from wastewater with solid sulfur deposition on the anode sulfate 9 8 Paracoccus... sulfide 9 8 Paracoccus... + anode sulfur (2) Applications of MFCs are currently limited because of low power densities generated. Utilization of higher surface area electrode materials appears to be a general trend for achieving increased power output (10). Activated carbon is notable as an odor adsorbent for wastewater treatment applications (11, 12). Adsorptive removal (13) and catalytic oxidation of hydrogen sulfide (14, 15) have also been successfully demonstrated with activated carbon cloth (ACC), which has the advantage of having a high specific surface area, mechanical integrity, and ease of handling. To the best of our knowledge, there has been no reported use of ACC as a MFC anode. In this study, three different carbon-based materials, ACC, carbon fiber veil (CFV), and graphite foil (GF), were evaluated for in situ electrochemical sulfide oxidation. The sulfate- reducing bacterium D. desulfuricans was used as biocatalyst for sulfate reduction in solution. A one-chamber, air- breathing cathode and continuous flow MFC was designed for obtaining high power output and achieving highly efficient sulfate removal. Experimental Section Anode Materials. Two different activated carbon cloths (ACCs), denoted as CTEX-20 and CTEX-27, were obtained from MAST Carbon Advanced Products Ltd. U.K. Graphite foil (GF) is commercially available from SGL Technologies GmbH, Germany. Carbon fiber veil (CFV) was supplied by Technical Fiber Products Limited, U.K. Electrode Preparation. ACC, CFV, and GF were cut to size for use as the working electrodes with titanium wires (Advent, 0.5 mm thick) inserted as the terminal. The reference electrode was a Ag/AgCl type (BASi, 3.0 mol dm -3 NaCl, +0.196 V versus standard hydrogen electrode at 25 °C). If not mentioned otherwise, all potentials reported were converted to the SHE reference scale. Platinum wire or CFV served as counter electrode in a standard electrochemical three- electrode chamber. * Corresponding author phone: +44 1483 682588; fax: +44 1483 686851; e-mail: R.Slade@surrey.ac.uk. † Chemical Sciences. ‡ Biological Sciences. Environ. Sci. Technol. 2008, 42, 4971–4976 10.1021/es8003766 CCC: $40.75  2008 American Chemical Society VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4971 Published on Web 06/03/2008