Electricity Generation from Synthetic Acid-Mine Drainage (AMD) Water using Fuel Cell Technologies SHAOAN CHENG, BRIAN A. DEMPSEY, AND BRUCE E. LOGAN* Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A Received May 23, 2007. Revised manuscript received August 27, 2007. Accepted September 12, 2007. Acid-mine drainage (AMD) is difficult and costly to treat. We investigated a new approach to AMD treatment using fuel cell technologies to generate electricity while removing iron from the water. Utilizing a recently developed microbial fuel cell architecture, we developed an acid-mine drainage fuel cell (AMD- FC) capable of abiotic electricity generation. The AMD-FC operated in fed-batch mode generated a maximum power density of 290 mW/m 2 at a Coulombic efficiency greater than 97%. Ferrous iron was completely removed through oxidation to insoluble Fe(III), forming a precipitate in the bottom of the anode chamber and on the anode electrode. Several factors were examined to determine their effect on operation, including pH, ferrous iron concentration, and solution chemistry. Optimum conditions were a pH of 6.3 and a ferrous iron concentration above ∼0.0036 M. These results suggest that fuel cell technologies can be used not only for treating AMD through removal of metals from solution, but also for producing useful products such as electricity and recoverable metals. Advances being made in wastewater fuel cells will enable more efficient power generation and systems suitable for scale-up. Introduction Acid-mine drainage (AMD) is a serious environmental problem caused by the biological oxidation of metal sulfides to metal sulfates. AMD is toxic to aquatic life because of its low pH and the solubilization of metals such as lead, copper, cadmium, and arsenic. The goals of AMD treatment are to raise the pH of the water and to achieve controlled removal of iron and other metals. Increasing the pH and provision of an oxidizing agent result in iron precipitation, which is shown by the following two reactions: Fe 2+ ) Fe 3+ +e – (1) Fe 3+ + 3H 2 O ) Fe(OH) 3 (s) + 3H + (2) Passive treatment of AMD is accomplished through an increase of pH (1, 2), oxygen transfer (3), and precipitation and sedimentation of Fe(III) (hydr)oxides, along with bio- logical processes for effluent polishing that also accomplish Mn(II) removal (4–7). Active treatment often involves a rapid increase in pH (lime, caustic, or other base) followed by agitation and forced aeration. Electrolytic oxidation processes have also been used (8, 9). Although these methods are effective, they can be costly, and recovery of ferric oxides for reuse is difficult (10, 11). Thus, conventional passive and active processes typically provide no other immediate benefit other than treatment. It is proposed here that AMD can be treated using new types of fuel cells being developed to generate electricity from wastewater. Typical fuel cells generate electricity from gases or liquid fuels such as hydrogen or methanol. However, new types of microbial fuel cells (MFCs) have been developed that are suitable for generating electricity from the bacterial oxidation of organic matter such as acetate, glucose, and domestic wastewater (12–22), and inorganic matter such as sulfides (23). The first requirement for using a fuel cell is that the overall reaction is energetically favorable. When ferrous iron is oxidized with oxygen, the overall reaction is that shown in eq 3: Fe 2+ + 1 / 4 O 2 + 5 / 2 H 2 O ) Fe(OH) 3 (s) + 2H + (3) Under standard conditions ([H + ] ) 1 M, pH ) 0), this reaction is thermodynamically favorable, but it has a small free energy (ΔG° )-27.15 kJ/mol). This reaction is strongly pH de- pendent, however, and therefore, increasing the pH makes the reaction more favorable. The overall reaction can be split into two half-cell reactions that can separately occur at the anode and cathode of a fuel cell, as shown by eqs 4 and 5: Fe 2+ + 3H 2 O ) Fe(OH) 3 (s) + 3H + + e – (Anode) (4) 1 / 4 O 2 + H + + e – ) 1 / 2 H 2 O (Cathode) (5) If AMD is treated using a fuel cell, ferrous iron is oxidized to ferric iron at the anode and precipitated (eq. 4), and O2 is reduced to water at the cathode (eq. 5) at an overall voltage of E cell 0 ) 0.28 V under standard conditions. The voltage under nonstandard conditions is given by eq 6, E ) E cell 0 - RT nF ln [H + ] 2 [Fe 2+ ]P O 2 (6) where E is the cell voltage, R is the universal gas constant, T is the temperature of the electrolyte, P O 2 is the partial pressure of oxygen, and [H + ] and [Fe 2+ ] are the activities of H + and Fe 2+ in solution, respectively. Assuming conditions (used in these experiments) of pH ) 6.3, P O 2 ) 0.21 atm, T ) 303 K, and [Fe 2+ ] ) 7 × 10 -3 M, the cell voltage could reach 0.899 V for hydrous ferric oxide (HFO) and a slightly greater voltage for goethite. The current generated in a fuel cell process cannot be predicted because of overpotentials at the electrodes and changes in solution chemistry in the cells and at the electrode surface. To demonstrate the feasibility of this new type of AMD fuel cell (AMD-FC) treatment process, we therefore examined the treatment of a synthetic AMD solution using a two-chambered MFC previously developed for electricity generation from domestic wastewater (24). We demonstrate here that we can achieve complete Fe 2+ removal and that we can generate electricity at power densities slightly lower than those achieved with organic substrates and bacteria. Several factors that could affect power production were examined, including pH, ferrous iron concentration, and solution chemistry. Experimental Section Fuel Cell Construction. The AMD-FCs were constructed based on a previous MFC design (24) from two plastic (Plexiglas) cylindrical chambers each 2 cm long by 3 cm in * Corresponding Author: Phone: 814-863-7908; Fax: 814-863-7304; E-mail: blogan@psu.edu. Environ. Sci. Technol. 2007, 41, 8149–8153 10.1021/es0712221 CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 23, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8149 Published on Web 10/19/2007