Reactivity of Ferric Oxides toward H 2 S at Low pH STEFAN PEIFFER* AND WINFRIED GADE BayCEER, Department of Hydrology, Universita ¨t Bayreuth, Universita ¨tsstrasse 30, D-95440 Bayreuth, Germany Goethite (gt), 2-line (2lfh), 6-line ferrihydrite (6lfh), and schwertmannite (shm) were reacted with H 2 S under steady- state conditions at pH 2.5-5 using a novel setup that consisted of an electrochemical sulfide generator transporting H 2 S into a suspension of oxides in a fluidized bed- reactor. The reactants were stoichiometrically converted into Fe 2+ and S°. Sulfate significantly inhibited the rates. Surface area normalized rates increased with pH. They depended on the mineral type following the order gt > 2lfh > 6lfh, contrasting previous observations at circumneutral pH with an inverse order. The rate of shm dissolution was highest probably due to direct reduction of dissolved ferric iron by H 2 S. A linear relationship between the observed rate and the surface species of H 2 S was derived from a surface complexation approach that allowed for the estimation of intrinsic rate constants k intr for the various oxides (38 min -1 , 1.4 min -1 , 0.29 min -1 for gt, 2lfh, and 6lfh, respectively). k int decreased with increasing ∆G f for the reactions and seems to depend on the reduction potential of the solution. From this observation we derived the hypothesis that k int is determined by the flat-band potential of the mineral/solute interface at low pH while being affected by surface interactions at higher pH. Introduction The interaction between ferric oxides (in the following the term oxides will be used synonymously for both oxyhydrox- ides and oxides) and H2S is of fundamental relevance for the cycling of sulfur in both marine and freshwater organic-rich sediments (1-3) and may play a key role for the burial of sedimentary pyrite (4, 5). The reaction mechanism has been the subject of a number of studies and is reasonably well understood (6-10). It is generally assumed that the reaction rate is proportional to a surface complex between the ferric oxide surface and a sulfide species. The reactivity toward dissolved sulfide varies significantly between ferric oxides (2, 8, 11). At the pH of seawater (pH 7.5), the rate of reductive dissolution of ferric oxide minerals by sulfide was found to increase in the order goethite < hematite < magnetite , lepidocrocite ≈ hydrous ferric oxides (11). The oxidation products formed upon reaction of H2S with ferric oxides are mainly elemental sulfur (6, 7, 9, 10, 12), which is regarded to play an important role in the recycling of sulfate in marine sediments by bacterial disproportionation (3, 13). Only recently, it could be demonstrated that also in an acidic environment affected by acid mine drainage cycling of sulfur occurs, the process being directly related to the alkalinity budget of the whole system (14). Sulfide generated through sulfate reduction is completely reoxidized by the high concentrations of solid-phase ferric iron in the sediments of an acidic mining lake, that consist mainly of goethite and schwertmannite, in a coupled chemical and microbial process (14, 15). Under these conditions, the reactivity appears to be fairly low. An empirical rate law was established for the pH range 5-7(9), according to which the oxidation rate with lepidocrocite decreases by 2 orders per unit of pH decrease. Moreover, the reductive dissolution of iron oxides may be significantly affected by the adsorption of dissolved species at the oxide surface (10-12, 16, 17). The presence of adsorbed seawater solutes has been shown to decrease sulfide oxidation rates during reaction with freshly precipitated hydrous ferric oxide (10) and with 2-line ferrihydrite (12). Reaction rates are significantly inhibited by competition with dissolved sulfate for available surface sites at lower pH (12), a process that should be relevant in an environment affected by acid mine drainage with its high sulfate concentrations. Hitherto, no systematic study exists that addresses the reactivity of H2S toward various ferric oxides at low pH. Moreover, a general concept to describe the dependency of the rate of reductive dissolution of ferric oxides by dissolved sulfide that accounts for surface and bulk properties of the various iron minerals is still missing. It is generally assumed that the rate depends on the free energy of formation of the ferric oxides (8, 11). The kinetic experiments, however, on which this assumption is based, had been performed far from equilibrium. In this study we aim to compare mineral reactivities of goethite, 2- and 6-line ferrihydrite, and schwertmannite at pH values between 2.6 and 5 and to identify the reaction products. In order to overcome drawbacks from initial-rate experiments previously used for most of the studies, a fluidized-bed reactor was developed that allows for experi- ments to be performed under steady-state conditions. Additionally, we aim to understand the effect of sulfate which occurs at high concentrations in acidic mining environments and which potentially may inhibit the reactivity of the minerals. Materials and Methods Experimental Setup. A fluidized-bed reactor was developed that allows for the determination of reaction rates at steady- state conditions. The experimental setup is described in full detail in the Supporting Information, and a brief summary will be given in the following. The experimental setup consisted of four components: (i) a carrier solution of constant pH, (ii) a sulfide source, (iii) the reactor vessel, and (iv) various measuring devices. Hydrogen sulfide was elec- trochemically generated from sulfate using a coulometric sulfide generator (AMT G 200, Germany). The generator was supplied with an acidic carrier solution that consisted of sulfuric acid (c ∼ 100 µmol L -1 ) and NH4BF4 (c ) 20 mmol L -1 ). The desired pH of the carrier solution was adjusted by adding suitable amounts of NH3 (up to 20 mmol L -1 to obtain a pH of 5). H2SO4 was completely converted into H2S under these conditions. The reactor consisted of a modified separating-funnel (V ) 125 mL) that was positioned upside down and to which a continuous flow of 3 mL min -1 of H2S containing carrier solution (c ) approximately 10 -4 mol L -1 ) was supplied. The H2S concentration (amperometric H2S sensor, AMT, Germany) and the pH (Int Lab 412 pH argental electrode with a silver ion barrier, Mettler Toledo, Germany) were continuously measured inside the reactor. At the outflow, samples were taken and analyzed with respect to Fe 2+ ,S 0 , SO4 2- , and S2O3 2- . Fe 2+ and periodically total Fe after addition * Corresponding author phone: ++49-921-552251; fax: ++49- 921-552366; e-mail: s.peiffer@uni-bayreuth.de. Environ. Sci. Technol. 2007, 41, 3159-3164 10.1021/es062228d CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 9, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3159 Published on Web 03/27/2007