Environ. Sci. Technol. 2001, 35, 1157-1163
In Situ Spectroscopic and Solution Analyses of the Reductive Dissolution of MnO2 by Fe(II) JOHN E. VILLINSKI,† PEGGY A. O’DAY,‡ TIMOTHY L. CORLEY,† AND M A R T H A H . C O N K L I N * ,† Department of Hydrology and Water Resources, The University of Arizona, P.O. Box 210011, Tucson, Arizona 85721-0011, and Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404
The reductive dissolution of MnO2 by Fe(II) under conditions simulating acid mine drainage (pH 3, 100 mM SO42-) was investigated by utilizing a flow-through reaction cell and synchrotron X-ray absorption spectroscopy. This configuration allows collection of in situ, real-time X-ray absorption near-edge structure (XANES) spectra and bulk solution samples. Analysis of the solution chemistry suggests that the reaction mechanism changed (decreased reaction rate) as MnO2 was reduced and Fe(III) precipitated, primarily as ferrihydrite. Simultaneously, we observed an additional phase, with the local structure of jacobsite (MnFe2O4), in the Mn XANES spectra of reactants and products. The X-ray absorbance of this intermediate phase increased during the experiment, implying an increase in concentration. The presence of this phase, which probably formed as a surface coating, helps to explain the reduced rate of dissolution of manganese(IV) oxide. In natural environments affected by acid mine drainage, the formation of complex intermediate solid phases on mineral surfaces undergoing reductive dissolution may likewise influence the rate of release of metals to solution.
Introduction One of the current challenges in geochemistry is to identify solid phases that influence the rates of environmentally important dissolution/precipitation reactions. These solids, which are often low in concentration, persist for seconds to hours or longer and are typically amorphous, poorly crystalline, or present as surface coatings. While it has been shown that reduced-Mn solid phases are formed during the reductive dissolution of MnO2 by As(III) (1), Cr(III) (2), oxalate (3), and Co(II)-EDTA (4), little is known about the reductive dissolution of MnO2 by Fe(II). The formation of MnOOH during the reductive dissolution of birnessite by Fe(II) was discounted based on solution chemistry (5). However, a nonstoichiometric release of Mn(II) to solution has been observed during the reductive dissolution of manganese oxide-coated alluvial materials by Fe(II) (6), with reduced manganese presumably incorporated into an amorphous solid undetectable by X-ray diffraction (XRD) (7). In this study, we use a spectroscopic method to examine the formation of * Corresponding author e-mail:
[email protected]; phone: (520)621-5829; fax: (520)621-1422. † The University of Arizona. ‡ Arizona State University. 10.1021/es001356d CCC: $20.00 Published on Web 02/16/2001
2001 American Chemical Society
an intermediate phase during the reductive dissolution of MnO2 by Fe(II). Synchrotron X-ray absorption spectroscopy (XAS) is a powerful in situ technique for probing geochemical or environmentally relevant samples that contain parts per million concentrations of the target element in a complex matrix, often in contact with an aqueous phase (8, 9). X-ray absorption near-edge structure (XANES) spectra can be obtained on the order of minutes, sufficiently fast to monitor changes in the oxidation state and local molecular structure of aqueous species and solid phases in many dissolution and precipitation reactions but not fast enough to identify very short-lived (
