PM07 Pollutant Chemistry Transport, Cycling and Fate
Exhibit Hall
8:00 AM - Monday
(PM082) Fe(II)-Catalyzed Oxidation of As(III) in Sediment Columns.
Rader, K1, 2, Bisceglia, K3, Farley, K2, 4, Carbonaro, R2, Mahony, J2, Di Toro, D1, 4, 1 University of Delaware, Newark, DE, USA2 Manhattan College, Riverdale, NY, USA3 The Johns Hopkins University, Baltimore, MD, USA4 HydroQual, Inc., Mahwah, NJ, USA
ABSTRACT- Recent field studies have shown that arsenic often exist as reduced arsenic (As(III) and methylated arsenic species) in oxic surface waters and as oxidized arsenic (As(V)) in anoxic bottom waters of stratified lakes and reservoirs. This behavior, which is directly opposite to our expectations based on chemical thermodynamics, has in part been explained by the reduction and methylation of arsenic by photosynthetic algae in the surface waters. The presence of As(V) in the bottom waters however is not well understood. One possibility is that As(III) is oxidized by transient oxidants (e.g., superoxide, Fe(IV) intermediates) that are produced during the oxidation of Fe(II). Although in batch systems this process is only effective in oxidizing 20-30% of the initial As(III) concentration, we hypothesized that in sediments there is likely to be a continuous supply of Fe(II) to drive the As(III) oxidation reaction to completion. We have therefore conducted sediment column experiments and a detailed modeling analysis of the transport processes and chemical reactions near the sediment-water interface (following the kinetics of Hug et al. 2003). Results of the study show that a large fraction of the As(III) is in fact oxidized during Fe(II) oxidation, and that the continuous diffusion of oxygen from the overlying water and the continual supply of Fe(II) by desorption and diffusion from deeper sediments are critical in driving the As(III) oxidation reaction to near completion. Based on these results, we believe that Fe(II)-catalyzed oxidation of As(III) plays an important role in the overall cycle of arsenic in lakes and reservoirs and needs to be considered in future assessment studies.
Key words: oxidation, arsenic, iron, sediment