Tuesday, 11 July 2006

Unearthing the Connection between Arsenic Mobility and Reductive Iron Transformations.

Katharine J. Tufano and Scott Fendorf. Stanford University, Braun Hall, Bldg 320, Room 118, Stanford, CA 94301

Iron oxides are strong sorbents of arsenic, thereby regulating its fate and transport. As a consequence, reductive iron transformations can have pronounced influences on arsenic partitioning. In fact, the primary process by which arsenic is displaced from solids appears to be a shift from aerobic to anaerobic conditions. Nevertheless, it is presently unclear whether arsenic mobilization under anaerobic conditions results from a transformation from As(V) to As(III) or from mineralogical changes as a consequence of iron and manganese reduction. Furthermore, the specific conditions that may lead to arsenic release within anaerobic environments have yet to be fully resolved. Here we examine desorption behavior of arsenic from ferrihydrite-, goethite- and hematite-coated sand within aerobic and anaerobic systems under hydrodynamic conditions. It is widely believed that reductive transformation, inclusive of dissolution and recrystallization, releases arsenic from the solid phase, thereby leading to its displacement into solution and subsequent mobilization. However, we observe that ferrihydrite transformation limits (rather than promotes) desorption of As(III) under conditions of intense Fe(III) reduction, potentially through precipitation of a secondary arsenic-bearing ferrous-iron phase. Formation of reduced iron oxide phases through the generation of high Fe(II) concentrations (>0.8 mM), whether abiotically or biologically introduced, severely retards As(III) desorption. Conversely ferrihydrite transformation resulting at lower Fe(II) concentration (<0.8 mM) does not appear to retard arsenite desorption from iron oxides—although they do not promote it either. Interestingly, we observe the greatest arsenite mobilization in the absence of Fe(II) and resulting mineralogical transformations of ferrihydrite. In contrast to ferrihydrite, desorption of As(III) from both goethite- and hematite-coated sands is not appreciably impacted by either bacterially-produced or abiotically-introduced aqueous Fe(II). Thus, long-term respiration on Fe(III) within goethite or hematite dominated systems would result in arsenic release to the dissolved state through a degradation of the principal sorbent. While As(III) desorption from hematite and goethite appear to fit the current paradigm of arsenic release under anaerobic conditions, (bio)mineralization of ferrihydrite may in fact stabilize As(III), limiting its mobility under hydrodynamic conditions.

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