| Abstract
| - Fe(II) present at surfaces of iron-containing minerals canplay a significant role in the overall attenuation ofreducible contaminants in the subsurface. As the chemicalenvironment, i.e., the type and arrangement of ligands,strongly affects the redox potential of Fe(II), the presenceof various mineral sorbents is expected to modulate thereactivity of surficial Fe(II)-species in aqueous systems. Ina comparative study we evaluated the reactivity offerrous iron in aqueous suspensions of siderite (FeCO3),nontronite (ferruginous smectite SWa-1), hematite (α-Fe2O3),lepidocrocite (γ-FeOOH), goethite (α-FeOOH), magnetite(Fe3O4), sulfate green rust (FeII4FeIII2(OH)12SO4·4H2O), pyrite(FeS2), and mackinawite (FeS) under similar conditions(pH 7.2, 25 m2 mineral/L, 1 mM Fe(II)aq, O2 (aq) < 0.1 g/L).Surface-area-normalized pseudo first-order rate constantsare reported for the reduction of hexachloroethaneand 4-chloronitrobenzene representing two classes ofenvironmentally relevant transformation reactions ofpollutants, i.e., dehalogenation and nitroaryl reduction.The reactivities of the different Fe(II) mineral systems variedgreatly and systematically both within and between thetwo data sets obtained with the two probe compounds. Asa general trend, surface-area-normalized reaction ratesincreased in the order Fe(II) + siderite < Fe(II) + iron oxides< Fe(II) + iron sulfides. 4-Chloronitrobenzene wastransformed by mineral-bound Fe(II) much more rapidlythan hexachloroethane, except for suspensions of hematite,pyrite, and nontronite. The results demonstrate thatabiotic reactions with surface-bound Fe(II) may affect oreven dominate the long-term behavior of reducible pollutantsin the subsurface, particularly in the presence of Fe(III)bearing minerals. As such reactions can be dominated byspecific interactions of the oxidant with the surface,care must be taken in extrapolating reactivity data of surface-bound Fe(II) between different compound classes.
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