The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions

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   Fig. 1.

   Arsenic K-edge x-ray absorption spectra. (a) XANES spectra for arsenic sulfide and oxide reference compounds compared with two sediment samples A and B. Total arsenic concentrations were 1.76 and 2.73 mmol·kg^–1 for sediment A and B, respectively. (b) EXAFS spectra and corresponding Fourier transforms for reference compounds and sediment sample B. Solid lines are data; dashed lines are nonlinear least-squares fits. (See Table 2, which is published as supporting information on the PNAS web site, for numerical fit results. Fit results for arsenopyrite and arsenic substituted into pyrite are published in ref. 12.)

   
   
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   Fig. 2.

   Iron K-edge EXAFS spectra. EXAFS spectra and corresponding Fourier transforms for sediment samples A and B compared with reference pyrite, freshly precipitated FeS and illite (see ref. 41 for quantitative fits), and a mechanical mixture of pyrite plus illite (50:50 wt % Fe). Solid lines are data, dashed lines are nonlinear least-squares fits, and the shaded areas indicate the fraction of the fit that is composed of pyrite (sulfide component). The nonsulfide component of the sediments is a composite of iron-bearing phyllosilicate and oxide phases (see Table 3 for numerical fit results).

   
   
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   Fig. 3.

   Biogeochemical model. Shown is a summary of adsorption and precipitation reactions that control arsenic uptake and release in the As-Fe-S system. Open arrows indicate adsorption reactions, filled arrows indicate dissolution/precipitation reactions, and curved arrows indicate reactions that may couple with microbial organic matter oxidation. Diagram shows the primary reactions for a system transitioning from oxidized to reducing conditions; reactions for oxidation pathways may differ.

   
   
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   Fig. 4.

   Reaction path model. Quantitative model for the schematic reactions shown in Fig. 3 illustrated as pe-log a _H2S stability diagram for the As-Fe-S-O-H system at 25°C and pH = 7. Log activity (S)_total = –2; iron mineral stability fields are shown for log activity (Fe)_total = –5 (stability fields shaded gray) and –7 (stability fields stippled); and arsenic mineral stability domains are calculated for log activity (As)_total = –3 to –6, as labeled. Reaction paths are shown for evolution of iron-rich (path A, solid red arrow) and iron-poor (path B, dotted blue arrow) sediments during oxidation of organic matter. Equilibrium reaction paths were calculated for evolution of groundwater containing 28 mmol·kg^–1 and 100 μmol·kg^–1 as during progressive oxidation of sedimentary organic matter in iron-rich (100 mmol·kg^–1 reactive Fe) and iron-poor (1 mmol·kg^–1 reactive Fe) sediment. At log activity (Fe)^total =–7, the stability field for green rust is absent and iron exists as Fe²⁺(aq).