Sulfate green rust (green rust in which sulfate is the interlayer anion; designated hereafter as GRSO4) was synthesized by the oxidation of a 1.0 M Fe(II)SO4 solution in air as described by O'Loughlin et al. . The experimental system consisted of 100 mL of GRSO4 suspension (5 g L -1 ) in 160-mL serum vials crimp-sealed with Teflon-lined rubber septa as described in detail by O'Loughlin et al. . The suspensions were amended with 100 µM of either Co(II), Cr(VI), Hg(II), Mn(II), Mo(VI), Ni(II), Pb(II), V(III), or Zn(II). All preparative work was performed in an anoxic glove box containing < 1 ppm O2 (Coy Laboratory Products, Grass Lake, Michigan). Reactions were initiated by the addition of 2 µmol of CT (2 µL of 1.0 M methanol in aqueous solution) and the vials were placed on a roller drum (rotating vertically as the bottle axis remained horizontal) in an incubator maintained at 25 °C. The pH of the experimental systems was 7.6 ± 0.1. The concentrations of CT and the products of its sequential reductive dechlorination (CF, DCM, CM, and methane; see Figure ) were monitored over time by measuring their headspace concentrations using a Hewlett-Packard 5890 Series II gas chromatograph with a GSQ column (0.53 mm id by 30 m) and an FID (240 °C) as described by O'Loughlin et al. . Samples for x-ray absorption spectroscopy (XAS) analysis of Mo(VI)-, Ni(II)-, Pb(II)-, or Zn(II)amended GRSO4 were prepared in 250-mL polypropylene centrifuge bottles with O-ring sealed caps. Unless otherwise indicated, sample preparation and handling were conducted in an anoxic glove box as described above. Reactions were initiated by adding 25 mL of 500 µM Mo(VI), Ni(II), Pb(II), or Zn(II) to 0.5 g of GRSO4 in 100 mL of high-purity water with constant mixing. After 12 h, the suspensions were centrifuged, and subsamples of the wet pastes were mounted in holes machined in Plexiglass sample holders (slide-mount sample holders) and the holes were covered with Kapton film held in place with Kapton tape. Mo K-edge (20,000 eV), Ni K-edge (8,333 eV), Pb LIII-edge (13,035 eV), and Zn K-edge (9,659 eV) x-ray absorption spectroscopy measurements were carried out at the MR-CAT/EnviroCAT insertion device beamline (Sector 10, Advanced Photon Source) . X-ray absorption near edge spectra (XANES) spectra from standards and the filtered/centrifuged solids were collected at room temperature in transmission and fluorescence mode. Sample filtration was done in the glove box as described above, and the hydrated solids were measured under a nitrogen atmosphere. Energy calibration was established by setting the inflection point in the foil spectrum of the corresponding values to the tabulated values, and maintained afterwards by collecting data from the foil simultaneously with the samples or approximately every two hours when samples were too thick. We monitored for radiation-induced changes by taking 2-3 consecutive quick XANES spectra at each location. No differences were observed between the spectra, so all scans from all locations on each sample were averaged to produce the final spectrum. Normalization and background removal of the data was done using the program AUTOBK . Linear combination fits of the XANES data were performed using the program ATHENA .