Arsenic (As) contamination in groundwater remains a pressing global challenge. In this study, we evaluated the potential of green rust (GR), a redox-active iron phase frequently occurring in anoxic environments, to treat As contamination at a former wood preservation site. We performed long-term batch experiments by exposing synthetic GR sulfate (GRso(4)) to As-free and As-spiked (6 mg L-1) natural groundwater at both 25 and 4 C. At 25 C, GRso4 was metastable in As-free groundwater and transformed to GlIcos, and then fully to magnetite within 120 days; however, GRso(4) stability increased 7-fold by lowering the temperature to 4 degrees C, and 8-fold by adding As to the groundwater at 25 degrees C. Highest GRso4 stability was observed when As was added to the groundwater at 4 C. This stabilizing effect is explained by GR solubility being lowered by adsorbed As and/or lower temperatures, inhibiting partial GR dissolution required for transformation to GlIcos, and ultimately to magnetite. Despite these mineral transformations, all added As was removed from As-spiked samples within 120 days at 25 C, while uptake was 2 times slower at 4 degrees C. Overall, we have successfully documented that GR is an important mineral substrate for As immobilization in anoxic subsurface environments.
