Mechanistic investigations of Se(VI) treatment in anoxic groundwater using granular iron and organic carbon: an EXAFS study.

The removal of aqueous Se(VI) from a simulated groundwater by granular iron (GI), organic carbon (OC), and a mixture of these reactive materials (GI-OC) was evaluated in laboratory batch experiments. The experiments were performed under anoxic conditions to simulate subsurface treatment. A total reaction time of 120 h (5 d) was chosen to investigate the rapid changes in speciation occurring over reaction times that are reasonable for permeable reactive barrier (PRB) systems. After 120 h, concentrations of Se decreased by >90% in the GI system, 15% in the OC system and 35% in the GI-OC mixture. Analysis of the materials after contact with Se using synchrotron-radiation based X-ray absorption spectroscopy (XAS) indicated the presence of Se(IV) and Se(0) on the margins of GI grains after 6h with evidence of SeO and SeSe bonding, whereas Se(VI) was not observed. After 72 h, Se(0) was the only form of Se present in the GI experiments. In the OC batches, the XAS analysis indicated binding consistent with sorption of aqueous Se(VI) onto the OC with only minor reduction to Se(IV) and Se(0) after 120 h. Selenium XAS spectra collected for the GI-OC mixture were consistent with spectra for Se(IV) and Se(0) on both the margins of GI grains and OC particles, suggesting that the presence of dissolved Fe may have mediated the reduction of sorbed Se(VI). The results suggest that the application of granular Fe is effective at inducing aqueous Se removal in anoxic conditions through reductive precipitation processes.