In Situ Groundwater Arsenic Removal Using Iron Oxide-Coated Sand

Abstract

In many regions of the world, groundwater is contaminated with a high level of arsenic that must be treated before it can be safely used as drinking water. In situ immobilization of arsenic from groundwater within subsurface environment could have major advantages over the conventional above-ground chemical coagulation-precipitation treatment process. In this study, we develop a novel technique that can in situ emplace iron oxides onto the sand grain surface of porous media under mild chemical and temperature conditions. The technique involves sequential injections of a preconditioned ferrous iron solution and an oxidant solution and then orchestrate the advective-diffusive transport of the two reagents in porous media to create an overlapped reaction zone where ferrous iron is oxidized and precipitated on the sand grain surfaces. We demonstrate through bench-scale column tests the feasibility of using this technique to create a large-scale iron oxide-enriched reactive barrier in subsurface environment for in situ removal of arsenic. A sand filter with a fresh iron oxide coating can treat thousands of pore volumes of water contaminated with dozens of ppb arsenic before the coating needs to be regenerated. Arsenic breakthrough curves through the sand filter suggest that both reversible adsorption and irreversible precipitation are responsible for removing arsenic from the water. Unlike conventional excavate-and-fill permeable reactive barriers, the treatment capacity of our in situ created barrier can be in situ regenerated and replenished with a fresh coating.