In Situ Precipitation of Hydrous Ferric Oxide for Remediation of Subsurface Iodine Contamination

Radionuclides and metals are an environmental concern in soil and groundwater due to their mobility, toxicity, and recalcitrance. Whereas traditional pump-and-treat approaches can be costly and produce high concentration wastes ex situ, in situ precipitation of Fe oxides, which are good sorbents for anionic aqueous species, may be viable approach for immobilizing radionuclides and metals in situ.

We conducted a series of 1D meter-long bench top column experiments packed with Hanford sediments (Hanford Site, WA), to test a potential in-situ Fe oxide precipitation approach for iodine immobilization under relevant Hanford site conditions. Single acidic ferric Fe solution (0.1 M, pH = 1.5) was pumped into water-saturated columns. Because carbonate and clay minerals are widely present in sediments, self-pH buffering of the injected acidic Fe(III) solution occurred due to mineral dissolution, leading to hydrous ferric oxide (HFO) precipitation under a neutral pH ~7. The iodate transport was strongly retarded, due to its removal via co-precipitation and adsorption to HFO. Our results indicate that HFO precipitated and was evenly distributed within the sediment matrix. Remobilization of the neo-formed HFO precipitates was limited (< 4 wt.% after 100 pore volumes of flushing). However, reversible adsorption of iodine on HFO was observed, which might limit its application to slow moving groundwater systems.