Conceptual Model of Subsurface Processes for Iodine at the Hanford Site

Abstract

death and lysis. These solid-phase interactions are relevant with respect to attenuating iodine transport and need to be considered with respect to fate and transport and remediation. • The gas phase is also relevant to the overall iodine cycle. In particular, microbial reactions that create methyl-iodine compounds are a mechanism of volatilization. This type of reaction can occur in Hanford sediments, so it should be considered as part of the potential fate for subsurface iodine. Current studies focused on groundwater conditions, but volatilization in the vadose zone during and after waste disposal is another potential ramification of these gas-phase iodine species. Volatilization is, therefore, another attenuation mechanism related to the fate and transport of iodine in the subsurface. For iodate in the vadose zone or groundwater, transport is attenuated by sorption and co-precipitation with calcite (and other calcium carbonate polymorphs such as aragonite) and iron oxides (e.g., ferrihydrite). These solid-phase interactions are not a permanent sequestration and iodate may be released back into the aqueous phase via desorption and/or dissolution. The solubility of iodate bearing calcite is similar to that of pure calcite. Ferrihydrite has been previously shown to have a high affinity for iodate adsorption. Data showed that iodate transport was significantly retarded in ferrihydrite-amended sediment, whereas iodide and organo-I species showed little adsorption to ferrihydrite. Organo-I complexes are present in the Hanford subsurface, predominantly in the aqueous phase. While the organo-I complexation structures in the 200-UP-1 site at Hanford could not be confidently identified, the results suggested that these compounds exist at Hanford. The differences in organic matter composition indicated that pore water samples have a higher abundance of aromatic compounds that are a better target for iodine complexation. Volatilization of iodine occurred in Hanford sediments. Iodomethane was produced by all of the sediment samples examined and was positively correlated with 129 I concentrations in sediment cores. Moreover, its production rate was insensitive to nutrient supplements. This result reflects a nominal capacity of Hanford subsurface sediments to volatilize iodine without additional perturbation of the subsurface system. The fate of iodomethane in the aquifer or the extent to which this process may have impacted iodine in the vadose zone during and after waste disposal have not been quantified. However, this transformation mechanism should be considered when interpreting potential pathways for iodine in the subsurface.

Time-Dependent Iodate and Iodide Adsorption to Fe Oxides

“…These species cannot be degraded or transformed to a less toxic form, and therefore, remedial strategies are required to decrease aqueous concentrations below the drinking water standard (i.e., 1 pCi/ L). 1,2 Past studies have demonstrated limited adsorption of IO 3 − and I − on sediments, especially in a subsurface environment low in organic matter. 3,4 Enhanced adsorption onto highaffinity sorbents may be a promising pathway to immobilize 129 I and reduce risks to human health and the environment.…”

“…Iodate (IO 3 – ) and iodide (I – ) are the two predominant aqueous inorganic I species in natural environments. These species cannot be degraded or transformed to a less toxic form, and therefore, remedial strategies are required to decrease aqueous concentrations below the drinking water standard (i.e., 1 pCi/L). , …”

Chromate Effect on Iodate Incorporation into Calcite

“…However, these studies have treated iodate and chromate incorporation independently and have not studied potential competitive effects that may exist over a range of concentration conditions. Previous studies investigating iodine distribution in Hanford site sediments have also shown that up to 39.4% of total iodine present is incorporated into calcite as iodate and suggest that iodate attenuation via sorption and co-precipitation with calcite is a viable retardation mechanism for this environment and possibly other contamination sites with similar geological properties. However, low concentrations of a co-located contaminant, e.g., CrO 4 2– , may interfere with an in situ remediation approach that enhances iodine attenuation via iodate incorporation into calcite.…”