PNNL

Abstract

Subsurface in situ remediation strategies require consideration of the complex geochemistry of contaminant mixtures, and the impact of chemical and physical sediment properties on the performance of remedies. Bismuth (Bi) materials can successfully sequester multiple contaminants that are present in the unsaturated zone and groundwater at Department of Energy legacy sites, such as the Hanford Site in Washington State. Adsorption experiments for both individual and co-located contaminants (chromium, iodine, technetium, and uranium) with two forms of Bi: (i) a commercially available Bi-subnitrate (BSN) material; and (ii) a laboratory synthesized Bi oxyhydroxide (BOH) material. Experiments revealed that the form of the Bi influenced hydrolysis and ion exchange interactions in aqueous solution, with subsequent impacts on solution pH, contaminant speciation, and the structural flexibility for contaminant uptake. Structural analysis demonstrated that Bi6O4(HO)46+ clusters, charged balanced by nitrate and hydroxide anions in the BOH structure, rapidly converted to bismutite, Bi2O2(CO3), in the presence of carbonate, and that both Bi clusters and bismutite removed contaminant ions from solution (e.g., up to 100%, 85%, 18%, and 100% for chromium, iodine, technetium, and uranium, respectively). The adsorption isotherms for the contaminants were fit with a Freundlich model that describes adsorption to Bi materials with dissimilar binding sites. Solid phase characterization after reaction confirmed structural rearrangement of the Bi materials and direct association of the contaminant ions with bismuth clusters, via different mechanisms including non-specific ion exchange for pertechnetate, specific sorption for all other contaminants, potential incorporation for iodate, and redox mechanisms for chromate and iodate. The BSN material underwent hydrolysis upon exposure to aqueous solutions, substantially reducing the pH and decreasing all uptake efficiencies relative to BOH, except for the non-specific interactions with pertechnetate. Sediment interactions with BOH buffered the pH at ~8. Small amounts of Bi material in sediments readily removed chromium and uranium from the aqueous phase, with more Bi material required for complete removal of technetium and iodine, likely due to competing reactions with other ions in solution, including Cl- and SO42-. Due to their flexible structure and multiple mechanisms of contaminant uptake, Bi materials removed comingled contaminants from solution under a wide range of geochemical conditions and in the presence of sediment, making them cost effective, candidates for sequestering contaminants in situ.