Mechanisms of interaction between bismuth-based materials and contaminants for subsurface remediation

Abstract

Successful deployment of in situ subsurface remediation strategies requires knowledge of contaminant geochemistry, and the impact of physicochemical sediment properties on remedy performance. Bismuth (Bi) materials can sequester multiple contaminants that are present in the unsaturated zone and groundwater at Department of Energy (DOE) legacy nuclear sites, such as the Hanford Site. Adsorption experiments for individual contaminants (chromate, iodate, pertechnetate, and uranyl carbonate) were conducted with two Bi materials: commercially available Bi-subnitrate (BSN); and laboratory synthesized Bi oxyhydroxide nitrate (BOH). The structure and composition of the Bi material influenced hydrolysis and ion exchange interactions in aqueous solution, with subsequent impacts on solution pH, contaminant speciation, and contaminant uptake. X-ray diffraction revealed that the disordered BOH structure, initially containing charge balancing nitrate and hydroxide anions, rapidly converted to bismutite, Bi₂O₂(CO₃), in the presence of carbonate. During this transformation, BOH removed most contaminant ions from solution. [Bi₆O₅(OH)₃]⁵⁺ clusters in BSN underwent hydrolysis upon exposure to aqueous solutions, substantially reducing pH, and transforming into several mineral phases, including a daubreeite (BiO(OH,Cl)) structure, and an unidentified mineral phase (unk-Bi(NO₃)_x(OH)_yO_z). This transformation decreased uptake efficiency relative to BOH, except for pertechnetate. 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 Bi mineral structures via different mechanisms, including anion exchange or outer-sphere complexation for pertechnetate, and inner-sphere adsorption for all other contaminants. Uranyl carbonate could substitute between the [Bi₂O₂]²⁺ layers, and some iodate was incorporated into a neo-formed δ-Bi₂O₃ phase. This remarkable versatility of Bi-based materials demonstrated here means that they are cost-effective materials with the potential to sequester co-located contaminants at DOE legacy sites.