Part II: Sediment Interactions with Layered Bismuth Materials and Implications for Subsurface Contaminant Remediation

To demonstrate the potential for bismuth(III) (Bi)-based materials to sequester subsurface contaminants in situ, aqueous batch experiments were performed to determine how sediments impact the sequestration of technetium (Tc), uranium (U), chromium (Cr), and iodine(I) with bismuth subnitrate (BSN) and bismuth oxyhydroxide (BOH). Results of these experiments demonstrated that Bi-based materials have the potential to rapidly remove colocated contaminants over a range of geochemical conditions representative of the Hanford Site and in the presence of Hanford subsurface sediments. In aqueous batch experiments without sediment, the hydrolysis of both BSN and BOH resulted in a lowering of the pH (to as low as 2.4 with BSN or 5.9 with BOH). In the presence of sediment, the pH was buffered at ∼7–8 for both systems (BOH and BSN). In the absence of sediment, BOH readily sequestered U(VI) from its 2.3 mg/L solution under challenging conditions of a high solution-to-solid material ratio of 1000 mL/g or even when the U(VI) concentration was increased to 150 mg/L at a solution-to-solid material ratio of 200 mL/g. Cr(VI) was likewise readily removed from the aqueous phase at a concentration of 0.05 mg/L. A greater quantity of BOH (i.e., at a solution-to-solid ratio of 100–200 mL/g) was required for complete removal of Tc(VII) and I(V); changes in solution chemistry suggest that this is due to competing reactions with other ions, including chloride (Cl–) and sulfate (SO42–). In the presence of sediment, removal of Tc(VII) and I(V) decreased by respectively 91% and 21% for BOH and 33% and 93% for BSN. The presence of sediment increased removal of U(VI) by BSN to nearly 100%, while only 17% of U was removed in the absence of sediment. This study demonstrates that Bi-based materials show promise as a remediation tool to remove multiple contaminants from contaminated sediments, with the removal efficiency dependent on the amount of Bi-based material present in the sediment.