Part I: Structural Transformation of Bismuth-Based Materials in Dynamic Aqueous Environments and Implications for Subsurface Contaminant Remediation

Bismuth (Bi) materials are advantageous for the subsurface remediation of contaminants due to its low toxicity and cost. Depending on the groundwater pH and ions present, bismuth materials undergo structural transformations, enabling interactions with aqueous contaminants at legacy nuclear sites. Here, the performance of bismuth oxyhydroxide (BOH) and bismuth subnitrate (BSN) was investigated with respect to the uptake of iodine-129 (iodate (IO 3 – ) or iodide (I – )), chromium (chromate (CrO 4 2– )), uranium-238 (uranyl carbonate complexes, (UO 2 (CO 3 ) x 2–2 x ) like UO 2 (CO 3 ) 3 4– ), and technetium-99 (pertechnetate, (TcO 4 – )) in arid and semiarid regions (e.g., western United States), specifically conditions representative of the geochemistry in the Central Plateau (200 Area) of the U.S. Department of Energy Hanford Site. The influence of solution chemistry on the time-dependent structural transformation of Bi-based materials between crystalline clusters and layered arrangements was assessed in experiments for up to 150 days using synthetic Hanford water. Aqueous environments, especially carbonate (CO 3 2– ), increase rates of Bi-based material structural transformation. Depending on solution pH and [CO 3 2– ], BOH, initially a disordered δ-Bi 2 O 3 -like phase ( dis -BiO w (OH) x (NO 3 ) y (CO 3 ) z ), transforms to a layered bismutite ( lay -Bi 2 O 2 (CO 3 )) with high affinities for IO 3 –, CrO 4 2–, and (UO 2 (CO 3 ) x 2–2 x ) complexes. The hydrolysis of BSN causes the pH to decrease from 7.98 to 3.38 such that an “unknown” phase with the general formula unk -Bi(NO 3 ) x (OH) y O z, charge-balanced by nitrate (NO 3 – ) is formed and has a high affinity for TcO 4 – . Overall, increasing concentrations of common groundwater anions result in smaller sized mineralogical transformation products with higher surface areas for contaminant sorption. The results corroborate that Bi-based materials are promising candidates for groundwater remediation at the Hanford Site.