The crystalline arrangements of bismuth(III) (Bi) mineral phases have potential for contaminant sequestration due to their electronic and spatial flexibility. Bi-based materials are particularly promising for application in subsurface remediation due to their low toxicity, low cost, and ability to uptake multiple contaminants. Before they can be deployed for contaminant remediation in the subsurface, the effectiveness of Bi-based materials must be demonstrated under site relevant geochemical conditions. Batch experiments were used to determine uptake of individual and mixed contaminants (iodine, chromium, uranium-238, and technetium-99) by lab-synthesized Bi oxyhydroxide ((BiO)x(OH)(NO3)m(CO3)n, BOH) and commercially available Bi subnitrate (Bi5O(OH)9(NO3)4, BSN) under geochemical conditions representative of the U.S. Department of Energy’s Hanford site, a former plutonium production site. A systematic study of BSN and BOH was conducted to evaluate relationships between the nanoscale structure and function of these Bi-based materials and the aqueous environments, increasing in complexity from pure water to aqueous solutions containing carbonate (CO32-), common groundwater constituents, individual contaminants (TcO4-, IO3-, U, and CrO42-), co-mingled contaminants, and sediments. Solid phase characterization techniques e.g., XRD, SEM-EDS, XPS, and XAS, together with solution analysis, were used to evaluate evolution of BOH and BSN morphology and composition over time. Contaminant remediation performance depends on the transformation product structure, which is driven by solution conditions. Depending on solution pH and CO32- concentration, BOH, initially present as a disordered δ-Bi2O3-like phase, transforms to disordered bismutite (Bi2O2(CO3)), a layered structure with high affinity for iodate (IO3-), chromate (CrO4-), and uranyl carbonate species. Due to hydrolysis of BSN, solution pH decreases such that there is limited CO32- available, and crystalline clusters with the general formula Bi6Ox(OH)8-x charge-balanced by nitrate are formed. These clusters have a high affinity for TcO4-. Overall, increasing anion concentration in solution results in mineralogical transformation products of smaller size and with higher surface area for anion sorption. These Bi-based materials are promising candidates for contaminant uptake and have potential for use in remediation of contaminated hotspots, such as the perched water zone in the Central Plateau at Hanford.

Nanostructured bismuth-based materials for contaminant remediation in the subsurface

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ACS Fall 2022

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