AS AND SE IMMOBILIZATION EXAMINED WITH OXYGEN ISOTOPE MEASUREMENTS

The mass arsenic occurrence in Southeast Asia and the widespread presence of selenium within the western U.S. are two examples of metals contamination that are affecting broad regions of water resources and, consequently, the health of people and ecosystems. Understanding how As and Se behave in surface waters and groundwaters, especially in regards to immobilizing As and Se, can lead to better assessment and management of affected waters. Both As and Se are commonly present as oxyanions of varying oxidation states that accordingly have differing mobilities and redox cycling. We have developed an isotopic method for exploring reaction processes as oxyanions interact with minerals and microorganisms in an effort to develop a tool for characterizing in situ As and Se oxyanion immobilization. The quantification method of δ¹⁸O in selenate, selenite, arsenate, and arsenite is based on thermochemolysis elemental analysis – isotope ratio mass spectrometry, similar to methods for other inorganic oxyanions such as phosphate. Dissolved oxyanions are recovered as silver-arsenic or barium-selenium solids within controlled pH ranges. Within laboratory experiments, the method was applied to confirm complete exchange of oxygen isotopes between arsenate and solvent water at pH 7 over the course of two days and reveals that arsenate δ¹⁸O is likely overprinted by that of water in the absence of any recent reactions such as sorption to metal oxides. In contrast, selenate does not exchange its oxygen with water under ambient conditions, indicating δ¹⁸O measurements may be useful for tracing chemical reaction pathways or hydrologic transport for these oxyanions. The immobilization of selenate via reduction to Se(0) by a green rust resulted in¹⁸O-enrichment within selenate (enrichment factor 20.9‰) which may be used for confirming selenate reduction in subsurface environments over time or along a plume length.