Release of arsenic and molybdenum by reductive dissolution of iron oxides in a soil with enriched levels of native arsenic

This study examined the release of arsenic (As), molybdenum (Mo), and iron (Fe) into the aqueous phase during reductive dissolution of iron oxides in an amended acid sulfate soil naturally enriched in arsenic. These acid sulfate soils occur worldwide and have a significant distribution in parts of Alberta and British Columbia. Reactions were studied in incubation vessels under controlled laboratory conditions. Soils were driven anaerobic by stimulation of indigenous bacteria using a glucose addition and then periodically analyzed for redox potential (Eh), pH, electrical conductivity, and aqueous concentrations of Fe, As, Mo, and other chemical species. Dissolution of iron oxides was pronounced when Eh values dropped to below +100 mV. With dissolution, concentrations of Fe, As, and Mo increased in the solution phase. Aqueous As concentrations increased from 0.87 to 119 μμg L^–1 during a reduction period of 48 d. Molybdate (MoO₄^2–), an oxyanion similar to arsenate, also closely followed the release of Fe and As into solution during the incubation. The results indicate both As and Mo behave similarly in response to the dissolution of soil iron oxides under reducing conditions and that their aqueous phase concentrations are largely controlled by dissolution and desorption from the iron oxide host. Key words: acid sulfate soil, arsenic, molybdenum, iron, redox potential, desorption.