Arsenic speciation, seasonal transformations, and co-distribution with iron in a mine waste-influenced palustrine emergent wetland

Arsenic is commonly associated with mined ores and thus may be detrimental to naturally occurring wetlands that reside in mine waste-impacted regions. Understanding the relationship between Fe and As in both the aqueous and solid phase is critical for assessing the risk As imposes on these sensitive ecosystems. Therefore, in this investigation, Fe (perceived to be a dominant factor in the cycling of As) and As were monitored seasonally in a mine waste-influenced wetland. Iron and As concentrations in the pore waters at the site were highly variable hut were at the highest levels during the spring and summer months. Using X-ray absorption near edge structure (XANES) spectroscopy, we confirmed that arsenite, arsenic sulfides, and arsenate species were present in the soil-solids, all of which varied seasonally within the site. Additionally, the reactivity of Fe and As solid phases, as measured by selective sequential extractions, changed seasonally. Iron and As are positively correlated in the sodium acetate/acetic acid (carbonates), HCl (amorphous materials), hydroxylamine-hydrochloride/acetic acid (crystalline oxide), and hydrofluoric acid (silicate) extractable fractions; these pools also comprised the largest portions of extractable Fe and As. Consequently, arsenate and arsenite species detected using XANES spectroscopy are likely to be associated with carbonates in the summer, with iron (hydr)oxides in the fall and winter, and with silicates in the spring with each of these components playing an important role in As sequestration and availability within the site.