Arsenic Mobilization Is Enhanced by Thermal Transformation of Schwertmannite

• Published in: Environmental science & technology Vol. 50; no. 15; pp. 8010 - 8019
• Main Authors: Johnston, Scott G, Burton, Edward D, Moon, Ellen M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 02.08.2016
• Subjects: Arsenic; biomineralization; crystal structure; Desorption; Environmental science; Ferric Compounds; fires; hematite; iron; Iron - chemistry; Nanocrystals; Oxidation-Reduction; particle size; seasonal wetlands; sulfates; surface area; temperature; Water Pollutants, Chemical; Water quality; Wetlands
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/acs.est.6b02618

Fires in iron-rich seasonal wetlands can thermally transform Fe­(III) minerals and alter their crystallinity. However, the fate of As associated with thermally transformed Fe­(III) minerals is unclear, as are the consequences for As mobilization during subsequent reflooding and reductive cycles. Here, we subject As­(V)-coprecipitated schwertmannite to thermal transformation (200, 400, 600 and 800 °C) followed by biotic reductive incubation (150 d) and examine aqueous- and solid-phase speciation of As, Fe and S. Heating to >400 °C caused transformation of schwertmannite to a nanocrystalline hematite with greater surface area and smaller particle size. Higher temperatures also caused the initially structurally incorporated As to become progressively more exchangeable, increasing surface-complexed As (AsEx) by up to 60-fold, thereby triggering enhanced As mobilization during incubation (∼70-fold in the 800 °C treatment). Although more As was mobilized in biotic treatments than controls (∼3–20×), in both cases it was directly proportional to initial AsEx and mainly due to abiotic desorption. Higher transformation temperatures also drove divergent pathways of Fe and S biomineralization and led to more As­(V) and SO4 reduction relative to Fe­(III) reduction. This study reveals thermal transformation of schwertmannite can greatly increase As mobility and has major consequences for As/Fe/S speciation under reducing conditions. Further research is warranted to unravel the wider implications for water quality in natural wetlands.