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Speciation and Biogeochemical Cycling of Arsenic in a Minerotrophic Peat Wetland

Speaker: Professor Ruben Kretzschmar, ETH Zurich

Time: 5:00p–6:00p

Location: 1-190

CEE C.C. Mei Distinguished Speaker Series

Organic-rich sediments are often enriched in arsenic (As), a potentially toxic and cancerogenic trace element, which can be released into groundwater or surface water under favorable biogeochemical conditions. The chemical speciation and biogeochemical behavior of As has been extensively studied in mineral soils and groundwater aquifers as well as hydrothermal environments. However, much less is known about the speciation and biogeochemical cycling of As in peat wetlands. In this talk, I will present our recent research on As speciation and biogeochemical cycling in a minerotrophic peatland in southern Switzerland, which is naturally enriched in As and other trace elements. Anoxic peat samples containing 106-1810 mg As kg-1 were analyzed by As and Fe K-edge X-ray absorption spectroscopy. The results revealed that, in deep peat layers, As was almost entirely present as As(III) coordinated to 2-3 S atoms of NOM. Peat incubation experiments showed that oxidation of As(III) to As(V) leads to mobilization of dissolved As in the peat. The surface layers of the peat contained several different As species including realgar (As4S4), As(III)-NOM complexes, and As(III)/As(V) adsorbed to Fe(III)-(hydr)oxides. Also NOM-rich flocs in surface water contained high concentrations of As, predominantly as As(V) sorbed to Fe(III)-oxyhydroxides (ferrihydrite, lepidocrocite). Possible implications of our findings for As cycling in wetland systems will be discussed.