Photooxidation of arsenic in pristine and mine-impacted Canadian subarctic freshwater systems

The redox transformations of arsenic (As) in lakes are thought to be mainly controlled by algal/microbial processes. We tested if photochemical oxidation could also be a significant redox process in northern freshwater systems. We exposed filtered natural water samples from four northern mine-impacted lakes (Northwest Territories, Canada) and three unimpacted ponds (Nunavik, Canada) to simulated solar radiation, and followed As(III) oxidation. During short-term incubation experiments (less than 8 h), all samples amended with As(III) displayed a rapid photooxidation (ca. 0.06-0.27 h−1) whereas no significant oxidation was observed in the dark. This photooxidation process was controlled by UV radiation, was influenced by short-lived oxidants and was partly driven by the concentrations of organic matter. Natural As complexes in unamended water samples were more rapidly photooxidized than in samples with As(III) amendments. Addition of goethite or simulation of snowmelt did not alter photooxidation rates significantly. Overall, these results imply that photooxidation of As should be considered as a potentially significant mechanism controlling inorganic As speciation in aquatic systems. This is particularly true for northern systems affected by mining activities and by changes of inputs of organic matter related to climate change.