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Eberle, Anne ; Besold, Johannes ; Leon Ninin, Jose M. ; Kerl, Carolin ; Kujala, Katharina ; Planer-Friedrich, Britta:
Potential of high pH and reduced sulfur for arsenic mobilization : Insights from a Finnish peatland treating mining waste water.
In: Science of the Total Environment.
Bd. 758
(2021)
.
- 143689.
ISSN 0048-9697
DOI: https://doi.org/10.1016/j.scitotenv.2020.143689
Abstract
Peatlands, used for purification of mining waste waters, have shown efficient solid-phase sequestration of contaminants such as arsenic (As). However, contaminant re-mobilization may occur related to management changes or chemical alteration of original peatland conditions. For a treatment peatland in Finnish Lapland, we here confirm efficient As retention in near-surface peat layers close to the mining waste water inflow, likely due to binding to FeIII-phases. Seven years into operation of the treatment peatland, there appears to be further retention potential, as large areas downstream still had solid-phase As concentrations at background levels. However, via depth-resolved pore water analysis we observed a hotspot 170 m from the inflow at 10–50 cm depth, where As pore water concentrations exceeded input concentrations by a factor of 20, indicating substantial As re-mobilization. At the same spot, a peak of reduced sulfur (S) species was found. Arsenic species detected were arsenite and up to 26% methylated oxyarsenates, 15% methylated and 7.9% inorganic thioarsenates. We postulate that As mobilization is a result of short-term re-equilibration to a changed inflow chemistry after installation of a process water treatment plant and a long-term consequence of changing pore water pH from acidic to near-neutral, releasing reduced S and As. We infer that the co-occurrence of reduced S and As leads to formation of methylated and/or thiolated As species with known low sorption affinity, thereby further enhancing As mobility. Laboratory incubation studies with two peat cores confirmed a high S-induced As mobilization potential, especially when As-Fe-rich, oxic surface layers were incubated anoxically at near-neutral pH. Highest risk of As re-mobilization from this treatment peatland is expected in a scenario in which mining waste water inflow has stopped but the peatland remains flooded, and near-surface layers transition from oxic to anoxic conditions.