Bachelor Thesis

Effects of freeze-thawing for As and Sb mobility from peat

Laura Richter (11/2018-12/2019)

Support: Britta Planer-Friedrich, Anne Eberle

In northern regions, peatlands are often used for wastewater treatment and have been shown to efficiently retain toxic contaminants like arsenic (As) and antimony (Sb). Although freezing and thawing are common phenomena in those regions, little research has been done about their effects on As and Sb mobility. To evaluate the influence of freeze-thaw cycles on As and Sb remobilization, peat samples were frozen and thawed alternately at different thawing temperatures and durations. Freezing and thawing showed little impact on Sb, but strongly affected As mobility. An As release of up to 11.5 μmol/L was observed for samples frozen and thawed in short cycles, whereas the samples which remained unfrozen during the experimental period and the samples frozen and thawed over longer durations showed almost no As mobilization (< 2.1 μmol/L). Arsenic mobilization increased with the number of freeze-thaw cycles conducted. Monitored chemical parameters like pH, sulfide and iron concentrations did not appear to be connected to the observed As release. Therefore, the exact mobilization mechanisms remain elusive. Possible mobilization processes include the competition of As with other chemical species, like phosphate or dissolved organic matter, released through freezing and thawing or the mobilization of As from the interior of aggregates, broken apart by ice crystals. Overall, the experiment showed a strong As mobilization from contaminated peat, if subjected to repetitive freeze-thaw cycles. Since global warming is expected to increase freezing frequencies this observation indicates a high potential for As mobilization from peatlands.