Master Thesis

Effect of freeze-thawing cycles on the mobilization of As and P from peat

Grethe Ploompuu (10/2019-10/2020)

Support: Britta Planer-Friedrich, José Miguel León Ninin

Wetlands in northern latitudes are often used as retention pools for pollutants in wastewater treatment. Nevertheless, the efficiency in cold climate conditions of organic soils and the changes they might be subjected to with climate change, are not entirely understood. With climate change, freeze and thawing cycles are expected to occur more often, leading to the possible mobilization of pollutants through different physical, chemical and biological processes. In the present study, mobilization of arsenic (As) and phosphorous (P) due to the freeze-thawing cycles was investigated in different laboratory incubations of peatland material that received mining wastewater. The control (C) treatment which was subject to cycling between the freezing and thawing temperatures of -15 and 5°C, respectively showed As mobilization from the peat material, but no corresponding increase in dissolved As concentrations in the porewater. This loss in the As budget was attributed to a possible formation of As-containing colloids. The treatment which was flash-frozen (FF) with dry ice, thus going through the highest physical disruption, showed comparable results with the C treatment. The treatment with inhibited bacterial activity by the addition of NaN3 showed higher concentrations of As both in porewater and peat material. In the treatment thawing at the room temperature (RT) the As concentrations increased in the porewater and decreased in the peat material. By normalizing the concentrations of the treatments with the respective thawing temperature, concentrations in the C and RT treatment decreased by the influence of the temperature, while in the FF and NaN3 treatment concentrations increased with number of freeze-thawing cycles. Net relative mobilization of P concentrations was only seen in the RT treatment, while none occurred in the C, FF and NaN3 treatments. Additionally, for analyzing chemical processes, glycerol was added to inhibit ice crystal formation and physical disruption, unfortunately leading to analytical difficulties due to high viscosity and C content, thus only porewater was analyzed and increased concentrations of As and P were seen in it. In conclusion, changes in temperature, increased occurrence of freeze-thawing cycles and concentrations of other elements present have an influence on the mobility of the investigated elements and can trigger As mobilization, which could end up turning the peatland from a sink of pollutants into a source.