Bachelor Thesis

Complexation of monothioarsenate to different size fractions of particulate and dissolved natural organic matter

Constantin Heitzer (03/2017-09/2017)

Support: Britta Planer-Friedrich, Johannes Besold

The mobility of arsenic (As) in sulfidic, organic rich systems such as peatlands is still uncertain. Recent studies demonstrated arsenite binding via sulfhydryl groups to natural organic matter. Thioarsenates, arsenic-sulfur-compounds forming at sulfidic conditions, can dominate the As aqueous phase speciation in peatlands. As of now however, the processes governing their mobility are still mostly unknown. To examine the complexation of monothioarsenate (MTA) with colloidal and dissolved organic matter (CDOM) laboratory experiments and a case study in Kittilä, Finland were performed, monitoring different particle size fractions. Moreover, treatments with and without prior sulfide treatment of CDOM as well as a comparison to the remobilization of MTA-loaded peat organic matter were conducted. Results showed MTA complexation with CDOM without prior sulfide treatment via carboxylic groups, while for prior sulfide treatment MTA was firstly reduced to arsenite and subsequently complexed to sulfhydryl groups of CDOM. The influence of CDOM particle size was negligible for the particle sizes larger than 0.1 µm. The <0.1 µm fraction contained the vast majority of As, where the presence of dissolved MTA was suggested over the formation of <0.1 µm As-CDOM complexes implying low complexation tendencies for thioarsenates and little influence of CDOM particle size. Similarly, prior sulfide treatment of CDOM as well as comparison to remobilized MTA-loaded peat organic matter portrayed no significant differences in MTA complexation. However, results have to be considered carefully due to possible methodological problems. In conclusion, this study suggests a higher mobility for thioarsenic species in sulfidic, organic rich systems.