Occurrence and Sorption Affinity of Methylated Oxy- and Thioarsenates to Natural Organic Matter in Peatlands
Lan Huong Pham (10/2020-10/2021)
Support: Britta Planer-Friedrich, Philipp Knobloch
Inorganic arsenic, namely arsenite and arsenate, have been effectively sequestered in peatlands by binding to natural organic matter (NOM) via oxygen (O) – containing groups or thiol and iron (Fe) bridging. A recent report on the occurrence of thiolated arsenates in the porewater of a Finnish mine-water impacted peatland raised questions whether these species occur ubiquitously in peatlands and are maybe not as readily sorbed to NOM as non-thiolated species. The present study on a minerotrophic peatland (Schloeppnerbrunnen I, Germany) with naturally low background arsenic (As) showed that up to 55% of all porewater As species were methylated thioarsenates, with dimethylmonothioarsenate (DMMTA) and dimethyldithioarsenate (DMDTA) as the dominants. Methylated thioarsenates were found to occur in highest fractions only under sulfidic and anoxic conditions. Laboratory sorption studies with DMMTA and its methylated oxy-analogs, including monomethylarsenate (MMA) and dimethylarsenate (DMA) were conducted in comparison at pH 4, 5.5, and 7 in background electrolytes of high ionic strength (30 mM NaCl) and low ionic strength (3 mM NaCl) for 96 h. Three types of “model” peats were used, including Federseemoor peat (as an example for peat low in sulfur and Fe), Finnish mine-water impacted peat (rich in Fe) and sulfide-reacted Federseemoor peat (rich in sulfur). Almost 90 to 98% of MMA, DMA or DMMTA spiked in were still present in the aqueous phase after 96 h, regardless of changes in peat characteristics, pH values, and ionic strengths. While MMA and DMA showed almost no transformation, DMMTA substantially transformed to DMA and DMDTA in all setups, especially at pH 7. Even though there were transformations among species, no sorption of the total As was observed, confirming the high mobility of both methylated oxy- and thioarsenates in peat systems. Based on the present data, a conceptual model is suggested for peatlands similar to Schloeppnerbrunnen I, in which methylated oxy- and thioarsenates could be preferably formed under sulfidic anoxic conditions and strong microbial activity. With high fractions of methylated oxy- and thioarsenates, As will be no longer effectively sequestered in peatlands, but will be extremely mobile with no direct bindings to peat functional groups.