Master Thesis

Influence of pH, temperature, and aerobic conditions on methylated thioarsenate stability

Philipp Knobloch (07/2019-09/2020)

Support: Britta Planer-Friedrich, Carolin Kerl

Methylated thioarsenates were detected in geothermal systems besides inorganic oxy- and thioarsenates and methylated oxyarsenates. To date, their environmental fate and stability when pH, temperature, and oxygen content changes, is unknown. Here, speciation changes were tested in laboratory experiments with synthesized standards of monomethylmonothioarsenate (MMMTA), monomethyldithioarsenate (MMDTA), dimethylmonothioarsenate (DMMTA), and dimethyldithioarsenate (DMDTA) without excess sulfide. Solid standards yielded purities of 64.8 % MMMTA, 58.3 % DMMTA, and 89.5 % DMDTA. MMDTA appeared as byproduct in the MMMTA synthesis (24.4 %). MMDTA and DMDTA were stable at pH > 6 and transformed to MMMTA and DMMTA, respectively, below pH 6. MMMTA was stable at pH > 4 and transformed to monomethylarsenate (MMA) below pH 4. DMMTA varied (54 to 80 % of sum of species) from pH 4 to 12. At pH 2, DMMTA completely transformed to dimethylarsenate (DMA). Addition of H2O2 as oxidizing agent completely transformed DMDTA to DMMTA and DMA, and MMDTA to MMMTA and MMA, while air purging for 24 hours induced no species transformations for any of the methylated thioarsenates. Exposition to high temperatures of up to 80°C over 24 hours did not lead to species transformations, either. Compared to the known rapid transformation of some inorganic thioarsenates, all four methylated thioarsenates were found to be more stable with regard to aeration and high temperatures. MMMTA was the most stable over the greatest pH range and is therefore the most likely one for intact long-distance transport out of geothermal systems.