posted on 2024-01-06, 14:09authored byPhilipp
V. T. Knobloch, Lan Huong Pham, Carolin F. Kerl, Qinghai Guo, Britta Planer-Friedrich
Peatlands
are known sinks for arsenic (As). In the present study,
seasonal As mobilization was observed in an acidic, minerotrophic
peatland (called Lehstenbach) in late summer, accompanied by a peak
in dissolved sulfide (S(-II)). Arsenic speciation revealed the lowest
seasonal porewater concentrations of arsenite and arsenate, likely
due to As(III)-S-bridging to natural organic matter. Arsenic mobilization
was driven by the formation of arsenite-S(-II) colloids and formation
of methylthiolated arsenates (up to 59% of the sum of As species)
and to a minor extent also of inorganic thioarsenates (6%–30%)
and oxymethylated arsenates (5%–24%). Sorption experiments
using a purified model peat, the Lehstenbach peat, natural (to mimic
winter conditions) and reacted with S(-II) (to mimic late summer conditions)
at acidic and neutral pH confirmed low sorption of methylthiolated
arsenates. At acidic pH and in the presence of S(-II), oxymethylated
arsenates were completely thiolated. This methylthiolation decreased
As sorption up to 10 and 20 times compared with oxymethylated arsenates
and arsenite, respectively. At neutral pH, thiolation of monomethylated
arsenates was incomplete, and As could be partially retained as oxymethylated
arsenates. Dimethylated arsenate was still fully thiolated and highly
mobile. Misidentification of methylthiolated arsenates as oxymethylated
arsenates might explain previous contradictory reports of methylation
decreasing or increasing As mobility.