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The influence of thioarsenic species formation on arsenic complexation to natural organic matter

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Term from 2016 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 291020486
 
Final Report Year 2020

Final Report Abstract

The high affinity of arsenic to reduced sulfur has previously been reported to cause formation of thioarsenates (HxAsVS-IInO4-nx-3, n = 1-4, x = 0-3) in aqueous environments with excess sulfide such as geothermal systems. Nothing was known about thioarsenates occurrence, mobility and stability in less extreme environments, specifically also not in environments rich in natural organic matter (NOM). In the first part of the project, we investigated the occurrence of inorganic thioarsenates in a naturally Ascontaminated peatland and potential consequences for As mobility. Up to 93% of aqueous As species were thioarsenates and the dominant species found, monothioarsenate (MTAs(V)), likely formed from reaction of arsenite with surface associated zero-valent S. Incubation experiments of MTAs(V), arsenate, and arsenite with model peat rich in oxygen (O)-containing (carboxyl and phenol) functional groups, demonstrated only little adsorption of MTAs(V) and arsenate to peat at slightly acidic to neutral pH. Arsenite substantially complexed via As(III)-O-organic carbon (Corg) bonds. Incubations of MTAs(V) and arsenite with thiol-rich model peat confirmed strong As(III)-S-Corg complexation of arsenite, increasing from slightly alkaline to slightly acidic conditions. For MTAs(V), As adsorption was observed from neutral to slightly acidic pH and was attributed to adsorption of arsenite, formed by acid-catalyzed dissociation of MTAs(V), which in turn had a high affinity to thiol groups of peat. At pH 8.5, when MTAs(V) was stable, no complexation of MTAs(V) with thiol groups was observed. Incubation experiments with Fe(III)-loaded peat to study MTAs(V) ternary complex formation showed that both MTAs(V) and arsenite can complex via As-O-Fe-Corg bonds. Extent of sorption for both As species was moderate and did not differ at a specific pH value, however, As sorption was clearly higher in both treatments at pH 7.0 compared to pH 4.5. In the second part, we focused on As mobilization mechanisms by reduced S species from laboratory to field conditions. Arsenic was mobilized through thioarsenate formation in laboratory incubations upon addition of dissolved sulfide and polysulfide from As-loaded thiol-rich model peat at neutral to slightly alkaline pH. A microbial influence of this process was observed using peat and sulfate-rich mining wastewater from a Finnish treatment peatland. Under field conditions in this peatland and by use of peat pore water samplers, we were able to show thioarsenate mobilization at the same depths where highest concentrations of dissolved sulfide and zero-valent sulfur were found. Incubations of this peat demonstrated further thioarsenate mobilization potential from peat layers with highest As concentrations. In column experiments with alluvial aquifer sands and different numbers of organic-rich interlayers, microbial sulfate-reduction was accompanied by thioarsenate mobilization and both clearly increased with an increasing number of organic-rich interlayers. In a side project, we explored binding mechanisms of antimonite to organic functional groups of model peat and under field conditions in the Finnish treatment peatland. Incubation experiments showed strong antimonite adsorption to peat by formation of Sb(III)-O-Corg bonds as revealed by spectroscopic analyses. Antimonite adsorption strongly increased with increasing peat thiol-group content. Spectroscopic data revealed that Sb adsorption occurred via Sb(III)-S-Corg coordination and inorganic Sb-S phases could be excluded. Solid-phase Sb speciation in a mine water impacted peatland confirmed the high affinity of antimonite to O-containing functional groups and Sb(III)-S-Corg coordination strongly increased with increasing peat depth. No organically complexed Sb(V) was found. Aqueous Sb speciation was dominated by antimonate. Antimonite concentrations were low. No thioantimonates where found. Hence, antimonate was very mobile while antimonite exhibited a very high affinity to peat functional groups. Overall, our findings imply that reduced S has complex influences on As and Sb mobility in organic-rich environmental systems. While thiol-bond formation and ultimately sulfide mineral precipitation of As and Sb under anoxic conditions and at acidic pH efficiently sequester arsenite and antimonite, formation of highly mobile thioarsenates with reduced S can turn solid NOM from an As sink to a source at already circumneutral pH.

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