PhD Thesis

Influence of reduced sulfur on arsenic and antimony mobility and binding to natural organic matter

Johannes Besold (10/2015-01/2020)

Support: Britta Planer-Friedrich

The fate of the potentially toxic metalloids arsenic (As) and antimony (Sb) in the environment is mainly controlled by iron (Fe) (oxyhydr)oxide minerals, which act as the main sinks for both elements under oxic conditions. In sub- to anoxic organic-rich environments like peatlands, Fe (oxyhydr)oxides are only thermodynamically stable at the oxic surfaces, but undergo reductive dissolution with increasing depth, accompanied by a release of adsorbed As and Sb. Simultaneously, sulfate-reduction produces reduced sulfur (S) species like dissolved sulfide, which interact with abundant solid natural organic matter (NOM), forming surface associated zero-valent S and thiol groups. Formation of As(III)-thiol bonds with solid NOM can sequester As in deep peat layers. Only little is known about the behavior of Sb in similar systems. Further, inorganic aqueous As(V)-S and Sb(V)-S species, so called thioarsenates and thioantimonates, have been found under sulfate-reducing conditions in geothermal waters and other terrestrial environments, but so far have never been reported in peatlands.

The aim of the present thesis was to investigate the occurrence of aqueous thioarsenates in peatlands and to elucidate the influence of reduced S on their formation, mobility, and binding to solid NOM incomparison to As oxoanions. A similar behavior is hypothesized for Sb, and therefore, we aimed to study Sb binding and mobility in a combination of laboratory and field experiments, to get new insights into the fate of Sb in sub- to anoxic organic-rich environments.

The first two studies investigated the occurrence of inorganic thioarsenates in a naturally As-contaminated peatland and its 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 modelpeat 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-richmodel peat confirmed strong As(III)-S-Corg complexation of arsenite, increasing from slightly alkaline toslightly 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. Thus, MTAs(V) and arsenate were very mobile, while arsenite showed high affinity to carboxyl/phenol and thiol groups of peat.

The following two studies focused on As mobilization potential by reduced S, when As(III) was previously bound to NOM within model peat and within organic-rich aquifer lenses. Addition of sulfide or polysulfides caused substantial As retention within peat compared to controls at acidic pH. Significant As mobilization occurred via thioarsenate formation in similar quantities at neutral to slightly alkaline pH. Spectroscopic analyses confirmed a decrease in As(III)-O-Corg binding, suggesting mobilization to proceed via arsenite desorption, reaction with reduced S, and formation of thioarsenates. Formed thioarsenates remained in solution without (re)adsorption to peat. Similarly, As mobilization downstream of organic-rich lenses within aquifer sands was observed compared to control columns without lenses. Sulfate-reduction drove production of sulfide and zero-valent S. Thiol groups partitioned As via As(III)-S-Corg bond formation to the solid phase within the lenses, while zero-valent S mobilized As again by thioarsenate formation under slightly alkaline experimental pH conditions.

The fifth and sixth study explored binding mechanisms of antimonite to organic functional groups of modelpeat and under field conditions in a mine water impacted peatland. Incubation experiments showed strongantimonite adsorption to peat by formation of Sb(III)-O-Corgbonds as revealed by spectroscopic analyses. Antimonite adsorption strongly increased with increasing peat thiol-group content. Spectroscopic datarevealed 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 ofantimonite to O-containing functional groups and Sb(III)-S-Corg coordination strongly increased withincreasing peat depth. No organically complexed Sb(V) was found. Aqueous Sb speciation was dominated by antimonate and 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. This knowledge has important implications for the safe and long-term management of contaminated peatlands and other organic-rich environments to keep As and Sb partitioned to the solid phase.