Preservation and analysis of sulfur isotopes in thioarsenates: New methods for the investigation of abiotic and biotic transformation processes
Maria Ullrich (04/2013-09/2017)
Support: Britta Planer-Friedrich
Sulfur is known to exhibit a high affinity towards metal(loid)s. The consequent formation of soluble thiometalloid species is increasingly being recognized as an important process within the biogeochemical sulfur cycle. Taking into account their environmental, but also toxicological impact, thioarsenates ([HxAsVS-IInO4-n]x-3, x = 1 - 3, n = 1 - 4) have gained special attention. However, exact pathways of thioarsenate formation and transformation are not fully resolved yet. In particular, speciation observed during thioarsenate oxidation still raises some fundamental questions regarding both abiotic and biotic transformation pathways.
Sulfur isotope analysis has been demonstrated to be a helpful tool for investigating the transformation of other sulfur species, such as thiosulfate or elemental sulfur. Yet, a suitable method for the isotopic analysis of thioarsenates does currently not exist. Therefore, the goal of the present study was to develop a method that would allow isotope analysis of thioarsenates, and apply this method to address open questions on thioarsenate transformation in order to elucidate their role within the sulfur cycle.
As the fundamental prerequisite to successful isotope analysis, methods of thioarsenate preservation were investigated first, focusing primarily on the challenges arising from natural, iron-rich waters. Established methods, namely the addition of mineral acids or flash-freezing, are unsuitable in this case because they induce arsenic sulfide or iron oxyhydroxide precipitation, inevitably changing the original thioarsenic speciation. In the present study, a new approach based on separating the anionic thioarsenates from cationic iron by solid phase extraction was investigated. Anion-exchange resin AG2-X8 was found to fully retain monothioarsenate and trithioarsenate, as well as sulfate, thiosulfate, and arsenate. Iron passed the resin without interaction, and was thus separated from the target species as hypothesized. Elution of the retained species was performed in flow-through mode with alkaline salicylate, which proved to conserve speciation and remove all investigated species quantitatively from the resin. However, complete recovery of the strongly bound trithioarsenate required repeated cycles of soaking the resin in salicylate prior to elution. Full recovery of the target species was also obtained by performing batch elution, which allowed lowering the elution volume, and consequently improved species enrichment in the eluates. Finally, the method was applied to iron-rich waters [BPF1] from a mineral spring in the Czech Republic and was found to preserve sulfur and arsenic speciation for up to 6 days.
For the subsequent determination of sulfur isotopes in thioarsenates, routine analytical methods were considered. Isotope analysis is commonly performed by isotope ratio mass spectrometry (IRMS), following precipitation of sulfur species from solution. However, this approach is not feasible since the high chemical similarity among thioarsenates prevents selective precipitation. Moreover, during standard precipitation of sulfide for IRMS analysis, monothioarsenate was found to co-precipitate, and thus impede correct δ34S determination of sulfide. To circumvent these issues, a new method was developed based on ion chromatographic species separation and online detection of sulfur isotopes by multi-collector inductively coupled plasma mass spectrometry (IC-MC-ICP-MS). This allowed simultaneous isotopic analysis of sulfide, sulfate, thiosulfate, and for the first time of all four thioarsenates.
The new IC-MC-ICP-MS method was applied to investigate oxidative thioarsenate transformation. Abiotic oxidation of monothioarsenate produced 34S-depleted sulfate, yielding a normal isotope effect of -6.1 ‰. Oxidation of tetrathioarsenate revealed that no isotopic fractionation is associated with the stepwise transformation to tri-, di-, and monothioarsenate. However, monothioarsenate was found enriched in 34S, which was proposed to result from further oxidation of monothioarsenate, but also intermolecular exchange with heavy sulfide. All these findings prove that oxidative monothioarsenate transformation causes a redox change in arsenic-bound sulfur, while all other thioarsenates release sulfidic sulfur in its original form. Furthermore, the current results support a previous hypothesis of thioarsenites occurring briefly as intermediates during formation, as well as decomposition of thioarsenates.
IC-MC-ICP-MS was further employed to investigate the question, if the reduced sulfur in thioarsenates can be utilized as an electron donor by chemolithotrophic bacteria. Previously obtained speciation data seemed to indicate direct microbial oxidation of monothioarsenate, yet abiotic transformation could not be excluded. Incubation experiments with the hyperthermophile Thermocrinis ruber performed in this study revealed a pronounced inverse isotope effect. The combination of δ34S values observed during abiotic and biotic oxidation indicate that arsenic-bound sulfur can in fact not be used directly as a substrate for microbial metabolism. An alternative model was proposed, in which monothioarsenate is first disproportionated abiotically to elemental sulfur and arsenite, followed by microbial oxidation of these intermediates to sulfate and arsenate.
Overall, the methods developed over the course of this study allow for the investigation of both abiotic and biotic pathways of thioarsenate transformation. Where speciation analysis alone did not yield conclusive results until now, isotope analysis was found to provide new, valuable insights. Thus, the findings of the present study assist in elucidating the processes that control the occurrence, stability, and utilization of thioarsenates within the biogeochemical sulfur cycle.