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

在缺氧条件下，砷（As）的三价形式（亚砷酸盐）已被报道通过巯基官能团与天然有机物（NOM）结合而完全螯合。实验室实验表明，即使当条件变成好氧，亚砷酸盐绑定到S（-II）-NOM的半衰期超过300天，其稳定性高于更常见的研究亚砷酸盐吸附到Fe（III）羟基氧化物。全球的影响是，有机丰富，硫化物环境，如湿地，是重要的定量作为汇。然而，迄今为止进行的所有机理研究都将亚砷酸盐暴露于预先形成的S（-II）-NOM，忽略了在含有As（III）、S（-II）和NOM的系统中，As（III）和S（-II）的络合将形成硫代亚砷酸盐（H2 AsIIIS-IInO 3-n-，n=1-3）和硫代亚砷酸盐（HAsVS-IInO 4-n2-，n=1-4）。我们的中心假设是，在溶液中的硫代砷物种形成的动力学是快于作为（III）或S（-II）吸附NOM，因此硫代砷物种确定的程度和动力学的有机物丰富，硫化物环境中的吸附。竞争吸附共存（Meta）稳定铁矿物相也将不同于已知的亚砷酸盐的行为。由于其不稳定性和缺乏纯标准，硫代亚砷酸盐的吸附行为是未知的，但。对于硫代砷酸盐，没有关于与NOM结合的信息，但已知与不同Fe（III）矿物的结合低于亚砷酸盐。我们假设硫代砷酸盐比亚砷酸盐对S（-II）-NOM的吸附更少更慢，因为硫代砷酸盐中的共价无机S键降低了S（-II）-NOM络合的亲和力。硫代砷酸盐与Fe（III）-NOM的结合应该更低且更慢，类似于已知的对Fe（III）矿物相的较低亲和力。我们还假设，硫化导致更快和更多的再活化比以前研究的氧化，因为非生物氧化是缓慢的，而S-络合与作为自发发生和不稳定的NOM和Fe矿物相结合。为了验证我们的假设，将进行批量实验，单和三硫代砷酸盐标准和砷硫化物混合物（形成硫代亚砷酸盐）在pH值为5，7和9与两个选定的NOM（Federseemoor泥炭和埃利奥特土壤腐殖酸;每个未反应，S（-II）和Fe（III）反应）。将确定吸附亲和力和动力学，并使用X射线吸收光谱法确定结合机制。在氧化条件下，以及在硫化条件下和二元体系中的优先吸附下，将研究负载（硫代）砷的NOMs的稳定性（铁羟基氧化物、Fe（III）-NOM、S（-II）-NOM和铁硫化物的组合）。总体目标是进一步阐明在含有S（-II）、Fe、和NOM，以帮助预测在何种条件下，作为汇将变成作为源。

Under anoxic conditions, arsenic (As) in its trivalent form (arsenite) has been reported to be completely sequestered by binding to natural organic matter (NOM) via sulfhydryl functional groups. Laboratory experiments showed that even when conditions turned oxic, arsenite bound to S(-II)-NOM had a half-life of more than 300 days and its stability was higher than that of the more commonly investigated arsenite sorbed to Fe(III) oxyhydroxides. The global implication is that organic-rich, sulfidic environments, such as wetlands, are important quantitative As sinks. However, all mechanistic studies carried out so far have exposed arsenite to pre-formed S(-II)-NOM, ignoring that in a system containing As(III), S(-II), and NOM, complexation of As(III) and S(-II) will form thioarsenites (H2AsIIIS-IInO3-n-, n=1-3) and thioarsenates (HAsVS-IInO4-n2-, n=1-4). Our central hypothesis is that kinetics of thioarsenic species formation in solution is faster than As(III) or S(-II) sorption to NOM and that therefore thioarsenic species determine extent and kinetics of As sorption in organic-rich, sulfidic environments. Competitive sorption to co-occurring (meta)stable iron mineral phases will also differ from known behavior of arsenite.Due to their instability and a lack of pure standards, sorption behavior of thioarsenites is unknown, yet. For thioarsenates, no information exists about binding to NOM, but binding to different Fe(III) minerals is known to be lower than that of arsenite. We hypothesize that thioarsenates show less and slower sorption than arsenite to S(-II)-NOM, because the covalent inorganic S bonds in thioarsenates decrease the affinity for S(-II)-NOM complexation. Thioarsenate binding to Fe(III)-NOM should be lower and slower in analogy to the known lower affinity for Fe(III) mineral phases. We also hypothesize that sulfidation causes faster and more As remobilization than the previously investigated oxidation because abiotic oxidation is slow, while S-complexation with As occurs spontaneously and destabilizes As binding to NOM and Fe mineral phases. To test our hypotheses, batch experiments will be conducted with mono- and trithioarsenate standards and an arsenite-sulfide mixture (forming thioarsenites) at pH 5, 7, and 9 with two selected NOMs (Federseemoor Peat and Elliott Soil Humic Acid; each non-reacted, S(-II)- and Fe(III)-reacted). Sorption affinity and kinetics will be determined and binding mechanisms will be identified using X-ray absorption spectroscopy. The stability of (thio)arsenic-loaded NOMs will be investigated under oxidizing but also under sulfidic conditions and preferential sorption in binary systems (combinations of iron oxyhydroxides, Fe(III)-NOM, S(-II)-NOM, and iron sulfides) will be determined.The overall goal is further elucidating As binding mechanisms in systems containing S(-II), Fe, and NOM to help predict under which conditions As sinks will turn into As sources.