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

在缺氧条件下，砷(As)的三价形态(亚砷酸盐)通过巯基官能团与天然有机物(NOM)结合而被完全隔离。实验室实验表明，即使在氧化条件下，与S(-II)-NOM结合的亚砷酸盐的半衰期也在300d以上，其稳定性高于更常研究的吸附在Fe(III)-羟基氧化物上的亚砷酸盐。这在全球范围内的影响是，富含有机物质的硫化物环境，如湿地，在数量上是非常重要的。然而，迄今进行的所有机理研究都是将亚砷酸盐暴露于预先形成的S(-II)-NOM，而忽略了在含有As(III)、S(-II)和NOM的体系中，As(III)和S(-II)的络合会生成硫代亚砷酸盐(H_2As_3IS-IInO_3-n-，n=1-3)和硫代亚砷酸盐(HAsVS-IInO_4-N_2-，n=1-4)。我们的中心假设是，硫代砷物种在溶液中的形成动力学比As(III)或S(-II)对NOM的吸附更快，因此，在富有机的硫化物环境中，硫代砷物种决定了As的吸附程度和动力学。亚砷酸盐对共生(偏)稳定铁矿物相的竞争吸附也将不同于已知的亚砷酸盐的行为。由于它们的不稳定性和缺乏纯标准，硫代亚砷酸盐的吸附行为尚不清楚。对于硫代砷酸盐，不存在与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。我们推测，由于硫代砷酸根中的共价无机S键降低了对S(-II)-NOM络合的亲和力，所以对S(-II)-NOM的吸附比亚砷酸盐表现出较小和缓慢的吸附。硫代砷酸盐与Fe(III)-NOM的结合速度应该较低且较慢，类似于已知的对Fe(III)矿物相的低亲和力。我们还假设，硫化作用比以前研究的氧化作用更快、更多地引起As的再动员，因为非生物氧化是缓慢的，而S-与As的络合作用是自发发生的，并且由于与NOM和Fe矿物相的结合而不稳定。为了验证我们的假设，将进行批量实验，使用单一和三硫代砷酸盐标准以及在pH 5、7和9的亚砷酸盐-硫化物混合物(形成硫代亚砷酸盐)与两个选定的NOM(联邦泥炭和埃利奥特土壤腐植酸；每个未反应，S(-II)-和Fe(III)-反应)进行批量实验。将确定吸附亲和力和动力学，并将使用X射线吸收光谱确定结合机制。我们将研究(硫代)砷的NOM在氧化和硫化条件下的稳定性，以及在二元体系(氢氧化铁、Fe(III)-NOM、S(-II)-NOM和硫化铁的组合)中的优先吸附。总体目标是进一步阐明S(-II)、铁和NOM体系中的结合机理，以帮助预测在哪些条件下汇将成为源。

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.