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

据报道，在缺氧条件下，砷（As）的三价形式（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结合的亚砷酸盐也有超过300天的半衰期，其稳定性高于与Fe（III）氧化氢氧化物吸附的亚砷酸盐。这对全球的影响是，富含有机物、硫化物的环境，如湿地，是重要的定量砷汇。然而，迄今为止进行的所有机制研究都将亚砷酸盐暴露于预形成的S(- ii)-NOM中，而忽略了在含有As（III）、S（- ii）和NOM的体系中，As（III）和S（- ii）的络合会形成硫代亚砷酸盐（H2AsIIIS-IInO3-n-, n=1-3）和硫代亚砷酸盐（HAsVS-IInO4-n2-, n=1-4）。我们的中心假设是，硫砷在溶液中形成的动力学比As（III）或S（-II）对NOM的吸附更快，因此硫砷的种类决定了富有机物、硫化物环境中As吸附的程度和动力学。对共发生的（元）稳定铁矿物相的竞争性吸附也将不同于已知的亚砷矿的行为。由于其不稳定性和缺乏纯标准，硫砷酸盐的吸附行为尚不清楚。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）矿物的结合比亚砷酸盐低。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，这是因为硫代砷酸盐中的共价无机S键降低了对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、更慢，这与已知的Fe（III）矿物相的亲和力较低类似。我们还假设，与先前研究的氧化相比，硫化导致更快、更多的As再活化，因为非生物氧化是缓慢的，而与As的s络合是自发发生的，并且破坏了As与NOM和Fe矿物相的结合。为了验证我们的假设，将采用单硫代砷酸盐和三硫代砷酸盐标准以及pH值为5、7和9的亚砷酸-硫化物混合物（形成硫代砷酸盐）和两种选定的无机氧化物(Federseemoor泥炭和Elliott土壤腐殖酸；每种无机氧化物均未反应，S（- ii)-和Fe(III)-反应）进行批量实验。吸附亲和和动力学将确定和结合机制将确定使用x射线吸收光谱。（硫）砷负载纳米粒子的稳定性将在氧化和硫化条件下进行研究，并在二元体系(铁氢氧化物、Fe（III)-NOM、S(-II)-NOM和硫化铁的组合）中优先吸附。总体目标是进一步阐明含S（-II）、Fe和NOM体系中的As结合机制，以帮助预测在何种条件下As汇将转变为As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.