Naturally occurring biogenic pyrite has been found in Holocene fluvial aquifers in the Uphapee watershed, Macon County, Alabama. The electron microprobe (EMP) analysis showed that the pyrite grains contain 0.20–0.92 weight% of arsenic (As). The scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) analysis confirmed a similar concentration of As in the pyrite that was consistent with the EMP analysis. The SEM analysis also confirmed the presence of additional trace elements such as cobalt (0.19 wt.%), and nickel (0.15 wt.%), indicative of pyrite’s capacity to sequester As and other trace elements. Pyrite grains were naturally formed and developed as large (20–200 μm) euhedral (i.e., cube, octahedron) crystals and non-framboid aggregates. However, the inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the As concentration in the groundwater was not high, and it was within the EPA drinking water standard for As (10 µg/L). These results indicate that dissolved As is sequestered in naturally formed pyrite found in the fluvial sediments. The groundwater was moderately reducing to slightly oxidizing (Eh = 46 to173 mV), and nearly neutral to slightly acidic (pH = 5.53 to 6.51). Groundwater geochemistry data indicated a redox sequence of oxidation, denitrification, Mn(IV) reduction, Fe(III) reduction, and sulfate reduction along the flow path in the fluvial aquifer. The downgradient increases in dissolved Mn and then Fe concentrations reflect increased Mn(II) and Fe(II) production via microbial competition as the aquifer becomes progressively more reduced. Bacterial sulfate reduction seems to dominate near the end of the groundwater flow path, as the availability of Mn- and Fe-oxyhydroxides becomes limited in sediments rich in lignitic wood where increasing sulfate reduction leads to the formation of biogenic pyrite. The groundwater is a Ca-SO₄ type and is not SO₄ limited; thus, sulfate may serve as an electron acceptor for the bacterial sulfate-reducing reactions that sequester As into pyrite, which in turn results in very low groundwater As concentration (<2 µg/L).

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阿拉巴马州梅肯县全新世河道含水层中地下水和天然存在的生物黄铁矿的地球化学

在阿拉巴马州梅肯县Uphapee流域的全新世河床含水层中发现了天然的生物黄铁矿。电子探针（EMP）分析表明，黄铁矿晶粒含有0.20-0.92重量％的砷（As）。扫描电子显微镜和能量色散光谱法（SEM-EDS）分析证实黄铁矿中As的浓度与EMP分析相一致。SEM分析还证实存在其他痕量元素，例如钴（0.19 wt。％）和镍（0.15 wt。％），表明黄铁矿具有螯合As和其他痕量元素的能力。硫铁矿晶粒是自然形成的，并发展为大的（20-200μm）自体（即立方，八面体）晶体和非蜂胶状聚集体。然而，电感耦合等离子体质谱法（ICP-MS）分析表明，地下水中的砷浓度不高，并且处于EPA饮用水中As的标准（10 µg / L）之内。这些结果表明，溶解态的As被隔离在河流沉积物中发现的天然形成的黄铁矿中。地下水适度还原至微氧化（Eh = 46至173 mV），而近乎中性至微酸性（pH = 5.53至6.51）。地下水地球化学数据表明，沿河流含水层中的流动路径氧化，反硝化，Mn（IV）还原，Fe（III）还原和硫酸盐还原的氧化还原顺序。溶解性Mn的下降梯度增加，然后随着含水层逐渐减少，通过微生物竞争，Fe浓度反映出Mn（II）和Fe（II）产量增加​​。硫酸盐细菌的还原似乎在地下水流径的尽头占主导地位，因为富含木质素木材的沉积物中Mn和Fe-羟基氧化物的可用性受到限制，其中硫酸盐还原的增加导致生物黄铁矿的形成。地下水是Ca-SO₄型，不受SO ₄限制；因此，硫酸盐可以作为将砷螯合到黄铁矿中的细菌减少硫酸盐还原反应的电子受体，从而导致地下水中砷的浓度非常低（<2 µg / L）。