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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >Pathways of coupled arsenic and iron cycling in high arsenic groundwater of the Hetao basin, Inner Mongolia, China: An iron isotope approach
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Pathways of coupled arsenic and iron cycling in high arsenic groundwater of the Hetao basin, Inner Mongolia, China: An iron isotope approach

机译：内蒙古河套盆地高砷地下水中砷铁耦合循环的途径：铁同位素方法

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摘要

High As groundwater is widely distributed all over the world, which has posed a significant health impact on millions of people. Iron isotopes have recently been used to characterize Fe cycling in aqueous environments, but there is no information on Fe isotope characteristics in the groundwater. Since groundwater As behavior is closely associated with Fe cycling in the aquifers, Fe isotope signatures may help to characterize geochemical processes controlling As concentrations of shallow groundwaters. This study provides the first observation of Fe isotope fractionation in high As groundwater and evaluation of Fe cycling and As behaviors in shallow aquifers in terms of Fe isotope signatures. Thirty groundwater samples were taken for chemical and isotopic analysis in the Hetao basin, Inner Mongolia. Thirty-two sediments were sampled as well from shallow aquifers for Fe isotope analysis. Results showed that groundwater was normally enriched in isotopically light Fe with δ~(56)Fe values between-3.40‰ and 0.58‰ and median of-1.14‰, while heavier δ~(56)Fe values were observed in the sediments (between-1.10‰ and 0.75‰, median +0.36‰). In reducing conditions, groundwaters generally had higher δ~(56)Fe values, in comparison with oxic conditions. High As groundwaters, generally occurring in reducing conditions, had high δ~(56)Fe values, while low As groundwaters normally had low δ~(56)Fe values. Although sediment δ~(56)Fe values were generally independent of lithological conditions, a large variation in sediment δ~(56)Fe values was observed in the oxidation-reduction transition zone. Three pathways were identified for Fe cycling in shallow groundwater, including dissimilatory reduction of Fe(III) oxides, re-adsorption of Fe(II), and precipitation of pyrite and siderite. Dissimilatory reduction of Fe(III) oxides resulted in light δ~(56)Fe values (around-1.0‰) and high As concentration (>50μg/L) in groundwater in anoxic conditions. Re-adsorption of isotopically heavy Fe(II) produced by microbially mediated reduction of Fe(III) oxides led to further enrichment of isotopically light Fe in groundwater (up to-3.4‰ of δ~(56)Fe) in anoxic-suboxic conditions. Arsenic re-adsorption was expected to occur along with Fe(II) re-adsorption, decreasing groundwater As concentrations. In strongly reducing conditions, precipitation of isotopically light Fe-pyrite and/or siderite increased groundwater δ~(56)Fe values, reaching +0.58‰ δ~(56)Fe, with a subsequent decrease in As concentrations via co-precipitation. The mixed effect of those pathways would regulate As and Fe cycling in most groundwaters.
机译：高地下水在世界各地广泛分布，这对数百万人的健康产生了重大影响。铁同位素最近已用于表征水环境中的铁循环，但尚无地下水中铁同位素特征的信息。由于地下水的As行为与含水层中的Fe循环密切相关，因此Fe同位素特征可能有助于表征控制浅层地下水As浓度的地球化学过程。这项研究首次提供了高砷地下水中铁同位素分馏的观察结果，并根据铁同位素特征评估了浅层含水层中的铁循环和砷行为。在内蒙古河套盆地采集了30个地下水样品进行化学和同位素分析。还从浅层含水层中取样了32个沉积物用于铁同位素分析。结果表明，地下水通常富含同位素轻铁，δ〜（56）Fe值在3.40‰至0.58‰之间，中位数为1.14‰，而沉积物中的δ〜（56）Fe值较高（在1.10‰和0.75‰，中位数+ 0.36‰）。在还原条件下，与有氧条件相比，地下水的δ〜（56）Fe值通常较高。通常在还原条件下发生的高砷地下水具有较高的δ〜（56）Fe值，而低砷地下水通常具有较低的δ〜（56）Fe值。尽管沉积物δ〜（56）Fe值通常与岩性条件无关，但在氧化还原过渡带中观察到了沉积物δ〜（56）Fe值的较大变化。铁在浅层地下水中循环的三个途径被确定，包括异化还原Fe（III）氧化物，Fe（II）的再吸附以及黄铁矿和菱铁矿的沉淀。 Fe（III）氧化物的异化还原导致在缺氧条件下地下水中的δ〜（56）Fe值较低（-1.0‰），As浓度较高（>50μg/ L）。在缺氧-缺氧条件下，微生物介导的Fe（III）氧化物的微生物还原而产生的同位素重Fe（II）的重新吸附导致地下水中同位素轻Fe的进一步富集（高达δ〜（56）Fe的-3.4‰） 。砷与铁（II）的再吸附预计会发生再吸附，从而降低地下水中砷的浓度。在强还原条件下，同位素轻铁黄铁矿和/或菱铁矿的沉淀增加了地下水的δ〜（56）Fe值，达到+ 0.58‰δ〜（56）Fe，随后通过共沉淀而降低了As浓度。这些途径的混合作用将调节大多数地下水中的砷和铁的循环。

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