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首页> 外文期刊>Geochemical Journal >Speciation study of Cr(VI/III) reacting with humic substances and determination of local structure of Cr binding humic substances using XAFS spectroscopy
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Speciation study of Cr(VI/III) reacting with humic substances and determination of local structure of Cr binding humic substances using XAFS spectroscopy

机译：Cr（VI / III）与腐殖质反应的形态研究及XAFS光谱法测定与Cr结合的腐殖质的局部结构

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X-ray absorption fine structure (XAFS) analysis was applied to elucidate Cr(VI/III) speciation during reaction with standard humic acid materials extracted from Japanese soils. The degree of reduction of Cr(VI) by humic acids differed for different degrees of humification. Adsorption of Cr(III) to humic substances was higher for higher carboxylic humic substance contents. After reduction experiments of Cr(VI) by humic acids, Cr binding with humic acids has a trivalent state. Nevertheless, a substantial amount of Cr(VI) remains in solution. The X-ray absorption near-edge structure (XANES) spectra of the Cr(VI) experimental system were almost identical to those of Cr(III) adsorbed onto humic acids in a low-pH condition. However, during adsorption of Cr(III) by humic acids, Cr was partly precipitated as Cr(OH)_3·nH_2O. Its concentration was higher for higher pH. XANES spectra of Cr(VI/III) reacted with different humic acids showed no systematic differences, indicating that the binding form of Cr to humic substances is common, irrespective of differences in functional group concentrations. Linear combination fitting using XANES spectra of reference compounds to HA-Cr samples suggests that 50% hydrated Cr(III) was adsorbed onto HA and that the remainder of Cr(III) was bound to carboxylic acid of HA. The local structure of Cr binding to humic acids was characterized using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS features of Cr binding to humic acids were similar to those of Cr~(3+) aquo ions having simple octahedral symmetry of the oxygen coordination sphere. They show no clear presence of carboxylic group of humic acids around the Cr site. Scattering from the C atom might not be visible because the backscatter amplitude of a C atom becomes weak steeply with increasing wavenumber (k) and because it is smaller than those of the O atom and the Cr atom. Considering the linear combination fitting results of XANES spectra, the structural mode of Cr binding with HA is R-COO-Cr-(OH_2)_5.
机译：X射线吸收精细结构（XAFS）分析用于阐明与从日本土壤中提取的标准腐殖酸物质反应过程中的Cr（VI / III）形态。腐殖酸对Cr（VI）的还原程度因腐殖化程度不同而异。对于较高含量的羧酸类腐殖质，六价铬对腐殖质的吸附较高。在通过腐殖酸还原Cr（VI）的实验后，Cr与腐殖酸的结合具有三价状态。尽管如此，溶液中仍残留大量的Cr（VI）。 Cr（VI）实验系统的X射线吸收近边缘结构（XANES）光谱与低pH条件下吸附到腐殖酸上的Cr（III）的光谱几乎相同。然而，在腐植酸吸附Cr（III）的过程中，Cr以Cr（OH）_3·nH_2O的形式部分沉淀。 pH越高，其浓度越高。与不同腐殖酸反应的Cr（VI / III）的XANES光谱显示没有系统的差异，表明Cr与腐殖质的结合形式是常见的，而与官能团浓度的差异无关。使用参考化合物的XANES光谱对HA-Cr样品进行线性组合拟合表明，有50％的水合Cr（III）吸附在HA上，其余Cr（III）与HA的羧酸键合。 Cr与腐殖酸结合的局部结构使用扩展X射线吸收精细结构（EXAFS）光谱进行了表征。 Cr与腐殖酸结合的EXAFS特征与氧配位域具有简单八面体对称性的Cr〜（3+）水族离子相似。它们显示在Cr位点附近没有明显的腐殖酸的羧基存在。从C原子的散射可能不可见，因为C原子的反向散射幅度随波数（k）的增加而急剧变弱，并且因为它小于O原子和Cr原子的反向散射幅度。考虑到XANES光谱的线性组合拟合结果，Cr与HA结合的结构模式为R-COO-Cr-（OH_2）_5。

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