EXAFS analysis of cadmium(II) adsorption to kaolinite
EXAFS analysis of cadmium(II) adsorption to kaolinite
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DOI：
10.1016/j.chemgeo.2008.01.001
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发表时间：
2008-04
期刊：
影响因子：
3.9
通讯作者：
I. F. Vasconcelos;E. Haack;P. Maurice;B. Bunker
中科院分区：
文献类型：
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作者：
I. F. Vasconcelos;E. Haack;P. Maurice;B. Bunker
用X射线吸收精细结构谱(XAFS)研究了不同pH和Cd溶液初始浓度([CDAQ]In)范围内高岭石吸附Cd的局部分子结构。在100μM和pH 7的条件下，高岭石样品的定量分析表明，Cd周围有一个单一的水化球体，与外球吸附络合物一致。在pH为9时，Cd-M配位(其中M代表Si或Al)在第二壳层距离处表明在边缘位置形成了内球吸附络合物(ES)。虽然吸附在Al边缘位置可能是首选的，但Al和Si原子序数的相似性和键长的微小差异使我们无法根据XAFS数据区分Si和Al边缘位置的络合。由于在空气(存在碳酸盐物种)和氮气气氛下制备的样品中Cd周围的局部环境相似，并且没有以Cd-Cd配位为特征的含Cd固相，因此排除了Cd固相析出的可能性。最后，对高岭石样品的分析表明，在pH为9时，3μM、10μM和100μM的高岭石样品的CD-M距离和德拜-沃勒因子随[CDAQ]In的变化一致，这表明形成的内球复合体类型随载荷的变化而变化。这是否意味着Al和Si边缘位置的相对重要性随着负载的变化而变化，或者Cd吸附到边缘位置是否改变了与其他附近位置的结合，或者其他一些因素，目前还没有确定。
The local molecular structure of Cd sorbed to kaolinite was examined with X-ray absorption fine structure spectroscopy (XAFS) over a range of pH and initial Cd solution concentrations, ([Cdaq]in). Quantitative analysis of kaolinite samples with [Cdaq]inof 100 μM and pH 7, shows a single hydration sphere around Cd, consistent with an outer-sphere adsorption complex. At pH 9, a Cd–M coordination (where M stands for Si or Al) at second shell distance indicates the formation of inner-sphere adsorption complex(es) on edge sites. Although adsorption to Al edge sites is probably preferred, the similarity in atomic numbers between Al and Si and the small difference in bond lengths prevents us from distinguishing complexation to Si- versus Al-edge sites based on the XAFS data. The possibility of precipitation of Cd solid phases was ruled out due to similar local environment around Cd in samples prepared under air (where carbonate species would be present) and N2atmospheres, and the absence of a Cd–Cd coordination characterizing a Cd-bearing solid phase. Finally, analysis of kaolinite samples with [Cdaq]inof 3 μM, 10 μM, and 100 μM at pH 9 shows a consistent change in Cd–M distance and Debye–Waller factor as a function of [Cdaq]in, suggesting a change in the type of inner-sphere complex being formed as a function of loading. Whether this means that there is a change in the relative importance of Al versus Si edge sites with loading, or whether adsorption of Cd to edge sites alters binding to other nearby sites, or some other factor, has not been determined at this time.