Morin Guillaume, Ona-Nguema Georges, Wang Yuheng, Menguy Nicolas, Juillot Farid, Proux Olivier, Guyot François, Calas Georges, Brown Gordon E
Institut de Minéralogie et de Physique des Milieux Condensés, UMR 7590, CNRS, Université Paris 6, Université Paris 7, IPGP, 140, rue de Lourmel, 75015 Paris, France.
Environ Sci Technol. 2008 Apr 1;42(7):2361-6. doi: 10.1021/es072057s.
Arsenic sorption onto maghemite potentially contributes to arsenic retention in magnetite-based arsenic removal processes because maghemite is the most common oxidation product of magnetite and may form a coating on magnetite surfaces. Such a sorption reaction could also favor arsenic immobilization at redox boundaries in groundwaters. The nature of arsenic adsorption complexes on maghemite particles, at near-neutral pH under anoxic conditions, was investigated using X-ray absorption fine structure (XAFS) spectroscopy at the As K-edge. X-ray absorption near edge structure spectra indicate that As(III) does notoxidize after 24 h in any of the sorption experiments, as already observed in previous studies of As(III) sorption on ferric (oxyhydr)oxides under anoxic conditions. The absence of oxygen in our sorption experiments also limited Fenton oxidation of As(III). Extended XAFS (EXAFS) results indicate that both As(III) and As(V) form inner-sphere complexes on the surface of maghemite, under high surface coverage conditions (approximately 0.6 to 1.0 monolayer), with distinctly different sorption complexes for As(III) and As(V). For As(V), the EXAFS-derived As-Fe distance (approximately 3.35 +/- 0.03 A) indicates the predominance of single binuclear bidentate double-corner complexes (2C). For As(III), the distribution of the As-Fe distance suggests a coexistence of various types of surface complexes characterized by As-Fe distances of approximately 2.90 (+/-0.03) A and approximately 3.45 (+/-0.03) A. This distribution can be interpreted as being due to a dominant contribution from bidentate binuclear double-corner complexes (2C), with additional contributions from bidentate mononuclear edge-sharing (2E) complexes and monodentate mononuclear corner-sharing complexes (1V). The present results yield useful constraints on As(V) and As(III) adsorption on high surface-area powdered maghemite, which may help in modeling the behavior of arsenic at the maghemite-water interface.
在基于磁铁矿的除砷过程中，砷吸附到磁赤铁矿上可能有助于砷的保留，因为磁赤铁矿是磁铁矿最常见的氧化产物，可能在磁铁矿表面形成涂层。这种吸附反应也可能有利于地下水中氧化还原边界处的砷固定。在缺氧条件下，于接近中性pH值的环境中，利用砷K边的X射线吸收精细结构（XAFS）光谱研究了磁赤铁矿颗粒上砷吸附络合物的性质。X射线吸收近边结构光谱表明，正如之前在缺氧条件下对三价砷吸附在铁（氢）氧化物上的研究所观察到的那样，在任何吸附实验中，三价砷在24小时后都不会氧化。我们吸附实验中没有氧气也限制了三价砷的芬顿氧化。扩展XAFS（EXAFS）结果表明，在高表面覆盖率条件下（约0.6至1.0单层），三价砷和五价砷都在磁赤铁矿表面形成内球络合物，三价砷和五价砷的吸附络合物明显不同。对于五价砷，EXAFS得出的砷-铁距离（约3.35±0.03 Å）表明单核双齿双角络合物（2C）占主导。对于三价砷，砷-铁距离的分布表明，以约2.90（±0.03）Å和约3.45（±0.03）Å的砷-铁距离为特征的各种表面络合物共存。这种分布可以解释为是由于双核双齿双角络合物（2C）的主要贡献，以及双齿单核边共享（2E）络合物和单齿单核角共享络合物（1V）的额外贡献。目前的结果对高比表面积粉末状磁赤铁矿上五价砷和三价砷的吸附产生了有用的限制，这可能有助于模拟砷在磁赤铁矿-水界面的行为。
