University of Connecticut, Civil and Environmental Engineering, Storrs, CT 06269, USA.
University of Connecticut, Civil and Environmental Engineering, Storrs, CT 06269, USA; National Technical University of Athens, School of Civil Engineering, Athens, Greece.
J Hazard Mater. 2015 Jan 8;281:56-63. doi: 10.1016/j.jhazmat.2014.05.067. Epub 2014 Jun 2.
Adsorption reactions play an important role in the transport behavior of groundwater contaminants. Molecular-scale information is needed to elucidate the mechanisms by which ions coordinate to soil mineral surfaces. In this study, we characterized the mechanisms of chromate adsorption on boehmite (γ-AlOOH) using a combination of extended X-ray absorption fine structure (EXAFS) measurements, in situ attenuated total reflectance Fourier transform infrared spectroscopy, and quantum chemical calculations. The effects of pH, ionic strength, and aqueous chromate concentration were investigated. Our overall findings were that chromate primarily forms outer-sphere complexes on boehmite over a broad range of pH and aqueous concentrations. Additionally, a small fraction of monodentate and bidentate inner-sphere complexes are present under acidic conditions, as evidenced by two sets of chromate stretching vibrations at approximately 915, 870, and 780cm(-1), and 940, 890, 850, and 780cm(-1), respectively. The bidentate complex is supported by a best-fit CrAl distance in the EXAFS of 3.2Å. Results from DFT also support the formation of monodentate and bidentate complexes, which are predicted to results in Gibbs energy changes of -140.4 and -62.5kJmol(-1), respectively. These findings are consistent with the intermediate binding strength of chromate with respect to similar oxyanions such as sulfate and selenite. Overall, the surface species identified in this work can be used to develop a more accurate stoichiometric framework in mechanistic adsorption models.
吸附反应在地下水污染物的迁移行为中起着重要作用。为了阐明离子与土壤矿物表面配位的机制，需要从分子尺度获取信息。在这项研究中，我们结合扩展 X 射线吸收精细结构（EXAFS）测量、原位衰减全反射傅里叶变换红外光谱和量子化学计算，对铬酸根在水铝石（γ-AlOOH）上的吸附机制进行了表征。考察了 pH 值、离子强度和水溶液中铬酸根浓度的影响。我们的总体研究结果表明，在较宽的 pH 值和水溶液浓度范围内，铬酸根主要在水铝石上形成外配位络合物。此外，在酸性条件下还存在少量的单齿和双齿内配位络合物，这可由两个铬酸盐伸缩振动谱带得到证实，分别位于约 915、870 和 780cm(-1) 以及 940、890、850 和 780cm(-1)。双齿络合物由 EXAFS 中的 CrAl 距离为 3.2Å 得到最佳拟合。DFT 结果也支持单齿和双齿络合物的形成，它们的形成吉布斯自由能变化分别为-140.4 和-62.5kJmol(-1)。这些结果与铬酸根与硫酸盐和硒酸盐等类似含氧阴离子的中等结合强度一致。总的来说，本工作中鉴定的表面物种可用于在机理吸附模型中建立更准确的化学计量框架。
