Laboratory for Waste Management, Paul Scherrer Institute, CH-5232, Villigen-PSI, Switzerland.
Environ Sci Technol. 2012 Jun 5;46(11):5713-9. doi: 10.1021/es204423k. Epub 2012 May 9.
Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono- and bidentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling.
粘土矿物是地球环境中重金属的有效汇。了解这些元素在粘土上的吸收机制有助于保护自然环境免受工业污染。在这项研究中，我们应用从头算分子动力学（MD）计算模拟了 Zn 在蒙脱石（一种二八面体粘土）边缘表面的吸收，并解释了吸附 Zn 的测量 K 边扩展 X 射线吸收精细结构（EXAFS）谱。这些实验是在高离子强度的 Na 背景电解质中进行的，这使得我们能够阻止阳离子交换过程，并将 Zn 的吸收限制在粘土边缘的吸附络合过程中。实验数据和模拟结果的分析表明，结构上掺入的 Zn 优先替代八面体层中反式对称位置的 Al(III)。在低负载时，Zn 掺入到（010）和（110）边缘的最外层反八面体中。在中等负载时，Zn 形成单齿和双齿内球表面络合物，附着在（010）和（110）边缘位点的八面体层上。从分子模拟推断出的内球吸附位的最大位密度与宏观研究和建模得出的表面络合位的位容量非常吻合。
