Goffinet Christoffer J, Mason Sara E
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA.
J Environ Monit. 2012 Jul;14(7):1860-71. doi: 10.1039/c2em30355h. Epub 2012 Jun 20.
The long-recognized risk to human health arising from arsenic-contaminated waters is known to be linked to partitioning reactions between arsenic and natural solid phases. Currently, the ability to predict As surface complexation is limited by the lack of molecular-level understanding of As-solid interactions. In the present study, we use density functional theory (DFT) to model mono-, bi-, and tri-dentate As(III) surface complexes on different (previously proposed) structural models for hydrated hematite, modeled as α-Fe(2)O(3)(0001)-water interfaces. One of the modeled hematite-water interfaces is terminated entirely by hydroxyl surface functional groups, comprised of hematite lattice oxygen atoms. The other hematite-water interface is an iron-terminated model in which the outermost oxygen functional groups are water (and water dissociation product) ligands. We report the DFT trends in adsorption energies in terms of As-hematite coordination, hematite surface geometry/stoichiometry, and oxygen functional group identity. The DFT energetics predict that a monodentate As(III) surface complex is preferred on both hematite-water structures, suggesting that the two structural models here employed do not sufficiently represent the true surface structure to reproduce the experimental observation of As(III) bidentate coordination. However, the results do elucidate fundamental concepts of interface reactivity: A key result, supported by electronic structure analysis, is that ligand oxygen functional groups cannot be treated on equal ground with true surface oxygen functional groups. For the systems modeled here the distinction between surface and ligand functional groups supersedes the differences in oxygen coordination with surface Fe. We discuss the impact of this finding on the application of bond-valence-based predictions of mineral-water reactivity, and use the results of this study to pose questions and directions for ongoing modeling efforts aimed at linking macroscopic reactivity with molecular-level understanding.
长期以来，人们认识到受砷污染的水对人类健康构成风险，这与砷和天然固相之间的分配反应有关。目前，由于缺乏对砷与固体相互作用的分子水平理解，预测砷表面络合的能力受到限制。在本研究中，我们使用密度泛函理论（DFT）对水合赤铁矿的不同（先前提出的）结构模型上的单齿、双齿和三齿As(III)表面络合物进行建模，将其模拟为α-Fe(2)O(3)(0001)-水界面。其中一个模拟的赤铁矿-水界面完全由羟基表面官能团终止，这些官能团由赤铁矿晶格氧原子组成。另一个赤铁矿-水界面是一个铁终止模型，其中最外层的氧官能团是水（和水离解产物）配体。我们根据As-赤铁矿配位、赤铁矿表面几何结构/化学计量以及氧官能团特性报告了吸附能的DFT趋势。DFT能量学预测，单齿As(III)表面络合物在两种赤铁矿-水结构上都是优选的，这表明这里采用的两种结构模型不能充分代表真实的表面结构，无法重现As(III)双齿配位的实验观察结果。然而，结果确实阐明了界面反应性的基本概念：电子结构分析支持的一个关键结果是，配体氧官能团不能与真正的表面氧官能团同等对待。对于这里模拟的系统，表面官能团和配体官能团之间的区别取代了氧与表面铁配位的差异。我们讨论了这一发现对基于键价预测矿物-水反应性应用的影响，并利用本研究的结果提出问题以及为正在进行的旨在将宏观反应性与分子水平理解联系起来的建模工作指明方向。
