Geosciences Institute, Johannes Gutenberg University , Becherweg 21, 55099 Mainz, Germany.
Environ Sci Technol. 2014 Oct 21;48(20):11803-10. doi: 10.1021/es502444c. Epub 2014 Oct 7.
Interaction between the goethite surface and 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide was studied using density functional theory (DFT) calculations combined with molecular dynamics (MD). The important step made here lies in the use of a periodic DFT method enabling the study of a mineral surface of different protonation states, in strong contrast with previous molecular modeling studies limited to single protonation state corresponding to the point of zero charge. Different surface OH groups and MCPA proton states were used to mimic the strong effects of pH on the outer- and inner-sphere surface complexes that are theoretically possible, together with their binding energies, and their bond lengths. Modeling both a solvated and a protonated (110) goethite surface provided a major breakthrough in the acidic adsorption regime. An outer-sphere complex and a monodentate inner-sphere complex with the neutral MCPA molecule were found to be the most energetically stable adsorbate forms. MD modeling predicted that the latter forms via the sharing of the carbonyl oxygen between the MCPA carboxylate group and a singly coordinated surface hydroxyl group, releasing an H2O molecule. All the other complexes, including the bidentate inner-sphere complex, had higher relative energies and were therefore less likely. The two most likely DFT-optimized structures were used to constrain a surface complexation model applying the charge distribution multisite complexation (CD-MUSIC) approach. The adsorption constants for the complexes were successfully fitted to experimental batch equilibrium data.
使用密度泛函理论（DFT）计算结合分子动力学（MD）研究了针铁矿表面与 4-氯-2-甲基苯氧乙酸（MCPA）除草剂之间的相互作用。这里的重要步骤在于使用周期性 DFT 方法，从而可以研究不同质子化状态的矿物表面，这与以前仅限于对应于零电荷点的单一质子化状态的分子建模研究形成鲜明对比。不同的表面 OH 基团和 MCPA 质子态被用来模拟 pH 值对理论上可能存在的外球和内球表面配合物的强烈影响，以及它们的结合能和键长。对溶剂化和质子化（110）针铁矿表面的建模在酸性吸附区取得了重大突破。发现中性 MCPA 分子的外球配合物和单齿内球配合物是最稳定的吸附物形式。MD 建模预测，后者形式通过 MCPA 羧酸盐基团和单配位表面羟基之间的羰基氧共享，释放出一个 H2O 分子。所有其他配合物，包括双齿内球配合物，都具有更高的相对能量，因此不太可能。使用两个最可能的 DFT 优化结构来约束应用电荷分布多位点络合（CD-MUSIC）方法的表面络合模型。成功地将配合物的吸附常数拟合到实验批量平衡数据。
