蒙脱石对几种典型有机污染物吸附机理的分子动力学模拟研究

Montmorillonite is one of the major natural geosorbents in soils responsible for the environmental fate and transport of many organic contaminants. However, the adsorption mechanisms of organic chemicals at montmorillonite-water interface are often complicated by many factors, including the structural features of montmorillonite, such as exchangeable cation type, interlayer swelling, heterogeneous distribution of surface charge, etc. The association of soil organic matter with montmorillonite at its broken edges/external surfaces strongly modifies the reactivity and accessibility of the adsorptive domains on the internal surfaces, which makes the adsorption mechanisms even more difficult to clarify by experimental techniques. To improve the understanding of the adsorption process at molecular level, we propose a large scale atomic model of the dynamic exchange process of different cationic and non-ionic organic contaminants between montmorillonite interlayer and bulk solution, or between montmorillonite/soil organic matter complex and bulk solution. High level quantum calculations are used to derive the classical forcefield parameters for those chemicals to better describe the non-bond interactions. Molecular dynamics of such model systems, coupled with modern free energy simulation techniques, are employed to study the adsorption mechanisms and associated energetics of selected organic chemicals. Validation of the model and simulation results is done by comparison with the adsorption free energies/enthalpies derived from adsorption isotherms at different temperatures. The model presented here would provide a theoretical tool for better understanding of the retention and fate of organic pollutants on clay minerals in soils.

蒙脱石是土壤的重要组成部分，能够强烈吸附许多类型的有机污染物，决定着这些污染物在土壤环境中的归趋。但蒙脱石复杂的结构以及土壤有机质对蒙脱石表面的包裹，造成蒙脱石与有机污染物的作用机制复杂，一直缺乏分子水平上的理解。本研究拟利用分子动力学技术，完善七类典型的非离子型和阳离子型有机污染物的分子力场参数；采用针对复杂过程的自由能计算新方法，结合蒙脱石层间-微孔分子结构模型，建立一个可以准确描述有机污染物-蒙脱石相互作用过程和吸附自由能的分子模拟平台；建立蒙脱石/模型土壤有机质分子复合体模型，考察土壤有机质对蒙脱石吸附有机污染物的影响。通过与吸附实验和光谱分析的对照研究，验证分子模拟计算能量和构型预测的可靠性，揭示有机污染物在蒙脱石微界面吸附过程的分子机制，为预测和评价有机污染物在土壤中的迁移和归趋提供重要的依据。

我们按照研究计划，开展了本基金项目的各项研究内容，目前已按计划全部完成了本青年基金项目的预期目标。在本基金的资助下，我们共发表科研论文4篇，其中SCI论文3篇，1篇收入于环境领域顶级期刊Environmental Science and Technology，完成了本青年基金预期的成果产出。取得了主要研究成果如下：基于量子化学计算生成专门描述蒙脱石-有机污染物的分子力场模型，参数具有一致性，首次解决不同力场模型混用造成的无法准确描述有机物与矿物间相互作用的问题；成功构建蒙脱石/有机改性蒙脱石层间-微孔分子结构模型，研究有机污染物在蒙脱石/有机改性膨润土上吸附迁移的过程，成功地预测了不同类型有机污染物，计算的到的吸附自由能ΔG平均比实验推导的吸附自由能ΔG的绝对值大0.4kcal•mol-1，平均相对误差为12.7％，与目前分子动力学方法可以达到的精度相符；成功地将宏观的吸附热力学实验和微观的分子动力学模拟两方面探讨蒙脱石矿物对PCBs的吸附相结合，从分子水平揭示了蒙脱石矿物对污染物的吸附机理，发现了疏水作用和空间位阻效应决定了不同结构有机污染物在蒙脱石上的吸附；成功地将有机溶剂助溶-外推方法成功的应用到本研究当中，解决低水溶解性有机污染物的定量吸附的难题。