蒙脱石对多氯联苯吸附的热力学及分子动力学模拟研究

发布时间：2018-05-12 23:45

本文选题：多氯联苯 + 蒙脱石　；参考：《安徽师范大学》2015年硕士论文

【摘要】：多氯联苯(PCBs)作为一类持久性有机污染物,能够通过食物链富集作用危害人类的身体健康,土壤或沉积物是环境中PCBs主要的汇。蒙脱石黏土矿物是土壤的重要组成部分,在土壤或沉积物环境中分布广泛且含量丰富,由于大比表面积、高阳离子交换量等结构特征,能够吸附多种类型的有机污染物,被广泛的用作于环境污染的治理材料。在实际土壤中,黏土矿物多与土壤有机质结合形成有机-无机复合体,二者很难单独作用于有机污染物,包裹在黏土矿物外表的有机质极大的影响了黏土的吸附特性,因其机理复杂,至今仍无法清楚解释。本论文利用不同阳离子饱和具有特定层间距的蒙脱石矿物和腐殖质-蒙脱石复合体,分别在有机溶剂助溶的混合体系内进行对一系列不同空间构型的1~3氯取代PCBs的等温吸附试验,利用相关推导的宏观热力学参数,结合分子动力学模拟技术,从层间阳离子类型、污染物分子的空间构型和土壤有机质角度解释污染物与黏土矿物的相互作用机制。主要研究结果如下:1)吸附热力学研究有机溶剂助溶-外推方法可以很好地解决低水溶解性有机污染物的定量吸附研究的困难。研究中所有吸附等温线均可用线性模型进行很好地拟合,溶液PCBs浓度是影响吸附量的主要因素,且吸附过程为自发放热的物理性吸附。蒙脱石层间域的疏水性硅氧烷表面是PCBs的主要吸附位点。K+、Cs+饱和蒙脱石(Mont)对2,4,4'-三氯联苯(PCB28)的吸附能力强弱为:K+-MontCs+-Mont,与层间阳离子的水合半径有关。尽管水合半径较小的Cs+能够提供更多层间疏水性的硅氧烷表面,但同时限制了黏土层间距的大小,无法提供足够的垂直吸附空间供非共平面构型PCB28插入;相反,水合半径较大的K+可提供最优的垂直间距和适当的黏土层表面吸附域,增强了PCB的吸附。腐殖质-蒙脱石复合体(HA-Mont)和K+-Mont对PCB28的吸附能力表现为:HA-MontK-Mont,表明土壤有机质能够极大的促进蒙脱石对PCBs的吸附。具体原因为两方面:一,有机质的分配作用;二,包裹在蒙脱石断面位置的有机质分子,通过限制了水分子的进入增大黏土层间的疏水性,从而增大蒙脱石对PCB28的吸附。非共平面的2-氯联苯(PCB1)和共平面的4-氯联苯(PCB3)在K+-Mont的吸附能力为:PCB3PCB1,对于非共平面的2,2',6-三氯联苯(PCB19)和共平面的3,3',5-三氯联苯(PCB36),也存在着相同的规律,原因主要为PCB分子的空间位阻效应。阻转类非共平面的PCB的旋转势垒和二面角较大导致分子体积更大,进入狭小的黏土疏水性层间需克服较大的势垒;相反,共平面类PCB分子厚度很小,既能自由插入黏土层间也可与黏土层疏水表面产生较大的接触面。PCB1、PCB3、PCB19和PCB36在HA-Mont上的吸附能力为:PCB36PCB19PCB3PCB1,表明除了疏水性的影响外,PCB分子构型因素在HA-Mont上同样是重要的影响因素。污染物分子构型因素是影响有机污染物在实际土壤中迁移转化的重要因素。2)分子动力学模拟研究构建不同含水量的SWy-2(蒙脱石)和PYRO(层间不带电荷的蒙脱石)两种水合黏土模型,通过对黏土水合系统中层间阳离子的运动轨迹的分析发现,不同含水量的黏土水合系统中阳离子的移动范围存在显著差异,黏土表面存在着相当大的区域未被阳离子占据,该区域大小相对恒定,对非极性有机污染物存在吸附亲和力。分析层间水密度分布发现,在SWy-2系统中,水分子与黏土表面存在H键作用,密度约为0.19?0.25个H键/黏土表面的氧原子。在PYRO系统中,层间水和表面氧的相互作用较弱,H键密度仅为0.06个H键/黏土表面的氧原子。PYRO层间表面较SWy-2表面的疏水性更强一些,可以作为后者层间阳离子未占据区域的理想特例;通过径向分布函数分析发现:所有水合SWy-2系统中,层间阳离子与层间水间的配合作用最强,层间阳离子与PCBs分子之间没有显著作用;受到水与阳离子间强相互作用的排挤,PCB分子被驱离到相对疏水的硅氧烷表面区域。通过对不同构型PCBs分子的芳环与层间二面角的分布分析发现,共平面PCB分子的两个芳环同时与黏土层表面直接接触,而非平面PCB分子只能有一个芳环接触黏土层表面,碍于空间位阻效应非平面的PCB分子很难进入层间,只可能吸附在黏土外表面。在黏土层间-微孔水两相模型中,运用加速分子动力学自适应偏置力方法(ABF)定量计算PCBs在黏土相和水相间的迁移自由能能,发现平面构型的PCB36和PCB 3的吸附自由能均比对应的非平面构型的PCB 19、PCB 28和PCB 1的吸附自由能的绝对值大1?3kcal·mol-1,与吸附试验观测到的趋势相一致,进一步证实了疏水作用和空间位阻效应决定了不同结构PCBs在黏土上的吸附。计算的到的吸附自由能ΔGABF平均比实验推导的吸附自由能ΔGw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。
[Abstract]:Polychlorinated biphenyls (PCBs), as a class of persistent organic pollutants, can harm human health through the enrichment of food chains. Soil or sediments are the main sinks of PCBs in the environment. Montmorillonite clay minerals are an important component of the soil, which are widely distributed in the soil or sediment environment, and are high in large specific surface area. The structure characteristics of cation exchange can be used to adsorb a variety of organic pollutants and are widely used as the treatment materials for environmental pollution. In the actual soil, clay minerals are combined with soil organic matter to form organic inorganic complexes. The two are difficult to act on organic pollutants alone and encapsulate the organic matter in the surface of clay minerals. The adsorption properties of clay are greatly influenced by the complex mechanism of the clay. The montmorillonite and humus montmorillonite complex with different cations are saturated with different cations, and a series of 1~3 chlorine substituted PCBs with different spatial configurations are carried out in the organic solvent assisted mixing system. The interaction mechanism of pollutants and clay minerals is explained from the interlayer cation type, the spatial configuration of contaminants molecules and the angle of soil organic matter. The main research results are as follows: 1) adsorption thermodynamics study on the solubilization and extrapolation of organic solvents The method can solve the difficulty of quantitative adsorption study of low water soluble organic pollutants. All adsorption isotherms in the study can be well fitted by linear model. The concentration of PCBs is the main factor affecting the adsorption amount, and the adsorption process is the physical adsorption of spontaneous exothermic. The hydrophobic siloxane in the interlayer of Montmorillonite. The surface is the main adsorption site.K+ of PCBs, and the adsorption capacity of Cs+ saturated montmorillonite (Mont) on 2,4,4'- three chlorobybenzene (PCB28) is: K+-MontCs+-Mont, which is related to the hydration radius of interlayer cations. Although Cs+ with smaller hydration radii can provide more interlayer hydrophobic siloxane surfaces, but at the same time limits the size of the clay layer spacing. Sufficient vertical adsorption space can not be provided for non common plane configuration PCB28 insertion; on the contrary, the K+ with the larger hydration radius provides the optimal vertical spacing and the appropriate surface adsorption domain of the clay layer, enhancing the adsorption of PCB. The adsorption capacity of the humus montmorillonite complex (HA-Mont) and K+-Mont to PCB28 is: HA-MontK-Mont, indicating the soil Organic matter can greatly promote the adsorption of montmorillonite to PCBs. The specific reasons are two aspects: first, the distribution of organic matter; two, the organic matter in the position of the montmorillonite section, by limiting the water molecules to increase the hydrophobicity between the clay layers, thus increasing the adsorption of the montmorillonite to the PCB28. The non coplanar 2- chlorinated biphenyl (PCB1) The adsorption capacity of the coplanar 4- chlorinated biphenyl (PCB3) at K+-Mont is PCB3PCB1. There is also the same rule for the non common plane 2,2', 6- three chlorinated biphenyl (PCB19) and the co plane 3,3', 5- three chlorinated biphenyl (PCB36), the main reason is the spatial steric effect of the PCB molecule. The rotating barrier and dihedral angle of the resistive non common plane PCB are caused by the larger dihedral angle. In addition, the thickness of the PCB molecules of the coplanar class is very small, which can either insert freely between the clay layer and produce a larger contact surface.PCB1 with the hydrophobic surface of the clay layer, and the adsorption capacity of PCB3, PCB19 and PCB36 on HA-Mont is PCB36PCB19PCB3PCB1, indicating that the hydrophobic surface is hydrophobic. The PCB molecular configuration factor is also an important factor on HA-Mont. The molecular configuration factor of the pollutants is an important factor affecting the migration and transformation of organic pollutants in the actual soil.2) molecular dynamics simulation study on the construction of two kinds of hydration of different water content SWy-2 (Meng Tuoshi) and PYRO (interlayer uncharged Meng Tuoshi). Clay model, through the analysis of the movement path of interlayer cations in the clay hydration system, it is found that there is a significant difference in the movement range of the cations in the clay hydration system with different water content, and there is a considerable area of the clay surface not occupied by the cation, the size of the region is relatively constant, and the adsorption of non polar organic pollutants exists. Affinity. The analysis of interlayer water density distribution shows that in the SWy-2 system, there is a H bond between the water molecules and the clay surface, and the density is about 0.19? 0.25 H bonds / clay surfaces. In the PYRO system, the interaction between the interlayer water and the surface oxygen is weak, and the H bond density is only 0.06 H bonds / the surface of the oxygen atom.PYRO on the clay surface than the SWy-2 table. The hydrophobicity of the surface is stronger, which can be used as an ideal case for the unoccupied region of the latter. Through the analysis of the radial distribution function, it is found that in all SWy-2 systems, the coordination of interlayer cations and interlayer water is the strongest, and there is no significant effect between the interlayer cations and the PCBs molecules; the strong interaction between water and cations is found. The PCB molecules were displaced to the surface of the relatively hydrophobic siloxane surface. Through the analysis of the distribution of the aromatic rings and interlayer dihedral angles of the different configuration PCBs molecules, it was found that the two aromatic rings of the co planar PCB molecules were directly exposed to the surface of the clay layer, while the non planar PCB molecules had only one aromatic ring to contact the surface of the clay layer, which was hindering space. The non planar PCB molecules are difficult to enter the interlayer and can only be adsorbed on the outer surface of the clay. In the interlayer microporous water two-phase model, the accelerated molecular dynamics adaptive bias method (ABF) is used to calculate the free energy of the migration of PCBs between the clay phase and the water phase, and the adsorption free energy of the PCB36 and PCB 3 of the plane configuration is found. The absolute values of the adsorption free energy of the PCB 19, PCB 28 and PCB 1 of the corresponding non planar configurations are 1? 3kcal. Mol-1, consistent with the trend observed by the adsorption test, and further confirm that the hydrophobic interaction and the spatial steric effect determine the adsorption of different structure PCBs on clay. The calculated adsorption free energy delta GABF average ratio is calculated. The absolute value of the adsorption free energy Gw is 0.4kcal. Mol-1, with an average relative error of 12.7%, which is consistent with the accuracy achieved by the present molecular dynamics method.

【学位授予单位】：安徽师范大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：X592

【参考文献】