Thermodynamics Mechanisms And Molecular Dynamics Simulation Study For Sorption Of Polychlorinated Biphenyls By Montmorillonite

Polychlorinated biphenyls(PCBs) are a kind of toxic organic compounds and endanger human’s health through bioaccumulation in food chains, and soil is a major reservoir for environmental PCBs. Montmorillonite is one of the most abundant clay minerals and widespread in soils which plays an important role for sorption of many natural organic chemicals due to its high surface area and high cation exchange capacity. Montmorillonite has been widely used as conventional adsorbents for environmental pollution remediation, and the clay interlayer surface of montmorillonite is the predominant sorptive locale. In the natural soil, clay mineral tends to be coated by soil organic matter to form stable humic substance-clay complex. The adsorption mechanism of organic contaminants on humic substance-clay complex is complicated and is not the simplesum of individual sorptions on the humic substance and clay mineral separately.. In this study, the interaction mechanism between a series of coplanar and non-planar PCBs and montmorillonite saturated with different exchange cations(K+- and Cs+-Mont) and humic acid-montmorillonite complex( HA-Mont) was examined using macroscopic sorption experiments and molecular dynamics simulations technology, and the relevant sorption mechanism of PCBs by clay minerals was elucidated with the detailed analysis of the impactsincluding interlayer exchange cations, PCB molecular planarity and HA. The main results were as follows:1)Macroscopic sorption experimentThe miscible organic solvents cosolvents-derived method was used to accurately quantify the sorption of substances with extremely low solubilities on montmorillonite in this study. All the sorption isotherms were well fit to linear adsorption model, and the sorption process was spontaneous and exothermic. The interlayer siloxane surface of montmorillonite was the main adsorption sites of PCBs. The driving forces of sorption of PCBs for K+-Mont and HA-Mont were surface adsorption and partition,respectively.Linear adsorption coefficient Kw of 2,4,4’-trichlorobiphenyl(PCB28) sorption on K+、Cs+- saturated montmorillonite(Mont) decreased in the order of K+-Mont>Cs+-Mont, where the hydration of interlayer cations was the controlling factor. Exchangeable cations with smaller hydration radii(Cs+), manifest lower hydration energy, and result in larger siloxane surface area unoccupied by water and cations, but the vertical domains of Cs+-Mont was less than K+-Mont because the non-planar PCB28 molecular is too large to enter the interlayer of montmorillonite, and K+-Mont can provide an optimal vertical domain and suitable lateral adsorption domain for adsorption of PCB28; Linear adsorption coefficient Kw of PCB28 sorption on K+-Mont, HA-Mont decreased in the order of HA-Mont>K+-Mont, indicating that the presence of HA promoted the sorption of PCB28 on montmorillonite. HA molecule was restricted to the external surface or edged faces of montmorillonite and unable to enter the interlayer due to the large HA molecular sizes. The hydrophobicity of clay interlayer increased as HA prevented water molecular from entering the interlayer. HA acted as a partitioning phase; the adsorption of coplanar 4-chlorobiphenyl(PCB3) by the K+-Mont was greater than that of non-planar 2-chlorobiphenyl(PCB1) despite the fact that both PCB1 and PCB3 had similar Kow values. Same trend was also observed for the trichlorobiphenyls including non-planar 2,2’,6-trichlorobiphenyl(PCB19) and coplanar 3,3’,5-trichlorobiphenyl(PCB36), mainly due to the spatialsteric effects.The molecular structure of organic contaminants was an important factor for the adsorption.Compared to nonplanar PCB1 and PCB19, coplanar PCB3 and PCB 36 had lower rotational barriers and smaller dihedral angles, therefore coplanar PCB can freely enter the interlayer. Linear adsorption coefficient Kw of PCBs sorption on HA-saturated montmorillonite decreased in the order of PCB36>PCB19>PCB3>PCB1, indicating that the spatialsteric effects also exists in the model humic substance-montmorillonite complex. Organic contaminant molecular structure was an important factor controlling the transportation and fate of contaminant in the natural soil.2) Molecular dynamics simulationA series of molecular dynamics simulations of PCBs intercalated in various hydrated montmorillonite were conducted to further investigate the molecular-scale sorption mechanism. The trajectories of interlayer cation in Swy-2 hydrates with different water content showed that the surface coverage by cationsshowed significant variation in the hydrated montmorillonite system and cation-free surface siloxane domains were observed whose size were relatively constant and would be sufficient to accommodate PCBs.The density distributions of interlayer water in both Swy-2(montmorillonite) and PYRO(montmorillonitewithout surface charge, and the entire interlayer region was cation-free surface siloxane domain) models were analyzed that stronger H bond interactions between interlayer water and surface O for SWy-2 were observed in SWy-2 system and weaker H bond interactions between interlayer water and surface O for PYRO, and density of H bond is 0.19?0.25 and 0.06 per surface O for SWy-2 and PYRO,respectively.The cation-free siloxane surfaces(PYRO)was more hydrophobic than SWy-2 surfaces,and the poorly hydrated basal surfaces could plausibly provide favorable sorption domains for PCBs.The radial distribution function analyses suggested that the interlayer cations were strongly complexed by interlayer water and surface O in the all hydrated SWy-2 systems, while the correlations between interlayer cations and PCB molecules were not significant, and the PCB molecules were driven to the relatively hydrophobic regions. The analysis of distributions of dihedral angles between the two PCB aromatic rings and clay basal surfaces indicated that both rings of coplanar PCB laid almost exclusively on one clay basal surface, but only one ring of non-planar PCB stayed parallel to the clay basal surface.Due to the spatialsteric effects, presumably most non-planar PCBs could not insert into the clay interlayer and could only be adsorbed on external surfaces of clay.The free energies associated with the transfer of PCBs molecules in the extended interlayer-micropore two-phase periodic model were directly computed by Adaptive Biasing Force(ABF)accelerated MD method. The free energies for sorption of coplanar PCB36 and PCB3 were 1 ? 3kcal·mol-1more favorable than those for non-planar PCB19, PCB28 and PCB1 in corresponding clayhydrates, which were in agreement with the observed sorptiontrends. The average calculated ΔGABFis ~0.4 kcal·mol-1 larger than the experimentally derived ΔGw, and the mean relative error is 12.7% consistent with the accuracy that the state-of-the-art molecular dynamics method could achieve.