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Theoretical investigation of congener-specific soil sorption of polychlorinated biphenyls by DFT computation and potent QSAR analyses

Zhu, Mengrong, Gu, Chenggang, Cheng, Yinwen, Ju, Xuehai, Bian, Yongrong, Yang, Xinglun, Song, Yang, Ye, Mao, Wang, Fang, Jiang, Xin
Journal of soils and sediments 2017 v.17 no.1 pp. 35-46
bioavailability, chlorination, electrostatic interactions, entropy, hydrophobicity, least squares, methodology, models, polychlorinated biphenyls, prediction, quantitative structure-activity relationships, soil, soil organic matter, sorption
PURPOSE: Few studies have been conducted to understand well the underlying soil sorptive mechanism due to the limited experimental determination for the enormous number of polychlorinated biphenyl (PCB) congeners. The objective of this paper was to obtain further insights into the soil sorption behavior of PCBs with exploration of the sorptive mechanism at the molecular level for sorption affinity, which could be anticipated to help explore the migration fates and assess the bioavailability of PCBs in soil. MATERIALS AND METHODS: Soil sorption coefficients of 52 kinds of PCB congeners were collected in this paper. The geometries of PCBs were fully optimized within Gaussian 03 suite of programs, and 27 molecular descriptors which describe the electronic and thermodynamic properties were finally determined with optimized structures after optimization. The quantitative structure–activity relationships (QSARs) for predicting the soil sorption of PCBs were developed by the combination of density functional theory (DFT) computation and partial least squares analyses, which maximized the correlation between the DFT-calculated properties and soil sorption of PCBs. The QSAR was critically validated with better performance in sensitivity, robustness, interpretation and prediction, and specific description of the applicability domain. RESULTS AND DISCUSSION: For the successfully developed QSAR, R y,cᵤₘ₍ₐdⱼ₎ ² and Q cᵤₘ ² were respectively recorded as 0.922 and 0.896, and R EXT ² and Q EXT ² were respectively recorded as 0.905 and 0.914, which demonstrated the stability and predictability of the model. The molecular electronegativity of PCBs by DFT was significantly indicative of a positive correlation with sorption potency, while polarizability had a negative correlation with it. QSAR analyses also revealed the favorable structural requirement of more chlorination at meta/para sites for soil sorption. It was implied that the soil sorption should be largely ascribed to the electrostatic interaction between PCBs and soil organic matter. Nevertheless, the thermodynamic stability and hydrophobicity related to the molecular entropy increment of PCBs were also beneficial to enhancing soil sorption. CONCLUSIONS: QSAR analyses particularly indicated the strong dependence of variation of soil sorption on molecular electronic properties, such as electronegativity and polarizability, which suggested the predominance of electrostatic interaction with soil organic matter. Meta/para chlorination was illustrated as a preferable structural requirement for soil sorption. In addition, the thermodynamic stability and hydrophobicity driven by entropy increment were also effective for soil sorption. These results contributed to predict the migration and fate of PCBs in soil system.