Characterization of Adsorption Mechanisms of Volatile Organic Compounds with Montmorillonite at Different Levels of Relative Humidity via a Linear Solvation Energy Relationship Approach

The sorption mechanisms of volatile organic compounds (VOCs) with montmorillonite, a clay, need to be studied adequately to predict the fate of VOCs in the environment and to fully employ the sorption characteristics of clays to control VOC pollution. The sorption of VOCs as vapors to a typical montmorillonite, STx-1, at different levels of relative humidity were characterized using a linear solvation energy relationship (LSER). The fitted LSER equation was obtained by a multiple regression of the partition coefficients of 22 probe chemicals against their solvation parameters. The coefficients of the five-parameter LSER equations show that montmorillonite interacts with VOC molecules mainly through dispersion, partly through dipolarity/polarizability and hydrogen bonds, and with negatively pi-/n-electron pair interaction under dry conditions. With an increase of relative humidity, hydrated montmorillonite behaves both as a hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) and interacts with chemicals by dispersion interactions and dipolarity/polarizability and also with negatively pi-/n-electron pair interaction. The related terms in LSERs suggest that the potential factors governing the sorption of VOCs are dispersion interactions on dry montmorillonite that become hydrogen-bond acidity and hydrogen-bond basicity interactions for hydrated montmorillonite. The dispersion interaction is recognized to be the predominant parameter for most VOCs, whereas the contributions of the other parameters depend on specific VOCs. The derived LSER equations successfully fitted the sorption coefficients of VOCs on montmorilloitite. The LSER approach coupled with inverse gas chromatography (IGC) has been used to characterize the sorption mechanism of VOCs with clay under different conditions.