Evaluating Nonlinear Sorption of Four Substituted Phenols to Agriculture Soils Using Expanded Polyparameter Linear Free Energy Relationship

Nonlinear sorption of substituted phenols (degradation products of several pesticides) onto soils was often observed. This sorption nonlinearity at low solute concentration ranges could result in higher soil organic carbon-water distribution coefficient (K ₒc) values than those predicted by their hydrophobicity (K ₒw). In this study, nonlinear sorption characteristic of four substituted phenols (2,6-dimethylphenol, 2-chlorophenol, 2-nitrophenol, and 2,4-dichlorophenol) onto two agricultural soils was investigated. The sorption nonlinearity gradually approached apparent saturation at low solute activity ranges (e.g., a ᵢ < 0.01). At high a ᵢ ranges, linear sorption was observed. Thus, partition and adsorption of solutes were successfully evaluated by a dual-mode sorption model. The concentrations of substituted phenols in the environment are pretty low (e.g., usually lower than 1 mg/L). According to our results, nonlinear adsorption is dominant in such low concentration ranges in the environment. To predict varied log K ₒc values resulted from nonlinear adsorption, especially for low a ᵢ range, an expanded polyparameter linear free energy relationship (pp-LFER) is established: log K ₒc = [(1.829 ± 0.488) + (3.481 ± 0.462) log a ᵢ)]E+ [(− 4.307 ± 0.466) log a ᵢ]S+ [(− 0.876 ± 0.138) log a ᵢ]A+ [(− 0.086 ± 0.529) + (1.209 ± 0.218) log a ᵢ]B+ (6.280 ± 0.649)V – (6.814 ± 0.917) (E, the excess molar refraction; S, the dipolarity/polarizability parameter; A, the solute H-bond acidity; B, the solute H-bond basicity; and V, the molar volume). This model can provide a better prediction (within 0.3 log unit) than previous models. This study provides essential parameters for predicting and understanding the environmental behavior of substituted phenols in agricultural soils. Graphical Abstract ᅟ