Correlations of nonlinear sorption of organic solutes with soil/sediment physicochemical properties

The estimation of solute sorptive behaviors is essential when direct sorption data are unavailable and will provide a convenient way to assess the fate and the biological activity of organic solutes in soil/sediment environments. In this study, the sorption of 2,4-dichlorophenol (2,4-DCP) on 19 soil/sediment samples and the sorption of 13 organic solutes on one sediment were investigated. All sorption isotherms are nonlinear and can be described satisfactorily by a simple dual-mode model (DMM): qe = KpCe + Q0 · bCe/(1 + bCe), where Kp (ml g−1) is the partition coefficient; Ce (μg ml−1) is the equilibrium concentration; Q0 (μg g−1) is the maximum adsorption capacity; Q0 · b (ml g−1) is the Langmuir-type isotherm slope in the low concentration (Henry’s law) range and b (ml μg−1) is a constant related to the affinity of the surface for the solute. Based on these nonlinear sorption isotherms and similar other nonlinear isotherms, it is observed that, for both polar 2,4-DCP and nonpolar phenanthrene, Kp, Q0 and Q0 · b are linearly correlated with soil/sediment organic carbon content (foc in the range of 0.118–53.7%). The results indicate that the nonlinear sorption of organic solutes results primarily from interactions with soil/sediment organic matter. The image, Qoc (Qoc = Q0/foc), Loc (Loc = Q0 · b/foc) and b for a given organic solute with different soils/sediments are largely invariant. Furthermore, image, logb and logLoc for various organic solutes are correlated significantly with the solute log Kow or logSw (logKow in the range of 0.9 to 5.13 and logSw in the range of −6.176 to −0.070). A fundamental empirical equation was then established to calculate approximately the nonlinear sorption from soil/sediment foc and solute Sw for a given solute equilibrium concentration.