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

Abstract

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): q e = K p C e + Q 0 · bC e/(1 + bC e), where K p (ml g −1) is the partition coefficient; C e (μg ml −1) is the equilibrium concentration; Q 0 (μg g −1) is the maximum adsorption capacity; Q 0 · 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, K p, Q 0 and Q 0 · b are linearly correlated with soil/sediment organic carbon content ( f oc 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 K oc ∗ ( K oc ∗ = K p / f oc ) , Q oc ( Q oc = Q 0/ f oc), L oc ( L oc = Q 0 · b/ f oc) and b for a given organic solute with different soils/sediments are largely invariant. Furthermore, log K oc ∗ , log b and log L oc for various organic solutes are correlated significantly with the solute log K ow or log S w ( log K ow in the range of 0.9 to 5.13 and log S w 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 f oc and solute S w for a given solute equilibrium concentration.