The influence of sorbate-organic matter interactions on sorption nonequilibrium

Literature data on nonequilibrium sorption of organic solutes by natural sorbents were compiled. The solutes included a broad spectrum of nonpolar, hydrophobic organic chemicals (Type I) and polar/ionizable organic chemicals (Type II). Values for equilibrium sorption coefficient (K_p) and sorption rate constant (k) were determined for the data and were analyzed with the Linear Free Energy Relationship approach. The compiled data spanned approximately seven orders-of-magnitude in K_p values and approximately six orders-of-magnitude in k values. An inverse linear relationship was found between log k and log K_p. The intercept value for the Type I chemicals was larger than that for the Type II chemicals, which suggests that the sorption dynamics for the Type II chemicals were constrained to a greater degree than that for the Type I chemicals. After examining the experimental conditions under which the data were collected, and considering the nature of the sorbents and the sorbates, it was postulated that the following processes were responsible for the observed sorption nonequilibrium: intraorganic matter diffusion for Type I chemicals; intraorganic matter diffusion and chemical nonequilibrium for Type II chemicals. The rationale for the elimination of other mechanisms is discussed. A regression equation, relating k and K_p, is presented that may be used to estimate approximate values of the sorption rate constant for organic solutes. This provides a means to evaluate nonequilibrium potential and to attempt to predict nonequilibrium behavior.