A distributed reactivity model for sorption by soils and sediments .4. Intraparticle heterogeneity and phase-distribution relationships under nonequilibrium conditions

Rates of phenanthrene sorption by four different types of soils and sediments were characterized by examining the time dependence of solute phase distribution relationships (PDRs) in completely-mixed batch reactors. Unlike conventional single-level concentration methods, the experiments were conducted using a range of concentrations to obtain a time series of nonequilibrium PDRs for each sorbent-sorbate system over reaction periods ranging from 1 min to 14 days. In all cases tested, the nonequilibrium PDRs changed from approximately linear form to increasingly nonlinear form as the time of reaction increased. A Freundlich-type relationship, q(t) = K-F(t)C(t)(n(t)), was used to relate values of measured temporal solid-phase solute concentrations, q(t), to corresponding solution-phase solute concentrations C(t). After a short ''initiation'' stage, the parameters n(t) and K-F(t) were observed to vary functionally with logarithmic time. A three-domain particle-scale model predicated on the existence of discrete soil components (exposed inorganic surfaces and amorphous and condensed soil organic matter) is invoked to explain the observed sorption behavior and functional relationships underlying the time dependence of the PDRs.