Mechanistic Modeling of Pollutant Mass Redistribution (Sorption/Desorption) in Heterogeneous Systems Explaining Unexpected Slow Kinetics

Redistribution of pollutants between different solid phases occurs frequently in field and laboratory settings. Examples include the input of urban particles carrying pollutants into soils or rivers with suspended particles or passive sampling. Since multiple mass transfer mechanisms are involved and natural particles typically are very heterogeneous, modeling of sorption/desorption kinetics is challenging. Here, we present a semi-analytical model formulated in the Laplace domain to simulate pollutant redistribution kinetics in heterogeneous systems. The model accounts for a coupled process governed by intraparticle and external boundary layer diffusion, and it considers the heterogeneity of various sorbents (e.g., geometric shape, size, sorption capacity coefficient, and solid and porous particles). The model is validated against data of two batch experiments: (i) the redistribution of phenanthrene in spherical polyethylene particles of different sizes and (ii) redistribution of anthracene-d10 and phenanthrene in a heterogeneous sediment suspension with polyethylene passive samplers. It allows to explain the temporary overshooting of concentrations in the aqueous phase due to different kinetic controls of various particles involved (fast desorption vs. slow sorption) as well as initial fast kinetics followed by surprising long tailing in batch experiments. The approach is very flexible and can be used for many different scenarios.