Dual-mode modeling of competitive and concentration-dependent sorption anddesorption kinetics of polycyclic aromatic hydrocarbons in soils

A radial dual-mode diffusion model is proposed for mass transfer of hydroph obic compounds in soil organic matter (SOM) that is able to predict competi tive and concentration effects on sorption and desorption rates. On the bas is of dual-mode sorption theory for glassy polymers the model assumes a pop ulation of specific adsorption sites ("holes") interspersed uniformly in th e dissolution (partition) domain of SOM. It further assumes Fickian diffusi on in the dissolution domain and immobilization in the holes, with microsco pic local equilibrium between the two domains. The model is solved numerica lly (Crank-Nicolson implicit method). Using parameters from single-solute e quilibrium and kinetic experiments, the model adequately predicts batch tra nsient sorption and desorption of phenanthrene (primary solute) as a functi on of pyrene (cosolute) concentration, and batch transient sorption of phen anthrene as a function of its own concentration, in two soils. The model sh ows that phenanthrene sorption approaches equilibrium faster with increasin g cosolute or self-concentration owing to the concentration dependence of t he apparent diffusivity, as predicted by a simple hole-plugging mechanism ( i.e., fewer and fewer holes are available). Simulations show the effect to be greatest under infinite bath uptake conditions. Under finite bath condit ions this positive effect on rate may be opposed by a batch process tempora l bias present when the water:soil ratio is kept constant in a series of ex periments. The bias is due to gradient driving force effects that slow the rate as a result of the decrease in percent of solute finally taken up by t he solid as cosolute or concentration increases.