Influence of Bisolute Competition on the Desorption Kinetics of Polycyclic Aromatic Hydrocarbons in Soil

Abstract

The dual-mode (partition/hole-filling) model of soil organic matter (SOM) as a heterogeneous polymerlike sorbent of hydrophobic compounds predicts that a competing solute will accelerate diffusion of the primary solute by blocking the holes, allowing the principal solute to move faster through the SOM matrix. We tested this hypothesis with phenanthrene as the primary solute and pyrene as the competing solute in two sterile soils of widely different organic carbon content (1.4 and 43.9%). Two- and 33-d isotherms of phenanthrene in both soils were nonlinear, indicating a heterogeneous distribution of sites. Pyrene suppressed phenanthrene sorption and increased the linearity of its isotherm. Uptake (adsorption) rates were measured in batch systems after preincubating with pyrene. Desorption was measured by the sequential dilution technique at constant pyrene concentration in the supernatant. The design of the experiment precluded comparison of adsorption rates, but desorption rates increased significantly (but not dramatically) as a function of pyrene concentration. Moreover, the effect was noticeable even at low and roughly equimolar concentrations of the two compounds (∼0.5 μmol/gOC). The effect was qualitatively similar in the two soils, implicating SOM as the matrix in which the effect takes place. The results suggest that the competing solute excludes the primary solute from less mobile sorption domains in SOM. Interpreted according to the polymer model, this exclusion is postulated to occur by a “hole-plugging” (competitive displacement) mechanism possibly in concert with penetrant-induced plasticization of SOM which leads to destruction of holes.