Influence of bisolute competition on the desorption kinetics of polycyclicaromatic hydrocarbons in soil

The dual-mode (partition/hole-filling) model of soil organic matter (SOM) a s a heterogeneous polymerlike sorbent of hydrophobic compounds predicts tha t a competing solute will accelerate diffusion of the primary solute by blo cking the holes, allowing the principal solute to move faster through the S OM matrix. We tested this hypothesis with phenanthrene as the primary solut e and pyrene as the competing solute in two sterile soils of widely differe nt organic carbon content (1.4 and 43.9%). Two- and 33-d isotherms of phena nthrene in both sails were nonlinear, indicating a heterogeneous distributi on of sites. Pyrene suppressed phenanthrene sorption and increased the line arity of its isotherm. Uptake (adsorption) rates were measured in batch sys tems after preincubating with pyrene. Desorption was measured by the sequen tial 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 funct ion of pyrene concentration. Moreover, the effect was noticeable even at la w and roughly equimolar concentrations of the two compounds (similar to 0.5 mu mol/gOC). The effect was qualitatively similar in the two soils, implic ating SOM as the matrix in which the effect takes place. The results sugges t that the competing solute excludes the primary solute from less mobile so rption domains in SOM. Interpreted according to the polymer model, this exc lusion is postulated to occur by a "hole-plugging" (competitive displacemen t) mechanism possibly in concert with penetrant-induced plasticization of S OM which leads to destruction of holes.