Reaction of toluene with soil organic matter

Toluene and a peat soil (93% organic matter content) were used to investigate time-dependence of isotherm nonlinearity and to gain a better understanding of sorption mechanisms of organic chemicals in soil organic matter (SOM). Isotherms were constructed using batch mixing technique at a series of contact time intervals. Toluene analysis was performed using liquid scintillation counting and sorption data were fitted with the Freundlich model (S = KFCN, where S and C are sorbed and solution concentrations, respectively, K-F and N are constants). Isotherms at all contact times were nonlinear (N = 0.84 - 0.93), indicating a distribution of site energies in SOM. The K-F values increased (from 54 to 75) with contact time. Nonlinearity also increased (i.e., N decreased) with increasing contact time, suggesting that energetic sites and condensed domains were relatively concentrated within the SOM matrix. These results are consistent with dual-mode model of sorption in SOM proposed earlier. In this model both solid-phase dissolution and site-filling (adsorption-like) mechanisms were shown to take place and the competition took place only in the site-filling (or condensed) domains.