Slowly and very slowly desorbing organic compounds in sediments exhibit Langmuir-type sorption

Desorption kinetics were determined for 1,2,4-trichlorobenzene (TCB), 1,3-dichlorobenzene (DCB), and trichloroethylene (TCE) in a sediment at various concentrations. The desorption data were interpreted with a (nonmechanistic) first-order three-compartment model. In this way, separate sorption isotherms could be constructed for rapidly, slowly, and very slowly desorbing sorbate, respectively. Slowly desorbing (rate constant k similar to 10(-3)/h) and very slowly desorbing (k = 10(-4) to 10(-5)/h) sorbate exhibited nonlinear Langmuir-type sorption, with capacities on the order of 4.6 to 19 and 0.54 to 1.5 mmol/kg organic carbon (OC) and affinity constants of 0.18 to 41 and 32 to 272 L/mmol, respectively. The affinity constants increased with increasing sorbate hydrophobicity. Rapidly desorbing (k = 10(-1)/h) sorbate showed linear sorption isotherms, with log K-oc (octonol-water partition coefficients) of 1.59 +/- 0.12 (TCE), 2.03 +/- 0.13 (DCB), and 3.13 +/- 0.03 (TCB), respectively. These results confirm the hypothesis that desorption is rapid from linearly sorbing organic matter, whereas it is slow and very slow from nonlinearly sorbing sites. Furthermore, the results also demonstrate the applicability of the desorption kinetic method in terms of experimentally separating an overall Freundlich-like isotherm in linear and nonlinear isotherms.