Temperature effects on very slow desorption of native chlorobenzenes from sediment to water

The temperature dependence of the kinetics of very slow desorption of eight chlorobenzenes was studied in laboratory batch experiments on a field-contaminated sediment from Lake Ketelmeer, The Netherlands. The observed rate constants for very slow desorption averaged (1.5 +/- 0.4) x 10(-4)/h at 10 degrees C, (2.9 +/- 1.0) x 10(-4)/h at 20 degrees C, (5.8 +/- 2.4) x 10(-4)/h at 35 degrees C, and (6.4 +/- 3.0) x 10(-4)/h at 45 degrees C over all eight compounds. Activation energies for desorption to water were close to the enthalpy for dissolution of the pure solid in water. The activation energies ranged from 18 to 53 kJ/mol with an average of 36 +/- 11 kJ/mol. These values confirm earlier reported activation energies for very slow desorption to water. They are much less than values for activation energies for desorption to the gas phase. That difference can be explained in terms of rate limiting desorption from carbonaceous adsorption sites.