Diffusion of volatile organic compounds in pressed humic acid disks

The slow sorption and desorption of volatile organic compounds (VOCs) in soils are assumed to be partly controlled by intraparticle diffusion and diffusion in soil organic matter (SOM) and influence soil remediation. In this study, humic acid, a component of SOM, is used to simulate SOM as a partition medium for VOCs. The sorption kinetics of VOCs in dry, pressed humic acid disks was investigated by tracking the weight change of the sorbent with a microbalance. A diffusion model successfully describes the kinetics of sorption and desorption. The apparent diffusivities of toluene, n-hexane, and acetone in humic acid disks range from 10(-8) to 10(-9) cm(2)/s; those of desorption, which increase with temperature, are about 10(-9) cm(2)/s. No significant amount of permanently bound sorbate residue could be observed. The enthalpy change for the sorption of toluene is -14 kcal/mol, and the activation energies for its diffusion during sorption and desorption are 10.1 and 15.7 kcal/mol, respectively. According to the results of this study, it is suggested that the sorption of VOCs in humic acid is a physical process and that the slow process of sorption in natural soils as reported in the literature is not entirely a consequence of diffusion in the humic acid.