Diffusion of Volatile Organic Compounds in Pressed Humic Acid Disks

Abstract

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 cm2/s; those of desorption, which increase with temperature, are about 10-9 cm2/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.

Sorption of toluene by humic acids derived from lake sediment and mountain soil at different pH

“…3 and 4). Similar studies by Chang et al [17] on the rate of sorption-desorption of VOCs by HS, revealed that the sorption of toluene onto HA, an integral member of a soil HS, is found to be reversible and diffusion controlled. Shih and Wu [39] reported that partitioning is believed to be the major mechanism of the sorption of toluene in HS.…”

“…Chang et al [17] investigated the sorption kinetics of toluene, n-hexane, and acetone using dry, pressed HA disks and they reported that the sorption of volatile organic compounds (VOCs) by HA is a physical process and that the slow process of sorption in natural soils is not entirely a consequence of diffusion. In this study we investigated the sorption of toluene by HAs extracted from Chengching Lake sediment and Meifong Mountain soil.…”

Kinetics of toluene sorption and desorption in Ca‐ and Cu‐montmorillonites investigated with Fourier transform infrared spectroscopy under two different levels of humidity

“…The gravimetric method with a microbalance has been used to trace the time courses of sorption of VOCs on soil organic matter [13][14][15] and on oven-dried clay [16,17], all under dry conditions. However, the major constraint associated with gravimetric methods for the investigation of sorption dynamics is the inability to observe the behavior at the molecular level or under a wet condition.…”

Distinctive sorption mechanisms of soil organic matter and mineral components as elucidated by organic vapor uptake kinetics

“…Volatile organic compounds (VOCs) are slowly sorbed into soil particles, and some of them could be retained in soil constituents over a long period of time [1][2][3]. This phenomenon has been attributed to the slow diffusion through soil micropore structures [5][6][7], through matrixes formed by condensed organic matter [8,9], and through the soil organic matter (SOM) [10][11][12]. This heterogeneity affects the prediction of the activity and fate of a pollutant.…”

“…The sorption of toluene in humic acid and humin extracted from Yangmingshan (Taiwan) soil has been found to be reversible and diffusion controlled [12,19]. Humic acid and kaolinite were selected as the two model soil components to represent SOM and soil inorganic matter (SIM).…”