Effect of water content on kinetics of volatile organic compounds mass transfer between gas and aqueous phases during gas transport in unsaturated sand.

摘要

The effect of changing water content on the air-water mass transfer kinetics of volatile organic compounds (VOCs) during gas transport in unsaturated sand was investigated. The kinetics of VOC adsorption at the air-water interface from the gas phase was explored using n-decane, whereas methylene chloride was used to examine the effect of air-water partitioning. Methane was used as the nonreactive tracer in VOC transport experiments conducted using a sand column installed in a gas chromatography system. The longitudinal dispersion of methane, estimated from methane breakthrough curve (BTC), was found to be constant in the water content range (0.07 to 0.18) examined in this study at a constant pore-gas velocity (vg), whereas an inverse proportionality between the longitudinal dispersion and vg was observed. The BTCs for methane and n-decane, measured at constant vg, were fairly symmetric, indicating that both gas diffusion and adsorption at air-water interfaces do not produce nonequilibrium transport. The shape of the methylene chloride BTC was gradually distorted at a constant vg as water content increased. Fitting methylene chloride BTCs with a modified two-region model showed that the water partitioning becomes more rate-limited as water content increases. The overall mass transfer coefficient for water partitioning of methylene chloride was found to be a direct function of specific air-water interfacial area..