Moisture effect on the transport of organic vapors in sand.

Vapor-phase affects the transport, lifetime, and fate of volatile organic contaminants in soils. Furthermore, it has an effect on the assessment and the selection of a remediation approach. The diffusion of dichloromethane (DCM), 1,1,1-trichloroethane (TCE) and carbon tetrachloride (CTC) in dry and wet sand media were investigated using a packed column at room temperature. The apparent diffusion coefficients, D(app), which accounts for effective diffusion and sorption of DCM, TCE and CTC were determined by fitting the experimental data to a diffusion model. The apparent diffusion coefficients of DCM, TCE and CTC for wet sand were found to be larger than the corresponding values for dry sand. The presence of water reduced retardation factors expressed in terms of the sorption capacity for each compound. The results indicated that dry mineral surface provided dominant sorption sites. This is reflected in greater amounts of organic vapors adsorbed under dry conditions. On the other hand, lower sorption capacities for organic compounds on vapor-water interface than on vapor-mineral interface and their low solubilities resulted in smaller retardation factors in wet sand. The experiments clearly demonstrate the role of humidity in sorption and transport of vapors in sand.