Mass transfer mechanisms in air sparging systems

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Braida, Washington
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Say Kee Ong
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Civil, Construction, and Environmental Engineering

The air-water mass transfer of VOCs during air sparging was investigated using a single-air channel air sparging setup and a 14 cm (51[over] 2 in) diameter soil column. Three different porous media and 10 VOCs were used in the study. Air velocities ranged from 0.2 cm/s to 2.5 cm/s. Experimental results for the single-air channel setup indicated that volatilization of VOCs during air sparging was a diffusion limited process. VOCs, in a thin layer of saturated porous media next to the air channels (identified as the mass transfer zone, MTZ), were found to deplete rapidly during air sparging resulting in a steep concentration gradient within this zone while the VOC concentrations outside the zone remained fairly constant. The rapid depletion was associated with faster initial volatilization of VOCs at the air-water interface as compared to the diffusive transport of VOCs to the air-water interface. The size of MTZ ranged from 17 to 41 mm or between 70 dp50 and 215 dp50 (dp50 = mean particle size of the porous media) depending on the VOC. A general correlation predicting the size of the MTZ was developed. The size of MTZ was found to be directly proportional to the aqueous diffusivity of the VOC, the mean particle size, and the uniformity coefficient. The size of MTZ was also found to decrease with increasing organic carbon content of the porous media. This effect was larger for VOCs with low solubilities and high partition coefficients;Air-water mass transfer coefficients (KG) for the volatilization of VOCs were estimated by fitting experimental data to a one-dimensional diffusion model. The air-water mass transfer coefficients ranged from 1.79x 10-3 cm/min to 3.85x 10-2 cm/min for the VOCs tested. Two empirical models were developed for the prediction of mass transfer coefficients by correlating the Damkohler and modified air phase Sherwood numbers with air phase Peclet number, Henry's law constant, and reduced mean particle size of porous media. The estimated lumped mass transfer coefficients (KGa) were found to be directly related to the air diffusivity of the VOC, air velocity, particle size, and inversely related to the Henry's law constant of the VOCs. Based on the two-resistance model, the liquid-side resistance accounted for more than 90% of the total resistance for the air-water interfacial mass transfer;Experiments with nonaqueous phase liquid (NAPLs) indicated that air sparging may control the spreading of NAPLs and were more effective for NAPLs with higher solubilities and lower densities. Removal efficiencies of NAPLs and dissolved VOCs were found to be greatly affected by the grain size of the porous media;The MTZ concept and the correlations developed for the single-air channel study were incorporated into a one-dimensional radial diffusion model and were found to successfully predict the air phase concentrations, final aqueous VOC concentrations, and total mass removed for a 51[over] 2 in diameter air sparged soil column.

Wed Jan 01 00:00:00 UTC 1997