NONEQUILIBRIUM PROCESSES AFFECTING FORCED VENTILATION OF BENZENE AND XYLENE IN A DESERT SOIL

A series of 23 unsaturated bench scale column experiments was carried out to study grain scale processes controlling the efficiency of soil vapor extraction for removal of volatile organic compounds (VOCs) from a desert soil sample (mass fraction of organic carbon, approximately 0.001). Experiments consisted of passing VOC-containing, humidified (> 95% relative humidity) air through an unsaturated soil column until br eakthrough occurred and then passing VOC-free air through the column u ntil the VOC removal was complete. Effluent VOC concentrations were me asured at frequent intervals. Experimental variables included VOC (ben zene and p-xylene), soil moisture content (0 to 18% by volume), concen tration of VOC in the inlet air stream, and interstitial velocity (0.2 to 0.6 cm s(-1)). Experimental breakthrough curves were modeled with an advection-dispersion model coupled with a first-order rate equation to describe mass transfer between phases. At 18% moisture, slow water -to-air mass transfer of benzene as the rate-limiting process explains the initial removal of benzene (C/C-0 > 0.1), but the desorption prof iles strongly suggest that a slower process (intraparticle pore diffus ion) becomes the dominant process at longer times. The breakthrough cu rves for p-xylene at 10 and 18% moisture also suggest that two sequent ial processes control removal of p-xylene, The time constant for the f aster process is comparable to that determined for the initial rate-li miting process for benzene at 18% moisture. The rate for the second pr ocess is the same order of magnitude for p-xylene at 10 and 18% and be nzene at 18%. Calculations of the mass distributions of the VOCs among air, water, and soil strongly suggest that the more hydrophobic p-xyl ene tends to accumulate at the air-water interface (up to 60% of the t otal mass) and that benzene primarily accumulates in the aqueous phase .