Air flow optimization and surfactant enhancement to remediate toluene-contaminated saturated soils using air sparging

This paper presents the results of laboratory experiments that investigate the removal of volatile organic compounds from saturated soils through the use of air sparging. Three series of experiments were performed in a column test apparatus using two different soils to represent actual field conditions, namely, a fine gravel and a medium to fine Ottawa sand (both obtained from sources near Chicago, Illinois, USA) contaminated with toluene, a major constituent of petroleum products. The results showed that toluene was removed from gravel very efficiently using air sparging; complete removal was achieved using a variety of air flow rates. However the toluene removal rates in tests using sand were significantly less. Even at the highest air flow rate used during testing, complete toluene removal took eight times longer than in comparable tests using gravel. With low air flow rates this was not achieved even after 17 hours of testing. It was further found that the injection of foams generated with surfactants, SDS and witconol SN70, at low air flow rates during the use of air sparging was found to accelerate the bulk removal of toluene in sand, but the use of surfactants did not facilitate the removal of residual levels of contamination. Introduction Subsurface contamination caused by the infiltration of hydrocarbons, including petroleum products, poses serious environmental problems. Hydrocarbon releases often occur initially within the vadose zone and then migrate downward through the soil, eventually reaching the water table. Light non-aqueous phase liquid (LNAPL) constituents of petroleum products tend to float on top of the water table and as a result may be transported vertically and horizontally due to water table fluctuations. Such fluctuations may be the result of seasonal variations, well pumping, or groundwater flow conditions. These LNAPLs will persist for decades within the subsurface unless remedial action is taken. A number of remedial technologies, including pump-and-treat, bioremediation and soil washing/flushing have been used to treat contaminated groundwater. However, in many cases, these methods have proven to be ineffective, expensive, unreliable, and often require extended time periods for adequate clean-up (Ardito and Billings, 1990; Leonard and Brown, 1992; NRC, 1994; Reddy et al., 1995). The soil vapor extraction (SVE) method has proven to be an effective and less expensive technique for the remediation of volatile organic compounds within the vadose zone; however, it cannot be used to treat saturated soil zone contamination. Therefore, a different remediation technique must be used for saturated soils and groundwater. In-situ air sparging is becoming an increasingly popular option within the USA for the remediation of saturated soils and groundwater contaminated with volatile organic compounds (VOCs). Previous field investigations have shown air sparging to be an efficient and effective method when applied to such contaminant conditions (Johnson et al., 1993; Reddy et al., 1995). Air sparging is most effective when applied to contaminants with Henry’s Law constants greater than 10–5 atm m3/mol and vapor pressures greater than 5mm Hg (Leonard and Brown, 1992). It is a simple method to apply; field implementation is performed using readily available equipment. Since no special equipment is required, air sparging has proven to be a cost-effective remedial method. In situ air sparging is performed by injecting air below the groundwater contamination plume, as shown in Figure 1. The injected air then travels towards the ground surface, creating air passageways. The contaminants are partitioned into the vapor phase through the action of different mechanisms, including volatilization, dissolution, and desorption, which result from the action of the injected air. The contaminantladen air continues to travel upward through the air passageways, eventually entering the vadose zone. The vapors are then collected using a soil vapor extraction system, as shown in Figure 1. Additionally, the injected air increases subsurface oxygen levels, aiding in the action of aerobic biodegradation. A detailed review of the mechanics of the air sparging process is provided by Reddy et al. (1995). Current air sparging design methods are heavily based on empirical approaches and limited field experience rather than a rational design basis because very little is understood regarding the mass transfer/transport/transformation mechanisms that occur during the use of air sparging. Additionally, little is known about the effects of control variables, including injection air flow rate and pressure. To provide a better understanding of these mechanisms and variables, a comprehensive air sparging research program has been underway at the University of Illinois at Chicago (UIC) since 1993. The research includes the performance of extensive onedimensional and two-dimensional laboratory experiments. Knowledge gained from the controlled laboratory experiments will be used to develop a conceptual model of the air sparging process as well as a comprehensive mathematical model that can serve as a predictive tool for the optimal design of air This study was partially funded by the University of Illinois at Chicago (UIC). The authors are thankful to John Gramsas for fabricating the column test setup.