Mechanisms controlling toluene removal from saturated soils during in situ air sparging

Abstract

In situ air sparging is an effective method for removing volatile organic compounds from saturated soils and groundwater. Removal efficiency levels as high as 98% are often reported, and the remediation time is significantly less than that required for conventional pump and treat technology. However, predictions of the time required for contaminant mass removal by air sparging have been approximate at best due to a lack of understanding of the relative importance of the various mechanisms that are responsible for this contaminant removal. Volatilization is considered the most dominant mass transfer mechanism during the air sparging process. Dissolution, desorption and biodegradation are the other major mechanisms that determine the rate at which contaminants are partitioned into different phases or transformed into nonhazardous substances. Additionally, advection, dispersion and diffusion are the transport mechanisms that dictate the overall contaminant removal efficiency. This paper first describes these different mechanisms along with the factors that affect these mechanisms. Then, experimental data is presented for toluene removal from Ottawa sand and fine gravel by means of air sparging. The tests performed included batch tests to characterize the adsorption characteristics of toluene on the Ottawa sand, and air sparging column tests on both the sand and the gravel to provide information on the effects of soil type and injected air flow rate on the overall air sparging remedial efficiency. These test results are assessed in light of the mechanisms affecting contaminant removal during air sparging.