Combined cloud point extraction and bioremediation for clean-up trichloroethylene in various soil types

Abstract

The contamination of trichloroethylene (TCE) in soil and groundwater has become an important environmental problem because of their toxicity and persistence. In this study, a combination of cloud point extraction and bioremediation was applied to enhance TCE removal efficiency from three soil types including sandy clay loam, sandy loam and clay soil. Dehydol LS7 TH, a nonionic surfactant synthesized in Thailand, was used in cloud point extraction technique for removing high TCE concentration out of the soil. Using an initial concentration of 90 mM Dehydol LS7 TH, the surfactant partition ratio on sandy clay loam and sandy loam were 14.61 and 10.71, respectively. Meanwhile, this surfactant could not separate well in clay soil. It was indicated that soil types can affect the phase separation. However the different in TCE removal efficiency from each soil types was minor and about 3% of the initial TCE was remained in those soil samples. The addition of 0.2 M sodium chloride (NaCl) was found to enhance the phase separation and increase the surfactant partition ratio of Dehydol LS7 TH in all three types of soil. The addition of NaCl also increased TCE partition ratio, in which only small amount of TCE was remained in the surfactant dilute phase. TCE removal efficiencies of this condition in all soil types were similar to the cloud point extraction without adding NaCl. The inoculum of Rhodococcus sp. L4 was later developed for bioremediation by immobilization technique i.e. attachment of cells on cumin seeds and encapsulation of cumin seed-attached cells in gellan gum beads. The immobilization was to enhance TCE removal and protect the cells from environmental stress. The result showed that cumin seed-Rhodococcus sp. L4 encapsulated in gellan gum was capable of degrading 60% of 10 ppm TCE. This encapsulated cell had higher TCE removal efficiency than that of cumin seed-attached cells alone and was more tolerant to high TCE concentration in the presence of surfactant and NaCl. The combination of cloud point extraction process using Dehydol LS7 TH in presence of NaCl and bioaugmentation with Rhodococcus sp. L4 inoculums were later performed to clean up 1,000 ppm TCE in those soil samples. After cloud point extraction, the residual TCE in soil was degraded by free cells of Rhodococcus sp. L4 and the residual TCE in surfactant dilute-phase was degraded by cumin seed-Rhodococcus sp. L4 encapsulated in gellan gum. The amounts of TCE after the combined treatment were decreased significantly to only 18-19 mg/kg in all three soil types and to 6-10 mg/L in the dilute phase surfactant. The results suggested that the combination of these techniques could be effectively applied for TCE removal from various soil types.