Emulsion-enhanced remediation of lindane and DDT in soils

Abstract

The agricultural lands in most Asian countries have been contaminated by pesticides for decades, especially γ-hexachlorocyclohexane (lindane) and dichlorodiphenyltrichloroethane (DDT). For most sites, the concentrations are in the parts per million level favoring bioremediation. However, due to high hydrophobicity, they are hard to be biodegraded in soils. In situ phase inversion emulsification and biological reductive dechlorination (ISPIE/BiRD) is proposed as an effective approach for the removal of lindane and DDT in soil. Three tests were conducted, i.e., a batch test, a column test, and a sandbox test. In the batch test, the Taguchi method was applied to define optimum environmental parameters, such as pH, water content, emulsion concentration, and soil organic matter level. A column test was performed using a real agricultural soil to define the removal efficiency of a single operation of ISPIE on lindane and DDT. A sandbox was employed to test if the remaining emulsion can assist in the biodegradation of lindane and DDT in a real agricultural soil. Next-generation sequencing (NGS) was performed to profile the microbial community for the best groups in the batch test and the sandbox test. The batch test results showed that water content and emulsion concentration are critical controlling factors, and the removal of lindane and DDT was 99.7% and 88.9%, respectively. A single operation of ISPIE removed 52.9% and 31.5% of DDT and lindane, respectively, and the difference is probably due to the log KOW. The best parameter combination was employed in the sandbox test and the best removal rates of DDT and lindane on the 28th day were 78.0 ± 1.2% and 99.7 ± 0.5%, respectively. NGS data results showed that agriculture soil has more abundant and diverse microorganisms than the acclimated sediment culture. NGS analysis showed two cohesive groups were competing against each other in the batch test and complex interactions existed in the sandbox test. These results suggest that ISPIE/BiRD is a highly viable remediation option for rice paddy farming lands contaminated by highly hydrophobic contaminants. ISPIE followed by BiRD can be finished in a 42-day period and is highly suitable for implementation during an annual break of paddy field farming. Some potential strong degraders and complex interactions require more research in the future.