Effect of surfactant on reductive dechlorination of trichloroethylene by zero-valent iron

Trichloroethylene (TCE) is a representative dense nonaqueous phase liquid (DNAPL) in groundwater. TCE as a form of free fluid has been treated by surfactant-enhanced aquifer remediation (SEAR). After application of SEAR, groundwater contains little amount of residual TCE and surfactant. Residual TCE could be treated by zero-valent iron (ZVI). However, surfactant might modify the surface property of ZVI. As a result, the reduction of TCE on the surface of ZVI might be affected by the presence of surfactant. In this study, the effect of surfactant types on reductive dechlorination of TCE was investigated using zero-valent iron. Cationic surfactant enhanced the dechlorination rate of TCE at a concentration below the critical micelle concentration (CMC) compared to pure ZVI. Electrostatic interaction between electronegative chloride group of TCE and positive head group of cationic surfactant was increased sorption of TCE on the surface of ZVI. However, anionic and nonionic surfactants inhibited the reductive dechlorination of TCE because of the decrease of TCE sorption on the surface of ZVI. The reduction rate of TCE in the presence of nonionic surfactant is dependent on the length of hydrophobic carbon chain in nonionic surfactant. Little amount of cationic surfactant could enhanced the reductive dechlorination rate of TCE. As a result, the time to treat TCE in groundwater will be shortened.

Cationic surfactants enhanced the TCE degradation rate, while reduction of TCE in anionic surfactant and nonionic surfactant decreased due to inhibition of TCE adsorption onto ZVI surface. As a result, cationic surfactants are most suitable to enhance the TCE dechlorination effect on the ZVI. However, residual surfactants in subsurface cause to contaminate groundwater, and cationic surfactants possess potential toxicity. Minimum amount of surfactant should be used to apply cationic surfactant to ZVI system even though it can enhance dechlorination rate of chlorinated compounds.