A shift in pathway of iron-mediated perchloroethylene reduction in the presence of sorbed surfactant--a column study

Surface modification of zero-valent iron (ZVI) to enhance its reduction rates for chlorinated ethanes and ethenes has recently attracted great attention. In this research, the enhancement of perchloroethylene (PCE) reduction by ZVI in the presence of sorbed micelles of the cationic surfactant hexadecyltrimethylammonium (HDTMA) was examined in a series of laboratory column tests with varying flow rates and input PCE concentrations. Model simulations using HYDRUS-1D showed that the overall pseudo first-order rate constants for PCE reduction by ZVI increased by a factor of four in the presence of sorbed HDTMA admicelles. The increase in reduction rate was attributed to a higher distribution coefficient (K(d)) for contaminant sorption on surfactant-modified ZVI (SM-ZVI) compared to untreated ZVI. Modeling results also showed that in the presence of HDTMA admicelles 58-100% of PCE reduction occurred via hydrogenolysis. In contrast, only 12-25% PCE underwent hydrogenolysis when HDTMA was absent. The significant increase in TCE production during PCE reduction by SM-ZVI verified a shift in reaction pathway previously observed in batch studies, most likely from beta-elimination to hydrogenolysis. Although this shift in reaction pathway resulted in a higher accumulation of TCE, the combined concentrations of chlorinated hydrocarbons in the effluent were 1.5-5 times lower when SM-ZVI rather than unmodified ZVI was used.