Synergistic effect of cationic surfactants on perchloroethylene degradation by zero-valent iron

Zero-valent iron (ZVI) as a permeable barrier material for degradation of chlorinated organic compounds has been extensively studied recently. One of the focal areas in ZVI studies is to increase the contaminant reduction rate. In this research, batch tests were performed to evaluate the synergistic effect of sorbed cationic surfactants on degradation of perchloroethylene (PCE). Sorption of cationic surfactants on ZVI was a function of hydrophobic chain length of the surfactanttail group. Minimal counterion sorption indicated that the sorbed surfactant molecules form a patchy monolayer on ZVI. Both PCE and trichloroethylene (TCE) degradation by ZVI with and without sorbed surfactant followed pseudo-first-order reaction kinetics. In general, the PCE degradation rate increases as the chain length of sorbed surfactant increases. Compared to unmodified ZVI, both apparent rate constants of PCE degradation and TCE accumulation increased by an order of magnitude when ZVI was modified by hexadecyltrimethylammonium. The rate of PCE degradation by ZVI modified to lower surfactant loading was relatively higher than that by ZVI modified to higher surfactant loading. It was speculated that longer chain length will result in better admicelle formations, and thus, promote PCE partition and increase PCE surface concentration or surface admicelle catalysis, while low surfactant loading makes significant amounts of surface reduction sites still available. The PCE reduction rate constants were not affected by solution ionic strength, but high initial solution pH, buffered by sodium carbonate and sodium bicarbonate, significantly reduced the PCE degradation rate.