Acceleration of microiron-based dechlorination in water by contact with fibrous activated carbon

Zero-valent iron (ZVI) is widely applied for reduction of chlorohydrocarbons in water. Since the dechlorination occurs at the iron surface, marked differences in rate constants are commonly found for nanoscale and microscale ZVI. It has already been shown for trichloroethene (TCE) adsorbed to activated carbon (AC) that the dechlorination reaction is shifted to the carbon surface simply by contacting the AC with highly reactive nanoscale ZVI particles. Transfer of reactive species to the adsorbed pollutant was discussed. The present study shows that even low price and very low reactive microscale ZVI can also be utilized for an effective dechlorination process. Compared to the reaction rate at the iron surface itself, an enormous acceleration of the dechlorination rate for chlorinated ethenes was observed, reaching activity levels such as known for nanoscale ZVI. When fibrous AC is brought into direct contact with microscale ZVI the iron-surface-normalised dechlorination rate constants increased by up to four orders of magnitude. This implies that the dechlorination reaction is fully transferred to the AC surface. At the same time, the anaerobic corrosion of the same material was not substantially affected. Thus, the utilization of iron's reduction equivalents towards dechlorination (dechlorination efficiency) can be considerably enhanced. A screening with various AC types showed that the extent of rate acceleration depends strongly on the surface chemistry of the AC. By means of temperature-programmed desorption, it could be shown that concentration and type of oxygen surface groups determine the redox-mediation properties. Quinone/hydroquinone groups were identified as being the main drivers for electron-transfer processes, but to some extent other redox-active groups such as chromene and pyrone can also act as redox mediators. AC overall plays the role of a catalyst rather than a reactant. The present study derives recommendations for practical application of the findings in water-treatment approaches.