Effect of cyclodextrins on iron-mediated dechlorination of trichloroethylene — A proposed new mechanism

Abstract

This paper evaluates the possibility of using cyclodextrins (CDs) as solubility-enhancing agents that could be employed to increase the rate of the mass delivery of trichloroethylene (TCE) to an iron wall where dechlorination will occur. Dechlorination of TCE by zero-valent iron was performed in the absence and in the presence of CDs in batch (static) and column (dynamic) systems. The presence of CDs slowed the overall degradation of TCE. The effect of various concentrations of different CDs on the rate of TCE degradation was evaluated, and it was found that as the CD concentration increased, reaction rate decreased. However, the overall decrease in reaction rate could not be explained solely on the basis of TCE:CD host-guest complexation. A novel mechanism involving CD binding to the iron surface, in addition to TCE:CD complexation, was proposed, and this enabled satisfactory interpretation of the kinetic data. A central feature of the proposal is that Fe:CD binding impedes electron transfer from the underlying iron metal to the TCE molecule. The overall rate of TCE degradation by zero-valent Fe in the presence of CDs is governed by several factors: the extent of binding of CD with the iron surface, the binding constant between CD and TCE, and the degradation of complexed TCE (TCE:CD), where CD is, in turn, associated with the iron surface. It appeared that CD binding to the iron surface depends largely on the type of substituent groups present in the β-CD derivatives as well as their degree of substitution. Association constants for the Fe:CD interaction were determined. The new hypothesis explained well both batch and column data, and the magnitude of Fe:CD binding constants was found to be similar in the two systems. Key words: cyclodextrin, trichloroethylene, zero-valent iron, reductive dechlorination.