Photoeffects on the Reduction of Carbon Tetrachloride by Zero-Valent Iron

Abstract

While reduction of chlorinated hydrocarbons by zero-valent iron in water is strongly influenced by the oxide layer at the metal−water interface, the role of the oxide in the dechlorination mechanism has not been fully characterized. In this paper, we investigate the semiconducting properties of the oxide layer on granular iron and show how the electronic properties of the oxide affect electron transfer to aqueous CCl4. Specifically, we determine whether conduction-band electrons contribute to the reduction of CCl4 by using light to increase the number of conduction-band electrons at the oxide surface and measuring how this treatment affects the rate and products of CCl4 degradation. We find that photogenerated conduction-band electrons do degrade CCl4 and, more importantly, shift the product distribution to more completely dechlorinated products that are indicative of two-electron transfer with a dichlorocarbene intermediate. Since the photogenerated electrons give different reduction products than the dark reducers, we conclude that the latter must not be conduction-band electrons. Further investigation of the reduction with photogenerated electrons is carried out by adding hole scavengers to the system. Isopropyl alcohol reacts with photogenerated holes to yield the α-hydroxyalkyl radical, which is known to reduce CCl4. With isopropyl alcohol present, we observe faster degradation of CCl4 with higher light intensity. Since no such increase is seen without isopropyl alcohol, the rate of CCl4 degradation by conduction-band electrons in water must not be limited by the number of photogenerated electron−hole pairs but rather by electron transfer from the oxide conduction band to CCl4.