PHOTOEFFECTS ON THE REDUCTION OF CARBON-TETRACHLORIDE BY ZERO-VALENT IRON

While reduction of chlorinated hydrocarbons by zero-valent iron in wat er is strongly influenced by the oxide layer at the metal-water interf ace, the role of the oxide in the dechlorination mechanism has not bee n fully characterized. In this paper, we investigate the semiconductin g properties of the oxide layer on granular iron and show how the elec tronic properties of the oxide affect electron transfer to aqueous CCl 4. Specifically, we determine whether conduction-band electrons contri bute 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, mor e importantly, shift the product distribution to more completely dechl orinated 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 investigatio n of the reduction with photogenerated electrons is carried out by add ing hole scavengers to the system. Isopropyl alcohol reacts with photo generated holes to yield the alpha-hydroxyalkyl radical, which is know n to reduce CCl4. With isopropyl alcohol present, we observe faster de gradation of CCl4 with higher light intensity. Since no such increase is seen without isopropyl alcohol, the rate of CCl4 degradation by con duction-band electrons in water must not be limited by the number of p hotogenerated electron-hole pairs but rather by electron transfer from the oxide conduction band to CCl4.