Ba. Balko et Pg. Tratnyek, PHOTOEFFECTS ON THE REDUCTION OF CARBON-TETRACHLORIDE BY ZERO-VALENT IRON, JOURNAL OF PHYSICAL CHEMISTRY B, 102(8), 1998, pp. 1459-1465
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.