Effects of Second Metal Oxides on Surface-Mediated Reduction of Contaminants by Fe(II) with Iron Oxide

This work examined the effects of two second metal oxides (SiO 2 and TiO 2 ) on the reductive reactivity of Fe(II)/goethite, an important natural reductant. SiO 2 significantly inhibited the reductive reactivity, as quantified by the reduction kinetics of p -cyanonitrobenzene ( p CNB) as a probe compound, while TiO 2 greatly enhanced the reactivity. Silicate showed comparable inhibitory effects as SiO 2 particles, indicating that the inhibition effect of SiO 2 was dominated by its dissolution. Pseudo-first-order rate constants ( k ) of Fe(II)/goethite + TiO 2 mixtures were higher than the sum of the k values of the respective single oxide. For the mixtures of Fe(II)/goethite + TiO 2, the k values followed rutile > TiO 2 –P25 > anatase, despite a different trend in the adsorbed Fe(II) amount. This reactivity trend agreed well with their conduction band potentials. Higher loadings of TiO 2 also led to higher reactivity. When TiO 2 was physically separated from goethite by confining it in a dialysis bag, k of Fe(II)/goethite + TiO 2 was comparable to the sum of the k values of the respective single oxide. We believe that the conduction bands of goethite and TiO 2 were used as conduits for electron transfer from Fe(II) through TiO 2 to goethite and eventually to reduce p CNB. This type of interparticle electron transfer was for the first time discovered for dark conditions, which might play a previously unrecognized yet important role in contaminant reduction and Fe(II)/Fe(III) redox cycling in the environment.