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 (SiO2 and TiO2) on the reductive reactivity of Fe(II)/goethite, an important natural reductant. SiO2 significantly inhibited the reductive reactivity, as quantified by the reduction kinetics of p-cyanonitrobenzene (pCNB) as a probe compound, while TiO2 greatly enhanced the reactivity. Silicate showed comparable inhibitory effects as SiO2 particles, indicating that the inhibition effect of SiO2 was dominated by its dissolution. Pseudo-first-order rate constants (k) of Fe(II)/goethite + TiO2 mixtures were higher than the sum of the k values of the respective single oxide. For the mixtures of Fe(II)/goethite + TiO2, the k values followed rutile > TiO2-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 TiO2 also led to higher reactivity. When TiO2 was physically separated from goethite by confining it in a dialysis bag, k of Fe(II)/goethite + TiO2 was comparable to the sum of the k values of the respective single oxide. We believe that the conduction bands of goethite and TiO2 were used as conduits for electron transfer from Fe(II) through TiO2 to goethite and eventually to reduce pCNB. 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.