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.