Impact of Interactions between Metal Oxides to Oxidative Reactivity of Manganese Dioxide

Manganese oxides typically exist as mixtures with other metal oxides in soil water environments; however, information is only available on their redox activity as single oxides. To bridge this gap, we examined three binary oxide mixtures containing MnO2 and a secondary metal oxide (Al2O3, SiO2 or TiO2). The goal was to understand how these secondary oxides affect the oxidative reactivity of MnO2. SEM images suggest significant heteroaggregation between Al2O3 and MnO2 and to a lesser extent between SiO2/TiO2 and MnO2. Using triclosan and chlorophene as probe compounds, pseudofirst-order kinetic results showed that Al2O3 had the strongest inhibitory effect on MnO2 reactivity, followed by SiO2 and then TiO2. Al3+ ion or soluble SiO2 had comparable inhibitory effects as Al2O3 or SiO2, indicating the dominant inhibitory mechanism was surface complexation/precipitation of Al/Si species on MnO2 surfaces. TiO2 inhibited MnO2 reactivity only when a limited amount of triclosan was present. Due to strong adsorption and slow desorption of triclosan by TiO2, precursor-complex formation between triclosan and MnO2 was much slower and likely became the new rate-limiting step (as opposed to electron transfer in all other cases). These mechanisms can also explain the observed adsorption behavior of triclosan by the binary oxide mixtures and single oxides.