Impact of Interactions
between Metal Oxides to Oxidative
Reactivity of Manganese Dioxide
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Abstract
Manganese oxides typically exist as <i>mixtures</i> with
other metal oxides in soil–water environments; however, information
is only available on their redox activity as <i>single</i> oxides. To bridge this gap, we examined three binary oxide mixtures
containing MnO<sub>2</sub> and a secondary metal oxide (Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub> or TiO<sub>2</sub>). The goal was
to understand how these secondary oxides affect the oxidative reactivity
of MnO<sub>2</sub>. SEM images suggest significant heteroaggregation
between Al<sub>2</sub>O<sub>3</sub> and MnO<sub>2</sub> and to a lesser
extent between SiO<sub>2</sub>/TiO<sub>2</sub> and MnO<sub>2</sub>. Using triclosan and chlorophene as probe compounds, pseudofirst-order
kinetic results showed that Al<sub>2</sub>O<sub>3</sub> had the strongest
inhibitory effect on MnO<sub>2</sub> reactivity, followed by SiO<sub>2</sub> and then TiO<sub>2</sub>. Al<sup>3+</sup> ion or soluble
SiO<sub>2</sub> had comparable inhibitory effects as Al<sub>2</sub>O<sub>3</sub> or SiO<sub>2</sub>, indicating the dominant inhibitory
mechanism was surface complexation/precipitation of Al/Si species
on MnO<sub>2</sub> surfaces. TiO<sub>2</sub> inhibited MnO<sub>2</sub> reactivity only when a limited amount of triclosan was present.
Due to strong adsorption and slow desorption of triclosan by TiO<sub>2</sub>, precursor-complex formation between triclosan and MnO<sub>2</sub> 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
