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

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 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

Effects of NOM on Oxidative Reactivity of Manganese Dioxide in Binary Oxide Mixtures with Goethite or Hematite

MnO₂ typically coexists with iron oxides as either discrete particles or coatings in soils and sediments. This work examines the effect of Aldrich humic acid (AHA), alginate, and pyromellitic acid (PA) as representative natural organic matter (NOM) analogues on the oxidative reactivity of MnO₂, as quantified by pseudo-first-order rate constants of triclosan oxidation, in mixtures with goethite or hematite. Adsorption studies showed that there was low adsorption of the NOMs by MnO₂, but high (AHA and alginate) to low (PA) adsorption by the iron oxides. Based on the ATR-FTIR spectra obtained for the adsorbed PA on goethite or goethite + MnO₂, the adsorption of PA occurred mainly through formation of outer-sphere complexes. The Fe oxides by themselves inhibited MnO₂ reactivity through intensive heteroaggregation between the positively charged Fe oxides and the negatively charged MnO₂; the low solubility of the iron oxides limited surface complexation of soluble Fe³⁺ with MnO₂. In ternary mixtures of MnO₂, Fe oxides, and NOM analogues, the reactivity of MnO₂ varied from inhibited to promoted as compared with that in the respective MnO₂ + NOM binary mixtures. The dominant interaction mechanisms include an enhanced extent of homoaggregation within the Fe oxides due to formation of oppositely charged patches within the Fe oxides but an inhibited extent of heteroaggregation between the Fe oxide and MnO₂ at [AHA] < 2–4 mg-C/L or [alginate/PA] < 5–10 mg/L, and an inhibited extent of heteroaggregation due to the largely negatively charged surfaces for all oxides at [AHA] > 4 mg-C/L or [alginate/PA] > 10 mg/L

Interactions in Ternary Mixtures of MnO₂, Al₂O₃, and Natural Organic Matter (NOM) and the Impact on MnO₂ Oxidative Reactivity

Our previous work reported that Al₂O₃ inhibited the oxidative reactivity of MnO₂ through heteroaggregation between oxide particles and surface complexation of the dissolved Al ions with MnO₂ (S. Taujale and H. Zhang, “Impact of interactions between metal oxides to oxidative reactivity of manganese dioxide” <i>Environ. Sci. Technol.</i> <b>2012,</b> 46, 2764–2771). The aim of the current work was to investigate interactions in ternary mixtures of MnO₂, Al₂O₃, and NOM and how the interactions affect MnO₂ oxidative reactivity. For the effect of Al ions, we examined ternary mixtures of MnO₂, Al ions, and NOM. Our results indicated that an increase in the amount of humic acids (HAs) increasingly inhibited Al adsorption by forming soluble Al–HA complexes. As a consequence, there was less inhibition on MnO₂ reactivity than by the sum of two binary mixtures (MnO₂+Al ions and MnO₂+HA). Alginate or pyromellitic acid (PA)two model NOM compoundsdid not affect Al adsorption, but Al ions increased alginate/PA adsorption by MnO₂. The latter effect led to more inhibition on MnO₂ reactivity than the sum of the two binary mixtures. In ternary mixtures of MnO₂, Al₂O₃, and NOM, NOM inhibited dissolution of Al₂O₃. Zeta potential measurements, sedimentation experiments, TEM images, and modified DLVO calculations all indicated that HAs of up to 4 mg-C/L increased heteroaggregation between Al₂O₃ and MnO₂, whereas higher amounts of HAs completely inhibited heteroaggregation. The effect of alginate is similar to that of HAs, although not as significant, while PA had negligible effects on heteroaggregation. Different from the effects of Al ions and NOMs on MnO₂ reactivity, the MnO₂ reactivity in ternary mixtures of Al₂O₃, MnO₂, and NOM was mostly enhanced. This suggests MnO₂ reactivity was mainly affected through heteroaggregation in the ternary mixtures because of the limited availability of Al ions

Spectroscopic Investigation of Interfacial Interaction of Manganese Oxide with Triclosan, Aniline, and Phenol

We investigated the reaction of manganese oxide [MnO_<i>x</i>(s)] with phenol, aniline, and triclosan in batch experiments using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and aqueous chemistry measurements. Analyses of XPS high-resolution spectra suggest that the Mn­(III) content increased 8–10% and the content of Mn­(II) increased 12–15% in the surface of reacted MnO_<i>x</i>(s) compared to the control, indicating that the oxidation of organic compounds causes the reduction of MnO_<i>x</i>(s). Fitting of C 1s XPS spectra suggests an increase in the number of aromatic and aliphatic bonds for MnO_<i>x</i>(s) reacted with organic compounds. The presence of 2.7% Cl in the MnO_<i>x</i>(s) surface after reaction with triclosan was detected by XPS survey scans, while no Cl was detected in MnO_<i>x</i>-phenol, MnO_<i>x</i>-aniline, and MnO_<i>x</i>-control. Raman spectra confirm the increased intensity of carbon features in MnO_<i>x</i>(s) samples that reacted with organic compounds compared to unreacted MnO_<i>x</i>(s). These spectroscopy results indicate that phenol, aniline, triclosan, and related byproducts are associated with the surface of MnO_<i>x</i>(s)-reacted samples. The results from this research contribute to a better understanding of interactions between MnO_<i>x</i>(s) and organic compounds that are relevant to natural and engineered environments