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 Fe(3+) 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.

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 MnO(2) and a secondary metal oxide (Al(2)O(3), SiO(2) or TiO(2)). The goal was to understand how these secondary oxides affect the oxidative reactivity of MnO(2). SEM images suggest significant heteroaggregation between Al(2)O(3) and MnO(2) and to a lesser extent between SiO(2)/TiO(2) and MnO(2). Using triclosan and chlorophene as probe compounds, pseudofirst-order kinetic results showed that Al(2)O(3) had the strongest inhibitory effect on MnO(2) reactivity, followed by SiO(2) and then TiO(2). Al(3+) ion or soluble SiO(2) had comparable inhibitory effects as Al(2)O(3) or SiO(2), indicating the dominant inhibitory mechanism was surface complexation/precipitation of Al/Si species on MnO(2) surfaces. TiO(2) inhibited MnO(2) reactivity only when a limited amount of triclosan was present. Due to strong adsorption and slow desorption of triclosan by TiO(2), precursor-complex formation between triclosan and MnO(2) 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.

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” Environ. Sci. Technol. 2012, 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.

We investigated the reaction of manganese oxide [MnO(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(s) compared to the control, indicating that the oxidation of organic compounds causes the reduction of MnO(s). Fitting of C 1s XPS spectra suggests an increase in the number of aromatic and aliphatic bonds for MnO(s) reacted with organic compounds. The presence of 2.7% Cl in the MnO(s) surface after reaction with triclosan was detected by XPS survey scans, while no Cl was detected in MnO-phenol, MnO-aniline, and MnO-control. Raman spectra confirm the increased intensity of carbon features in MnO(s) samples that reacted with organic compounds compared to unreacted MnO(s). These spectroscopy results indicate that phenol, aniline, triclosan, and related byproducts are associated with the surface of MnO(s)-reacted samples. The results from this research contribute to a better understanding of interactions between MnO(s) and organic compounds that are relevant to natural and engineered environments.

To better understand the oxidative reactivity of iron oxides in the fate of contaminants in acidic environments, we examined the reactivity of goethite in binary mixtures with Al 2 O 3 by carrying out oxidation experiments of hydroquinone (HQ) in the presence of goethite and/or Al 2 O 3 at pH 3. Kinetic results revealed inhibiting effects of 0.2−20 g/L of three different types of Al 2 O 3 on the oxidative reactivity of goethite. Surprisingly, soluble Al ions of 0.18−18 mM had a negligible impact on the reactivity. It turned out that the Fe 3+ dissolved from goethite partly contributed to the observed HQ oxidation and the Al 2 O 3 adsorbed the Fe 3+ to lead to the slower HQ oxidation. The observed pseudo-first-order rate constants in HQ oxidation had a strong linear correlation with Fe 3+ concentration in various goethite and Al 2 O 3 mixtures. Separate experiments confirmed the reactivity of Fe 3+ toward HQ and the linear correlation between [Fe 3+ ] and HQ oxidation reactivity. Finally, sedimentation experiments showed negligible heteroaggregation between goethite and AluC−Al 2 O 3 or nAl 2 O 3 but intensive heteroaggregation between goethite and Alu 65−Al 2 O 3 , which explained the observed highest inhibition effect of Alu 65. Overall, oxide mixtures are very complex whose reactivity is determined by many factors such as oxide dissolution.