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” 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.