Aggregation Kinetics of Manganese Oxides Formed from permanganate activated by (Bi)sulfite: Dual Role of Ca2+ and MnII/III

Aqueous aggregation kinetics of manganese oxides, the solid products formed during water treatment and subsurface remediation with permanganate, are crucial for its application. In this study, manganese oxides nanoparticles were in situ formed in a permanganate/(bi)sulfite system, which was found to have excellent oxidation ability. Aggregation kinetics of such manganese oxides (i.e., MnOx-1.5, MnOx-2.5 and MnOx-5; the number represents the molar ratio of (bi)sulfite to permanganate) were evaluated by employing time-resolved dynamic light scattering under various aquatic conditions. In NaNO3 solution, the stability of manganese oxides decreased in the order of MnOx-1.5 > MnOx-2.5 > MnOx-5, indicated by their critical coagulation concentrations (CCCs). X-ray photoelectron spectroscopy (XPS) and zeta potential measurements indicated that Mn-II/III were responsible for the decreased stability due to their charge neutralization effects. However, in Ca(NO3)(2) solution, three manganese oxides had similar CCCs, probably due to the relatively great charge neutralization ability of Ca2+. Suwannee River fulvic acid (SRFA), through electrosteric interaction, suppressed the aggregation of MnOx-1.5 in Ca(NO3)(2) solution, but had no such effect in NaNO3 solution. Comparatively, the stability of MnOx-5 was markedly enhanced with SRFA in NaNO3 solutions. It was proposed that Ca2+ and Mn-II/III could increase the adsorption of SRFA through charge neutralization and cation bridging. This study highlights the dual role, dependent on either presence or absence of SRFA, of Ca2+ and Mn-II/III in controlling the aggregation of manganese oxides nanoparticles. (C) 2019 Elsevier Ltd. All rights reserved.