Bisulfite triggers fast oxidation of organic pollutants by colloidal MnO₂

Colloidal MnO₂ is the most reactive phase of Mn(IV) while HSO₃ ⁻ is a common reductant in water treatment. This study shows that the presence of HSO₃ ⁻ resulted in significant increase in the decomposition rate of organic contaminants by colloidal MnO₂. The degradation rate of contaminants in the MnO₂/HSO₃ ⁻ process dropped with elevating pH and a proper MnO₂/HSO₃- molar ratio was critical for efficient decomposition of contaminants. The time-resolved spectroscopy of manganese species, the influence of pyrophosphate on UV absorbance spectra, and the relative rate constants of contaminants oxidation in MnO₂/HSO₃ ⁻ process suggested that the synergetic effect of HSO₃ ⁻ and colloidal MnO₂ arose from the generation of Mn(III)_aq, which could oxidize contaminants rapidly. The presence of pyrophosphate, ethylenediaminetetraacetic acid, and humic acid depressed the degradation of contaminants in MnO₂/HSO₃ ⁻ process by complexing with Mn(III)_aq, buffering the solution or competing with contaminants for Mn(III)_aq, and/or inhibiting the consumption of bisulfite. However, Ca²⁺ and Mg²⁺ accelerated the oxidation of contaminants in MnO₂/HSO₃- process by enhancing the reduction of MnO₂ by HSO₃-. The good negative correlation of the O/N or H Mulliken charges of organic contaminants with their removal in MnO₂/HSO₃ ⁻ process suggested that organic contaminants were oxidized by Mn(III)_aq via electrophilic attack.