Roles of MnO₂ Colloids and Mn(III) during the Oxidation of Organic Contaminants by Permanganate

Abstract

Although intermediate manganese species can be generated during the reactions of permanganate (Mn(VII)) with organic pollutants in water, the role of the in situ generated MnO2 colloids in the Mn(VII) oxidation process remained controversial and the contribution of Mn(III) was largely neglected. This study showed that the apparent second-order rate constants (k app) of Mn(VII) oxidation of methyl phenyl sulfoxide and carbamazepine remained constant with time. However, the degradation of four selected phenolic contaminants by Mn(VII) exhibited an autoaccelerating trend and a linear trend at pH 3.0–6.0 and pH 7.0–9.0, respectively. Multiple lines of evidence revealed that the occurrence of the autoaccelerating trend in the Mn(VII) oxidation process was ascribed to the oxidation of the phenolic organics by MnO2 colloids. The influence of pyrophosphate on the oxidation of different organic contaminants by MnO2 colloids suggests that Mn(III) was also responsible for the autoaccelerating oxidation of organic contaminants by Mn(VII) under specific reaction conditions. The kinetic models revealed that the overall contributions of MnO2 colloids and Mn(III) ranged within 6.6–67.9% during the autoaccelerating oxidation of phenolic contaminants by Mn(VII). These findings advance the understanding of the roles of MnO2 colloids and Mn(III) in the Mn(VII) oxidation process.

In Situ Regulation of MnO2 Structural Characteristics by Oxyanions to Boost Permanganate Autocatalysis for Phenol Removal

“…Previous research has revealed the critical role of Mn­(III) aq for phenol removal by Mn­(VII), especially under acidic conditions . Therefore, soluble Mn­(III) aq is another possible candidate for the rapid degradation of phenol in the HCO 3 – /Mn­(VII) process.…”

“…MnO 2 colloids formed from Mn­(VII) decomposition generally exhibit excellent reactivity in removing contaminates through oxidation and/or catalytic processes. , Figure S13 unravels that phenol degradation by ectopically prepared MnO 2 colloids cannot be accelerated by HCO 3 – , suggesting that the enhanced removal of phenol is not associated with an increase in the oxidation ability of MnO 2 . Moreover, a previous study demonstrated the limited role of MnO 2 in phenol removal at pH 3.0–8.0 . Therefore, MnO 2 oxidation is likely to play a minor role in the HCO 3 – /Mn­(VII) system.…”

“…5 mM PP did not significantly affect phenol removal, and the absorption peak of Mn­(III) aq –PP at 258 nm was not observed (Figures c and S12), indicating the limited involvement of Mn­(III) aq at pH 5.0–8.0. This finding contrasts with previous results reported in the literature for the Mn­(VII) alone system, which may be attributed to the relatively limited production of MnO 2 in a shorter time frame or the potential complexation of HCO 3 – to alter the oxidizing activity of Mn­(III) aq . The cyclic voltammetry (CV) curve of Mn 2+ shows two oxidation peaks at 0.53 and 0.99 V, corresponding to the transformation of Mn­(II) to Mn­(III) and Mn­(III) to Mn­(IV) on the electrode surface, respectively (Figure d) .…”

“…Previous research has revealed the critical role of Mn(III) aq for phenol removal by Mn(VII), especially under acidic conditions. 24 Therefore, soluble Mn(III) aq is another possible candidate for the rapid degradation of phenol in the HCO 3 − / Mn(VII) process. 5 mM PP did not significantly affect phenol removal, and the absorption peak of Mn(III) aq −PP at 258 nm was not observed (Figures 1c and S12), indicating the limited involvement of Mn(III) aq at pH 5.0−8.0.…”

“…The abundant Mn­(III) s and large surface area of MnO 2 colloids induced by oxyanions allow them to strongly contact with Mn­(VII), boosting interfacial electron transfer and improving the oxidizing activity of Mn­(VII)* to expedite phenol removal. The significant advancement facilitates the comprehension of the role of oxyanions as well as provides mechanistic insights into the autocatalytic process in Mn­(VII) oxidation, which has not been fully elucidated in previous studies. , Considering the essential role of MnO 2 in governing the overall efficiency of Mn­(VII) oxidation, further research can concentrate on the conversion behavior of MnO 2 to enhance Mn­(VII) treatment performance, such as in situ immobilization of colloidal MnO 2 on fixed reactors to achieve a long-lasting catalytic process.…”