Oxidation of Microcystins by Permanganate: pH and Temperature-Dependent Kinetics, Effect of DOM Characteristics, and Oxidation Mechanism Revisited

Abstract

Oxidative degradation of six representative microcystins (MCs) (MC-RR, -LR, -YR, -LF, -LW, and -LA) by potassium permanganate (KMnO; Mn(VII)) was investigated, focusing on the temperature- and pH-dependent reaction kinetics, the effect of dissolved organic matter (DOM), and the oxidation mechanisms. Second-order rate constants for the reactions of the six MCs with Mn(VII) ( k) were determined to be 160.4-520.1 M s (MC-RR > -LR ≈ -YR > -LF ≈ -LW > -LA) at pH 7.2 and 21 °C. The k values exhibited activation energies ranging from 15.1 to 22.4 kJ mol. With increasing pH from 2 to 11, the k values decreased until pH 5, and plateaued over the pH range of 5-11, except for that of MC-YR (which increased at pH > 8). Species-specific second-order rate constants were calculated using predicted p K values of MCs. The oxidation of MCs in natural waters was accurately predicted by the kinetic model using k and Mn(VII) exposure (∫[Mn(VII)]dt) values. Among different characteristics of DOM in natural waters, UV, SUVA, and the abundance of humic-like substances characterized by fluorescence spectroscopy exhibited good correlation with ∫[Mn(VII)]dt. A thorough product study of MC-LR oxidation by Mn(VII) was performed using liquid chromatography-mass spectrometry.

Permanganate Oxidation of Microcystin-LA: Kinetics, Quantification, and Implications for Effective Drinking Water Treatment

“…Our rate constant values for MC-LR and MC-RR are consistent and in the range observed by other researchers and follow the trend MC-RR > MC-LR > MC-LA (Table 1). Correcting for the temperature difference between our conditions and those reported by Kim et al [21] or Ding et al [26] would increase our values by eight percent. Sample data are shown in Figure 2.…”

“…Combining experiments (N=9) yielded an average second order rate constant at these conditions of 118 ± 9 M −1 · s −1 . This is approximately 25% lower than previously reported values by Kim et al [21] and Ding et al [26]. We also conducted a limited number of experiments with MC-RR and MC-LR.…”

“…In the early literature, the basic permanganate mechanism was believed to be the production of vicinal dihydroxyl or carbonyl products [20]. More recent work by Kim et al [21] describes the permanganate oxidation progressing through vicinal hydroxyl-carbonyl pathways more like ozone oxidation [22, 23] rather than vicinal dihydroxy OPs produced via chlorine oxidation [24]. Because of its selective oxidation, permanganate treatment may be useful early in the treatment process for degradation of MCs as part of a multibarrier oxidation process to achieve the new low concentration guidance for total MCs.…”

Natural organic matter-cations complexation and its impact on water treatment: A critical review

“…Specific UV absorbance (SUVA), ascertained as the UV absorbance of a solution at a given wavelength normalized for DOC concentration, is strongly correlated with the degree of aromaticity of DOM (Weishaar et al, 2003). This indicator of DOM aromaticity is often calculated at 254 nm (SUVA 254 ) (Kim et al, 2018;Lee et al, 2018). Numerous studies have shown that the variation of the SUVA 254 descriptor is directly bound to metal complexation.…”

Unexpected bias of freeze-drying on the performance assessment of chemical oxidation of soils contaminated by polychlorinated biphenyls

“…As the soil has an alkaline pH, an increase of OHconcentration with the freezing concentration effect may take place. This pH increase might also affect the action of persulfate, since it is more efficient under acidic conditions (Fang 2013;Tang 2015), but not that of permanganate which is pH independent (Kim 2018;Waldemer 2006). In the case of ferrate, a pH increase may affect the molecular speciation of ferrate(VI).…”