Simultaneous photometric determination of oxidation kinetics and average manganese valence in manganese products in situ formed in the reactions of aqueous permanganate with model organic compounds and natural organic matters

Abstract

In this study, a colorimetric method using 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) was adopted to investigate oxidation kinetics by permanganate (Mn(VII)) in trace levels (lower than 1 mg/L or 6 µM) and measure average manganese valence in manganese product bulks in (i) synthetic waters containing selected model compounds with electron-rich moieties, (ii) natural organic matters (NOM) isolate solution, and (iii) real water samples including river water, reservoir water and secondary wastewater effluent. The second-order rate constant for Mn(VII) oxidation of phenol determined using ABTS method was demonstrated to be comparable to that measured by traditional high-performance liquid chromatography (HPLC) method at pH 5–9. This method was applied to measure the rate constants of the reactions between Mn(VII) and model compounds with electron-rich moieties. Average manganese valence in manganese product bulks from Mn(VII) oxidation of selected model compounds was lower than 4 at acidic pH, suggesting that Mn(III) was likely to be generated during Mn(VII) oxidation. Possible mechanisms for Mn(III) formation via one/two-electron process were also proposed. Mn(VII) reaction with NOM isolate sample followed two-phase kinetics with rate constants of 3.47 × 10-5 s−1 (mg C/L)-1 and 6.94 × 10-6 s−1 (mg C/L)-1 in the initial and secondary phases at pH 8. Similarly, two-phase kinetics were also found in the consumption of Mn(VII) in real waters. Mn(VII) consumption rates in different real waters followed the order of their dissolved organic carbon levels. In contrast, chlorine decay rates were mainly based on the order of their dissolved organic nitrogen levels. Moreover, average valence in manganese products in real water samples was determined as around 4, which was mainly ascribed to the one-electron-transfer processes of Mn(VII) with NOM constituents in real water samples such as phenols and amines at ambient weak alkaline pH.