Determination Of Reaction Kinetics Of Permanganate And Valence State Of Manganese Oxides Formed In Situ By An ABTS Spectroscopic Method

Many studies have already been conducted to evaluate the performance of permanganate (Mn(Ⅶ)) for the oxidative removal of contaminants in aqueous solution. However, the kinetic data are still lacking. The consumption dynamics of Mn(Ⅶ) in real waters are rarely known in these studies either, due to the lack of sensitive methods for the quantification of Mn(Ⅶ) in trace levels therein. In addition, it is usually assumed that the solid product formed upon Mn(Ⅶ) reduction is MnO2(i.e., the valence state of Mn in the the solid product is+4). Nevertheless, several recent studies have shown that the valence state of Mn in the solid product is less than4. In this work, a simple, convenient, and sensitive method (i.e., an ABTS spectroscopic method) for the determination of trace levels of Mn(Ⅶ) in aqueous solution was developed. By using this analytical method, the reaction kinetics of Mn(Ⅶ) with several electron-rich moieties were determined over a wide pH range. Also, the valence states of manganese intermediates/solids in situ formed upon Mn(Ⅶ) reduction were analyzed. Moreover, the consumption rates of Mn(Ⅶ) in several natural waters were measured and the effectiveness of Mn(Ⅶ) for the oxidative removal of micropollutants in these waters were assessed.Mn(Ⅶ) could rapidly oxidize ABTS (2’2-azino-bis(3-ethylbenzothiazoline-6-sulfonate)) in excess at acidic pH, where ABTS lost one electron to form a green radical cation ABTS·+, while Mn(Ⅶ) was reduced to Mn(Ⅱ) by obtaining five sequential electrons. The radical ABTS·+was very stable and could be measured spectrophotometrically at415nm with the molar coefficiency of (3.16±0.05)×104M-1cm-1.Mn valence states in the solid products formed during the reaction of Mn(Ⅶ) with several organic electron moieties such as phenols, anilines, olefins, and sulfoxides, as well as inorganic As (Ⅲ) were determined. It was shown that Mn valence states gradually increased with pH and finally approached4at alkaline pH. Mn solids showed high reactivity toward phenols and anilines under slightly acidic solutions, leading to the relatively much lower valence states therein. In contrast, pH had no effects in the cases of olefins and sulfoxides, due partly to the negligible reactivity of these compounds with Mn solids. The common ions Ca2+, Mg2+, and HCO3-did not exert significant effect in all the cases.By using the developed ABTS method, the reaction kinetics of Mn(Ⅶ) with four typical organic pollutants were determined. The results showed that second-order rate constant(k) of phenol with Mn(Ⅶ) increased with pH from18.9to109.7M-1s-1over the pH range of4-10. For aniline, k decreased with pH increasing from99.0to6.5M-1s-1. However, pH had no significant effect in the cases of olefins and sulfoxides. Moreover, in the cases of phenol and aniline, the oxidation dynamics showed significant autocatalysis under slightly acidic conditions, suggesting the important role of manganese oxides as mild oxidants/catalysts. The hardness ions Ca2+and Mg2+could accelerate the reaction kinetics of Mn(Ⅶ) with aniline.The background matrices in several natural waters could slowly consume Mn(Ⅶ) along the reaction time of several hours. The coagulation processes had negligible effect on oxidant consumption rates. This result suggested that the background composition of these waters transferred from aqueous solution to the floc surface did not significantly affect their reaction with Mn(Ⅶ). Powdered activated carbon (PAC) could consume Mn(Ⅶ) appreciably but pH had negligible effect, suggesting that phenolic hydroxyl functional groups on the surface of PAC were not the major constituents responsible for the consumption of Mn(Ⅶ). Moreover, Mn(Ⅶ) was much more effective than ozone and chlorine for the oxidative removal of phenolic endocrine disrupting chemicals such as bisphenol A and estradiol, although Mn(Ⅶ) showed comparable reactivity toward these two compounds to that of chlorine and much lower reactivity than that of ozone. This was mainly ascribed to the relatively high stability of Mn(Ⅶ) and thus larger oxidant exposure in natural waters.