Performance And Mechanism Of Ru~Ⅲ-catalyzed Permanganate Oxidation Of Emerging Micropollutants In Water

With the development of measuring and testing techniques and people’s awareness of environmental protection in this decade, the emerging micropollutans (EMs) in aquatic environment are becoming a new concern to the worldwide researchers. Emerging micropollutants are those that are suspected to have harmful effects-e.g. endocrine disruption and antibiotic drug resistance-but are not routinely monitored or regulated against. Research on emerging micropollutants is further complicated as there is an extensive array of different chemicals entering the aquatic environment, their volume is increasing and new compounds are continually being introduced to the market. Emerging micropollutants have been found in the effuent of wastewater treatment plants, which constitutes a major limitation for their potential reuse. Hence, the development of new strategies to deal with this concern is needed. Permanganate has several obvious advantages, such as relatively low cost, ease of handling, effectiveness over a wide pH range, and comparative stability. Its reductant manganese dioxide can also remove the pollutants by adsoption, oxidation and coagulation. Considering the unpleasant color of permanganate, only very low inlet concentration was allowed to avoid the appearance of chromaticity in the treated water. Thus, catalyzing this process is becoming a necessity to achieve high emerging pollutants removal with lower permanganate dosage.This study proved that the addition of homogeneous Ru enhanced the performance of permanganate oxidation of emerging micropollutant at environmentally relevant pH. The second order rate constants of permanganate oxidation of bisphenol A (BPA) were increased by1.2to8.4fold at pH4.0-8.0in the presence of RuIII. The mineralization rate of BPA was much higher in RuIII catalyzed permanganate oxidation than its uncatalytic counterpart. Ru is rather expensive and the addition of RuIII into permanganate oxidation system is far from practical application, since it is troublesome to remove or/and Ru from the efuents. The heterogeneous catalyst (Ru/CeO2) was prepared for reclamation andrecycling. The catalytic mechanism of Ru/CeO2in permanganate oxidation of emerging micropollutant was explored. The XANES analysis revealed that (i) Ru was deposited on the surface of CeO2as RuIII;(ii) RuIII was oxidized by permanganate to its higher oxidation state RuVI and RuVII, which acted as the co-oxidants in butylparaben (BP) oxidation;(iii) RuVI and RuVII were reduced by BP to its initial state of RuIII. Therefore, Ru/CeO2acted asan electron shuttle in catalytic permanganate oxidation process.The kinetics of BP degradation by Ru/CeO2catalyzed permanganate was determined as functions of pH, permanganate concentration, catalyst dosage and temperature. In the heterogeneous catalysis, the oxidation potential of permanganate increased with decreasing pH over the pH range of4.0-8.0, which would result in more efective RuIII oxidation to RuVII and RuVI and thus more rapid BP oxidation at lower pH. More signifcant enhancement in BP removal rate due to the presence of Ru/CeO2catalyst at lower permanganate concentration was observed. With the increase of Ru/CeO2dosage from0.13to2.0g L-1, the rate constant of BP degradation increased linearly from6.7to74.7M-1s-1.The second order rate constants of Ru catalyzed permanganate correlated well with the loading of Ru on the catalysts, indicating that the performance of heterogeneous catalysts dependented on their coorespounding Ru loading. The stability of heterogeneous catalysts was highly depended on the BET suface area of the support. The higher BET suface area would lead to higher amount of MnO2deposition onto the surface of catalyst, thus the catalytic effect of Ru/CeO2catalyst would decrease due to the masking of the active sites.The regenerated Ru/ZSM-5A by hydroxylamine hydrochloride performed as good as the newly prepared one. But the ascorbic acid could not totally remove the deposited MnO2. It should be noted that hydroxylamine hydrochloride was toxic, thus Ru/ZSM-5A regenerated by hydroxylamine hydrochloride should be flushed for some times before next run in water treatment.Permanganate catalyzed by Ru/TiO2is a selective oxidant, which can oxidize emerging micropollutants containing electron-rich organic moieties. Ru catalyzed permanganate oxidation was effective not only for eliminating emerging micropollutants but also for detoxification.The oxidation rates of these three emerging micropollutants by Ru/TiO2catalyzed permanganate were not decreased due to the presence of other emerging micropollutants, indicating that the oxidation ability of Ru/TiO2catalyzed permanganate was much stronger than permanganate. Therefore, Ru/TiO2catalyzed permanganate could remove target emerging micropollutants effectively in the presence of a variety of other emerging micropollutants at trace concentration.In real water, the oxidative removal of emerging micropollutants by Ru/TiO2catalyzed permanganate is a promising method due to its higher selectivity. In catalytic process, the removal of emerging micropollutants was enhanced in the presence of lower concentration humic acid, while some reductants and higher concentration humic acid in real water may compete for the oxidant with target organics and thus lower the performance of Ru catalyzed permanganate oxidation. No bromate byproduct was formed during the catalytic process. Therefore, Ru-catalyzed permanganate oxidation was an effective and promising peroxidation process in water treatment.