Research On The Effect Of Natural Organic Matters On The Oxidation By Manganese Oxidants And Ozone And Their Transformation Mechanisms

Recently,micropollutants in drinking water and secondary effluent of wastewater plants has seriously affected the water quality and safety.Therefore,more and more attention has been paid to the advanced treatment for the abatement of micropollutants.Conventional water treatment is difficult to effectively remove micropollutants.Chemical oxidation technology(manganese oxidants(KMnO4 and MnO2)and ozone),as a typical advanced water treatment technology,can effectively remove micropollutants.Meanwhile,during the application of chemical oxidation technology to treat micropollutants in synthetic waters containing natural organic matter(NOM)isolates and real water samples,the NOM in the background component of the water will compete with micropollutants for the oxidant,and then affect the oxidation efficiency of micropollutants removal.Therefore,the effect of NOM in water on the oxidant oxidation process,and the transformation of NOM during oxidation needs further studies.In this study,the effect of model NOM compound,model humic constituents,humic substances,and NOM in real water samples,on the a batement of typical pollutants by mild oxidants(KMnO4 and MnO2)and strong oxidant(ozone)were studied.Meanwhile,the transformation of NOM during oxidation was also studied.NOM in water has a significant enhancing effect on the oxidative removal of phenolic compounds by KMnO4.Previous studies showed that the complexing agent in humic acid could stabilize highly active intermediate manganese and further promote the removal of phenols.However,a large amount of electron shuttle is also present in NOM.In this study,model NOM compound(ABTS),a 1-e-electron shuttle,could catalyze the oxidation of phenolic compounds by KMnO 4.This was attributed to the fact that these substituted phenols could be readily oxidized by the stable radical cation(ABTS·+),which was quickly produced from the oxidation of ABTS by KMnO4.The second-order rate constants varied negligibly from p H 5.0 to 9.0(9.44 × 104 M-1 s-1).ABTS·+ produced from the reaction between KMnO4 and ABTS can quickly react with phenolic compounds.The reaction of ABTS·+ with phenol showed biphasic kinetics.The second-order rate constants for the reactions of ABTS·+ with substituted phenols obtained in the initial phase were strongly affected by p H(p H 5.0-9.0: 5.4 × 102-9.57 × 105 M-1 s-1),and they were several orders of magnitude higher than those for the reactions of Mn(VII)with substituted phenols at each p H.In addition,similar enhancing effect of ABTS on Mn(VII)oxidation of other substituted phenols was found.This result also indicated that enhancing effect of humic acid on Mn(VII)oxidation of phenolic compounds,on one hand followed the law of high-activity intermediate manganese.On the other hand,electron shuttle in humic acid(HA)and its oxidation product(HAox)could form redox pairs(HAox/HA),which could efficiently remove phenolic compounds.Syringaldehyde(SA)was selected as model humic constituents.T he enhancing effect of SA on the oxidation of phenolic and amine compounds by KMnO4 was negligible.However,SA could enhance MnO2 transformation process.a MnO2-mediated oxidation model is proposed.SA was oxidized by MnO2 forming oxidized SA(SAox),including phenoxy radical(SA·)and 2,6-dimethoxy-1,4-benzoquinone(DMBQ).The cross-coupling of SAox and typical phenolic or amine compounds occurred,which could improve the efficiency of micropollutants removal.Model humic constituents could not catalyze the oxidation of phenols and amines by KMnO4.This result was attributed to the fact that KMnO4 could readily oxidize with model humic constituents containing phenolic moieties forming organic acids rather than quinones.Different types of metal cations(such as Mn2+,Ca2+,and Mg2+)inhibit the removal efficiency of MnO2-SA system in different degrees,because oxidative ability of MnO2 for organic compounds was inhibited by metal ions via occupying reactive sites on negatively charged MnO2 surface via electrostatic interactions,and then metal ions play a role in competition with SA to occupy the active site on MnO2 surface.The inhibitive effect of the metal ions basically followed the order of Mn2+ > Ca2+ > Mg2+,which may partially be ascribed to their affinity to MnO2 surface.Mn2+ as manganese reduction product in MnO2-SA system significantly reduced the oxidation ability of MnO2,and then inhibited the oxidation performance of MnO2-SA system.In this study,KMnO4 was used to regenerate the inactivated MnO2 active site,which could recover the oxidative behavior of MnO2 as before.During oxidation of micropollutants by strong oxidizing agent,ozone(O3),NOM in water could compete with micropollutants for ozone,and then ozone was quickly decayed into ·OH.Finally,micropollutants and NOM were oxidized by the combination of O3 and ·OH.In this study,ozonation of dissolved organic nitrogen(DON)and UV254 were studied in synthetic waters containing natural organic matter(NOM)isolates and three real water samples.NO3-could be generated during ozoantion of DON.At the same ozone dosage,NO3-formation increased with the increase of p H value.Based on the kinetics of ozonation of DON,DON transferred into intermediate(X)with R-NO moiety via oxygen transfer,and then X was ozonated into NO3-.The reaction of ozone with X was the rate-limiting step.The relative residual UV254 decreased as a function of the specific ozone dose in two linear phases,which was not related to p H changes.During the initial phase,organic moieties with high ozone reactivity(e.g.,olefins,phenols,and other activated aromatic moieties)lead to an efficient abatement of UV254.During the secondary phase,relatively higher specific ozone doses are required for a similar abatement of UV254 due to organic moieties with lower ozone reactivity,such as non-activated aromatic compounds(e.g.,benzenes,alkyl benzenes,etc.)NO3-formation and UV254 abatement were investigated as novel surrogate parameter to evaluate the abatement of micropollutants during ozonation of synthetic waters containing NOM isolates,a natural water and secondary wastewater effluents.The abatement of the selected micropollutants(i.e.,17a-ethinylestradiol(EE2),carbamazepine(CBZ),bezafibrate(BZF),ibuprofen(IBU),and p-chlorobenzoic acid(p CBA))showed linear relationships with NO3-formation and UV254 abatement.And its efficiency depended strongly on the reactivity of the selected compounds with ozone.The differences between the correlations for these micropollutants were in agreement with the relative order of their ozone reactivities: EE2 > CBZ > BZF > IBU > p CBA.During ozonation of a wastewater sample,the N-nitrosodimethylamine formation potential(NDMA-FP)during chloramination decreased with increasing specific ozone doses.A good correlation was obtained between NDMA-FP abatement and NO3-formation.Therefore,the results of this study suggest that NO3-formation in combination with UV254 abatement during ozonation of real waters may be a good surrogate for assessing the abatement of micropollutants and the by-product formation potential.