Degradation Kinetics And Products Of Typical Organic Pollutants By Peroxymonosulfate Oxidation

With the development of society and the improvement of people's concerns of environmental protection,control of emerging organic pollutants in water environment will be more and more important.Advanced oxidation processes?AOPs?based on persulfate has become a hot topic in recent years,which has great potential in the treatment of environmental organic pollutants.In the past,researchers generally focused on the development of activation method and improvement of the decontamination capacity.However,they overlooked the strong oxidizing capacity of peroxymonosulfate?PMS?,which may showed great reactivity toward emerging organic pollutants.If we can use PMS direct oxidation to replace the PMS AOPs,it can improve the utilization efficiency of PMS and prevent the potential risk of heavy metals release during AOPs.However,there is insufficient understanding of the oxidation characteristics of PMS.The oxidizing efficiency of PMS direct oxidation is not clear.In this study,phenols,aromatic amines,and aliphatic amines were selected as model structure compounds for emerging pollutants,and their oxidation efficiency by PMS were explored.Then steroid estrogens,quinolones,and tetracycline antibiotics,which contained phenol,aromatic amine,and aliphatic amine groups,were selected as typical emerging contaminants.Oxidation kinetics and mechanisms of these selected compounds by PMS were investigated and the effect of PMS oxidation on the estrogenic activity or antibiotic activity were examined.The degradation of phenol by PMS showed an obvious autocatalysis.The degradation rate of phenol at pH 10 was much higher than that at pH 8.5.It was found that the position of substituents had significant effect on the autocatalysis degradation of phenols.The degradation of ortho-and meta-substituted methylphenols/methoxyphenols showed autocatalysis,but those para-substituted phenols didn't.The degradation rates of para-substituted phenols were much lower than those of ortho-and meta-substituted methylphenols/methoxyphenols.For dihydroxybenzenes,only the degradation of hydroquinone showed autocatalysis.Experimental results showed that quinone intermediates generated during the degradation of phenols resulted in the autocatalysis,i.e.,quinones could activate PMS for the accelerated degradation of phenols.Quinone intermediates were detected during the degradation of phenol,other-and meta-substituted methylphenol/methoxyphenol,and hydroquinone,while in the cases of para-substituted phenols,catechol,and resorcinol no quinone intermediates were detected.Benzoquinone was used as a model quinone compound to investigate the mechanism of quinone activation of PMS.Radical quenching and chemical trapping experiments showed that it was singlet oxygen,rather than hydroxyl radical and sulfate radical,was generated during the reaction between benzoquinone and PMS.Activation of PMS by benzoquinone was a non-radical process with the generation of singlet oxygen.This process was based on the generation of a doxirane-intermediate between PMS and benzoquinone,and the yield of singlet oxygen was 0.5.Kinetic model that proposed based on this mechanism can well describe the benzoquinone catalyzed decomposition of PMS.The oxidation of aromatic amines by PMS was different from phenols.There was no obvious autocatalysis during the degradation of aromatic amines.The type and position of substituents had significant influence on the oxidation rates of aromatic amines,which was attributed to the substituent effect.The degradation rates were in the order of methylaniline>aniline>chloroanilines,because chlorine was electron-withdrawing substituent group which inactivate the aniline ring.The change of pH value would affect the degradation rates of anilines by affecting the ionization of PMS and aromatic amines.The degradation rates of aliphatic amine were higher than those of aromatic amines.The degradation of trimethylamine was faster than dimethylamine at each pH.These two aliphatic amines deprotonated gradually with the increase of pH,and thus the degradation rate was significantly accelerated.The reaction rates between PMS with steroid estrogens,the typical phenol emerging contaminates,were relatively slow.The reaction between steroid estrogens and PMS followed second-order kinetics and the second order rate constants were in the range of0.01-0.57 M^-1s^-1.PMS readily oxidized the phenolic group of steroid estrogens,leading to the generation of hydroxylated products.The further oxidation of hydroxylated products led to the generation of ring-opening products which contained hydrophilic functional groups such as carboxyl group and carbonyl group.The estrogenic activity of oxidation products was significantly lower than that of the parent compounds.PMS showed strong oxidative capacity toward quinolone antibiotics,ciprofloxacin?CF?and enrofloxacin?EF?,which contained both aromatic and aliphatic amine groups.The reactions between PMS with CF and EF all followed second-order rate law and the second order rate constants were in the range of 0.10-33.17 M^-1s^-1.The apparent second-order rate constants showed a great pH-dependency,where the reaction rates increased with the increase of solution pH.Transformation of FQs by PMS yielded hydroxylated,N-oxide,and dealkylated products via oxidation of tertiary and secondary aliphatic N4amines on the piperazine ring.The oxidation products had a much lower antibacterial activity than parent FQs suggesting that PMS oxidation efficient for the treatment of quinolone antibiotics.PMS also showed strong oxidative reactivity toward tetracycline antibiotics.The reaction between PMS with tetracyclines showed a strong pH-dependency,where the degradation rates in alkaline conditions were much higher than those in the cases of acidic conditions.The second order rate constants were in the range of 0.06-150.10 M^-1s^-1.The oxidation mechanisms of tetracycline?TTC?,oxytetracycline?OTC?,and chlortetracycline?CTC?were similar.PMS oxidation mainly underwent oxygen-addition pathway and dealkylation pathway.Oxygen-addition mainly occurred at D-ring of the structure and C4 tertiary amine group with the generation of some isomers.The D-ring oxygen addition product could be further transformed to quinoid product.Dealkylation occurs at the C4 tertiary amine group.The tertiary aliphatic amine at C4 position could be transformed to secondary amine and amino and finally could be oxidized into carbonyl.PMS oxidation could significantly reduce their antibiotic activity.In simulated natural water,the degradation kinetics of CF and EF by PMS were similar to those in pure water,and the removal of CF and EF by PMS was significantly better than that by Mn?VII?.For CF,the removal efficiency of PMS is slightly less than that of Fe?VI?,while in the case of EF,PMS is better than that of Fe?VI?.O₃ was the most efficient one for quinolone antibiotics degradation among these four oxidants.The degradation of TTC by PMS in simulated natural water was faster than that in pure water,and this might be attributed the complexing of calcium and magnesium ions with TTC.The removal of TTC by PMS was comparable to O₃,higher than Mn?VII?,but lower than Fe?VI?.Comparing with Mn?VII?,Fe?VI?and O₃,PMS oxidation showed more selectivity and its consumption by natural organic matter was limited.PMS oxidation showed negligible effect on the UV254,three-dimensional fluorescence intensity and DOC values of natural water.