Oxidation Degradation Of Naphthalene And Its Derivatives In Water By Cerium（Ⅲ）-Activated Peroxymonosulfate

Organic pollution caused by polycyclic aromatic hydrocarbons（PAHs）in aquatic environments has attracted much attention.Among them,naphthalene is listed as a priority control pollutant in water in China.The wastewater from the chemical industries containing naphthalene and its derivatives is discharged into water bodies in large quantities every year,which poses a grave environmental threat in both ecological and human health aspects.There is an urgent need to develop feasible technologies for its effective treatment.In recent years,the persulfate-based advanced oxidation processes based on SO4·-have gradually attracted much attention due to its advantages such as high efficiency,low toxicity and selectivity,strong oxidation capacity,multiple activation methods,and environmental friendliness.Its mechanism involves the degradation of organic pollutants into smaller molecule intermediates before the eventual conversion into CO2 and H2O by activating oxidants to generate reactive oxidative species.Peroxymonosulfate（PMS）is more easily activated than other oxidants due to its asymmetric molecular structure.The conventional physical activation methods have higher energy consumption and cost,while the transition metal ion activation methods could be performed at room temperature and pressure.Among them,the rare-earth cerium ion has an excellent catalytic performance,and its activation of PMS produces SO4·-with high efficiency and many oxidative species,which is expected to degrade organic pollutants effectively.However,at present,there are few reports on the activation of PMS by cerium ion.In this study,a Ce3+/PMS homogeneous catalytic oxidation system was constructed to degrade naphthalene,1-naphthylamine,1-methylnaphthalene,1-nitronaphthalene,and 1-naphthylamine in water.This work investigated the degradation efficiency of the system for naphthalene and its derivatives and analyzed the effects of temperature,catalyst and oxidant dosage.Based on the identification of reactive oxidative species in the Ce3+/PMS system,the degradation mechanism and degradation pathway were revealed.Finally,the effects of co-existing substances such as humus and inorganic anions in natural water on the degradation of naphthalene and its derivatives in the Ce3+/PMS system were simulated.The main results obtained are as follows:（1）The activation of PMS by Ce3+could efficiently degrade naphthalene and its four derivatives.The degradation efficiency of each pollutant is ranked as follows:1-naphthalamine>1-naphthol>1-nitronaphthalene>1-methylnaphthalene>naphthalene.Within 300 min,1-naphthylamine was most rapidly degraded with a removal rate of up to 100%,followed by 1-naphthol and 1-nitronaphthalene whose final degradation reaction rates were 98.90%and 97.02%respectively.But 1-naphthol was degraded faster and its apparent rate constant（0.0570 min-1）of 1-naphthol was nearly 5 times that of 1-nitronaphthalene（0.0119 min-1）.The removal rates of 1-methylnaphthalene and naphthalene were 93.10%and 92.47%respectively,and their degradation reaction rates were comparable（around 0.0080 min-1）.Naphthalene derivatives with electron-donating groups（-NH2，-OH,and-CH3）were more favorable for degradation,while the electron-withdrawing group（-NO2）did not exhibit a dulling effect on naphthalene,and its degradation efficiency was even better than that of 1-methylnaphthalene.The increase of reaction temperature helped to enhance the degradation reaction rate of each pollutant,and the order of influence strength was generally:naphthalene>1-nitronaphthalene>1-methylnaphthalene>1-naphthol>1-naphthylamine.With the increasing dosage of catalyst and oxidant,the degradation efficiency of several pollutants generally showed a trend of increase.The optimum conditions for the degradation of naphthalene and its four derivatives（10 mg/L）were 25℃,5.0 mmol/L of initial concentration for both catalyst and oxidant.（2）The mechanism of degradation of naphthalene and its derivatives in water by Ce3+-activated PMS was expounded.The Ce3+/PMS system degraded naphthalene and its derivatives through both radical and non-radical pathways.Four reactive oxidative species,SO4·-,-OH,O2·-and 102,were produced during the catalytic decomposition of PMS by Ce3+.For naphthalene,1-nitronaphthalene and i-methylnaphthalene,SO4·-and 1O2 played a leading role in oxidation and a small amount of ·OH and O2·-made oxidation contributions for their degradation.The oxidation of a small amount of SO4·-,102,and ·OH resulted in the efficient degradation of 1-naphthol,while 1-naphthylamine required only a small amount of O2·-to be completely degraded.Various reaction forms,including dealkylation,dehydrogenation,hydrogenation,ring opening,chain breaking,cyclization,and isomerization,occurred during the degradation process,and the pollutants were finally mineralized into inorganic small molecular substances.（3）The effects of co-existing substances such as humus and inorganic anions on the degradation of naphthalene and its derivatives in water by the Ce3+/PMS system were revealed.It was found that inorganic anions such as NO3-Cl-,HCO3-and natural organic humus（HA）had weak effects on the degradation of these five pollutants by the Ce3+/PMS system.With the increasing concentration of co-existing substances,the degradation efficiency of the pollutants（including naphthalene,1-methylnaphthalene,1-nitronaphthalene and 1-naphthol）degraded by the non-radical pathway through 1O2 was hardly affected or slightly promoted.While the degradation efficiency of 1-naphthylamine by the radical pathway was slightly inhibited when co-existing substances were at low or medium concentrations.The final degradation rates of naphthalene and its derivatives were not significantly different from that of the original,and the absolute values of the removal rates of all pollutants were within 16%.The above results indicated that the Ce3+/PMS system in this study reduced the effects of water environmental background substances to a certain extent,and broadened the application of persulfate-based advanced oxidation processes based on cerium（Ⅲ）-activated PMS to treat organic wastewater.