Degradation Of Organic Pollutants By UV/KMnO₄:Kinetics And Mechanism

Organic dyes and organic micro-pollutants（OMPs）are common organic pollutants in wastwater,due to their complex components and refractory degradation,they have attracted extensive attention and research because of their harmness.KMnO₄,as a common oxidant,can generate hydroxyl radicals（·OH）and reactive manganese species（RMn S）to effectively degrade organic pollutants when it is excited by ultraviolet（UV）.Based on,This topic attempts to construct a UV/KMnO₄system for the study of two representative organic dyes methylene blue（MB）and micro-pollutants benzoic acid（BA）.By examining the influence of operating parameters,complexing agents and common anions and cations in water on the degradation efficiency of target pulltants,the factors affecting the degradation efficiency were studied.In addition,the degradation pathway and reaction mechanism of methylene blue by UV/KMnO₄system were further explored.The research results are as follows:（1）Compared to UV or KMnO₄alone,the removal rate of UV/KMnO₄process was about66.3%which was much higher than UV and KMnO₄alone with 4.1%and 14.3%,respectively.By adding tert-butanol（TBA）as a radical scavenger,the efficient degradation of methylene blue was mainly from the contribution of·OH and RMn S.Increasing the dosages of KMnO₄,reducing the concentrations of MB and increasing the solution temperature would improve the reaction efficiency;Under acidic conditions（p H=4.0）,the decolorization rate of MB can reach 98.6%,which was significantly higher than that under neutral and alkaline conditions.This was because reducing the solution p H would make the reaction proceed in the direction of promoting the generation of active components,and also enhanced the autocatalysis of KMnO₄and the dissociation of methylene blue.In addition,the addition of cations Ca²⁺and Mg²⁺,complexing agents EDTA（0-100μM）and HA（0.5-3 mg C/L）could significantly inhibit the progress of the reaction.Furthermore,the addition of anions Cl^-and SO₄^2-did not cause significant changes in the degradation efficiency of MB;CO₃^2-/HCO₃^-quenched part of·OH so that the degradations of the target were weakly inhibited;while the addition of NO₃^-would generate reactive nitrogen species（RNS）,thereby increasing the degradation rate by 18.1%.（2）Neither UV nor KMnO₄systems could effectively degrade the organic micro-pollutant BA,but the removal rate of BA can reach 95%in 20 min when KMnO₄was excited by UV.The degradation rate increased with the increase of the oxidant concentrations,but when the dosage reached 150μM,the degradation rate was no longer accelerated due to the competition of the degradation products of BA and the inner filter effect of KMnO₄;increasing the concentration of BA and decreasing temperature of the solution both would prevent the reaction from proceeding;the solution p H was affected by redox potential and dissociation constant,which was negatively correlated with the degradation rate of BA.The addition of Ca²⁺and Mg²⁺and complexing agents EDTA and HA would also lead to the reduction of the oxidation ability of the UV/KMnO₄system.Cl^-generated a variety of active chlorine radicals with different selective oxidation properties in the system,when the dosage was more than 5 m M,the generated·Cl₂^-will inhibit the reaction,resulting in the phenomenon that the removal rate of BA first increased and then decreased with the increase of Cl^-dosages.Moreover,SO₄^2-did not significantly change the effect of the system to degrade BA;but CO₃^2-/HCO₃^-would inhibit.（3）In this study,it could be seen from UV-Vis spectral scanning that the decolorization of MB was mainly due to the cleavage of the chromophore group（-S-）and the auxochrome group（-CH₃）and the breakage of the phenothiazine structure.The co-response of Azure A,Azure B,Azure C and Thionin were formed.DMPO was used to capture free radicals,and the presence of·OH in UV/KMnO₄system was confirmed by EPR detection,the contrabutions of UV,·OH and RMn S to dedradation of MB was caculated to be 2.0%,49.1%and 48.9%,respectively.The degradation path of methylene blue degradation by UV/KMnO₄system was analyzed by LC-MS,and the results showed that,on the one hand,MB decolorized through the reaction of demethylation and hydroxylation,and then was attacked by RMn S to form phenolic oxygen radicals,C-O/C-C radicals were coupled,through polymerization and decomposition,the thioether bond of methylene blue is broken.As a result,the phenothiazine structure was broken;on the other hand,it was speculated that MB also decolorized by the occurrence of ring-opening and polymerization reactions.In addition,TOC analysis of the solution after MB reaction proved that about 27%of the target was completely mineralized and decomposed into CO₂and H₂O during the degradation process;the other part was decomposed into small molecules or polymers.