Comparative Study On The Removal Performance And Mechanisms Of Nitrogen Heterocyclic Compounds By Fenton And KMnO₄/NaHSO₃

Nitrogen heterocyclic compounds?NHCs?widely existed in industrial wastewater are difficult to degrade,and still have high concentration after secondary biological treatment.With high toxicity and other characteristics,nitrogen heterocyclic compounds will have adverse effects on the ecological environment and human health.Therefore,it is very important to remove nitrogen heterocyclic compounds from industrial wastewater.The nitrogen heterocyclic compounds were belong to the refractory pollutants and were biodegradable.Advanced oxidation process has the advantages of high removal efficiency,fast oxidation speed and no secondary pollution to treat refractory organic pollutants.But on different organic material,advanced oxidation technology efficiency may be different.Therefore,it is necessary to study the degradation efficiency of different advanced oxidation technologies on nitrogen-containing heterocyclic compounds and clarify their removal mechanism,which can provide data support for the optimal selection and regulation of nitrogen-containing heterocyclic compounds removal technologies in wastewater.This article selected Fenton and KMnO₄/NaHSO₃ as nitrogen heterocyclic compounds degradation technology.In addition,quinoline and indole were selected as two kinds of typical nitrogen heterocyclic compounds to study the processing performance of these two oxidation technology,initial pH value,reagent-dosing ratio and the concentration and reaction time on treatment effect were concluded.The mechanism of degradation of nitrogen-containing heterocyclic compounds by two techniques was discussed.The main research conclusions are as follows.?1?Fenton oxidation system was used to remove the quinoline solution with the initial concentration of 0.5mmol/L,and the optimal conditions were c?H₂O₂?=6.0mmol/L,n(H₂O₂/Fe²⁺)=6,and pH=3.Under these conditions,the highest removal rate of pollutants was98.2%,and 30.1%TOC was removed.KMnO₄/NaHSO₃ oxidation system was used to remove the quinoline solution with the initial concentration of 0.5mmol/L.The optimal conditions were n?KMnO₄/NaHSO₃?=1:5,c?KMnO₄?=6.0mmol/L,pH=4 and the pollutant removal rate was 75.6%.?2?Fenton oxidation system was used to remove the indole solution with the initial concentration of 0.5mmol/L.The optimal conditions were c?H₂O₂?=7.5mmol/L,n(H₂O₂/Fe²⁺)=3,and pH=3.The removal rate of pollutants was up to 97.4%and and 14.4%TOC was removed.KMnO₄/NaHSO₃ oxidation system was used to remove the indole solution with the initial concentration of 0.5mmol/L.The optimal conditions were n?KMnO₄/NaHSO₃?=1:4,c?KMnO₄?=5.0mmol/L,pH=4,and the highest removal rate of pollutants was 94.3%.?3?The analysis of first-order reaction kinetics showed that,for the degradation of quinoline,the maximum K value of reaction rate constant was 0.0194 min^-1 of Fenton system and 0.2865min^-1 of KMnO₄/NaHSO₃ system.For the degradation of indole,the maximum K value was 0.0172 min^-1 of Fenton system and 0.7462 min^-1 of KMnO₄/NaHSO₃ system.?4?Ultraviolet total scanning and GC-MS analysis were used to investigate the intermediates in the oxidation process of organic matter.According to the analysis of intermediates,it was speculated that the removal path of degradation of quinoline by Fenton system was to react with H⁺to generate quinoline cation first,and then introduce oxygen group to oxidize to intermediate products,such as 2?1H?-quinolinone,etc.Pyridine ring was first opened.Then further decompose them into small molecules of organic matters,such as organic acids?formic acid,acetic acid,oxalic acid,etc.?.Finally,the organic matters were degraded into CO₂ and H₂O.The removal path of degradation of indoles by Fenton oxidation was to firstly open the azacyclic ring and oxidize it into intermediate products,and further decompose it into small molecules of organic compounds,such as organic acids.Finally,the organic matters were degraded into CO₂ and H₂O.?5?Ultraviolet total scanning and GC-MS analysis were used to investigate the intermediates in the oxidation process of organic matter.The main active substance in the KMnO₄/NaHSO₃ process is Mn?III?and Mn?III?are electrophilic.In the process of degradation of nitrogenous heterocyclic compounds,and organic substances will undergo-OH substitution reaction with Mn?III?-OH.KMnO₄/NaHSO₃ degrades quinoline,which first attacked the carbon atoms on the benzene ring to form 5-hydroxyquinoline or 8-hydroxyquinoline.Some quinoline is oxidized to 2?1H?-quinolinone due to its acidic environment,and then further degraded to CO₂,H₂O and NO₃^-.?6?The degradation path of KMnO₄/NaHSO₃ degrades indoles was similar to that of Fenton,but the mechanism was different.-OH first replaced the hydrogen at the C7 and C8positions on the nitrogen heterocyclic ring,and then was oxidized to 2-indosterone,which was further oxidized to indiginine then into small molecular substances.After the benzene ring was opened,the C=C double bond was opened,and aldehydes and ketones were formed,and some may form amides without removing the amino group.Finally organic matter was converted into CO₂,H₂O and NO₃^-.In this paper,there are 46 figures,11 tables and 82 references.