Study On The Degradation Efficiency And Mechanism Of Organic Dyes By UV/NO₃^- System

Effluents from dyestuffs are typically wastewater containing large organic pollutants.It is estimated that more than 280,000 tons of dye wastewater is discharged into natural water bodies each year without treatment,leading to a significant threat to the health of environment and biological.Nitrate(NO₃^-)is one of the major sources of hydroxyl radicals in natural water environment,existing in natural water and industrial wastewater with a certain concentration.NO₃^-,as a photosensitizer,can degrade organic pollutants indirectly by generating active components such as hydroxyl radicals(·OH)and reactive nitrogen species(RNS,·NO₂,·NO,ONOO^-,etc.)when it is excited by ultraviolet(UV).However,there has been no clear research on the efficacy and mechanism of degradation of dyes by UV photolysis of nitrate(UV/NO₃^-)system.This study investigated the efficacy and mechanism of UV/NO₃^-system to degrade organic dyes by using congo red and methylene blue as target pollutants.It was mainly investigated the effects of background composition of water and main operating parameters of system on degradation,analyzing the reasons affecting degradation efficiency and comparing the similarities and differences of these effects.In addition,the contributions of different active ingredients were calculated,the sites of reaction between dyes and active ingredients were determined,the possible intermediates in the degradation process were analyzed,and the degradation pathways were inferred;the degradation efficacy of the system for dyes in real water background was evaluated.The detailed results of the study are as follows:(1)Compared to UV degradation alone,congo red and methylene blue could be degraded effectively by the UV/NO₃^-system with 84.00%and 56.65%,respectively.The degradation of congo red was dominated by·OH and·NO₂,and the degradation of methylene blue was dominated by UV,·OH and RNS by the addition of·OH scavenger(methanol(Me OH))and nitrogen dioxide radical(·NO₂)scavenger(ferulic acid(FA)).In addition,the observed reaction rate constants(k_obs)for the degradation of congo red and methylene blue increased from 0.004min^-1 to 0.045 min^-1 and 0.002 min^-1 to 0.020 min^-1 when the addition of NO₃^-was increased from1 m M to 200 m M,respectively.However,increasing the concentration of congo red or methylene blue(10-40?M)reduced the proportion of UV incidence,resulting in a decrease in the k_obs of congo red and methylene blue from 0.046 min^-1 to 0.013 min^-1 and 0.022 min^-1 to 0.007 min^-1,respectively.Under acidic conditions,the degradation of methylene blue could be enhanced due to the promotion of NO₃^-photolysis,but the degradation of congo red was inhibited due to the protonation of congo red.Under alkaline conditions,the degradation of congo red and methylene blue was weakened due to the inhibition of NO₃^-photolysis and the reduction of the redox potential of·OH.Furthermore,the chloride radicals generated by the capturing of·OH by chloride ions(Cl^-)could oxidize congo red,so that the injection of Cl^-(0.5-7 m M)had no significant effect on the degradation of congo red,but slightly inhibited the degradation of methylene blue due to the capturing of·OH.The carbonate radicals generated by the scavenging of·OH by carbonate/bicarbonate(CO₃^2-/HCO₃^-)oxidized congo red,and the addition of CO₃^2-/HCO₃^-improved·NO₂ production,so that the addition of CO₃^2-/HCO₃^-(0.5-7 m M)enhanced the degradation of congo red,but prevented the degradation of methylene blue due to the scavenging of·OH.Natural organic matter(NOM)could scavenge the active components produced in UV/NO₃^-system and decrease the proportion of UV incident flux irradiated to NO₃^-,therefore,the degradation of congo red and methylene blue was significantly inhibited with the addition of NOM(0.5-10 mg C/L),respectively.Moreover,congo red and methylene blue could be degraded in real water,but the degradation efficiency was affected by the presence of ions and NOM in water matrix.(2)The contributions of UV,RNS and·OH to the degradation of congo red and methylene blue was calculated to be 0.63%,17.46%and 81.90%and 6.21%,15.93%and 77.86%,respectively.It could be seen from ultraviolet visible(UV-vis)scan that the decolorization of congo red was mainly caused by the breakage of the chromophore(azo group,-N=N-),and the decolorization of methylene blue was caused by the shedding of the auxochrome(methyl,-CH₃)and the breakage of the chromophore(thioether bond,-S-).Eight possible degradation products of Congo red were obtained by liquid chromatography-mass spectrometry(LC-MS/MS)analysis,mainly including biphenyl products,naphthalene products and some small molecule aromatic compounds,so the main degradation pathway of congo red was that the active ingredient destroyed the azo group through addition,hydrogen extraction or electron transfer to produce biphenyl products and naphthalene products,and some of which could be further oxidized to aromatic products and inorganic compounds.On the other hand,14 intermediate products of methylene blue were detected,mainly including dealkylation products,hydroxylation products,nitration products and aromatic products,etc.Thus,the main degradation pathways of methylene blue were the shedding of methyl groups to produce dealkylation products,the addition of·OH and·NO₂ to methylene blue and/or dealkylation products to produce hydroxylation and/or nitration products.Additionally,these intermediate products could be further destroyed by radicals through addition or hydrogen extraction to produce aromatic products,such as the breakage of thioether or nitrogen-hydrogen bonds.And a slight decrease in the total organic carbon(TOC)of the reaction system was detected,indicating that some of the products were completely mineralized to inorganic substances.