Study On The Degradation Efficiency And Mechanism Of Sulfathiazole By UV/NO_x^- System

Sulfonamides exist widely in the water environment and are difficult to treat thoroughly,which seriously endanger the ecological environment.Nitrate（NO₃^-）and nitrite（NO₂^-）are significant nitrogen-containing anions in nature,especially in industrial wastewater.They can be excited by UV light to produce reactive oxygen species(ROS:·OH,O^·-,O（~3P）,O₂^·-,etc.)and reactive nitrogen species（RNS:·NO₂,·NO,ONOO^-,etc.）,which play important roles in promoting the degradation of organic matter in water.However,the degradation mechanism of sulfonamides in the UV/NO_x^-system is not clear enough.Sulfathiazole（STZ）was selected as the target pollutant,and a series of experiments were conducted to obtain the efficiency and mechanism of UV₂₅₄/NO₃^-system and UV₃₆₅/NO₂^-system on the degradation of STZ.The main work includes:the effects of various water background components on the degradation efficiency under the two systems were studied,and the degradation effects in actual waters were studied,the active species were identified and contribution rates of each part were calculated,the degradation products in the system were detected and the reaction pathways were deduced,the changes of toxicity in the degradation process of the two systems were compared and the risks were assessed,and the electrical energy per order(E_EO)of the two systems under different conditions were compared.（1）It was found that UV light of 365 nm did not degrade STZ basically,and 254 nm UV light alone had a certain degradation effect on STZ through the experiment of degradation efficiency.After adding 5 m M NO₂^-and 10 m M NO₃^-to the solution under the two kinds of UV light,the degradation rates of STZ were increased to 81.2%and 82.2%respectively.The degradations increased steadily as NO₂^-and NO₃^-dosage increased.The degradation rate in UV₂₅₄/NO₃^-system decreased to 42.4%when the concentration of STZ increased from 2μM to15μM,while the degradation rate of STZ in UV₃₆₅/NO₂^-system was not affected by its concentration.The increase of solution p H will increase the proportion of STZ in the form of anions that are more degradable,so the degradation rates of STZ in both systems increased significantly with the increase of p H.CO₃^2-/HCO₃^-can quench hydroxyl radical（·OH）to produce carbonate radical(CO₃^·-)to degrade STZ and promote the formation of nitrogen dioxide radical（·NO₂）,so it could accelerate the degradation in both systems.However,the phenomena of adding Cl^-in the two systems were different.And the degradation was slightly inhibited in UV₃₆₅/NO₂^-system,but not significantly inhibited by adding Cl^-（1～10 m M）in UV₂₅₄/NO₃^-system.The active species could be scavenged and the penetration of UV light could be reduced by natural organic matter（NOM）,so the addition of NOM could inhibit the degradation of STZ in the two systems to varying degrees.In all kinds of actual waters,the degradation effects of STZ were better than that in deionized water,mainly because the p H values of actual waters were higher than that of deionized water and p H values were higher than p K_a,2 of STZ,which were more conducive to degradation.（2）By adding tert-butanol（TBA）and sinapic acid（SA）to the solution to capture active species,the existence of active species such as·OH and·NO₂ were identified.By using nitrobenzene（NB）as the probe compound of·OH,the steady-state concentration of·OH([·OH]_ss)in UV₂₅₄/NO₃^-and UV₃₆₅/NO₂^-systems were calculated as 3.10×10^-15 M（10 m M NO₃^-）and 1.15×10^-14 M（5 m M NO₂^-）.The calculation results showed that the light irradiation and RNS contributed the most in the UV₂₅₄/NO₃^-system,and the proportion of light irradiation decreased with the increase of NO₃^-concentration.However,in the UV₃₆₅/NO₂^-system,RNS and·OH played major roles,and when NO₂^-increased from 0.1 m M to 5 m M,the contribution rate of RNS increased from 70%to more than 90%.（3）Through full-wavelength scanning and total organic carbon（TOC）detection of STZ during the degradation process,it was determined that a small part of the products was mineralized into inorganic substances.The intermediate products degraded by STZ were detected by LC-MS/MS.17 and 16 products were analyzed during the degradation of STZ by UV₂₅₄/NO₃^-system and UV₃₆₅/NO₂^-system,respectively.And the degradation pathways were speculated.The degradation pathways were divided into hydroxylation and nitration on the original molecule,and further reactions such as hydroxylation and nitration after the chemical bonds（C-S,S-N,N-C）in STZ were broken,resulting in a series of hydroxylation products,nitration products,and a small number of intermediates were mineralized into inorganic substances.（4）The changes in the solution toxicity during the degradation process were analyzed by the changes of the luminescence value of the Luminescent bacterium.The results showed that the toxicity of the UV₂₅₄/NO₃^-system first increased and then weakened.However,due to the toxicity of NO₂^-itself,the toxicity of the solution increased slowly during the degradation of STZ in UV₃₆₅/NO₂^-system.By comparing the E_EO under the influence factors of the two systems,the E_EO of UV₃₆₅/NO₂^-system was lower when the STZ concentration and the NOM concentration in the solution were higher.In the solution with higher alkalinity,the UV₂₅₄/NO₃^-system had more advantages,and the two systems had no obvious advantages and disadvantages in terms of E_EO under other operating parameters.In this study,UV/NO_x^-system was proposed as a method to degrade sulfonamides,which has great application potential in water with high concentrations of NO₃^-and NO₂^-.These results will provide effective solutions to reduce the adverse impact of antibiotics on sewage biological treatment.