Catalyst-free Activation Of Permanganate Under Visible Light Irradiation For Aqueous Sulfamethazine Degradation

Sulfonamide antibiotics（SAs）are widely used because of their remarkable effects in preventing or treating human and animal diseases,promoting the growth of livestock and crops.However,in recent decades,the abuse of SAs has caused it to be frequently detected in water bodies and induced the generation of"super drug-resistant bacteria",which has caused great harm to the ecological environment and human health and safety.Advanced oxidation processes（AOPs）have attracted widespread attention in the field of water treatment because of their good stability and thorough degradation of refractory organic pollutants.It is particularly important to explore sustainable,efficient and economical emerging AOPs,because SAs parent and their incomplete mineralization products exhibit potential bio-toxicity.In this study,sulfamethazine（SMT）was selected as the target pollutant,and the degradation behavior of visible light activated potassium permanganate（VL/PM）on SMT in water was investigated,the degradation mechanism was also studied.The primary achievements were as follows:（1）We studied and compared the degradation efficiency of SMT by potassium permanganate under different conditions.According to the experimental results,the single visible light irradiation and the single potassium permanganate oxidation system presented poor removal efficiency for SMT,while the combined technology（VL/PM）can achieve a good removal efficiency for SMT.Compared to single visible light irradiation and single potassium permanganate oxidation system,the degradation efficiency of SMT in the VL/PM process increased by 83.47%and 50.87%,respectively（Reaction condition:[SMT]₀=10 mg/L,[PM]₀=50μM,reaction time=90 min）.In addition,the degradation process of SMT in the VL/PM system can well fit the pseudo-first-order reaction kinetics（R²=0.99）,and the reaction rate constant(k_obs)was 0.0217 min^-1.（2）The different influence factors on SMT degradation under the VL/PM system was investigated,such as PM dosage,solution pH,humid acid（HA）concentration,common inorganic anions(CO₃^2-,SO₄^2-,Cl^-and NO₃^-)and different water bodies.The experimental results indicated that the removal efficiency of SMT gradually increased with the increase of the PM dosage,while the removal efficiency decreased as the solution pH rises.Besides,the removal rate of SMT was only 2.93%when the value of solution pH was 10.85.HA had two sides to the degradation of SMT in the VL/PM system.Low-concentration levels of HA（<5 mg/L）could promote the degradation of SMT,while further increasing HA concentration began to inhibit the degradation of SMT.For the common inorganic anions in natural water bodies,except for CO₃^2-which had a significant inhibitory effect,SO₄^2-,Cl^-and NO₃^-presented a certain promotion effect on the degradation of SMT.Comparing the removal efficiency of SMT in ultrapure water,tap water,Xiangjiang River water and Taozi Lake water,we observed that the degradation of SMT was inhibited to a certain extent in tap water,Xiangjiang River water and Taozi Lake water,indicating that the soluble substances in the water have a significant negative effect on the degradation of SMT in the VL/PM system.（3）Through radical quenching experiment and electron spin resonance（ESR）experiment,we found that the main reactive oxygen species（ROS）in the VL/PM system was·O₂^-.The effect of pyrophosphate（PP）on SMT degradation and the UV-Vis scanning spectrum proved that active Mn（III）played a vital role in the degradation of SMT in the VL/PM system.（4）Based on the detection results of SMT degradation intermediate products by high-performance liquid chromatography-mass spectrometer（HPLC-MS）,and combined with Density Functional Theory（DFT）calculation to find out the reactive sites of SMT,two degradation pathways of SMT in the VL/PM system were inferred:（i）Smiles rearrangement and SO₂ removal of SMT molecules;（ii）S–N bond breakage in SMT molecules.Finally,we adopted DFT calculations to study the potential energy of each degradation pathway of SMT,and found that Smiles rearrangement and SO₂removal was the result of being attacked by·OH,while the breaking of the S–N bond was attributed to the attack of·O₂H.Meanwhile,the former was more likely to occur than the latter’s degradation pathway.In summary,the VL/PM process has a good removal effect on SMT.Furthermore,the study helps to clarify the degradation mechanism of SMT in the VL/PM system.This study would provide a theoretical basis and practical support for the remediation of antibiotics and other refractory organic pollutants.