Degradation Of Sulfadiazine By Persulfate Activated By Fe(Ⅲ) Complex Of Aminopolycarboxylic Acids And Its Mechanism

The treatment of new pollutants is one of the key contents of ecological environment protection in the 14th Five-year Plan.Antibiotics are among the new pollutants.In recent years,the abuse of sulfa antibiotics leads to frequent detection in water environment,posing a potential threat to human health and ecological environment stability.Therefore,it is necessary to study its removal processes and technologies.In the traditional Fe（Ⅱ）and photo/Fe（Ⅲ）activated persulfate Advanced Oxidation Processes,the Fe（Ⅲ）/Fe（Ⅱ）cycle is too slow to improve the efficiency.In order to overcome this problem,two environment-friendly aminopolycarboxylic acids,ethylenediamine-N,N’-disuccinic acid（EDDS）and nitrotriacetic acid（NTA）,were used to form complexes with Fe（Ⅲ）to activate two persulfates（PDS and PMS）to degrade the target pollutant sulfadiazine（SD）.The degradation efficiency and mechanism were studied.The main research contents and conclusions were as follows:Firstly,under irradiation,the Fe（Ⅲ）-EDDS/PDS/hv system was constructed to degrade SD by using EDDS and Fe（Ⅲ）complexes to activate PDS,and its feasibility was discussed.The results showed that SD could be degraded completely by Fe（Ⅲ）-EDDS/PDS/hv system in 90 min.By exploring the effects of Fe（Ⅲ）and EDDS molar ratio,Fe（Ⅲ）-EDDS concentration and PDS concentration on SD degradation,the optimal degradation conditions of SD degradation in Fe（Ⅲ）-EDDS/PDS/hv system were obtained.The effects of four common inorganic anions(Cl^-,CO₃^2-,SO₄^2-,NO₃^-)and humic acids on the degradation of SD in Fe（Ⅲ）-EDDS/PDS/hv system were analyzed.The results showed that the degradation of SD was almost unaffected by the addition of Cl^-in Fe（Ⅲ）-EDDS/PDS/hv system,and was slightly affected by the addition of NO₃^-and humic acid.The degradation of SD was significantly inhibited by the addition of SO₄^2-and CO₃^2-.The quenching of radicals by anions and humic acids was the main reason for the decrease of SD degradation.Secondly,Fe（Ⅲ）-NTA/PMS system was constructed to degrade SD by Fe（Ⅲ）-NTA/PMS complex activating PMS,and its feasibility was discussed.The results showed that SD could be degraded completely by Fe（Ⅲ）-NTA/PMS system in90 min.By discussing the effects of Fe（Ⅲ）/NTA molar ratio,Fe（Ⅲ）-NTA concentration and PMS concentration on SD degradation,the optimal degradation conditions of SD in Fe（Ⅲ）-NTA/PMS system were obtained.The effects of four common inorganic anions(Cl^-,CO₃^2-,SO₄^2-,NO₃^-)and humic acid on SD degradation in Fe（Ⅲ）-NTA/PMS system were analyzed.The results showed that the Fe（Ⅲ）-NTA/PMS system had a good effect on SD degradation with the addition of four anions and humic acids,and even with the addition of CO₃^2-,the degradation efficiency of SD was improved.In the Fe（Ⅲ）-NTA/PMS system,the non-radical reaction was less interfered by coexisting substances,which was an important reason why SD could maintain a high degradation efficiency.Finally,the degradation mechanisms of SD in Fe（Ⅲ）-EDDS/PDS/hv system and Fe（Ⅲ）-NTA/PMS system were analyzed.The results showed that the Fe（Ⅲ）/Fe（Ⅱ）cycle in the iron activated persulfate system was effectively accelerated by the introduction of aminopolycarboxylic acids,improving the SD degradation efficiency.In Fe（Ⅲ）-EDDS/PDS/hv system,Electron Spin Resonance（ESR）and trapping experiments showed that·OH and SO₄^·-were the main active substances for SD degradation,and the reaction process of the system was mainly radical reaction.The intermediate products of Fe（Ⅲ）-EDDS/PDS/hv system in the process of SD degradation were analyzed by Liquid Chromatography-Mass Spectrometer（LC-MS）,and the degradation path of SD was proposed.In Fe（Ⅲ）-NTA/PMS system,through ESR,trapping experiment,theoretical calculation and LC-MS analysis,it was found that SD was degraded by the interaction of radical and non-radical process in Fe（Ⅲ）-NTA/PMS system.The main intermediates of SD degradation were obtained by LC-MS,and the degradation path of SD in Fe（Ⅲ）-NTA/PMS system was proposed.