Study On The Mechanism Of Activated PS By Reduction Of Ferric Iron Induced By Dissolved Organic Matter Of Secondary Effluent Under Visible Light

Fe²⁺-activated persulfate（PS）is one of the most promising advanced oxidation technologies of persulfate,having the advantages of easy implementation,low cost,green and non-toxic.However,during the activation of PS by Fe²⁺,Fe²⁺is rapidly oxidized to Fe³⁺,which is difficult to be spontaneously reduced to Fe²⁺under conventional conditions,leading to the rapid accumulation of Fe³⁺and severely inhibiting the treatment efficiency of the system.In this study,based on the charge transfer effect of photoligands to metals,a new photo-assisted Fenton-like reaction(DOM-Fe³⁺-PS-Vis)composed of visible light,secondary effluent soluble organic matter（DOM）,Fe³⁺,and PS was constructed to accelerate Fe³⁺reduction by utilizing the organic matter’s own electrons.In order to improve the activation efficiency of PS and the yield of free radicals,and finally to realize the efficient self-degradation of organic matter,the following research contents around the catalytic reaction efficiency and mechanism were carried out:（1）Using humic acid（HA）,fulvic acid（FA）and actual secondary effluent as DOM model pollutants,Fe³⁺reduction,PS activation and DOM oxidative degradation efficiency in DOM-Fe³⁺-PS-Vis system and other related reaction systems were investigated.The influence of DOM content,Fe Cl₃dosage and other key factors on the catalytic performance of the system was analyzed.The results show that:DOM-Fe³⁺-PS-Vis system can effectively reduce Fe³⁺(Fe²⁺steady-state concentration in HA and FA reaction systems is up to 0.034 mmol·L^-1),and thus significantly improve the activation efficiency of PS（decomposition rates of PS in HA and FA reaction systems are up to 54.81%and 53.53%,respectively）.Compared with DOM-PS-Vis,the improvement is 5 times.The TOC removal rates of the DOM-Fe³⁺-PS-Vis system for HA,FA and secondary effluent were up to 80.02%,28.42%and 41.99%,respectively,much higher than those of the DOM-Fe³⁺-Vis（75.71%,13.22%,28.56%）and DOM-PS-Vis（27.18%,5.43%,13.25%）.The catalytic efficiency and oxidation capacity of persulfate can be improved by increasing the dosage of DOM and Fe Cl₃in a certain range.In addition,DOM-Fe³⁺-PS-Vis system not only achieves oxidative degradation of DOM,but also has a good degradation effect on co-existing refractory organic compounds（such as bisphenol A and phenol）.（2）HA and FA were used as DOM model pollutants to identify the main active substances in DOM-Fe³⁺-PS-Vis system,and the relative content of each active substance in different systems was analyzed according to the relative intensity of characteristic signal peaks,and the yield of each active substance was quantified through degradation of probe compounds.The results showed that the concentration of active free radicals was the highest in DOM-Fe³⁺-PS-Vis system,and the signal intensity of SO₄^-·was higher than that of·OH and ¹O₂.The SO₄^-·yield of DOM-Fe³⁺-PS-Vis system was the highest(the SO₄^-·yield of HA and FA system was2.19 and 2.08μmol L^-1s^-1,respectively).It was significantly higher than DOM-Fe³⁺-PS-Dark(0.87 and 0.64μmol L^-1s^-1),Fe³⁺-PS-Vis(0.12 and 0.12μmol L^-1s^-1)and DOM-PS-Vis(0.98 and 0.57μmol L^-1s^-1).It is proved that the main active free radical in the system is SO₄^-·.（3）Taking HA,FA and the actual secondary effluent as targets,the changes in the distribution of organic compounds in the system before and after the reaction were compared and analyzed,as well as the correlation between the catalytic performance and the relative content of various organic compounds.The main organic compounds in characteristic DOM components were identified through DOM classification characterization.The results showed that HPO（strong hydrophobic organic matter）was the main component,and there were more humic substances and benzene ring carbon.Meanwhile,the catalytic oxidation performance of DOM-Fe³⁺-PS-Vis system composed of HPO was better than the other two components.The relative content of benzene ring carbon and organic matter in DOM showed a high correlation with the catalytic performance,and its proportion would decrease after reaction,which was the main active site to promote the removal efficiency of the system.