Efficiency And Synergistic Mechanism Of Fenton-like System Based On Vacuum Ultraviolet For Degradation Of Emerging Organic Contaminants

Emerging organic contaminants（EOCs）in the effluent of municipal wastewater treatment plants have attracted much attention because of their adverse effects on the aquatic environment and water safety.Currently,wastewater treatment plants cannot effectively remove EOCs and EOCs were frequently detected in the secondary effluent.Therefore,there is an urgent need to develop high-efficiency technologies combined with existing wastewater treatment processes to degrade EOCs.Ultraviolet（UV）irradiation was commonly used for wastewater disinfection,but the removal of EOCs by UV was very limited.Compared with UV,vacuum ultraviolet（VUV）can split water to produce reactive oxygen species（ROS）and quickly remove EOCs,and VUV irradiation can be achieved by simply replacing the UV lamp.The uses of new VUV-based oxidation technologies in the UV disinfection process not only retain the advantages of UV but also efficiently degrade EOCs,which are of great significance to the advanced treatment of sewage and wastewater.In this paper,fluoroquinolones（norfloxacin（NOR）,ciprofloxacin（CIP）and pefloxacin（PEF））,sulfadiazine（SDZ）,and tetracycline（TTC）were used as model pollutants for EOCs.Based on commercial VUV lamps,for the purpose of improving process oxidation performance and broadening applicable p H,VUV-based Fenton-like oxidation systems were gradually constructed to reveal the ROS conversion pathways and synergistic mechanisms in these systems,and to explore the applicability of the constructed systems.First of all,in view of the inefficient removal of EOCs by UV,the mechanisms of in-situ generation of ROS by VUV irradiation to accelerate EOCs removal were explored.Then,due to the in-situ accumulation of H₂O₂during the water splitting by VUV irradiation,Fe²⁺was introduced to construct the VUV/Fe²⁺process to improve the oxidation performance.Next,since VUV/Fe²⁺process was suitable for acidic p H,oxalic acid（Ox）was introduced to construct VUV/Fe²⁺/Ox process to broaden the reaction p H and improve the oxidation efficiency.Further,since the introduction of Ox in VUV/Fe²⁺/Ox increased DOC concentration of influent water and VUV/Fe²⁺/Ox process was suitable for near neutral p H,a new type of VUV/Fe²⁺/H₂O₂process was constructed to broaden the reaction p H and improve the mineralization efficiency.Finally,since the oxidation performance of sulfate radical(SO₄^·-)was better than that of hydroxyl radical（HO^·）,VUV/Fe²⁺/persulfate process with a wider applicable p H and stronger oxidation performance was constructed.The main conclusions of this work were as follows:（1）The degradation efficiencies of NOR,CIP and PEF by VUV irradiation were significantly higher than that of UV.The in-situ generation of ROS from VUV irradiation splitting water was the key to accelerate EOCs degradation.The in-situ generated H₂O₂ was mainly formed through VUV photons cracking of dissolved oxygen,the self-recombination reaction of HO^·,and the self-recombination reaction of hydroperoxy radical（HO₂^·）/superoxide radical(O₂^·-).HO^·was mainly generated through the splitting of water by VUV photons,the decomposition of H₂O₂ by UV,and the reaction of H₂O₂with hydration electrons(e^-_aq).And HO₂^·/O₂^·-mainly came from the disproportionation reaction between H₂O₂ and HO^·.The in-situ generation of H₂O₂ and ROS under VUV irradiation were in a dynamic equilibrium state of mutual conversion.H₂O₂ generated in situ could not directly remove EOCs,but it was an important source of ROS.（2）Compared with VUV irradiation,the VUV/Fe²⁺process accelerated the degradation and mineralization efficiencies of NOR,CIP,and PEF at acidic p H,and saved 37.12%of energy consumption.The VUV/Fe²⁺process had a significant synergistic effect,and the synergistic factors for degradation of NOR,CIP,and PEF were 1.74,1.98,and 1.87,respectively.The acidic p H and a certain amount of dissolved oxygen were beneficial to the degradation of EOCs by VUV/Fe²⁺process.Through fluorescent molecular probes and free radical capture experiments,it was proved that HO^·and HO₂^·were the main ROS in VUV/Fe²⁺process,and the relative contributions of HO^·and HO₂^·were 68.62%and 28.15%,respectively.The degradation pathways of NOR were also recommended,and the addition of Fe²⁺accelerated the defluorination and the conversion of organic nitrogen to inorganic nitrogen during NOR degradation by VUV irradiation.（3）Compared with its sub-processes,VUV/Fe²⁺/Ox process accelerated the degradation and mineralization of NOR,and the addition of Ox did not increase the DOC concentration in the effluent.The optimized conditions for VUV/Fe²⁺/Ox process were 180μM Ox,45μM Fe²⁺,and p H=5.Ox promoted the in-situ generation of H₂O₂from VUV irradiation,which was utilized by Fe²⁺to improve the performance of VUV/Fe²⁺/Ox process.HO^·and O₂^·-were identified as the main ROS in VUV/Fe²⁺/Ox process,and the relative contributions of HO^·and O₂^·-were 73.17%and 20.91%,respectively.In VUV/Fe²⁺/Ox process,the degradation pathways of NOR included piperazine ring conversion,quinolone ring conversion and defluorination,and the toxicity evaluation of by-products of NOR showed that VUV/Fe²⁺/Ox process reduced the environmental toxicity of NOR.The coexisting Cl^-,NO₃^-,SO₄^2-and DOM would inhibit NOR degradation as the concentration increased.（4）Compared with its subsystems,VUV/Fe²⁺/H₂O₂process can not only increase the pseudo-first-order reaction rate constant of NOR removal by 2.3-14.9 times and increase the mineralization by 20.4%-59.4%,but also reduce the residual ratio of H₂O₂by 19.9%-70.1%and reduce total cost by 20.0%-68.0%.The synergistic effect in the VUV/Fe²⁺/H₂O₂process was attributable to the fact that VUV irradiation promoted the iron redox cycle and H₂O₂ decomposition.The optimized parameters of VUV/Fe²⁺/H₂O₂process were determined to be 3 m M H₂O₂ and 90μM Fe²⁺.Through free radical capture and fluorescent molecular probe methods,HO^·and O₂^·-were identified as the main ROS,and their relative contributions to NOR degradation were79.07%and 18.47%,respectively.The by-products of NOR degradation and their environmental toxicity were also studied.The coexisting ions Cl^-,NO₃^-,SO₄^2-,Na⁺,Ca²⁺,Mg²⁺（0-50 m M）basically did not affect NOR degradation in VUV/Fe²⁺/H₂O₂process.（5）Compared with its sub-processes,the VUV/Fe²⁺/PS process accelerated the degradation and mineralization of NOR,SDZ and TTC at neutral p H.The optimized conditions of VUV/Fe²⁺/PS were 1.5 m M PS and 90μM Fe²⁺,and VUV/Fe²⁺/PS process had excellent performance in a wide range of p H（3-10）.In VUV/Fe²⁺/PS process,Fe²⁺,e^-_aq,and VUV synergistically activated PS and in-situ H₂O₂.Both VUV and e^-_aq can accelerate the conversion of Fe³⁺to Fe²⁺.HO^·and SO₄^·-were identified as the main free radical species,singlet oxygen and Fe（IV）were identified as the main non-radical species,and the main free radical species(HO^·and SO₄^·-)had a dominant contribution（over 60%）to the degradation of NOR,SDZ and TTC in VUV/Fe²⁺/PS process.During NOR degradation in VUV/Fe²⁺/PS process,F^-,NO₃^-and NH₄⁺were the main inorganic products,and the degradation pathways of NOR were piperazine ring conversion,quinolone ring conversion and defluorination.The toxicological evaluation of degradation by-products showed that VUV/Fe²⁺/PS process significantly reduced the environmental toxicity of NOR.（6）Considering the application results of various oxidation processes in actual water,VUV irradiation was expected to replace traditional UV in water and wastewater treatment.VUV/Fe²⁺process was suitable for advanced treatment of acidic organic wastewater,and the VUV/Fe²⁺/Ox process would rapidly degrade NOR at near neutral p H.The VUV/Fe²⁺/H₂O₂ and VUV/Fe²⁺/PS processes have realized the rapid removal of EOCs in the secondary effluent of wastewater treatment plant at neutral and weakly alkaline p H,and the total iron and sulfate content are far below the threshold in the national standard GB/T 31962-2015 of China.Therefore,the secondary effluent of wastewater treatment plant is an ideal application scenario for VUV/Fe²⁺/H₂O₂ and VUV/Fe²⁺/PS processes.In order to solve the problems that the wastewater treatment plant cannot effectively remove EOCs and the EOCs were frequently detected in the secondary effluent,new VUV-based Fenton-like oxidation systems that are easy to be combined with the UV disinfection unit have been constructed.In the processes without adding oxidant(VUV,VUV/Fe²⁺,VUV/Fe²⁺/Ox process),the process mechanisms of the in-situ generation of ROS and H₂O₂ from VUV irradiation splitting water were revealed.In the systems with external oxidant(VUV/Fe²⁺/H₂O₂,VUV/Fe²⁺/PS process),the mechanisms of synergistic activation of oxidant by VUV and Fe²⁺were elaborated.In addition,the influence mechanisms of typical process parameters on the constructed VUV-based Fenton-like oxidation systems were clarified,and its applicability to EOCs degradation in complex water matrices were evaluated.The VUV/Fe²⁺/H₂O₂ and VUV/Fe²⁺/PS processes were recommended for advanced treatment of secondary effluent from wastewater treatment plants.