Degradation Of Tetracycline Hydrochloride By Electro-coupled Nanoscaled Magnetic CoFe₂O₄ Activated Persulfate Oxidation

In recent years,the residue of emerging pollutants such as pharmaceuticals and personal care products?PPCPs?in the environment has grown an increasingly serious problem.Among these,tetracycline antibiotics wastewater and its inhibition of microorganisms limited the application of traditional biological treatment processes.Sulfate radicals-based advanced oxidation processes?SR-AOPs?are the promising processes for the treatment of bio-refractory organic wastewater.Persulfate(PDS,S₂O₈^2-)is activated by means of transition metal ions(Fe²⁺,CO²⁺,etc.),heat and ultraviolet irradiation to generate sulfate radicals?SO₄�^-?.The SO₄�^-has a high redox potential?2.5-3.1 V?,which can degrade most organic pollutants.Homogeneous system of transition metal ion activated persulfate has most significant economic cost effectiveness.However,some problems remain unresolved,such as low utilization of catalyst because of metal ions easily oxidized or precipitated,secondary pollution to the environment because of excessive dosing,and the only acidic conditions.Therefore,it is urgent to solve these problems and thus improve the overall performance of catalytic oxidation system.In view of the problems above,this paper focuses on two aspects:the activity of the catalyst itself and persulfate activation method.On one hand,the nanoscaled CoFe₂O₄ particles are prepared as heterogeneous catalysts for the persulfate activation system to avoid the problems that the metal ion solution in the homogeneous system is easily oxidized and the pH application range is limited.The magnetic property of CoFe₂O₄ catalysts is utilized to simplify the catalyst separation process in the later stage of wastewater treatment.On the other hand,a new type of electro-activated persulfate oxidation is used to improve the catalytic oxidation activity of the reaction system.The synergistic effect in electro-coupled persulfate oxidation system is also emphatically revealed.In addition,this paper introduces electrochemistry into heterogeneous system of CoFe₂O₄ activated persulfate oxidation,in order to improve the utilization of catalysts and oxidants.The mechanism of electro-coupled CoFe₂O₄activated persulfate oxidation is revealed.Finally,the CoFe₂O₄ catalysts are applied to the self-made continuous flow tube reactor and continuous cell electrocatalytic reactor to investigate its actual treatment effect on tetracycline wastewater,which would provide guidance for the promotion and application of CoFe₂O₄ catalysts and the practical application of tetracycline wastewater treatment.The main contents and the relevant results of this paper are as followes:?1?Nanoscaled magnetic CoFe₂O₄ catalysts are prepared and its catalytic activity in activated persulfate oxidation system are evaluated.Combined with the metal ions dissolution from CoFe₂O₄ and the removal efficiency of tetracycline?TCH?in PDS/CoFe₂O₄ system,it is concluded that 600�C is the best preparation temperature of CoFe₂O₄ by sol-gel method,aiming to maintain its catalytic activity.The results from X-ray diffraction?XRD?,fourier transform infrared spectroscopy?FT-IR?,X-ray photoelectron spectroscopy?XPS?,scanning electron microscopy?SEM?,specific surface area?BET?,and H-M hysteresis loops indicate the successful preparation of nanoscaled magnetic CoFe₂O₄ catalyst.The prepared CoFe₂O₄ catalyst has high stability and catalytic activity in the activated persulfate oxidation system,and its magnetic property facilitates the recycling of the catalyst.?2?The effectiveness,mechanism and application of TCH degradation by CoFe₂O₄ activated persulfate oxidation are studied systematically.Firstly,the prepared nanoscaled magnetic CoFe₂O₄ catalysts are applied to the activated persulfate system to investigate the effect of main operation parameters on the degradation of tetracycline.The results show that the optimal conditions for 25 mg L^-1of tetracycline wastewater treatment are 4.20 mM of initial PDS concentration,1 g L^-1of catalyst dosage and natural pH?4.42?.The reaction system maintains the high removal efficiency of TCH in a wide range of pH?3-8?.The increase in temperature promotes the treatment effect of tetracycline wastewater,and the activation energy required for TCH oxidation in PDS/CoFe₂O₄ system is 14.95 kJ mol^-1,which is much less than the activation energy in heat-activated PDS system(75.55 kJ mol^-1),indicating that the oxidative degradation process in PDS/CoFe₂O₄ system is easier to carry out.Secondly,the recycling and stability of nanoscaled magnetic CoFe₂O₄catalysts are evaluated.Results show that the catalysts still maintain high catalytic activity and stable spinel structure after repeated use for 10 times,indicating that the catalysts have highly stable activity and can be recycled and reused.Thirdly,the catalytic mechanism of CoFe₂O₄ activated persulfate oxidation system is systematically discussed.The results show that the CoFe₂O₄ activated persulfate oxidation system is mainly a heterogeneous reaction process,and the primary radical species in the PDS/CoFe₂O₄ system are sulfate radical?SO₄�^-?and hydroxyl radical?�OH?.The XPS spectra of the surface elements of the catalyst before and after the reaction indicate that the low-valent metal ions on the surface of catalyst provide electron to activate PDS and thus generate SO₄�^-,and the participation of catalyst surface crystal oxygen(O^2-)and adsorbed oxygen?O²?ensures the recycling of Co²⁺-Co³⁺-Co²⁺on catalyst surface.Finally,CoFe₂O₄ catalysts are applied to the self-made continuous flow tube reactor to achieve rapid immobilization and application of the catalyst,and it achieves the good treatment effect on tetracycline wastewater.The removal efficiency of 25 mg L^-1 of tetracycline wastewater is stable at more than 90%with 31 min of hydraulic retention time.?3?The effectiveness and mechanism of TCH degradation by electro-activated persulfate oxidation are studied systematically.Firstly,this new method of persulfate activation is used to investigate the activation effect and the treatment effect on tetracycline wastewater,and results indicate that a significant synergistic effect is found by coupling electrolysis with persulfate oxidation,which is mainly due to the electron activation of persulfate to produce strong oxidant SO₄�^-.Secondly,the influence of various operation parameters,including current density,initial persulfate concentration and initial pH are investigated systematically.Results show that increasing current density and PDS dosage in a certain range can increase the TCH removal efficiency,but excessive current density and PDS dosage are not conducive to the removal of TCH.Peracid or overbase is not conducive to the removal of TCH.Under the optimal condition:25�C,natural pH?4.42?,12.6 mM of initial PDS concentration and 13.33 mA/cm² of current density,the TCH and TOC removal efficiency reached 82.0%and 31.3%respectively after 4h reaction,this indicating that electro-activated persulfate process can mineralize TCH to CO₂,which is a feasible method for the treatment of TCH polluted wastewater.Thirdly,the electrochemical reaction mechanism of synergistic oxidation of tetracycline in EC/PDS system is also revealed.The tests using ethanol?EtOH?and tert-butanol?t-BuOH?to capture free radicals indicate that SO₄�^-plays a dominant role in the EC/PDS system.Finally,the energy consumption of TCH degradation in the EC/PDS system is evaluated.In this paper,EE/O is defined as the electrical energy?in kilowatt hours?required to reduce the concentration of a pollutant by one order of magnitude?90%?in 1m³ of contaminated water.The EE/O value in EC/PDS system is only 11.48 kW h/?m³order?,which is much less than that in EC system?54.38 kW h/?m³ order??.?4?The effectiveness,mechanism and application of TCH degradation by electro-coupled CoFe₂O₄ activated persulfate oxidation are studied systematically.Firstly,electrochemistry is introduced into PDS/CoFe₂O₄ system to investigate the effect of current density,oxidant dosage,catalyst dosage,initial TCH concentration and initial pH on TCH removal.The results show that the removal efficiency of 50 mg L^-1 TCH solution reaches 87.0%after 4 h of reaction time under the conditions:20�C,natural pH?4.42?,8.4 mM of initial PDS concentration,6.67 mA/cm² of current density and 0.3 g L^-1 of CoFe₂O₄.Relative to PDS/CoFe₂O₄ system,the introduction of electrochemistry significantly improves the TCH removal,and the dosage of the catalysts required for the degradation of the same amount of tetracycline is significantly reduced.Relative to EC/PDS system,the addition of CoFe₂O₄ catalysts greatly improves the TCH removal,and the oxidant dosage and current density required for the degradation of the equivalent amount of tetracycline are significantly reduced.The energy consumption of TCH degradation in the EC/CoFe₂O₄/PDS system gets smaller,and the related EE/O value is 3.14 kW h/?m³ order?,this indicating that the EC/CoFe₂O₄/PDS system can more effectively achieve the treatment of tetracycline wastewater.Secondly,the electrochemical reaction mechanism of synergistic oxidation of tetracycline in EC/CoFe₂O₄/PDS system is releaved.The primary radical species in the EC/CoFe₂O₄/PDS system is sulfate radical?SO₄�^-?.The XPS spectra of the surface elements of the catalyst before and after the reaction indirectly prove cathodic reduction of metal ions on the surface of catalyst.This also indicates that the introduction of electrochemistry promotes the recycling of metal ions on the catalyst surface and improves the utilization of the catalyst.Thirdly,the stability of nanoscaled magnetic CoFe₂O₄ in the EC/CoFe₂O₄/PDS system is evaluated,and results show that the catalysts still maintain high catalytic activity after repeated use for 10 times.Finally,CoFe₂O₄catalysts are applied to the self-made continuous cell electrocatalytic reactor to analyze the actual treatment effect on tetracycline wastewater.The results show that the removal efficiency of 50 mg L^-1 of tetracycline wastewater is stable at more than90%with 42 min of hydraulic retention time,and meanwhile the related TOC removal efficiency reaches 58.2%,indicating that the electro-coupled CoFe₂O₄ activated persulfate technology is an efficient and feasible wastewater treatment technology.