Preparation Of CuFe₂O₄/SEP And Its Multi-effect Catalytic Performance On Degradation Of Nitrogen Heterocyclic Compounds

Coal chemical wastewater contains many nitrogen heterocyclic compounds such as quinoline,indole,pyridine,carbazole and their derivatives,which are not easily removed by biological metabolism,so that nitrogen heterocyclic substances still exist in secondary biochemical effluent of coal chemical wastewater.At present,the focus of coal chemical wastewater treatment has been gradually transformed from the past standard discharge to advanced treatment and reuse,i.e.,the effluent still needs advanced treatment after A/A/O biochemical treatment.Advanced oxidation technology has attracted extensive attention as many highly active free radicals can be produced in the process,which can oxidize or even mineralize organic pollutants rapidly.Ozone oxidation technology,Fenton oxidation technology and photocatalysis technology are commonly used in water treatment,disinfection,reuse and so on.The development of highly efficient and stable catalysts is the key to improving ozone catalytic oxidation,heterogeneous degradation and photocatalytic oxidation efficiency.A large number of ozone catalysts,heterogeneous catalysts and catalysts for enhanced photocatalytic oxidation have been developed.However,the catalytic performance of these catalysts is relatively fixed/single:Ozone catalysts are mainly used for catalytic ozone decomposition to produce free radicals;the Fenton catalysts are mainly used to catalyse the decomposition of hydrogen peroxide to produce free radicals;and the catalysts with enhanced photocatalytic activity are only suitable for photocatalytic technology.Once these catalysts are applied to other advanced oxidation processes,they will lose original catalytic capacity.Therefore,it is very important to develop a new catalyst which can be applied in all three processes:ozone catalytic oxidation,heterogeneous degradation and photocatalytic oxidation.In this paper,quinoline,a highly polluted organic substance in coal chemical wastewater,was studied as a target polhutant,and a catalytic ozone oxidation system was used to screen the catalysts.CuFe2O4/SEP composite material,which can be applied to catalytic ozonation system,catalytic UV system and photocatalytic system,has been developed.The preparation conditions of the catalyst were optimized and the CuFe2O4/SEP composites were characterized.The effects and mechanisms of CuFe2O4/SEP catalysed ozonation of quinoline,CuFe2O4/SEP catalysed UV-Fenton degradation of quinoline and CuFe2O4/SEP assisted photocatalytic oxidation of quinoline were studied.The operating parameters of each system were optimized,the reaction kinetics was analysed,the intermediate products of quinoline degradation in each system were identified,and the possible degradation pathways of quinoline in different processes were described.Four nano-spinels CuFe2O4,MnFe2O4,ZnFe2O4 and NiFe2O4 were prepared by sol-gel method.Among them,CuFe2O4 and NiFe2O4 catalysed ozone degradation of quinoline was more efficient.Six kinds of supported spinel ferrites were prepared by loading CuFe2O4 and NiFe2O4 on attapulgite,sepiolite and bentonite in a 1:1 loading ratio.Among them,CuFe2O4/SEP composite exhibited better catalytic performance.The preparation parameters such as loading ratio,calcination temperature,calcination time and ammonia dosage were optimized.CuFe2O4/SEP composites have better catalytic efficiency and higher mineralization degree when calcination temperature is 400?,calcination time is 3h,ammonia dosage is 10 mL and loading ratio is 1:1.CuFe2O4/SEP was used to catalyse ozonation of quinoline for 30 min.The removal rate of TOC was 90.2%.In CuFe2O4/SEP catalytic ozonation system,the quinoline degradation and TOC removal could be well fitted to pseudo first order kinetics.The apparent rate constant of quinoline degradation is 0.8931 min-1,and the apparent rate constant of TOC removal is 0.0885 min-1.The XRD,BET,SEM,EDS,XPS,FTIR and vibrating sample magnetometer were used to characterize the morphology,structure and element composition of the catalyst.The results show that CuFe2O4/SEP composites are mainly composed of elements such as Cu,Fe,Ca,Si,Mg and O.The surface of the composites is rough,compact and porous,and they also have good magnetic properties.Compared with pure CuFe2O4,CuFe2O4/SEP has larger specific surface area,abundant surface hydroxyl groups and more surface acid sites.It can improve catalytic performance by providing more active sites and promoting mass transfer of pollutants and ozone in the process of catalytic ozonation.The catalytic activity of CuFe2O4/SEP composite remained high after five times reuse in the catalytic ozonation of quinoline system.The removal rate of TOC reached 80.8%in 30 minutes,which is 4.8 times higher than that of using ozone alone,and is significantly higher than that of using CuFe2O4 as catalyst.The result shows that CuFe2O4/SEP composite material has good recyclability and stability.Compared with ozonation alone,CuFe2O4 catalyzed ozonation and sepiolite catalyzed ozonation,CuFe2O4/SEP catalyses ozonation degradation of quinoline with higher efficiency and faster reaction rate.The effects of ozone concentration,ozone flow rate,catalyst dosage,initial pH value of solution,initial concentration of quinoline and inorganic ions on the catalytic ozonation of quinoline by CuFe2O4/SEP were investigated.The TOC removal rate of CuFe2O4/SEP catalyzed ozonation of quinoline increased with the increase of catalyst dosage,ozone concentration and ozone flow rate,and decreased with the increase of initial quinoline concentration.CuFe2O4/SEP has a high removal efficiency of TOC under neutral and weak alkaline conditions,and its activity is inhibited slightly under acidic conditions,but it still has a significant catalytic activity.The Cl-,SO42-and NO3-had no significant effect on CuFe2O4/SEP catalytic ozonation system,while CO32-,HCO3-and PO43-had significant inhibition.Surface hydroxyl groups on CuFe2O4/SEP are active sites in catalytic ozonation.The effects of ozone concentration,catalyst dosage and initial pH value of solution on TOC removal in CuFe2O4/SEP catalytic ozonation of quinoline system were studied by three-factor and three-level orthogonal test,and the significance analysis was carried out to optimize the process parameters.Among them,the amount of catalyst has the most significant effect,followed by the initial pH value of the solution,and then the concentration of ozone.The operation parameters of CuFe2O4/SEP catalytic ozonation process were optimized by selecting the catalyst dosage of 2.0 g/L,ozone concentration of 6.7 mg/L and initial pH value of the solution as the original pH value.Concentrations of oxalic acid,acetic acid and formic acid in the system of ozonation of quinoline were determined by ion chromatography.The results show that the small molecule carboxylic acid could not be further oxidized in the ozonation system alone,resulting in the accumulation of small molecule acids,which greatly limited the removal of TOC.CuFe2O4/SEP has good catalytic activity for ozonation of saturated carboxylic acid,which is helpful to improve the mineralization efficiency of ozonation.It is confirmed by the free radical quenching experiment that the active oxygen substances in the process of CuFe2O4/SEP catalytic ozonation mainly include·OH,O3 and·O2-.The intermediate products of quinoline degradation in CuFe2O4/SEP catalytic ozonation system were identified by GC-MS,and the degradation pathway of quinoline was speculated.Quinoline is degraded by·OH,O3 and·O2-to form intermediates such as 2-pyridineformaldehyde,3-pyridineformaldehyde,N-phenylformamide and phenylpropanal.These intermediates oxidize and ring-opening to form small molecular acids such as oxalic acid,acetic acid and formic acid.Small molecules of acids can be further oxidized by·OH and·O2-to produce CO2 and H2O.The TOC concentration of secondary biochemical treatment of coal coking wastewater by CuFe2O4/SEP catalytic ozonation process for 60 minutes decreased from 66.90 mg/L to 28.22 mg/L.The corresponding TOC removal rate was up to 57.81%,which was 2.9 times higher than that of the ozonation system alone,and the natural fluorescence components in secondary biochemical effluent of coal coking wastewater could be completely degraded.Quinoline degradation efficiency of CuFe2O4/SEP UV process,UV hydrogen peroxide process,CuFe2O4/SEP UV-Fenton process and CuFe2O4/SEP heterogeneous phase process were compared.The kinetic data of each oxidation process could be well fitted to pseudo first order kinetics.The degradation efficiency of quinoline is 95.9%and the degradation rate is 0.1077 min-1 under the near neutral condition.The degradation rate of CuFe2O4/SEP is higher than that of other technologies.The effects of concentration of hydrogen peroxide,dosage of catalyst,initial pH value of solution and initial concentration of quinoline on UV-Fenton oxidation of Quinoline Catalyzed by CuFe2O4/SEP were analysed.The removal efficiency of quinoline in UV-Fenton system catalyzed by CuFe2O4/SEP increases with the increase of the concentration of hydrogen peroxide and the amount of catalyst added,and decreases with the increase of the initial pH value of solution and the initial concentration of quinoline.A three-level orthogonal experiment was conducted to determine the concentration of hydrogen peroxide,the dosage of catalyst and the initial pH value of the solution.The significance of the three factors was analyzed and the process parameters were optimized.The effect of initial pH value of solution on UV-Fenton oxidation of Quinoline Catalyzed by CuFe2O4/SEP was significant,but the effect of concentration of H2O2 and dosage of catalyst was relatively insignificant.Considering the cost of the process,the initial pH of the solution was chosen as the original pH of the sewage,the dosage of catalyst was 0.5 g/L,and the concentration of hydrogen peroxide was 10 mmol/L.The process parameters were optimized.After five consecutive runs of CuFe2O4/SEP in the UV system,the removal efficiency of quinoline by 30 min is still as high as 86.9%.It proves that CuFe2O4/SEP has good catalytic activity and stability in UV system.Tert-butanol was used as ·OH inhibitor,benzoquinone as·O2-scavenger,silver nitrate as e-quencher,and disodium ethylenediaminetetraacetate as h+capture agent.According to the results of free radical quenching experiments,the active substances in UV-Fenton system catalyzed by CuFe2O4/SEP mainly include·OH,·O2-,e-and h+.Among them,·OH contributes most to the UV-Fenton system catalyzed by CuFe2O4/SEP,followed by h+,·O2-,and finally e-.The intermediate products of quinoline degradation in CuFe2O4/SEP catalyzed UV-Fenton system were identified by GC-MS and ion chromatography.And the degradation pathway of quinoline was analyzed according to the intermediate products.In CuFe2O4/SEP catalyzed UV-Fenton system,the degradation process of quinoline is similar to that of CuFe2O4/SEP catalyzed ozonation.In the quinoline degradation process,intermediate products are produced such as phenylpropanal,2-pyridylformaldehyde,3-pyridylformaldehyde,N-phenylformamide,oxalic acid,acetic acid and formic acid.The intermediate products can be further oxidized to CO2 and H2O under the action of active substances.Ultraviolet irradiation can directly accelerate the decomposition of hydrogen peroxide to hydroxyl radicals,and induce the reaction between catalyst and quinoline with electrons producing.And electrons can accelerate the reduction of Fe3+to Fe2+on the surface of catalyst in solution.Moreover,ultraviolet irradiation can also promote the cycle of Cu(?)/Cu(?)and Fe(?)/Fe(?)which indirectly accelerates the decomposition of hydrogen peroxide and the production of active substances.The CuFe2O4/SEP composite has higher photocatalytic activity than CuFe2O4 and sepiolite.After 90 minutes of photocatalytic degradation of quinoline,the removal rate of CuFe2O4/SEP composite is 89.1%,and the corresponding apparent rate constants are 0.0246 min-1.With the increase of catalyst dosage,the number of active sites and free radicals in CuFe2O4/SEP-assisted photocatalytic system increased,which resulted in higher degradation rate of quinoline.However,too much catalysts also caused the scattering loss of light incident radiation.With the increase of the initial concentration of quinoline,the path length of photons entering the solution decreases,and quinoline adsorbs on the surface of CuFe2O4/SEP occupying the active sites,which limits the interaction between photons and CuFe2O4/SEP,leading to the decrease of photodegradation efficiency.When the pH value increased,the active substances produced increased,and the quinoline removal efficiency of CuFe2O4/SEP-assisted photocatalytic system increased.The effect of catalyst dosage on the activity of photocatalytic system is the most significant,followed by the initial pH value of the solution and the concentration of quinoline.The optimum technological conditions are catalyst dosage of 2 g/L,the original pH and concentration of wastewater are maintained considering the economic cost.According to the quenching experiments of active substances,four active substances were produced during the photocatalytic degradation of quinoline by CuFe2O4/SEP,including·OH,·O2-,e-and h+.Among them,·O2-and h+contributes a lot.·OH attacks the benzene ring with high charge density in quinoline molecule and produces quinoline-5-alcohol.·O2-attacks the pyridine ring with low electron density in quinoline molecule and produces intermediate products such as 2-aminobenzaldehyde,quinoline-4(1 H)-one and quinoline-2(1 H)-one.Macromolecular intermediates are further oxidized to ring-opening and small molecule acids are formed,then small molecule acids are thoroughly oxidized producing CO2 and H2O.Under visible light irradiation,the energy of light excites the valence band electrons of CuFe2O4,which jumps to the conduction band,resulting in the formation of vacancies in the valence band.CuFe2O4 loading on sepiolite increases the dispersion of CuFe2O4 and reduces the electron-hole recombination.Electron-hole react with water molecules,hydroxide ions and oxygen molecules,meanwhile·OH,·O2-and H2O2 are formed.Under the action of these active substances,organic pollutants are degraded.CuFe2O4/SEP composites still exhibit excellent photocatalytic properties after five cycles of experiments.