Preparation Of MgFe₂O₄-SiC Complex Catalyst And Microwave-induced Catalytic Degradation Of Direct Black BN In Aqueous Solutions

In recent years,with the rapid development of printing and dyeing industry,a large number of dyestuff wastewater is discharged into environmental water,not only leading to the decline in environmental water quality,but also posing a potential hazard to human health.As one of advanced oxidation technology,microwave-induced catalytic oxidation technology has received more and more attention.Due to its rapid reaction,high catalytic efficiency and no secondary pollution,it is widely used in the treatment of dye wastewater.And the catalyst has an important influence on the reaction.Thus,it is very important to select the catalyst with the capability to absorb microwave energy strongly in the microwave induced catalytic reaction.A novel and efficient microwave catalyst—magnesium ferrite supported on silicon carbide,is prepared in this study.And azo dye Direct Black BN is chosen as the target to investigate microwave-induced catalytic degradation property of the material.The main research contents are as follows:(1)MgFe2O4and MgFe2O4-SiC were successfully synthesized by sol-gel method.MgFe2O4-SiC for the best catalytic activity was prepared by optimizing the preparation conditions.XRD analysis showed that the corresponding characteristic diffraction peaks of MgFe2O4 and MgFe2O4-SiC appeared,and the particle sizes of MgFe2O4 and MgFe2O4-SiC were 25.39 nm and 19.58 nm,respectively.As can be seen from SEM and TEM images,MgFe2O4 was chain-like spherical particles.MgFe2O4was uniformly supported on the surface of SiC and a large number of pores existed in MgFe2O4-SiC.Moreover,the specific surface areas calculated by BET equation for MgFe2O4 and MgFe2O4-SiC were found to be 26.2 and 105.3 m2/g,respectively,and their pore size belonged to the mesoporous range.Zeta potential measurements indicated that the isoelectric points of MgFe2O4 and MgFe2O4-SiC were in the vicinity of pH 4.5 and 3.6,respectively.XPS analysis showed that the two phases of MgFe2O4-SiC were well combined together,and the valence state of the elements were not changed.In addition,the dielectric constant(real and imaginary)of MgFe2O4-SiC were much higher than that of MgFe2O4.The dielectric loss of the composite was greatly enhanced when MgFe2O4 was loaded on SiC.The permeability(real and imaginary)of MgFe2O4 and MgFe2O4-SiC were high in the low frequency band and the values decreased with the frequency increased.The materials exhibited certain magnetic loss.(2)Degradation experiments of azo dye Direct Black BN showed that after 5 minutes of reaction the removal of DB BN in four systems(separate microwave radiation,MgFe2O4-SiC,microwave-MgFe2O4 system and microwave-MgFe2O4-SiC system)were 6%,17.7%,42.3%and 96.5%,respectively.MgFe2O4-SiC presented higher degradation efficiency of DB BN and RBR X-3B(Brilliant Red X-3B)than that of MnFe2O4-SiC.The effects of initial pH,MgFe2O4-SiC dosage and MW power on DB BN degradation were investigated,respectively.It can be viewed that MgFe2O4-SiC presented a broad pH range;the optimal conditions for dosage of MgFe2O4-SiC and microwave power were 1.5 g L-1 and 800 W,respectively.The removal of TOC was 65%in this system,and most of DB BN molecules were degraded.The change of the biological toxicity of the reaction solution showed that the toxicity of this system obviously decreased,and the inhibiting effect to Photobacteriumphosphoreum T3 reached 11%far below the initial inhibition of 32%.Moreover,DB BN degradation efficiency after 10 runs still reached 80.2%,and the crystal structure of MgFe2O4-SiC after 10 runs hardly changed.MgFe2O4-SiC presented high removal for five different kinds of dyes under microwave radiation,Therefore,MgFe2O4-SiC possessed excellent microwave catalytic ability.(3)Based on the results of microwave absorbing properties of the materials,the reflection loss of MgFe2O4 were greater than 5 dB in the whole frequency range,while the maximum reflection loss of MgFe2O4-SiC was less than-10 dB.The maximum absorption value was 13.32 dB with 5.0 mm thickness and the corresponding frequency was 2.57 GHz,which was close to microwave frequency of 2.45 GHz.Hence,the microwave absorbing capacity of MgFe2O4-SiC was significantly enhanced.The microwave absorbing ability of MgFe2O4-SiC was mainly due to the dielectric loss of the material and MgFe2O4-SiC absorbed more than 90%of the microwave energy for the degradation of DB BN,so that the degradation and mineralization efficiency increased.The maximum reflection loss and efective bandwidth of MgFe2O4-SiC were higher than MnFe2O4-SiC.Thus MgFe2O4-SiC had better microwave absorbing performance compared with MnFe2O4-SiC,which was in accord with the result of microwave catalytic degradation.(4)To determine the main active oxygen species during the degradation,tert-butyl alcohol and sodium oxalate as·OH scavenger and h+scavenger were added in the reaction solutions,respectively.The results showed that ·OH and h+were the main active species in the reaction system,playing an important role in the degradation of DB BN.Moreover,·OH played a more critical role compared with h+ in this system.They can react with DB BN,thus promoting its rapid and efficient degradation to be small molecular substances and inorganic ions.By means of ion chromatography,GC-MS and LC-MS analysis,the degradation products of DB BN were determined,and the degradation pathway of DB BN in the microwave-induced catalytic system was analyzed.(5)Based on the above research results,the degradation mechanism of DB BN in the microwave-induced catalytic system was proposed.With microwave radiation,SiC,which is known as the typical dielectric,can strongly absorb microwave energies.Then,these energies could be quickly transferred to the MgFe2O4 particles on the surface of SiC.In consequence,lots of hot spots were generated on its surface simultaneously with the surface temperature of MgFe2O4-SiC increasing,causing numerous activated sites and holes.Hence,the electron hole pairs can be generated to react with O2 and H2O in this system to form active species(e.g.·OH),which participated in the degradation of DB BN.Therefore,the microwave-induced catalytic degradation of DB BN could be enhanced significantly.