Microwave Hydrothermal Synthesis Of Fe???-ZnS/g-C₃N₄ And Its Visible-light Photocatalytic Performnace And Mechanism

Photocatalysis is a green,safe and effective method for the removal of refractory organic pollutants from water.However,several defects,such as low catalytic efficiency under visible light irradiation,severe loss of catalyst powders,and ambiguous photocatalytic mechanism,need to be solved in the traditional photocatalysis technology.This study aimed to develop a novel kind of visible-lightresponsive Fe???-ZnS/g-C₃N₄?FZCN?hybrid catalyst with high catalytic activity through Fe doping and g-C₃N₄ loading on the basis of zinc sulfide?ZnS?,when pnitrophenol?PNP?,as a typical refractory organic pollutant in water,was used as the target pollutant.The effects of microwave hydrothermal synthesis and modification approach on the structure and property of the hybrid were systematically investigated.Moreover,a visible-light photocatalysis-microfiltration treatment process was constructed for synchronous efficient degradation of PNP and rapid separation of the catalyst powders.In addition,the PNP degradation mechanism by the FZCN hybrid catalyst under visible light irradiation was elucidated according to the analyses of degradation intermediates and reactive oxygen species?ROS?.The visible-light responsive Fe???-ZnS photocatalyst was successfully synthesized by the microwave hydrothermal method.In comparison with the synthetic time of traditional hydrothermal method?10 h?,the Fe???-ZnS catalyst was synthesized by the microwave hydrothermal method at 180 ??the same temperature in the traditional method?with synthetic time of only 60 min.Thus,the synthetic efficiency was improved greatly.In the meantime,the catalyst obtained by microwave heating had larger specific surface area and higher photocatalytic activity than that by the traditional heating,indicative of the superiority of microwave hydrothermal method on the syntheses of inorganic nano-sized catalytic materials.Results showed that the optimal synthetic conditions included Fe doping amount of 4%,microwave hydrothermal reaction temperature of 180 ?,and reaction time of 60 min,with PNP as the target pollutant.The optimal Fe???-ZnS catalyst exhibited the highest PNP degradation efficiency of 93.2% with a reaction rate constant of 0.0239 min^-1 and an energy consumption of 1495 k Wh·m^-3·order^-1 within 60 min under visible light irradiation when the H2O2 concentration was 34 mg·L^-1,catalyst dosage was 0.8 g·L^-1,initial PNP concentration was 10 mg·L^-1,and initial pH was approximately 6.The experimental results in combination with quantum chemical calculations demonstrated that Fe doping did not change the crystallinity pattern of ZnS,which were all cubic sphalerite structures.But an impurity level was introduced into the ZnS bandgap,which could play a transitional role during electron transition and reduce the required excitation energy.Thus,its absorption edge redshifted to the visible region,indicating its visible-light responsive capacity.The bandgap was decreased from 3.44 eV to 2.53 eV.The photo-induced electrons of Fe???-ZnS promoted the redox cycle of Fe3+/Fe2+ with both visible-light irradation and H2O2 in the system.The ROS was generated through the visible-light photocatalytic and Fenton-like reactions,thus the PNP removal rate was increased.To further improve the catalytic activity of Fe???-ZnS and reduce the energy consumption accordingly,the visible-light responsive Fe???-ZnS/g-C₃N₄?FZCN?hybrid photocatalyst was successfully synthesized by the microwave hydrothermal method with Fe???-ZnS loading onto the g-C₃N₄ surface.The PNP removal rate was 90.7% with a reaction rate constant of 0.0406 min^-1 and decreased energy consumption of 840 k Wh·m^-3·order^-1 by the FZCN-0.20 sample when the g-C₃N₄ loading mass ratio of 0.20 under the same reaction conditions as mentioned before.The g-C₃N₄ loading effectively decreased the aggregation of Fe???-ZnS nanoparticles and improved its surface area from 23.50 m²·g^-1 to 39.26 m²·g^-1,which increased the reactive sites for the contact between catalyst and pollutant,thus improved the PNP removal.According to the investigation of influencing factors on PNP degradation efficiency,the PNP removal rate within 60 min by the FZCN-0.20 hybrid catalyst was 96.0% with further reduced energy consumption of 616 k Wh·m^-3·order^-1 under the conditions of initial PNP concentration of 10 mg·L^-1,initial pH of approximately 6,H2O2 concentration of 68 mg·L^-1,and catalyst dosage of 0.8 g·L^-1.Due to the features of the polyvinylidene fluoride hollow fiber microfiltration membrane module,such as proper pore size,resistence to light irradiation and oxidation,and good mechanical property,a visible-light photocatalysismicrofiltration treatment process was constructed.When the influent PNP concentration was 10 mg·L^-1,initial pH was 5,catalyst dosage was 1.0 g·L^-1,H2O2 concentration was 170 mg·L^-1,aeration rate was 0.50 m3·h^-1,and operation time was 4 h,the PNP removal rate was higher than 90% under visible light irradiation.The rejection rate of the FZCN-0.20 hybrid catalyst by the microfiltratin membrane was 100%,which realized the rapid separation of the suspended catalyst powders,and avoided the catalyst loss and secondary pollution.The mechanism of PNP degradation by the FZCN hybrid catalyst under visible light irradiation was elucidated based on the analyses of ROS and degradation pathway.According to the Pearson's correlation analysis,the strong correlation between the surface area of FZCN and the reaction rate constant indicated that the increase of surface area was crucial for improving the removal efficiency of PNP.The band structure analyses of g-C₃N₄ and Fe???-ZnS showed that a terraced electron transfer mode formed inside the hybrid catalyst?type-? heterojunctions?,which enabled the electrons of g-C₃N₄ to be accepted by Fe???-ZnS,and facilitated the redox cycle of Fe³⁺/Fe²⁺ and the generation of ROS.The electron spin resonance results,fluorescence detection and trapping experiments verified that the main ROS of PNP degradation by the FZCN hybrid catalyst was hydroxyl radical,but superoxide radical,electrons,and holes contributed to the reaction as well.On the basis of the detection of degradation intermediates by high performance liquid chromatograph tandem mass spectrometer?HPLC-MS/MS?and the quantum chemical calculations of molecular active sites,the decomposition of PNP molecules involved denitrification and hydroxylation reactions.The possible intermediates included hydroquinone,p-benzoquinone,1,2,4-trihydroxybenzene,p-nitrocatechol,fumaric acid,maleic acid,glyoxylic acid,and acrylic acid.The toxicity prediction results of PNP and its intermediates demonstrated that the toxicity of the water sample was decreased after the degradation treatment.