Photocatalytic Reduction And Mechanism Of Hexavalent Chromium By ZnIn₂S₄ Based Composites

With the rapid development of society and economy,the pollution of industrial waste water,especially the heavy metal pollution in the production of chemical products,has become a hidden danger to the environment worldwide.The environmental form of heavy metal ion pollution of hexavalent chromium is becoming more and more serious,so the development of efficient and green reduction technology of hexavalent chromium wastewater has become an indispensable research direction.In this paper,ZnIn₂S₄ was selected as a photocatalyst,and two kinds of modified indium zinc sulfide materials,g-C₃N₄/ZnIn₂S₄ and Zn Fe₂O₄/ZnIn₂S₄,were prepared by constructing z-type heterostructure.With hexavalent chromium as the target pollutant,the removal performance of hexavalent chromium in the solution of two kinds of composite materials was studied.XRD,SEM,BET,XPS and other characterization methods were used to analyze the surface structure and elemental composition of the photocatalytic composite.The influence of material morphology,composition,specific surface area and elemental valence on the reaction process was investigated.The mechanism of photocatalytic reduction of Cr（VI）wastewater by composite materials was explored,and the reusability of photocatalytic materials was explored based on the needs of practical application.The main research achievements are as follows:（1）g-C₃N₄/ZnIn₂S₄ composites with different mass ratios were prepared by two-step hydrothermal method.In the dark reaction time of 6 h,the composite can adsorb 35.1%Cr（VI）,showing good Cr（VI）adsorption performance.Under visible light irradiation,the catalyst exhibits good reduction efficiency of Cr（VI）,and the experiment shows that the reduction efficiency of Cr（VI）can reach 100%in 1 hour when the mass ratio is 1:1.Combined with the results of inhibition experiments,it is found that·OH and·O₂^-exist and play an important role in photocatalytic reduction system.（2）Zn Fe₂O₄/ZnIn₂S₄ heterostructures with different proportions were successfully synthesized by two-step solvothermal method,and the prepared composite was flower-like structure.The adsorption and reduction properties of Cr（VI）were studied.The experimental results show that the composite with 40%mass ratio can adsorb 51.6%Cr（VI）at p H=3 for 90 min.The photocatalytic reduction efficiency of the composite material is also the highest at p H=3,and 99.9%Cr（VI）can be reduced under 50 min simulated visible light irradiation of Cr（VI）.The mechanism of photocatalytic reduction of composite materials was analyzed by electron spin resonance（ESR）spectral data combined with the results of inhibition experiment.（3）Oxalic acid was introduced into the system during the photocatalytic reduction of Cr（VI）of two ZnIn₂S₄ modified composite materials,and oxalic acid was used to activate the above two composite materials.The results showed that oxalic acid played an important role in the photocatalytic reduction of Cr（VI）.According to ESR technology and the quenching experiment of active substances,It is found that the addition of oxalic acid can produce·CO₂^-radical,which can not only reduce Cr（VI）to Cr（III）,but also promote the production of·OH and·O₂^-.For Zn Fe₂O₄/ZnIn₂S₄composites,in addition to the above reactions,organic ligands of Fe and oxalic acid are also generated,resulting in the formation of Fe（II）active substances in the system,which participate in the reduction process of Cr（VI）.