Photocatalytic Reduction Of Cr⁶⁺ By G-C₃N₄/Mesoporous TiO₂ Under Sunlight Simulation

Heavy metal ion Cr⁶⁺mainly comes from mining,metallurgy,electroplating,tanning,dichromate chemical production and chromium slag treatment and other industries.In the process of chemical production or waste slag treatment,a large amount of chromium-containing wastewater will be produced in these industries.The discharge of chromium-containing wastewater that is not up to the standard will cause serious pollution to the water environment,soil environment and ecological environment.TiO₂ is an environmentally friendly photocatalytic material.Under ultraviolet irradiation,extremely reductive photogenerative electrons are generated on the surface,which can efficiently reduce highly toxic Cr⁶⁺to non-toxic Cr³⁺with good environmental compatibility.In the photocatalytic reaction of TiO₂ reduction of Cr⁶⁺,there is no need to add other reagents to the reaction system,so the reaction cost,follow-up treatment and environmental secondary pollution and other potential hazards are greatly reduced.Although TiO₂ has strong photocatalytic performance,it still has the following two fatal shortcomings;（1）Can only absorb 5%of the ultraviolet light in the sunlight,visible light utilization rate is low;（2）Photoelectron-hole pairs are easy to recombine,and the photocatalytic efficiency is reduced.These shortcomings limit the practical application of photocatalytic reaction in wastewater treatment.In order to establish an efficient photocatalytic technology for the treatment of chrome-containing wastewater,we designed a g-C₃N₄-doped TiO₂ catalyst（g-C₃N₄/TiO₂）for the photocatalytic reduction of Cr⁶⁺in water.g-C₃N₄ was prepared by thermal condensation method.The optimal experimental conditions were obtained by single factor experiment:the calcination temperature was450℃,the calcination time was 3 h and the heating rate was 10℃/min.Mesoporous TiO₂ was prepared by sol-gel method.The optimal experimental conditions were obtained by orthogonal experiment:calcination temperature 450℃,calcination time 2h,CTAB addition 0.15 g,pH 2,and the order of influence of experimental conditions on photocatalytic reduction efficiency was D pH>A calcination temperature>C CTAB addition>B calcination time.g-C₃N₄/mesoporous TiO₂ was prepared by sol-gel blending method,and the optimal doping ratio between g-C₃N₄ and mesoporous TiO₂ was 1:5.Through the characterization information of FTIR,XPS,SEM and TEM,it was found that mesoporous TiO₂ was successfully supported on the surface of g-C₃N₄,and the crystal structure of mesoporous TiO₂ was not affected.XRD patterns showed that TiO₂ in the prepared mesoporous TiO₂ and g-C₃N₄/mesoporous TiO₂belonged to anatase type,and the ultraviolet-visible diffuse reflection spectra confirmed that the spectral response of g-C₃N₄/mesoporous TiO₂ showed obvious redshift phenomenon,with a band gap of 2.8 e V.The BET calculation shows that the specific surface area of g-C₃N₄/mesoporous TiO₂ is 137.061 m²/g,the pore volume is0.512 cm³/g,and the average pore size is 7.47 nm.Transient photocurrent,open-circuit potential,electrochemical impedance spectroscopy and linear voltammetry scanning curve test results all show that g-C₃N₄/mesoporous TiO₂ has higher photocurrent density,higher photoconversion efficiency,higher photocarrier separation efficiency,smaller charge transfer resistance and better conductivity.Mott-schottky curve analysis shows that the photogenerated carrier density of g-C₃N₄/mesoporous TiO₂ is 1.01×1024 cm^-3.The influence of different reaction conditions on the photocatalytic efficiency of g-C₃N₄/mesoporous TiO₂ simulated reduction of Cr⁶⁺under daylight was studied,and the optimal reaction conditions were obtained:The initial pH of the reaction solution was 3,no inorganic anions existed,the dosage of g-C₃N₄/mesoporous TiO₂ was 1.6g/L,and the initial concentration of Cr⁶⁺was 0.2 mg/L.Under the optimal photocatalytic reaction conditions,the photocatalytic degradation efficiency of Cr⁶⁺was up to 98%.The co-existence of humic acid can improve the photocatalytic reaction efficiency,and the acidic environment can improve the photocatalytic reduction efficiency of Cr⁶⁺by g-C₃N₄/mesoporous TiO₂.The pH of solution increases slightly with the increase of reaction time.The cyclic service life of g-C₃N₄/mesoporous TiO₂ was tested,and it was found that the photocatalytic activity of g-C₃N₄/mesoporous TiO₂ decreased with the increase of The Times of catalyst reuse,and the reduction efficiency of Cr⁶⁺was still more than 80%after 7 times of reuse.The mechanism of photocatalytic reduction of Cr⁶⁺simulated by g-C₃N₄/mesoporous TiO₂ under sunlight was explored:（1）Cr⁶⁺was adsorbed on the surface of g-C₃N₄/mesoporous TiO₂,（2）g-C₃N₄/mesoporous TiO₂ produced photoelectron-holes under simulated solar irradiation,（3）Cr⁶⁺adsorbed on the surface of g-C₃N₄/mesoporous TiO₂ was reduced to Cr³⁺by photoelectron.The kinetic equation of photocatalytic reduction of Cr⁶⁺by g-C₃N₄/mesoporous TiO₂ under sunlight was established,and the influence of reaction conditions on the reaction rate constant was studied.the effects of three conditions on the apparent total reaction rate constants were investigated.The results are as follows:k=1.2825×10^-3C_H+^3.1649n^1.5113C₀^-1.9633...