Preparation Of G-C₃N₄ Composite And Adsorption And Photocatalytic Treatment Of Crystal Violet Dye Wastewater

Graphic Carbon Nitride?g-C₃N₄?is widely used in the fields of adsorption and photocatalysis because of its graphite-like layer structure,lower band gap?².7 eV?,high chemical stability,easy-get raw materials and simple preparation conditions.The modification of g-C₃N₄ by surface control,doping,recombination,etc.can effectively solve the defects of low adsorption efficiency and high photogenerated electron-hole recombination to improve its adsorption and photocatalytic properties.At present,the recombination of g-C₃N₄ is mostly to add a semiconductor to reduce the recombination rate of photogenerated electron-holes,but there are few reports on regulating the adsorption and photocatalytic performance with non-semiconductor composites.In this paper,non-semiconductor alkaline earth metal carbonates?MCO₃=magnesium carbonate?MgCO₃?,calcium carbonate?CaCO₃?,strontium carbonate?SrCO₃?,barium carbonate?BaCO₃??and melamine?C₃H₆N₆?are used as raw materials to prepare g-C₃N₄ based alkaline earth metal carbonate composite by one-step calcination.By changing the amount of MCO₃ added,the adsorption and photocatalytic properties of MCO₃/g-C₃N₄ composites for crystal violet?CV?were effectively regulated.The high addition composites showed high adsorption performance on CV,while low addition composites showed photocatalytic performance on CV,which provides a new strategy for the efficient treatment of dyes.The main research contents and conclusions are as follows?1?Using melamine?C₃H₆N₆?and alkaline earth metal carbonates?MCO₃=MgCO₃,CaCO₃,SrCO₃,BaCO₃?as raw materials to preparation g-C₃N₄ based alkaline earth metal carbonate composite by one step calcination at different calcination times?t=3 h,4 h,5 h?,calcination temperatures?T=500°C,550°C,600°C,650°C,700°C?and mass ratios of raw materials?MCO₃:C₃H₆N₆=3:1,2:1,1:1,1:2,1:3?.X-ray Diffraction?XRD?,Fourier Transform Infrared Spectra?FT-IR?,Transmission Electron Microscopy?TEM?,Scanning Electron Microscopy?SEM?,N₂ adsorption-desorption curve?N₂-BET?,X-ray Photoelectron Spectra?XPS?,Zata potential were used to characterize the prepared adsorbent materials,and the structural characteristics of the materials were determined.The results of the characterization proved that g-C₃N₄ based alkaline earth metal carbonate composite can be successfully synthesized with alkaline earth metal carbonate and melamine by one-step calcination method,and the alkaline earth metal carbonate is complexed with g-C₃N₄ by non-covalent interaction.Among them,CaCO₃/g-C₃N₄,SrCO₃/g-C₃N₄ and BaCO₃/g-C₃N₄ adsorbent are binary complex of carbonate?CaCO₃,SrCO₃,BaCO₃?and g-C₃N₄;due to the pyrolysis of MgCO₃,MgCO₃/g-C₃N₄ contains a small amount of MgO in addition to MgCO₃ and g-C₃N₄.?2?The adsorption properties for crystal violet?CV?solution?10 mg/L?of the composite under varied calcination temperature?T?,calcination time?t?and raw material ratio were investigated to determine the best adsorption material.The results showed that the calcination temperature and the mass ratio of raw materials have great influence on the adsorption properties of the prepared composites,while the calcination time has almost no effect on the adsorption performance.For MgCO₃/g-C₃N₄,the optimal conditions for the adsorption performance are at a calcination temperature of550°C,a calcination time of 4 h,and a raw material mass ratio of 1:2?MgCO₃:C₃H₆N₆?.And for CaCO₃/g-C₃N₄,the optimal adsorption performance is exhibited at a calcination temperature of 650°C,a calcination time of 4 h,and a raw material mass ratio of 1:2?CaCO₃:C₃H₆N₆?;for SrCO₃/g-C₃N₄ and BaCO₃/g-C₃N₄,the optimum conditions for the adsorption performance are at a calcination temperature of 600°C,a calcination time of 4 h,and a mass ratio of 1:1?SrCO₃:C₃H₆N₆?and?BaCO₃:C₃H₆N₆?;?3?The effects of the reaction time,dosage of adsorbent and initial pH of the solution on the adsorption of CV for the best performed materials in each group were systematically investigated.The results showed that within 140 min,the composite could reach the adsorption equilibrium for different concentrations of CV.Among them,CaCO₃/g-C₃N₄ showed the best adsorption performance.Under the condition of 0.5 g/L dosage,the removal rate of CV at 1600 mg/L with CaCO₃/g-C₃N₄ was close to 100%in20 min.Under the condition of 0.5 g/L dosage,the removal rate of CV at 1600 mg/L with SrCO₃/g-C₃N₄ was close to 98%in 120 min.The optimum dosage of the other two composite adsorbent materials was 1g/L,the removal rate of CV at 600 mg/L with BaCO₃/g-C₃N₄ was 90%at 80 min;the removal rate of CV at 300 mg/L with MgCO₃/g-C₃N₄ was 95%at 140 min.The initial pH of the solution has little effect on the adsorption process of CV,indicating that the composite material is suitable for adsorption treatment of CV solution with a wide range of acid and alkali solutions.Due to the different composition of the composite adsorbent,MgCO₃/g-C₃N₄ and CaCO₃/g-C₃N₄ were selected as the typical g-C₃N₄ based alkaline earth metal carbonate adsorbent.The models of Langmuir,Freundlich,Tenkin and D-R were used to simulate the isothermal adsorption processes of composites on CV;the models of pseudo-first-order,pseudo-second-order and the intra-particle diffusion kinetic models were used to fit the the kinetics of the adsorption process;and the thermodynamic analysis was carried out on the adsorption process.The results show that the D-R model can better describe the adsorption behavior of MgCO₃/g-C₃N₄ on CV and the adsorption rate of the adsorption process is controlled by intragranular diffusion.The Tempkin model can better describe the isothermal adsorption of CaCO₃/g-C₃N₄ on CV and the pseudo-second-order kinetic model can better describe the adsorption kinetic of CV;the thermodynamic analysis of adsorption of CV by two materials indicates that the process is a spontaneous endothermic reaction;the reusibility of four composite adsorbents is not ideal,which could further indicate that the adsorption of CV on the composites is chemical adsorption.The FT-IR and LC analysis of the four composite adsorbents before and after CV can show that the four composite adsorbents have electrostatic attraction with CV.In summary,the g-C₃N₄ based alkaline earth metal carbonate exhibits excellent chemisorption properties to CV because CO₃^2-in the alkaline earth metal carbonate destroys the hydrogen bond between the g-C₃N₄ layers and g-C₃N₄ is recombined by non-covalent interaction,which makes the MCO₃/g-C₃N₄ composite material negatively charged,which enhances the electrostatic attraction with the positively charged coloring group of CV.?4?Calcium carbonate?CaCO₃?was selected as the raw material to inveatigate the effect of the amount on the adsorption and photocatalytic properties of MCO₃/g-C₃N₄composites.Because the source of CaCO₃ was more widely available than other alkaline earth metal carbonates,and the CaCO₃ was the best for CV adsorption performance of the modified composit.Through adsorption experiments and UV-vis DRS,it was found that with the increase of CaCO₃ addition,the adsorption performance of CaCO₃/g-C₃N₄ composites increased first and then decreased.The UV-vis DRS pattern analysis showed that the CaCO₃/g-C₃N₄ composites with low CaCO₃ addition exhibited higher visible light response.Prepared a photocatalytic material by mixing a small amount of CaCO₃ and C₃H₆N₆,and investigated the effects of different raw material mass ratio?CaCO₃:C₃H₆N₆=1%,3%,5%?and calcination temperature?T=550°C,600°C,650°C?on the prepared materials.X-ray Diffractometry?XRD?,Fourier Transform Infrared spectroscopy?FT-IR?,Transmission Electron Microscopy?TEM?,Scanning Electron Microscopy?SEM?,N₂ adsorption-desorption curve?N₂-BET?,X-ray Photoelectron energy Spectra?XPS?,Ultraviolet-visible Diffuse Reflectance?UV-vis DRS?and Photoluminescence Spectra?PL?were used to characterize the structure of the prepared composite photocatalytic material,and determine the phase composition,microstructure,chemical bonds and chemical groups,specific surface area,surface electrons and other structural features.The results showed that the composite photocatalytic material with low addition of CaCO₃ has higher visible light response than g-C₃N₄.The CaCO₃/g-C₃N₄ with mass ratio of 3.0%has the best visible light response capability.?5?The photocatalytic profermence of composite under different calcination temperatures?T=550°C,600°C,650°C?and different raw material ratios?1.0%,3.0%,5.0%?were investigated with the crystal violet?CV?dye solution.The photocatalytic active groups and photocatalytic degradation products were determined by free radical trapping experiments,electron spin-resonance spectroscopy?ESR?and liquid chromatography-mass spectrometry?LC-MS?.The results showed that CaCO₃/g-C₃N₄with a calcination temperature of 600°C and a mass ratio of 3.0%exhibited the highest photocatalytic degradation performance to 20 mg/L CV,the degradation rate reached75.65%,which was 3.5 times and 3.3 times than that of g-C₃N₄ and calcined CaCO₃.The repeatability experiments of the materials showed that CaCO₃/g-C₃N₄photocatalytic material has good stability and good cycle performance for degradation of CV.It was confirmed that the main active group of CaCO₃/g-C₃N₄ photocatalytic degradation of CV was hydroxyl radical?·OH?;the photocatalytic degradation product is 1,3-dimethoxy-5-pentylbenzene(Olivetol dimethyl ether,C₁₃H₂₀O₂)and4-hydroxybenzaldehyde?C₇H₆O₂?.The photocatalytic mechanism is as follows:the low addition of CaCO₃ promotes the directional migration of g-C₃N₄ photogenerated carriers,and CaCO₃ acts as a hole?h+?acceptor.Partial migration to CaCO₃ reduces the recombination rate of photogenerated electrons?e-?and holes?h+?,resulting in higher visible light catalytic performance of CaCO₃/g-C₃N₄ on CV.