Removal Of Nitrate From Water By Vacancy Polyoxometalate Titanium Dioxide Composite

Due to the rapid development of industry and agriculture,groundwater nitrate pollution is increasingly serious,and nitrate nitrogen chemical properties are stable,it is difficult to remove it from water by general methods.At present,there are chemical,biological denitrification and physical methods to remove nitrate nitrogen from groundwater,among which chemical methods can be divided into metal reduction method and catalytic reduction method.In this study,two new photocatalysts were prepared by sol-gel method using catalytic reduction method,and the performance,influencing factors and reaction mechanism of photocatalytic reduction of nitrate nitrogen were discussed,aiming at opening up a new way to remove nitrate nitrogen in groundwater.In this study,two kinds of vacant polyoxometalates(Na[α-Si W₉O₃₄]and K₈[α-Si W₁₁O₃₉])were synthesized and the two kinds of deficient polyoxometalate titanium dioxide composite catalysts(Si W₉/Ti O₂/Cu and Si W₁₁/Ti O₂/Cu)were prepared by sol-gel method.The effects of different conditions on the photocatalytic nitrate nitrogen reduction were investigated by photocatalytic nitrate solution photocatalytic test under ultraviolet light.The main research results are as follows:（1）Automatic Specific Surface Area and Porosity Analyzer（BET）,Scanning Electron Microscope（SEM）and Energy Dispersive Spectrometer（EDS）were used to characterize the structure of the self-made catalyst.The results showed that Na[α-Si W₉O₃₄],K₈[α-Si W₁₁O₃₉]and Cu²⁺were successfully supported on Ti O₂.The elements are evenly distributed in the composite catalyst.Si W₉/Ti O₂/Cu and Si W₁₁/Ti O₂/Cu are mesoporous structures.The performance of the self-made catalyst was characterized by UV-Vis DRS,X-ray Diffraction（XRD）,Fourier Transform Infrared Spectroscopy（FT-IR）,X ray Photoelectron Spectroscopy（XPS）and Electrochemical Impedance Spectrum（EIS）.The results showed that the composite catalyst maintained the Keggin structure of polyoxate,and Ti O₂ was the structure of titanium oxide.Compared with pure Ti O₂,the composite catalyst widens the spectral response range,decreases the crystal size and the band gap width,and effectively improves the photocatalytic performance of the composite catalyst.The structure of the catalyst remains unchanged after use,while the vacant polyoxomethoate still maintains the Keggin structure.（2）Through the photocatalytic treatment experiment of nitrate in water,the effects of different factors on the photocatalytic reduction of nitrate by composite catalyst（vacancy polyoxometalate loading,catalyst dosage,initial concentration of nitrate in solution,type of hole scavenger,anion and anion in solution,protective gas）were investigated.The results showed that the removal rate of nitrate nitrogen increased firstly and then decreased with the increase of the missing polyoxoate loading capacity.The removal rate of nitrate nitrogen was the highest when the loading capacity of Na[α-Si W₉O₃₄]and K₈[α-Si W₁₁O₃₉]were 10%.With the increase of catalyst dosage,the removal of nitrate nitrogen firstly increased and then decreased.When the dosage of Si W₉/Ti O₂/Cu was 1.6 g·L^-1,the removal rate of nitrate nitrogen was 76.5%and the nitrogen selectivity was 88.4%.When the dosage of Si W₁₁/Ti O₂/Cu was 1.2g·L^-1,the nitrate nitrogen removal rate was 96.5%,and the nitrogen selectivity was 77.6%.The removal rate of nitrate nitrogen decreases with the increase of the initial concentration of nitrate nitrogen in solution.The hole scavenger had great effect on the photocatalysis of nitrate nitrogen,and formic acid had the best effect.Ions in water have a great effect on the photocatalytic reduction of nitrate,and cations can promote the removal of nitrate nitrogen in the following order:Fe³⁺>K⁺>Na⁺.Anions can inhibit nitrate nitrogen removal in the order of SO₄^2->PO₄^3->Cl^-.Nitrogen was introduced as a protective gas to reduce the effect of oxygen in solution on the photocatalytic reduction of nitrate nitrogen.（3）The photocatalytic reduction of nitrate by Si W₉/Ti O₂/Cu and Si W₁₁/Ti O₂/Cu were in accordance with first-order reaction kinetics.Si W₉/Ti O₂/Cu and Si W₁₁/Ti O₂/Cu have better removal effect on nitrate nitrogen than pure Ti O₂,and Si W₁₁/Ti O₂/Cu has better removal effect than Si W₉/Ti O₂/Cu.Electron spin resonance（EPR）was used to detect the carbon dioxide anion radical in the system,and the reduction mechanism of nitrate nitrogen was investigated.（4）Filtered,washed and dried the used catalyst,and repeat the test for 3 times under the best test conditions to evaluate the stability of the catalyst(Si W₉/Ti O₂/Cu,Si W₁₁/Ti O₂/Cu).The nitrate nitrogen removal rate of Si W₉/Ti O₂/Cu decreased from 76.5%to 65.4%,and the nitrate nitrogen removal rate of Si W₁₁/Ti O₂/Cu decreased from 96.5%to 78.4%.The photocatalytic performance of the catalyst decreased after using,because the surface of the catalyst became rough and it was not easy to adsorb nitrate nitrogen.The wide band gap of the composite catalyst increases,which weakens the absorption of ultraviolet light and reduces the utilization rate of light.The vacant polyoxometalate and Cu²⁺were successfully supported on titanium dioxide by sol-gel method.The composite catalyst POMs/Ti O₂/Cu has a good removal effect on nitrate nitrogen,and has the characteristics of easy preparation,mild reaction conditions,high catalytic activity,low dosage,no pollution,etc.Therefore,the composite catalyst POMs/Ti O₂/Cu has a good application prospect.