Treatment Of Nitrate Nitrogen In Water By Polyoxometalate Titanium Dioxide Composites

In recent years,due to the excessive application of nitrogen fertilizer,the disposal of domestic sewage and industrial wastewater containing nitrogen is unreasonable,which makes nitrate concentration in some areas show an upward trend.Polyoxometalate catalyst has good activity,high selectivity,mild reaction conditions and environmental protection.It is a kind of multifunctional catalytic material,which can promote photocatalysis after being compounded with titanium dioxide.It is of great significance to study the nitrate nitrogen test in water by using the polyoxometalate titanium dioxide composite material,to increase the removal of nitrate nitrogen in the water,and to explore the use of the polyoxometalate titanium dioxide composite material.In this experiment,four kinds of titanium dioxide supported polyoxometalates were prepared as photocatalysts by sol-gel method,using Cu and Ni as metals and phosphotungstic acid and phosphomolybdic acid as polyacids.The microstructure,structure and optical properties of the catalysts were characterized and studied.At the same time,the experiment of photocatalytic treatment of nitrate nitrogen was carried out under different conditions.After the reaction was finished,the composite catalyst was washed with water and centrifuged to recover,and the experiment was repeated under the optimal conditions.The feasibility of using polyoxometalate titanium dioxide composite to treat nitrate nitrogen in water under photocatalytic conditions was discussed,and the reaction mechanism of removing nitrate under photocatalytic conditions was studied.The effects of different reaction conditions on the removal efficiency of nitrate nitrogen and the changes of structure and function of polyoxometalate titanium dioxide composite catalyst before and after the reaction were explored.The main results of the experiment are as follows:?1?Through the analysis and characterization of XRD,SEM and UV-vis DSP before and after the catalyst is used,it was concluded that the catalyst successfully supported Keggin structure polyoxometalate on titanium dioxide.The wavelength range of ultraviolet absorption spectrum is red shifted to the right than TiO₂,and the reaction wavelength range is wider.The structure of the polyoxometalate titanium dioxide composite catalyst before and after use is unchanged,the crystal grains become larger,and the catalytic performance is reduced.?2?Through the photocatalytic treatment of nitrate nitrogen in water,we can know that the catalyst Cu-H₃PW₁₂O₄₀/TiO₂ has the best removal effect of nitrate nitrogen,59.60%,under90 mg/L nitrate concentration,0.8 g/L catalyst and 60 min treatment time.With 120 mg/L nitrate concentration,0.8 g/L catalyst and 30 min treatment time,Ni-H₃PW₁₂O₄₀/TiO₂ has the best removal effect on nitrate nitrogen,which is 54.58%.Cu-H₃PMo₁₂O₄₀/TiO₂ has the best removal effect of nitrate nitrogen,53.71%,with 120 mg/L nitrate concentration,1.2 g/L catalyst and 60 min treatment time.With 120 mg/L nitrate concentration,1.2 g/L catalyst and120 min treatment time,catalyst Ni-H₃PMo₁₂O₄₀/TiO₂ has the best removal effect of nitrate nitrogen,57.33%.Taken together,Cu-H₃PW₁₂O₄₀/TiO₂ has the best effect on nitrate nitrogen removal among the four catalysts.?3?The removal rate of the catalyst Cu-H₃PW₁₂O₄₀/TiO₂ decreased from 59.60%to43.94%that of Ni-H₃PW₁₂O₄₀/TiO₂ decreased from 54.58%to 40.16%after four photocatalytic tests on four kinds of polyoxometalate titanium dioxide composite catalysts.The removal rate of catalyst Cu-H₃PMo₁₂O₄₀/TiO₂ decreased from 53.71%to 38.62%,while the removal rate of catalyst Ni-H₃PMo₁₂O₄₀/TiO₂ decreased from 57.33%to 41.20%.After the prepared catalyst is used for four times in a row,although the photocatalytic reduction test of nitrate nitrogen can be continued,the removal efficiency of nitrate nitrogen is continuously reduced by about 15%,so the preparation process of the catalyst needs to be optimized to improve the stability of the catalyst and the number of repeated uses.