Photocatalytic Oxidation Of High-concentration Ammonia Nitrogen Wastewater By Polyoxometalates/Titanium Dioxide Composite

High concentration ammonia nitrogen wastewater can do harm to the environment,and it is difficult to treat.How to better deal with high concentration ammonia nitrogen wastewater has been a problem of great concern.Biological methods are common approaches for the treatment of ammonia nitrogen wastewater,but these methods are more suitable for the treatment of wastewater with a low concentration of NH₄⁺/NH₃.When the concentration of NH₄⁺/NH₃ is high,the biodegradability of wastewater is poor,the carbon/nitrogen?C/N?ratio is low,and the nitrification process is inhibited.Therefore,an additional carbon source is needed,and the treatment efficiency is low.The high concentration of ammonia nitrogen wastewater is treated preliminarily,so that the concentration of ammonia nitrogen can be reduced to the range that can be treated by microorganisms,so as to reduce the load for subsequent biological treatment and improve the efficiency of biological treatment.This is of great significance for the basic work of photocatalytic treatment of high concentration ammonia nitrogen wastewater.In this paper,copper and two kinds of polyacids?phosphotungstic acid and silicotungstic acid?were introduced.Two kinds of TiO₂ supported polyoxometalates composite catalysts(Cu-H₃PW₁₂O₄₀/TiO₂,Cu-H₄SiW₁₂O₄₀/TiO₂)were prepared by sol-gel hydrothermal synthesis method.Under ultraviolet?UV?light,the photocatalytic oxidation of a high-concentration NH₄⁺/NH₃ solution was carried out to explore the effects of different influencing factors on the photocatalytic effect and to optimize the reaction conditions.The main research results of this paper are as follows:?1?The composite catalysts were characterized by X-ray diffraction?XRD?,UV-vis-diffuse reflectance spectroscopy?UV-Vis-DRS?,fourier transform infrared spectroscopy?FT-IR?,scanning electron microscopy?SEM?and energy dispersive spectrometer spectroscopy?EDS?,which evaluated the morphology,structure,grain size and optical properties of the catalyst.XRD and FT-IR showed that the composite catalysts not only kept the anatase structure of TiO₂,but also kept the Keggin type of polyoxometalate,and the structure of the catalyst did not change after use;SEM and EDS showed that both phosphotungstic acid and silicotungstic acid were loaded on TiO₂,and all elements were evenly distributed in the composite catalysts,and the particle size of the catalyst after use increased;UV-Vis DRS showed that compared with TiO₂,the UV absorption spectrum of the composite catalysts moved to the long wave direction and had stronger absorption in the near-infrared band.?2?Through the photocatalytic treatment of ammonia nitrogen solution,the influence of different factors?loading capacity of polyoxometalates,initial pH of ammonia nitrogen solution,initial concentration of ammonia nitrogen in the solution,dosage of catalyst and aeration volume?on the photocatalytic oxidation of ammonia nitrogen by composite materials was investigated.With the increase of the loading capacity of polyoxometalates,the removal of ammonia nitrogen increased first and then decreased.When the loading capacity of Cu-H₃PW₁₂O₄₀0 and Cu-H₄SiW₁₂O₄₀0 was 40%and 30%respectively,the removal rate of ammonia nitrogen was the highest;With the increase of dosage of catalyst,the removal of ammonia nitrogen increased first and then decreased.When the dosage of Cu-H₃PW₁₂O₄₀/TiO₂ and Cu-H₄SiW₁₂O₄₀/TiO₂ were 1.5g·L^-11 and 0.9g·L^-1,the removal rate of ammonia nitrogen was the highest;The pH of the solution had a great impact on the photocatalytic oxidation of ammonia nitrogen,which was found in 7h,the removal rate of ammonia nitrogen increased first and then decreased with the increase of pH value.When the pH of solution was 11,the removal effect of Cu-H₃PW₁₂O₄₀/TiO₂ was the best,when the pH of solution was 12,the removal effect of Cu-H₄SiW₁₂O₄₀/TiO₂ was the best;With the increase of initial concentration of solution,the removal rate of ammonia nitrogen increased first and then decreased.When the initial concentration of ammonia nitrogen was 300 mg·L^-1,the removal rate of ammonia nitrogen was the highest.A proper amount of aeration can increase the removal rate of ammonia nitrogen,but if the amount of aeration is too large,part of ammonia in the solution will be blown out.The reduction of ammonia nitrogen in photocatalytic reaction results in the removal rate of ammonia nitrogen.?3?The catalyst was recovered and tested for 4 times under the optimum conditions to evaluate the stability of the catalyst.The removal rate of ammonia nitrogen by Cu-H₃PW₁₂O₄₀/TiO₂ decreased from 82%to 69%,and the removal rate of ammonia nitrogen by Cu-H₄SiW₁₂O₄₀/TiO₂ decreased from 68%to 50%.The photocatalysis performance of the catalyst decreased after use,because the band gap energy increased,the absorption of UV light decreased,and the utilization of light decreased.?4?Based on the free radical inhibition test,the principle of photocatalytic oxidation of ammonia nitrogen by composite materials was discussed.In the process of photocatalytic oxidation of ammonia nitrogen,hydroxyl radicals?·OH?,hole?h⁺?and superoxide radicals?·O₂^-?played the role of oxidation of NH₄⁺/NH₃,but·O₂^-and h⁺were the main active substances;NH₄⁺/NH₃ could be oxidized to N₂,NO₂^-and NO₃^-,very little NO₂^-and NO₃^-were produced,and N₂ was the main product.