| Excessive nitrate nitrogen in water will threaten human health,cause blood system diseases,nervous system diseases,tumor diseases,etc.,and also harm the ecological environment,and even destroy the ecological balance in serious cases.Nitrate nitrogen is easy to dissolve and diffuse,leading to a wide range of pollution.How to better treat nitrate nitrogen in water has become an urgent and challenging task.Membrane treatment method has high cost and produces high concentration of nitrate nitrogen wastewater.Biological treatment method has high requirements and needs to add other nutrients Efficient nitrate nitrogen treatment method has very important theoretical and practical significance.In this paper,the nitrate nitrogen in the water was reduced by ultraviolet light irradiation to explore the effects of catalyst dosage,hole scavenger dosage,composite catalyst loading,and aeration on the photocatalytic effect at low(30 mg·L-1),medium(60 mg·L-1)and high(90 mg·L-1)nitrate nitrogen concentrations,and to optimize the reaction conditions by using two kinds of catalysts,Ti O2and Ti O2/SiO2.The main conclusions are as follows:(1)The particle morphology,grain size,structure and optical properties of the two catalysts were evaluated by characterization and comparative analysis.The results of X-ray diffraction(XRD)and Fourier infrared spectroscopy(FT-IR)showed that the Ti O2in the Ti O2/SiO2composite catalyst retained the anatase structure and had a smaller particle size.The anatase crystal form did not change but the particle size increased after the catalyst was used.Scanning electron microscopy(SEM)and energy dispersive analysis(EDS)proved that Ti O2was successfully loaded on SiO2and the elements were uniformly distributed.The particle size of the catalyst in the scanning electron microscope image became larger after use,which was consistent with the FT-IR conclusion.UV-Vis diffuse reflectance spectroscopy(UV-Vis DRS)results showed that compared with Ti O2,the ultraviolet absorption spectrum of the Ti O2/SiO2composite catalyst shifted to the long-wave direction.After the two catalysts were used,the band gap energy increased and the light absorption performance decreased.(2)In the experiment of catalytic degradation of nitrate nitrogen solution under ultraviolet light irradiation,different influencing factors were carried out on low(30 mg·L-1),medium(60 mg·L-1)and high(90 mg·L-1)nitrate nitrogen solutions with different concentrations.The gradient test of nitrate nitrogen removal rate and nitrogen conversion rate are used to illustrate the experimental results.The results showed that the two catalysts had better effect on treating low concentration nitrate nitrogen wastewater,and the treatment effect decreased slightly with the increase of concentration.In 30 mg·L-1nitrate solution,when the dosage of formic acid was 0.6 ml·L-1,CO2aeration was 0.1 m3·h-1,and the dosage of Ti O2was 0.9 g·L-1,the removal rate of nitrate nitrogen and the selectivity of nitrogen in the product were the highest,which are 99.93%and 81.21%.When the loading rate of 40%Ti O2/SiO2was 1.4 g·L-1,the removal rate of nitrate nitrogen and the selectivity of nitrogen in the product were 83.48%and 98.80%.Ti O2/SiO2was more effective in treating medium concentration nitrate nitrogen wastewater,in 60 mg·L-1nitrate solution,when formic acid dosage was 1.1 ml·L-1and CO2aeration was 0.1 m3·h-1,when Ti O2dosage was 1.0 g·L-1,the removal rate of nitrate nitrogen and the selectivity of nitrogen in the product were the highest,which were 97.99%and 71.83%.When 40wt%Ti O2/SiO2dosage was 3.0 g·L-1,the removal rate of nitrate nitrogen and the selectivity of nitrogen in the product were the highest,which were 87.53%and 88.81%.The best removal rates of Ti O2and 40wt%Ti O2/SiO2in 90 mg L-1nitrate nitrogen solution were 85.6%and 82.7%.Compared with Ti O2,Ti O2/SiO2composite catalyst had higher nitrogen selectivity in the product.(3)In order to verify the stability of the catalyst,three experiments were carried out under the best experimental conditions after the recovery of the catalyst.The removal rate of nitrate nitrogen by Ti O2decreased from 99.93%to 63.25%,and the removal rate of nitrate nitrogen by Ti O2/SiO2decreased from 83.48%to 65.79%.After repeated washing,the physical morphology of the catalyst changed,the band gap energy increased,the light absorption rate decreased and the catalytic effect decreased.(4)Based on the experimental results and combined with previous studies,explore the mechanism of Ti O2as an effective catalyst to degrade nitrate nitrogen in water under photocatalytic conditions.In the process of photocatalytic reduction of nitrate nitrogen,both electrons(e-)and free radicals·CO2-can play a role in reducing nitrate nitrogen.NO3-can be reduced to N2,NO2-and NH4+.However,the final product is mostly N2. |
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