| With the rapid development of economy and the vigorous of industries, the eutrophication of water has become an environmental problem that can not be ignored. Therefore, controlling the content of total nitrogen in water is to become one of the hottest points in environmental protection. Nitrate is an important component of total nitrogen, which can convert from nitrite and ammonia beside the outside sources. Therefore, effective technology must be taken not only to reduce the nitrate content, but also further control the selectivity of nitrate reduction products to translate nitrate into nitrogen, which thereby reduce the total nitrogen content and ultimately repair the polluted water. Photocatalytic reduction is a new water treatment technology and this method has won widespread concern because of its advantages:green, biodegradable, high efficiency, high selectivity for N2 and relatively simple operating conditions. The photocatalytic reduction of nitrate is considered one of the most promising water treatment technologies by domestic and foreign researchers, which is also one of nitrate removal technologies concerned in recent years.In this paper, different contents of metal ions doped TiO2 catalysts were prepared using P25 as the carrier by photo reduction method, metal ion species including noble metal Palladium (Pd), Ruthenium (Ru), Silver (Ag) and transition metal (Fe). Using formic acid as the hole scavenger, the photocatalytic activities and the selectivity for N2 were investigated under the wavelength of 365nm UV irradiation for photocatalytic reduction of the initial concentration of 30mgN/L nitrate solutions. Comparing all the results, it can be seen that Ag-doped TiO2 catalysts displayed the best performance on photocatalytic nitrate reduction at the 365nm wavelength when the same content of metal ions were doped on TiO2 in the same reaction system. Further study on the effect of Ag doping content on photocatalytic nitrate were carried out, including conversion of nitrate, forming nitrite and ammonia concentration, and the selectivity for N2. The results demonstrated that the best photocatalytic activity for nitrate reduction with presence of introduced Argon gas and 0.03 mol/L formic acid was obtained when the doping Ag content was 5.0 wt% and the catalyst is 0.1 g. After 6 h under 365 nm UV light irradiation, the conversion of nitrate reached 90%; low concentration of generated nitrite was detected; the generation rate of ammonia was about 10%; and the selectivity for nitrogen is 90%. The effect of nitrate reduction of Ag-TiO2 under visible light irradiation was carried out in the optimum experimental conditions above. The results shown that about 12% nitrate was reduced under visible light; no nitrite generated; and the selectivity for nitrogen is about 79%.The catalyst of 5.0wt% Ag-TiO2 was characterized using XRD, XPS, UV-vis and electrochemical technology. It can be seen from the measured results that the Ag-TiO2 catalysts prepared by photo reduction were mainly composed of anatase TiO2. There was no obvious difference in catalyst particle size with the increaseing Ag loading content, which was all about 20nm. Seen from the UV-vis results, the edge of Ag-TiO2 photocatalysts has a red shift compared with P25, indicating that Ag-doped photocatalysts favor the absorption of visible light. However, the change of the loading content of Ag ions has an effect on the response range. The XPS results demonstrated that loading Ag on the TiO2 mainly presented as the metallic silver. Electrochemical properties, such as electron transfer of Ag-TiO2 in the nitrate reduction process were studied by cyclic voltammetry, linear sweep voltammetry and Tafel polarization. The influence of initial nitrate concentration on the electrochemical behavior was investigated, and the results shown that the charge carrier concentration will increase with the increase of nitrate concentration, and correspondingly electron transfer speed up and the reaction velocity of nitrate reduction also increase. |
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