Research On Nitrogen Removal Via Coupled Oxidation And Reduction Using Metal Doping TiO₂ And Photocatalysis

In this study, Pt/TiO₂, Ag/TiO₂ and Fe/TiO₂ photocatalyst were prepared using a photocatalytic reduction method;phosphotungstic acid/TiO₂ photocatalyst was prepared using impregnation. Also these photocatalysts were characterized with various analysis methods. The activities of the prepared photocatalysts on removal of inorganic nitrogen from water via ammonia oxidation and coupled ammonia oxidation and nitrite reduction, as well as the influence of several key factors were investigated.(1) Pt/TiO₂ and Ag/TiO₂ were characterized with XRF, TEM and XRD and the result showed that Pt doping or Ag doping had no effect on crystal structure of TiO₂ and Pt and Ag are distributed on surface of TiO₂ evenly with small grain size, some even reached l²nm.(2) Catalyst activity of Pt/TiO₂ was obviously increased in oxidizing ammonia in water, only a little nitrite being produced compared with pure TiO₂. N₂ production was best with concentration of ammonia at 50mg/L.(3) Factors that influence coupled ammonia oxidation and nitrite reduction using Pt/TiO₂ and photocatalysis were researched. Coupled photocatalysis could be achieved with or without Pt doping, but activity of Pt/TiO₂ was higher with removal rate of total nitrogen being doubled. Effect of pH value on catalyst activity was obvious and basic condition favored the coupled ammonia oxidation and nitrite reduction. Nitrite reduction rate was highest with 0.5% Pt doping. Addition of Fe²⁺ under basic condition was also in favor of reduction. Conversion rate of nitrogen species was higher with protection gas N₂ than with argon gas. But addition of carbon source inhibited oxidation while promoting reduction. Without regard to the action of adsorption and blow, the optimal removal rate reached as high as 51%.(4) The catalysis activities of Ag/TiO₂ catalyst varied with Ag doping and presented an optimum value. Nitrite reduction rate and the nitrogen removal rate were highest with 1.0% Ag doping in this study. Influence of irradiation time during catalyst preparation on catalyst activity was obvious, coupled photocatalytic redox reaction can be carried out with catalyst prepared with 1.5h irradiation. Effect of different protection gas and Addition of Fe²⁺ was similar to that with Pt doping. The optimal removal rate reached as high as 64%.(5) Comparison showed that oxidation ability of Pt/TiO₂ was stronger, reduction ability of Ag/TiO₂ was better and oxidation activity and reduction activity of Fe/TiO₂ were almost equal, with more nitrite produced and less total nitrogen removed.(6) Reduction activity of PW/T1O2 catalyst was quite low with only presence of nitrite and got better for nitrite reduction with presence of ammonia. Coupled catalysis can be achieved with PW/T1O2 under strong basic condition. Effects for N removal were different when mixed with different proportion of pure TiO2 and were best when ratio of PW/TiCh to pure T1O2 was 1:4.