Study On Two-stage Catalytic Denitrification Efficiency Of High Concentration Nitrate Wastewater

High-concentration nitrate wastewater that produced during industry process is one of the main sources of nitrogen environmental nitrogen pollutants.Due to the high concentration of nitrate nitrogen（NO₃^--N）in the wastewater,the application of trational biogical denitrification and physicochemical technology is limited.Therefore,the research on the treatment technology of high concentration NO₃^--N wastewater has been the direaction of the efforts of technicians.Chemical catalytic reduction can reduce NO₃^--N to N₂ in water and achieve the purpose of complete nitrogen removal.It has shown good technical advantages and development prospects in the treatment of high-concentration NO₃^--N wastewater.But its nitrogen removal efficiency still needs to be further improved.In this paper,based on the basic principle of catalytic nitrate reduction,and a two-stage catalytic denitrification process was proposed by regulating the reaction p H.In order to improve the removal efficiency of nitrate and the selectivity of N₂products,the first-stage reaction converts nitrate to nitrite and nitrogen under alkaline conditions,and the second-stage reaction converts nitrite to nitrogen under acidic conditions.In this paper,the self-built catalytic denitrification unit was used to investigate the efficiency,mechanism and stability of the two-stage catalytic denitrification by a semi-batch experiment.On the basis of analyzing the water quality conditions of typical high-concentration nitrate wastewater,the effects of initial nitrate concentration and coexisting ions on the efficiency of two-stage catalytic denitrification were investigated.The main contents and results of this study are as follows:（1）On the basis of commercial 5%Pd/C catalyst,Pd-Cu/C bimetallic catalysts with different metal ratios were prepared by surface reduction method.The morphology and structure of catalysts were characterized by transmission electron microscope,X-ray diffractometer,and other techniques.The results showed that with the increase of Cu loading,the particle size of the catalyst increases（no agglomeration）,and the diffraction peak of Pd-Cu bimetallic structure increases,indicating the successful preparation of Pd-Cu/C bimetallic catalyst.The results of catalytic reduction efficiency of nitrate show that the ratio of Pd-Cu metal has a significant effect on catalytic reduction efficiency of nitrate.under the conditions of Pd:Cu=6:1 and p H=10,the Pd-Cu/C bimetallic catalyst reached an optimal mass catalytic activity of 7.95 mg/gcat·min,and the selectivity of N₂,NO₂^--N and NH₄⁺-N were 36.50%,45.88%and 17.62%,respectively.（2）Based on the above research,the 5%Pd₆Cu₁/C（Pd:Cu=6:1）was selected as the first-stage reaction catalyst,and the commercial 5%Pd/C was used as the second-stage reaction catalyst for the catalytic reduction of high concentration nitrate.The results show that under the conditions of reaction p H=10,the dosage of Pd₆Cu₁/C catalyst 1.0 g/L,and the initial concentration of NO₃^--N 500 mg/L,the removal rate of NO₃^--N can reach90.72%after reaction for 50 min.The selectivity of NO₂^--N,NH₄⁺-N and N₂ was 53.83%,13.04%and 33.13%,respectively.Under the conditions of reaction p H=5.5,Pd/C catalyst dosage 0.6 g/L and NO₂^--N initial concentration 300 mg/L,the NO₂^--N removal rate reached 97.38%after 25min of reaction,and the selectivity of NH₄⁺-N and N₂ were99.86%and 0.14%,respectively.The results of the comprehensive efficiency of the two-stage catalytic denitrification process showed that the nitrate removal rate reached82.05%at the reaction time of 60min,and the selectivity of by product ammonia nitrogen was only 4.63%.The denitrification efficiency was much better than that of the single-stage catalytic denitrification process（nitrate removal rate was less than 50%,and the selectivity of ammonia nitrogen was more than 20%）.（3）Taking chlor-alkali industrial wastewater and electroplating industrial wastewater as examples,the water quality conditions and coexisting ions of typical industrial wastewater with high concentration of nitrate were analyzed,and the influence of related factors on the denitrification efficiencies of two-stage catalytic denitrification process were studied.The results show that when the initial concentration of nitrate is1250 mg/L and the reaction time is 120 min,the removal rate of nitrate can still maintain at more than 86%,and the by-products of ammonia nitrogen decreased,which further indicates that the two-stage catalytic denitrification process has a good effect on the treatment of high concentration of nitrate.In terms of the influence of co-existing ions,the co-existing Cu²⁺will deposit on the surface of the Pd during the reduction reaction,resulting in catalyst deactivation,which decrease the efficiency of the two-stage catalytic denitrification process.Therefore,the Cu²⁺should be firstly removed and before the treatment of nitrate by two-stage catalytic denitrification.Cl^-ion will compete with nitrite for Pd active site,which has a certain influence on the second-stage reaction efficiency.SO₄^2-had no significant effect on the efficiency of two-stage catalytic denitrification.