Removal Of Nitrate From Water By UV Activated High Safety Organic Matter

Due to the impact of anthropogenic activities such as agricultural runoff,nitrogen-based fertilizers,animal manure and urban sewage,excessive nitrate pollution in water bodies has been caused.At present,the reducing agents for removing nitrate from carbon dioxide free radicals produced by advanced reduction method(CO₂^·-)are formic acid,formate and oxalate.If the reaction is not complete,residues in water will cause secondary pollution risk.This subject proposes to use ethanol and citric acid as high safety reducing agents to produce CO₂^·-reduces nitrate.The specific research results are as follows:（1）UV activated ethanol system（UV/C₂H₅OH/N₂）has high reduction ability for nitrate under nitrogen atmosphere.When n_NO-₃:n_C2H5OH=5:7 the best nitrate nitrogen reduction rate and total nitrogen removal rate can be achieved.Increasing the intensity of ultraviolet light can improve the reduction rate of nitrate nitrogen,but reduce the removal rate of total nitrogen.The initial p H value has no obvious effect on the reduction rate of nitrate nitrogen,but when the system is acidic（p H=2）,nitrate nitrogen tends to be converted into ammonia nitrogen,thus reducing the total nitrogen removal rate.Under ultraviolet light,chloride ions（Cl^-）and sulfate ions(SO₄^2-)will generate reactive chlorine and sulfate radical(SO₄^·-)and other oxidizing radicals to consume too much CO₂^·-,and oxidizes ammonia nitrogen,while bicarbonate ions（HCO₃^-）will consume hydroxyl radicals（·OH）and interact with CO₂^·-reaction will reduce ammonia nitrogen in the system and improve the removal rate of total nitrogen.Humic acid will quench free radicals and compete with ultraviolet light to reduce nitrate reduction rate and total nitrogen removal rate.（2）In UV activated citric acid system（UV/C₆H₈O₇/N₂）,increasing the dosage of citric acid will accelerate the reduction rate of nitrate nitrogen and have little effect on the reduction rate of nitrate nitrogen,which is about 90%,but will reduce the removal rate of total nitrogen.The reduction rate of nitrate nitrogen and the removal rate of total nitrogen will increase with the increase of UV intensity.The optimal reduction p H of the system is 3.54～6.The over acidic environment will promote the transformation of nitrate nitrogen to ammonia nitrogen,while the over alkaline environment will delay the transformation of nitrite nitrogen into nitrogen-containing gas,which is easier to stay in the form of nitrite nitrogen.Prolonging the illumination time can increase the input energy and continuously generate CO₂^·-participate in the reaction and improve the reduction rate of nitrate nitrogen and the removal rate of total nitrogen at the same time.Cl^-and SO₄^2-excited by ultraviolet light will generate oxidative free radicals and CO₂^·-reaction to reduce the nitrate nitrogen reduction rate and total nitrogen removal rate,while the nitrate nitrogen reduction rate and total nitrogen removal rate first increased and then decreased with the increase of bicarbonate ion.（3）The reduction effect of nitrate nitrogen in UV/C₆H₈O₇/N₂ is better than UV/C₂H₅OH/N₂.On the one hand,the amount of ethanol added in the experiment is more than that of citric acid,resulting in excessive generation of CO₂^·-.On the other hand,because the molecular formula of ethanol is simpler than that of citric acid,it is easier to break chemical bonds to generate CO₂^·-.Therefore,UV/C₂H₅OH/N₂ is easier to generate ammonia nitrogen by-products,The reaction rate was higher than UV/C₆H₈O₇/N₂.（4）The reduction process of nitrate nitrogen in UV/C₂H₅OH/N₂ and UV/C₆H₈O₇/N₂ conforms to the first-order reaction kinetics,and the reaction rate constants are between 0.0402～0.1657 min^-1 and 0.0122～0.0556 min^-1,respectively.Methyl viologen was added to the two systems to quench the CO₂^·-,which reduced the reduction rate of nitrate and determined that the active substance was CO₂^·-.The transformation pathway of nitrate nitrogen is generally carried out according to NO₃^-NO₂^-N₂ON₂ and NO₃^-NO₂^-NH₄⁺.UV activated high safety organic matter as the basis of CO₂^·-this new advanced reduction system based on high-efficiency electron donor has the advantages of rapid reaction,less secondary pollution and strong regulation ability.It is worthy of further exploration and research.