Study On Reduction Of Nitrate Nitrogen In Water By Formic Acid Induced By Ultraviolet Light

Research shows that the rate at nitrogen produced by humans entering fresh water is almost three times the value of"safe"level in China.Many areas of groundwater nitrate concentrations in China rose to more than 15 mg/L,and nitrates can cause disease of methemoglobin when ingested,stomach,diseases such as hypertension,conventional life drinking water from process is difficult to remove nitrate,given the groundwater for raw water drinking water security hidden danger.At present,the commonly used removal methods of nitrate pollutants in water have such problems as slow reaction rate,complex reaction process and insufficient carbon source,etc.Due to the particularity of nitrogen element,conventional removal methods are also associated with the risk of producing ammonia nitrogen,nitrite nitrogen and other by-products.In recent years,Advanced Reduction Processes（ARPs）can effectively reduce and remove pollutants in water by using strong reducing free radicals,which has the advantages of fast reaction rate,simple reaction process and low energy consumption.The common ARPs systems reported include ultraviolet/sulfite,ultraviolet/bisulfite,ultraviolet or thermal activated persulfate coupled formic acid,etc.,which are mainly used for reducing halogenated organic matter,relatively high oxidation capacity of inorganic pollutants（represented by hexavalent chromium salt）and a few new pollutants.In this paper,UV light is used to directly activate formic acid to produce·CO₂^-free radicals,which can be used to reduce NO₃^--N in water.Besides,degradation efficiency,influencing factors,reaction mechanism and reaction kinetics are studied.The main conclusions are as follows:1、The reduction efficiency of nitrate nitrogen in different degradation systems was compared,and the basic reaction conditions for the efficient degradation of nitrate nitrogen pollutants were determined to be UV/HCOOH/N₂.When the 125 W high-pressure mercury lamp was used as the light source,the initial concentration of NO₃^--N in the reaction system was 50 mg/L,the initial concentration of HCOOH was 9 mmol/L,and the initial pH value was 3-4,the degradation rate and gas conversion rate of NO₃^--N reached 99.9%and 99.8%respectively after the reaction for 90 min.Increasing concentration of formic acid and increasing intensity of light will improve the reduction efficiency of NO₃^--N.The change of cations in the substrate and the presence of Cl^-,HCO₃^-/CO₃^2-and H₂PO₄^-have no significant influence on the reduction result of NO₃^--N.The initial pH and HA will have a great influence on the reduction effect of nitrate nitrogen.2、The reaction mechanism of reducing nitrate nitrogen from formic acid induced by ultraviolet light was studied by the analysis of reducing products,the transformation process of intermediate products and the determination of active free radicals.It was found that the reduction and degradation of nitrate by UV/HCOOH/N₂system would be accompanied by the formation of nitrite and ammonia nitrogen,but the final result would not bring the harm of nitrite and ammonia nitrogen.In the reaction of reducing nitrate nitrogen in this system,NO₃^--N is directly converted into nitrogen,instead of into NO₂^--N and then NO₂^--N is further reduced into nitrogen.Through the competitive reaction after adding MV and isopropanol and EPR detection of the reaction system,it can be confirmed that the degradation of nitrate nitrogen in the UV/HCOOH/N₂system is mainly caused by the production of·CO₂^-by HCOOH/HCOO^-in water induced by ultraviolet radiation,and the production of·CO₂^-is the result of the effect of·OH.3、In the UV/HCOOH/N₂system,the degradation of NO₂^--N as the target pollutant conforms to the zero-order reaction kinetics model,and the corresponding rate constant is 1.54 min^-1.The degradation kinetics of nitrate nitrogen was divided into two stages:In the early stage of the reaction,when the removal rate of NO₃^--N is less than 60%,the reaction kinetics is zero order,and the rate constant is 1.03 min^-1;The first order reaction kinetics at the later stage,the corresponding rate constant is 0.08 min^-1.The increase of the temperature of the reaction system is conducive to the degradation of nitrate nitrogen,and the apparent activation energy of the reaction system is E_a=3.08 k J/mol.4、In the application discussion,it was found that NO₃^--N in the theoretical reaction system could achieve efficient and convenient harmless removal when the molar ratio between HCOOH and NO₃^--N reached 2.5 and the reduction time was long enough,other reaction conditions were the same and there was no external interference.The secondary pollution of TOC will not occur if the removal rate of NO₃^--N is not strict.When potassium sorbate was used as a food additive to replace formic acid as a reducing agent,the degradation effect was also higher,but its reduction rate and gas conversion rate were lower.During the reaction process,a large number of ammonia nitrogen by-products were generated,and potassium sorbate was not fully mineralized after participating in the reduction reaction,and there were still olefin by-products generated.In this paper,UV light is used to directly activate formic acid to produce·CO₂^-free radicals,which can be used to reduce NO₃^--N in water.It has the advantages of high reduction efficiency,good gas selectivity and mild reaction conditions.It is worthy of further engineering research and provides a new path and method for the removal of nitrate pollution in water in China.