A Detection Method For Dissolved Organic Nitrogen Using Ultraviolet Photolysis Pretreatment

Dissolved organic nitrogen is an important part of nitrogen element in aquatic environment.Dissolved organic nitrogen can not only cause eutrophication but also threaten the safety of drinking water and affect human health.Routine detection methods of dissolved organic nitrogen,including ultraviolet oxidation,persulfate oxidation,and high temperature combustion,are either difficult to measure samles of low concentration or inappropriate because of poor recoveries,high detection limit and other issues.In order to solve these problems,the conversions of dissolved organic nitrogen under ultraviolet irradiation at 254 nm and 185 nm were studied in this work,in which low-pressure mercury lamps were used to be the light source.The dissolved organic nitrogen can be converted into inorganic nitrogen effectively.The products were mainly ammonia nitrogen and nitrate nitrogen.In order to achieve trace detection of dissolved organic nitrogen using ion chromatography,the oxidation method of ammonia nitrogen to nitrate nitrogen has also been studied in this work.Based on the traditional ultraviolet method,conversions of dissolved organic nitrogen under ultraviolet and peroxide hydrogen system were investigated under various conditions.The results showed that most of organic matter had poor recoveries under ultraviolet and peroxide hydrogen system.After 60 min photolysis,only amino acid was converted into inorganic nitrogen completely.Peroxide hydrogen dosage,p H and chlorine ions had weak effects on the conversions of these substances.The results of ammonia nitrogen oxidation in ultraviolet and peroxide hydrogen system showed that ammonia can react with hydroxyl radicals quickly to produce nitrate.Ammonia nitrogen was oxidized to nitrate nitrogen completely.In order to further improve the conversion efficiencies of dissolved organic nitrogen,vacuum ultraviolet light at wavelength of 185 nm which has higher photon energy was used.The results showed that dissolved organic nitrogen can be photo-converted into inorganic nitrogen effectively.The recoveries of amino acid,urea and monochloroacetonitrile(with peroxide hydrogen)were all up to 100%,and recovery of methyl orange was approximately to 63% under irradiation for 60 min.The photolysis products of dissolved organic nitrogen were mainly ammonia nitrogen and nitrate nitrogen,the conversion efficiency increased with the increase of light intensity,and decreased slightly with the increase of p H.The average recoveries using vacuum ultraviolet photolysis,persulfate oxidation and high temperature combustion method were 89 ± 16%,87 ± 18% and 86 ± 20% by determining 16 kinds of organic compounds,respectively.The high temperature combustion method had poor recoveries for proteins.The recoveries of heterocyclic nitrogen compounds such as melamine were poor for vacuum ultraviolet photolysis and persulfate oxidation method.All these methods showed poor performance to detect matters with N=N bond.Vacuum ultraviolet photolysis had good performance for samples at concentration below 2 mg/L and p H range of 4.0 to 8.6.When the irradiation time was set to 1.5 h,ammonia nitrogen nitrogen and nitrate nitrogen were required to be detected,and the relative error and relative standard deviation were less than ± 5% and 7%,respectively.However,only nitrate nitrogen need to be detected when irradiation time was set to 12 h,and the relative error and relative standard deviation were less than ± 5% and 7% respectively.