Research On Conversion And Detection Of Dissolved Organic Nitrogen In Trace Level By Vacuum Ultraviolet

Dissolved organic nitrogen（DON）has been in the spotlight in recent years for it can cause eutrophication and generate high toxic nitrogen-containing disinfection byproducts during chlorination.However,traditional methods can not accurately detect trace DON in water due to the interference of dissolved inorganic nitrogen（DIN）and high method detection limit（MDL）.In order to overcome these problems,we have developed a method for the determination of trace DON based on electrodialysis（ED）and vacuum ultraviolet（VUV）.In this method,DIN was first removed by ED,and then DON was digested to DIN（i.e.ammonium,nitrite,and nitrate）by VUV.Finally,the concentrations of each DIN were determined.The concentration of DON in the sample is the sum of the concentrations of DIN.Although this method can detect samples with a trace amount of DON,it is time-consuming to operate.Moreover,adding three nitrogen fractions may accumulate measurement errors.To solve these problems,this study aims to develop a vacuum ultraviolet nitrification method,which can completely convert DON into nitrite and nitrate（NO_x^-）in a short time.To meet this goal,four VUV-based advanced oxidation methods,including VUV,VUV/K₂S₂O₈,VUV/Na Cl O,and VUV/H₂O₂,are selected.The conversion effects on the ammonia of four methods are evaluated from three aspects:ammonia removal,nitrogen recovery,and DIN change.Although VUV/Na Cl O method has the best ability to remove ammonia,and dissolved nitrogen recovery is only 78%.The total dissolved nitrogen is not conserved in this reaction.In contrast,the dissolved nitrogen recovery of the VUV and VUV/K₂S₂O₈ methods is 100±10%,meaning the total dissolved nitrogen is conserved during the process.However,both methods reach ammonia removal of 85%at 60 minutes.Besides,VUV/K₂S₂O₈ would generate byproducts which may interfere with nitrate and nitrite detection.VUV/H₂O₂ not only convert ammonia into nitrate completely within 60 minutes but also reach 100±10%dissolved nitrogen recovery.Without introducing other ions,VUV/H₂O₂ is the most suitable method for DON digestion.Glutamic acid,glycine,adenine,and 2,3-Butanedione monoxide with high ammonia nitrogen yield are selected as typical DON to optimize VUV/H₂O₂performance.The optimized performance parameters include p H,H₂O₂ dosage,initial concentration of DON,and common inorganic ions in water.The results show that VUV/H₂O₂ can effectively nitrify DON.The increase of the initial p H or H₂O₂ dosage and the decrease of the initial concentration of the DON will promote ammonia conversion.In addition,a stepwise H₂O₂ dosage,the existence of Cl^-and SO₄^2-in water samples do not affect DON conversion,while a high concentration of CO₃^2-will inhibit DON conversion.To evaluate the feasibility of the new method,VUV/H₂O₂,persulfate oxidation,and high temperature combustion method were used to analyze 11 model DON compounds with an initial concentration of 0.5 mg-N/L,respectively.The average nitrogen recovery of the VUV/H₂O₂ is 87%,which is similar to the persulfate oxidation（PO method）and high temperature combustion（HTC method）.The result shows that all three methods can be used to detect DON in real water samples.Compared with other methods,VUV/H₂O₂ has the highest precision and lowest MDL,enabling the detection of trace DON.Compared with our previous method,VUV/H₂O₂ has a better nitrification ability.The average NO_x^-yield of 11 DON is 80.6%.VUV/H₂O₂ has an advantage of converting most DON into NO_x^-completely within 1 h,thus DON can be detected by ion chromatography only.In summary,this study develops a robust method to measure DON by VUV/H₂O₂,which can achieve accurate quantification of trace DON.