Study On The Effect And Mechanism Of Ultraviolet Enhanced Chlorine Oxidation Of Sodium Diatrizoate

lodinated X-ray Contrast Media（ICM）is a kind of iodine-containing organic matter widely used in medical imaging of organs and blood vessels.Because of its high water solubility and chemical inertia,the traditional sewage treatment process has poor removal effect,so ICM is a new pollutant frequently detected in surface,underground and even drinking water at home and abroad in recent years.ICM itself is not toxic to human body,but ICM residues and incomplete transformation are the main reasons for the formation of disinfection by-products,so it is of great scientific research and practical significance to control ICM pollution in water environment.In this paper,ionic ICM（Sodium Diatrizoate,DTZ）was taken as the research object.Based on the comparative study of ultraviolet photolysis and NaOCl oxidation,the enhancement effect,influencing factors,main products and degradation pathway of ultraviolet to chlorine oxidation DTZ were explored.The main contents of this paper include:（1）The hydrolysis of NaOCl to HCIO（E⁰HCIO=1.45V）and CIO’（E⁰ClO-=0.97V）have a certain oxidation effect on DTZ.The degradation rate of 0.015 mmol/L DTZ was 23% at pH=6.5（±0.2）,25℃,NaOCl concentration 0.15mmol/L,40 min.The degradation rate of DTZ increased with increasing NaOCl concentration（0.05-0.20 mmol/L）.The initial pH range of 5.30-11.09 had no obvious effect on the NaOCl oxidation of DTZ,and the degradation rate was 28.5% when pH=3.20.The oxidation of DTZ decreased with the increase of its initial concentration（0.0075-0.0300 mmol/L）.The increase of temperature in the range of 15-45℃ is beneficial to the oxidation of NaOCl in DTZ,and the calculated activation energy of the reaction is 63.04 kJ/mol.Different ions in water(Cl^-,SO₄^2-,NO₃^- and Br^-)have different effects on DTZ oxidation by NaOCl.（2）The degradation rate of DTZ was 41.3%,which was oxidized with NaOCl under the light source of 6-W low pressure mercury lamp.The effect of initial pH（3.20-11.09）on the photolysis of DTZ was not obvious.The increase of temperature（15-45℃）was favorable for the photolysis of DTZ,and the reaction activation energy was 18.99 kJ/mol.The UV photolysis of DTZ mainly includes direct photolysis and secondary photochemical processes involving active species.The direct photolysis accounts for 20.08%,and the contribution rates of HO·,¹O₂ and h+are 6.89%,38.36%and 34.66%,respectively.Cl^-,Br^-and SO₄^2- had no significant effect on the photolysis of DTZ,while NO₃^-inhibited the photolysis of DTZ,and the inhibition effect was independent of its concentration.（3）UV/NaOCl significantly enhanced the oxidation effect of DTZ.At the initial pH=6.5（10.2）and 25℃,DTZ and NaOCl concentration were 0.015 mmol/L and 0.15 mmol/L,respectively.After 40 min,the degradation rate of DTZ reached 100%.The oxidation process of DTZ follows the pseudo-first-order reaction kinetics（R²=0.9927）,and the first-order reaction rate constant k is 4.999×10^-2 min^-1,They were 6.65 times and 3.44 times of NaOCl oxidation(k=7.52×10^-3 min^-1)and UV photolysis(k=1.451×10^-2 min^-1)under the same reaction conditions.The results of free radical analysis showed that,compared with HO·and Cl·,ClO·and Cl₂^-· were the main active species for UV/NaOCl oxidation of DTZ,and the contribution rate of DTZ degradation was 50.37%.The contributions of NaOCl oxidation and DTZ photolysis were 15.04%and 29.03%,respectively.The degradation rate of DTZ decreased with the increase of initial DTZ concentration.The optimum concentration of NaOCl（0.15mmol/L）was found during the reaction.When pH=6.50,DTZ removal rate was the highest,which was consistent with the contribution rate of ClO· and Cl₂^-·（50.37%）.The degradation rate of DTZ increased with the increase of temperature（15-45℃）,and the reaction activation energy was 21.464 kJ/mol.Different ions in water(Cl^-,SO₄^2-,NO₃^-and Br^-)have different effects on DTZ oxidation process.（4）The analysis of I^- and IO₃^-showed that DTZ was deiodized in all three reaction systems.The results of UPLC-QTOF-MS/MS showed that NaOCl oxidized DTZ in two ways.A total of 5 products were detected by UV photohydrolysis of DTZ,and 6 products were detected by UV/NAOCl degradation of DTZ,including deiodination,decarboxylation,electrophilic reaction and intramolecular cyclization.（5）Based on toxicogenomic biotoxicity assessment,from the perspective of stress response gene expression changes,the dynamic changes of toxicity of DTZ degradation products of the three systems were analyzed.At 5 min,the toxicity of products of the three systems was NaOCl>UV and UV/NaOCl,15min,30 min,NaOCl>UV/NaOCl>UV,UV/NaOCl>NaOCl>UV at 40 min.