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环境化学

Photochemical fate of Metribuzin in aqueous solution under simulated sunlight irradiation

摘要：以氙灯为光源模拟太阳光,研究了农药嗪草酮(MT)在水溶液中的光降解;探讨了pH值以及天然水体中广泛存在的光敏剂腐殖酸、硝酸根和Fe³⁺等对嗪草酮间接光解的影响;测量了嗪草酮量子产率及其与活性氧类物种(ROS)^®OH和¹O₂的反应速率常数.结果表明,在pH值3.5～9.5范围内,嗪草酮直接光降解速率随pH值升高而降低.pH=7时,腐殖酸具有光屏蔽作用而抑制嗪草酮光降解,硝酸根和Fe³⁺对嗪草酮间接光降解基本没有作用.嗪草酮与¹O₂反应速率常数为0mol^-1·L·s^-1,与^®OH反应速率常数为5.7×10⁹mol^-1·L·s^-1.在天然水体中嗪草酮与^®OH反应最小半衰期为68h,远远大于北纬40°(40°N)下直接光解半衰期.以上研究结果表明,嗪草酮直接光解占主导作用,其量子产率为0.01.预测在40°N春、夏、秋、冬时嗪草酮直接光解半衰期分别为1.44、1.08、2.58和5.10h,室外太阳光照实验结果显示,在40°N秋季嗪草酮的半衰期为2.98h,与理论预测值接近.

Abstract：Photodegradation of Metribuzin(MT) was investigated in aqueous solution under simulated sunlight irradiation.The influence of initial pH value and photosensitizers such as humic acid(HA),NO₃^- and Fe³⁺ in water are discussed in detail.The quantum yield for metribuzin was determined and reaction rate constants with reactive oxygen species(ROS),^®OH and ¹O₂,were also measured.The results demonstrate that the photodegradation efficiency of MT decreased with increasing solution pH over the range of 3.5～9.5.Addition of humic acid suppresses metribuzin photodegradation,while the photolysis of metribuzin is not significantly affected by NO₃^-,Fe³⁺.The reaction rate constants with ¹O₂ and ^®OH for MT are 0 and 5.7×10⁹ mol^-1·L·s^-1.The lowest possible half-life time for reaction with ^®OH is 68 h.When compared to the direct photolysis half-lives at 40°N latitude,it is clear that indirect photochemical processes cannot compete with direct photolysis.The quantum yield of the direct photodegradation for MT is 0.01 at pH = 7.0.Calculated half-lives for the direct photolysis of MT at 40°N latitude in spring、summer、fall and winter season are 1.44、1.08、2.58 and 5.10 h respectively.Experiments carried out under solar irradiation confirmed that the half-life of Metribuzin at 40°N in fall season is 2.98 h,which is in good accordance with the calculated values.