关键词: 阿特拉津, 甲醇, 紫外光, 高级氧化工艺, 降解速率, 降解机理

Stock solutions of organic micro-pollutants with low water solubility are commonly prepared using organic solvents in laboratory studies on degradation of these organic compounds. Dilution of the stock solution unavoidably introduces a small amount of organic solvent into the experimental working solutions. This could possibly affect the estimation of the degradation rate constants (k_obs) of these organic micro-pollutants by UV-based oxidation processes. To demonstrate this problem, the effect of organic solvents on the degradation rate of atrazine (ATZ) has been investigated in the sole-UV, UV/H₂O₂ and UV/TiO₂ process at the concentration levels that would likely be derived from stock solutions. Organic solvent methanol (MeOH) commonly used for stock-solution preparation was selected. The degradation of ATZ was investigated under ultraviolet irradiation (253.7 nm). The reaction was conducted in an annular photochemical reactor, in the axis of which a low-pressure mercury lamp (LPUV) was installed. The photon flux into the solution from the LPUV was determined to be at 1.18×10^-7 Einstein/s. A magnetic stirrer was located at the bottom of the reactor to maintain homogeneity of the reacting solution. A thermostatic water recirculation system was used to control the solution temperature at 20±0.5℃. Prior to irradiation, the mercury lamp was ignited for 30 min for a stable output. UV photo-oxidation was performed with ultrapure water containing an initial 0.1 or 5 mg/L ATZ and different volume ratio of methanol. Solution pH value of 4.0, 7.0 and 10.0 was buffered using phosphate or borate. Determination of ATZ using ultra-performance liquid chromatography-electrospray-triple quadrupole mass spectrometry coupled with an ACQUITY^TM UPLC BEH C8 separation column. The results show that the reaction rate of ATZ in UV/TiO₂ process could be affected significantly by the presence of MeOH, even at a concentration well below that possibly introduced during the preparation of working solutions from the organic solvent stock solutions (e.g. 0.01%, V/V). With the increase of MeOH concentration, the k_obs of ATZ in UV/TiO₂ process gradually decreases. The organic solvents having a stronger reaction activity with·OH tend to impose a greater effect on the k_obs of ATZ. However, MeOH does not affect k_obs of photolysis of ATZ in sole-UV process, and a small effect for the k_obs of ATZ in UV/H₂O₂ process. In addition, MeOH in the reaction system does not affect the speciation and degradation pathway of ATZ under different UV-based oxidation processes. The findings here provide a plausible explanation for the discrepancies in the reaction rate constants reported in the literature for some organic micro-pollutants during the UV-based oxidation processes.

Key words: atrazine, methanol, ultraviolet, advanced oxidation process, degradation rate, degradation mechanism