Influence of Solution pH on Photolysis Intermediates and DegradationPathway of Diazinon during UV Irradiation Treatment
Received date: 2015-07-30
Online published: 2015-09-15
Supported by
Project supported by the National Natural Science Foundation of China (No. 51308437) and the Foundation of Shaanxi Educational Committee of China (2013JK0980).
The direct photolysis of an organophosphorus pesticide, diazinon, was investigated under monochromatic ultraviolet (UV) irradiation (253.7 nm) at different solution pH. Photolysis was conducted in an annular photochemical reactor, in the axis of which a low-pressure mercury UV lamp was installed. The photon flux into the solution from the UV lamp was determined to be at 1.18×10-7 Einstein·s-1. 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 photolysis, the UV lamp was ignited for 30 min for a stable output. UV photolysis was performed with ultrapure water containing an initial 5 mg/L diazinon. Solution pH (4.0, 7.0 and 10.0) was buffered using 2 mmol/L phosphate and/or borate buffers. The analyses of diazinon and its UV photo-degradation intermediates were conducted by using ultra-performance liquid chromatography-electrospray-tandem mass spectrometry (UPLC-ESI-MS/MS) coupled with an ACQUITYTM UPLC BEH C8 separation column. Full-scan spectra were acquired from 50 to 500 m/z in both positive electrospray ionization (ESI+) mode and negative electrospray ionization (ESI-) mode. To obtain further information for analyzing the structure of UV photo-degradation intermediates of diazinon, collision induced dissociation (CID) experiments were also conducted. The collision energy for each intermediate was optimized in the range from 15 to 35 eV. Argon was used as collision gas and its flow rate was at 0.12 mL/min. The results show that solution pH affected significantly on the photo-degradation rate of diazinon. The rate was lower at acidic pH 4 (the pseudo-first-order rate constant (k) is 0.0193 min-1) than those at neutral and alkaline pH values of 7.0 and 10.0 (k=0.0234 min-1 and k=0.0236 min-1, respectively). More than 90% of diazinon were degraded within 120 minutes of UV irradiation at all the three pH values. The photolysis intermediates of diazinon were found to vary with solution pH. There are 5, 8, and 6 major species of photolysis intermediates for diazinon after 60 minutes of UV irradiation in water at pH of 4.0, 7.0 and 10.0, respectively. Moreover, the amount of a certain intermediate formed was very different under different pH conditions. A systematic qualitative and quantitative analyses of the photo-degradation intermediates of diazinon under different pH conditions were conducted. The molecular structure of the intermediates were deduced based on the information of mass spectrometry; the photo-degradation pathways of diazinon under different pH conditions were also proposed.
Key words： diazinon; pH; ultraviolet; photolysis intermediates; degradation pathway
Liu Yucan , Duan Jinming , Li Wei . Influence of Solution pH on Photolysis Intermediates and DegradationPathway of Diazinon during UV Irradiation Treatment[J]. Acta Chimica Sinica, 2015 , 73(11) : 1196 -1202 . DOI: 10.6023/A15070520
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