Determination of rate constants of low volatile, particle-bound, long-lived, persistent pollutants by OH-radicals

Field: Environmental Science (Physical Sciences)

Understanding the behavior of persistent organic pollutants (POPs) under environmental conditions is fundamental in shaping environmental engineering and science. POPs are organic compounds not only of substantial atmospheric life time and low vapor pressure, but also with certain chemical and physical properties which are harmful to the environment and the health. To date, research on the significance of POPs is impaired by the lack of kinetic data for assessing their environmental relevance. The present study examines the degradation by indirect photolysis of selected semi-volatile Biocides, as terbuthylazine (TBA), lindane, and simazine. The laboratory experiments were performed with OH-radicals under normal atmospheric conditions. The experiments were carried out in a reaction chamber, where layers (mono and multi) of investigated substances deposited on nano-sized silica (Aerosil) were exposed to OH-radicals. H2O2 was used as the best available OH-radical source for the experimental setup. The progress of the photolysis was monitored using mainly Raman spectroscopy. IR spectroscopy data, NMR and mass spectroscopy were used as complementary methods uniquely to characterize the reaction products. The obtained results could be rationalized by a Retro- Diels-Alder ring cleavage as an important and complementary step of herbicide degradation. The spectrum calculation was performed after structure optimization and frequency calculation with the help of the Quantum Chemistry code Turbomole or Gaussian. Achieved results can be summarized as follows: \- In order to estimate the concentration of OH-radials formed in the reaction chamber, the reaction of terbuthylazine (TBA) with OH-radicals was considered as a reference for the indirect photolysis reaction. The OH rate constant has been calculated from the fitted rate constant at 1255 cm-1 as follows: kOH (CH2) = λ / COH = (3.9 ± 0.59) x 10-3 / (163 ± 31)x106 = (2.73163 ± 0.645)x10-11 cm3∙ s-1 The corresponding OH-radical concentration value is the following (taking into account the lower value): C[OH] = (163± 31)x106 molecules.cm-3 = (2.7 ± 0.5) x10-13 Mol.L-1 Using TBA for quantifying OH- radicals, we succeeded in estimating the rate constant of indirect photolysis of TBA. the results of laboratory experiments investigating the process of OH- radical formation via photolysis of aqueous hydrogen peroxide (H2O2), which occurs in an open storage within a reaction chamber. Other precursors for the formation of OH-radicals included nitrate (NO3-), ozone (O3) and water (H2O). However, O3 could not be used as OH-radical source in the experimental setup used in our work because of the possibility of ozonolysis to occur. Also, the obtained levels of OH-radicals from H2O photolysis are high in humid air. -We showed that the Raman spectra of TBA do not practically change with time when the irradiation lamp in the reaction chamber is turned off. -Using the Igor Pro software, we found the important physical parameter λ from the time- dependence of the intensities or areas of the observed Raman bands. It followed from the detailed analysis of the Raman bands that the overall rate constant could be realistically determined by the time-dependent behavior of the Raman band at 524 cm-1. The confidence range for the overall rate constant was defined by using the following formula: 0.0018 ± 0.0003 ≤ λ (524) ≤ 0.0020 ± 0.0001 s-1. The corresponding range for the OH-radical concentration by considering TBA as a reference substance was determined as follows: (163± 31) x106≤ C[OH] in cm-3 ≤ (184 ± 9) x106. The rate constant of the reaction in the case of simazine was obtained from the fitted rate constant at the band 1255 cm-1: kOH (CH2) = λ / COH = (3.9 ± 0.59) x 10-3 / (163 ± 31)x106 = (2.73163 ± 0.645) x10-11 cm3∙ s-1 The rate constant of lindane for the 2992.8 cm-1 area was estimated as following: kOH =λ / COH = (2.555 ± 0.0376) x 10-3/ (163± 31)∙106 = (156.442± 12.129)∙10-13 cm3 ∙ s-1. The present results allow us, not only to better understand the Raman spectroscopic method for the monitoring of TBA photolysis, but also to extend the method to the case of other pesticides under study.