Study of the phototransformation of imazaquin in aqueous solution: a kinetic approach

The photodegradation of imazaquin, [2-(4,5-dihydro-4-methyl-4-(methylethyl)-5-oxo-1-imidazol-2-yll-3-quinolinecarboxylic acid (CAS RN: [81335-37-7], an herbicide belonging to the family of imidazolinone, was investigated using steady-state irradiation in aqueous solutions at different pH values, different wavelengths and different oxygen concentrations. Using spectrometry techniques (UV-vis and fluorescence), pK(a) values for the ground state, pK(a)(S-o), and for the lowest excited singlet state pK(a)(S-1), were found to be 3.6 and 7.5, respectively. The degradation of imazaquin appeared to be more effective at pH > pK(a)(S-o) and in the absence of oxygen. The use of laser flash photolysis has shown the presence of a triplet state in the region from 320 to 550 nm with a maximum centred at 380 nm. By comparing the quenching rate constant, k(q), obtained from the quenching of the triplet state by molecular oxygen (k(q) = 9.31 x 10(8) mol(-1) L s(-1)) and from the Stern-Volmer relation (k(q) = 1. 13 x 10(8) mol(-1) L s(-1)), we deduced that the singlet state also seems to play a role in the photodegradation process. According to the experimental results, it has been found that imazaquin is a rather stable molecule in the dark or at low pH. In pure water, imazaquin degrades slowly and the photodegradation leads to the formation of four photoproducts. Three photoproducts were tentatively identified by HPLC-MS analyses and another one was proposed by taking into account the literature. (C) 2004 Elsevier B.V. All rights reserved.