Oxidation reactions of carbaryl in aqueous solutions

Abstract

Hydroxyl radical induced oxidation of carbaryl has been studied using steady state photolysis followed by high-resolution mass spectrometry (LC-Q-TOF-MS), pulse radiolysis, and theoretical (DFT) calculations. The reaction of ●OH with carbaryl resulted in a number of hydroxylated adduct radicals (λmax - 330 nm and 390 nm; k2 - 1.2 × 1010 dm3 mol-1 s-1). The DFT calculations and results obtained from LC-Q-TOF-MS analysis shows the possible addition of ●OH at C1 (energetically most stable) and C7 positions of carbaryl leading to the generation of resonance stabilized hydroxycyclohexadienyl-type radicals as the immediate intermediates, which eventually converted into naphthol and a hydroxylated naphthols. LC-Q-TOF-MS results also revealed the formation of other hydroxylated derivatives and naphthoquinones that are most likely originated from the consecutive ●OH attack on the initially formed products. Naphthoquinones are found to undergo ring opening and the corresponding products are identified. The reaction of SO4●- with carbaryl, on the other hand, results the radical cation of parent molecule (λmax - 320 nm and 390 nm) which exhibits reasonable stability in the pulse radiolysis timescale. Total organic carbon (TOC) analysis after H2O2/UV photolysis revealed that nearly 70% of the organic content is mineralized after 35 min of irradiation, which demonstrates the potential application of oxidative methods towards the degradation of carbaryl.