Abstract
The reaction of three chloronicotinoid insecticides, namely Imidacloprid (IMD), Thiacloprid (THIA) and Acetamiprid (ACT), with carbonate radicals (CO3−) was investigated. The second order rate constants (4 ± 1) × 106, (2.8 ± 0.5) × 105, and (1.5 ± 1) × 105 M−1 s−1 were determined for IMD, THIA and ACT, respectively. The absorption spectra of the organic intermediates formed after CO3− attack to IMD is in line with those reported for α-aminoalkyl radicals. A reaction mechanism involving an initial charge transfer from the amidine nitrogen of the insecticides to CO3− is proposed and further supported by the identified reaction products. The pyridine moiety of the insecticides remains unaffected until nicotinic acid is formed. CO3− radical reactivity towards IMD, ACT, and THIA is low compared to that of HO• radicals, excited triplet states, and 1O2, and is therefore little effective in depleting neonicotinoid insecticides.
Highlights
Carbonate radical (CO3À) is a selective one-electron oxidant, E0 (CO3À/CO23À) 1⁄4 1.78 V vs. NHE (Wu and Linden, 2010), capable of initiating the oxidation of many organic compounds
Photolysis of air-saturated solutions of pH 7.7 containing 0.025 M S2O28À and 1 M HCOÀ3 showed the formation of a transient species absorbing in the wavelength range from 400 to 670 nm whose spectrum taken immediately after the flash of light is in agreement with that reported for CO3À (Behar et al, 1970; Busset et al, 2007)
Taking c(l) values obtained from the fitting of the traces at 600 nm to Eq (1) and considering ε600(CO3À) 1⁄4 2000 Æ 100 MÀ1 cmÀ1, (Zuo et al, 1999) it results that 2k3 1⁄4 (1.1 Æ 0.1) Â 107 MÀ1 sÀ1 for a reactive mixture of 1.08 ionic strength
Summary
Carbonate radical (CO3À) is a selective one-electron oxidant, E0 (CO3À/CO23À) 1⁄4 1.78 V vs. NHE (Wu and Linden, 2010), capable of initiating the oxidation of many organic compounds. Carbonate radical reactivity is high for electron-rich N-containing chemicals such as heterocycles, fenuron, carbendazim and phenylurea herbicides (Busset et al, 2007; Mazellier et al, 2007). It is reactive with sulphur-containing compounds such as thioanisole, dibenzothiophene, fenthion, and. Carbonate radicals in natural waters are mainly formed by the reactions of carbonate/bicarbonate ions with either hydroxyl radicals or aromatic-ketones triplet excited states as those contained in dissolved organic matter, DOM (Canonica et al, 2005; Huang and Mabury, 2000b; Lam et al, 2003; Vione et al, 2009a; Wu and Linden, 2010).
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