OH-Radical induced degradation of ethylenediaminetetraacetic acid (EDTA) in aqueous solution: a pulse radiolysis study

Hydroxyl radicals have been generated pulse radiolytically in N2O- and N2O–O2-saturated solutions. In their reaction with N,N,N′,N′-ethylenediaminetetraacetic acid (EDTA) they give rise to an absorption spectrum with maxima at 275 and 480 nm. The 480 nm band decays (k = 3.3 × 104 s–1) with a concomitant increase at around 290 nm. The 480 nm intermediate does not react with O2 and has been attributed to an N-centered radical cation bridged to the second nitrogen (N˙+N–EDTA). Using strong reductants as probes, e.g.N,N,N′,N′-tetramethylphenylenediamine, G(N˙+N–EDTA) = 1.6 × 10–7 mol J–1 has been obtained. Besides generating N˙+N–EDTA, the OH radicals produce C-centered radicals by H-abstraction. They have reducing properties and react rapidly with tetranitromethane forming the strongly absorbing nitroform (trinitromethane) anion [NF–; k = 109 dm3 mol–1 s–1; G(NF–) = 4.2 × 10–7 mol J–1]. This rapid formation of NF– is followed by a slower one [G(NF–) = 1.5 × 10–7 mol J–1] showing the same kinetics as the decay of the 480 nm absorption. It is hence concluded that the N˙+N–EDTA species decays into reducing C-centered radicals by deprotonation or electron transfer from a carboxylate group. The C-centered radicals react rapidly (k = 7.6 × 108 dm3 mol–1 s–1) with O2, and subsequent fast O2˙– elimination. The Schiff-bases thus formed hydrolyze yielding the final products.In the presence of oxygen the following products (G values in units of 10–7 mol J–1 in parentheses) have been observed after γ-radiolysis: formaldehyde (1.6), CO2 (2.5), formic acid (0.7), glyoxylic acid (3.6), iminodiacetic acid (2.1), ethylenediaminetriacetic acid (detected, not quantified).