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-O-2-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 x 10(4) s(-1)) with a concomitant increase at around 290 nm. The 480 nm intermediate does not react with O-2 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 x 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 = 10(9) dm(3) mol(-1) s(-1); G(NF-) = 4.2 x 10(-7) mol J(-1)]. This rapid formation of NF- is followed by a slower one [G(NF-) = 1.5 x 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 x 10(8) dm(3) mol(-1) s(-1)) with O-2, and subsequent fast O-2(.-) 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 gamma-radiolysis: formaldehyde (1.6), CO2 (2.5), formic acid (0.7), glyoxylic acid (3.6), iminodiacetic acid (2.1), ethylenediaminetriacetic acid (detected, not quantified).