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

Abstract

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).