Kinetic Interactions Between the Emerging Pollutant Tungsten(VI), Hydrogen Peroxide, and Biological Amino Acids

ABSTRACT

The kinetics of the reduction of tungstate ion by hydrogen peroxide in aqueous media containing phosphate ions, both in the absence and presence of amino acids as oxidizable organic substrates, have been monitored spectrophotometrically at 225–230 nm. Three long‐lived tungsten intermediates have been detected: the first two being monoperoxo and diperoxo tungsten(VI) complexes, and the third believed to be W(IV). The kinetics of decay of the peroxo complexes has been studied by the use of the initial rate method, since it showed a deceleration much more accused than expected for a pseudo‐first order process. The reaction in the absence of organic compounds was of first order in both tungstate ion and hydrogen peroxide, and of a non‐integral order (0.71 ± 0.10) in hydrogen ion (acid catalysis). The oxidation of amino acids was of first order in the amino acid, with a non‐integral kinetic order (1.7 ± 0.2) in hydrogen peroxide, and showed the coexistence of both acid catalysis and base catalysis. The oxidation of eight biological amino acids was studied, the less reactive being the ones with no additional functional group (alanine, monosodium glutamate, glycine, and proline), a little more reactive being the amino acids containing a hydroxyl group (threonine, serine, and hydroxyproline), whereas substitution of the latter group by a thiol group (cysteine) resulted in a dramatic increase in the reactivity. The proposed mechanism involves four different oxidation states of tungsten(VI, V, IV, and III), as well as two free radicals, one inorganic (hydroperoxyl) and the other organic, the strong deceleration of the reaction being explained by the accumulation in the solution of a long‐lived intermediate, presumably W(IV).