Search

The reaction of aqueous iron(vi) (FeVIO42−, Fe(vi)) with ethylenediaminetetraacetic acid (EDTA) was studied kinetically as a function of pH (1.98–12.40) and temperature (15–45 °C) using a stopped flow kinetic technique. The rate law for the reaction of Fe(vi) with EDTA was found to be first-order with respect to each reactant over the entire studied pH range. The observed rate constants, k, decrease with an increase in pH, varying from 4.19 × 104 to 8.60 × 10−2 M−1 s−1 over the pH range. The speciation of Fe(vi) (H3FeO4+, H2FeO4, HFeO4−, and FeO42−) and EDTA (H4Y, H3Y−, H2Y2−, HY3−, and Y4−, Y = EDTA) species was used to explain the pH dependence of the k values. From the temperature effect on k at pH 5.4, 7.1, and 9.2, activation parameters, ΔS‡ and ΔH‡, were obtained for the reactions of Fe(vi) with EDTA. The values of ΔS‡ for the reactions were found to be negative, implying a highly ordered transition state in the reaction. The ΔH‡ for the reaction at pH 7.1 and 9.2 showed similar values within experimental error. Using the observed enthalpy parameters and the enthalpy of deprotonation of HFeO4− and EDTA species (HEDTA3− and H2EDTA2−), the enthalpy of deprotonation of H2FeO4 (ΔH0H2FeO4) was determined as 5.7 ± 3.0 kJ mol−1. The reactivity of Fe(vi) with aminopolycarboxylates (APCs) was also studied in alkaline medium. The order of reactivity was determined as primary > secondary > tertiary, which suggests that FeVIO42− attacks at the nitrogen atom sites of APCs. [ABSTRACT FROM AUTHOR]

Amino acids and peptides may form potentially harmful disinfection byproducts during the conventional treatment of water and wastewater. Removal of these parent compounds by the use of the environmental-friendly oxidant, ferrate(VI) (Fe(VI)O(4)(2-), Fe(VI)) was assessed by studying the kinetics of the oxidation of glycine (NH(3)(+)CH(2)COOH, Gly) and glycylglycine (NH(3)(+)CH(2)CONHCH(2)COOH, Gly-Gly) as a function of pH (4.0-12.4) at 25 degrees C. This study with Gly-Gly represents an initial investigation of oxidation of peptides by Fe(VI). Generally, the second-order rate constant (k) increased with decreased pH in the basic pH region, but this trend was reversed in the acidic pH range. Consideration of the reactivity of three oxidants (H(2)FeO(4), HFeO(4)(-), and FeO(4)(2-)) with three species of Gly and Gly-Gly (positive, neutral, and negative) reasonably explained the pH dependence of the rates. At pH 9.0, the molar consumption of Fe(VI) was nearly equal to that of Gly. The reaction of Fe(VI) with Gly at molar ratios of 1.0 and 2.0 ([Fe(VI)]:[Gly]) produced ammonia, carbon dioxide, and acetate. A reaction scheme is proposed which explains the formation of these products. The values of k for oxidation of iminodiacetate and nitriloacetate at pH 7.0 were also determined in order to compare oxidation of amines by Fe(VI). The calculated half-lives at neutral pH for the oxidation of primary and secondary amines were in seconds while decomposition of tertiary amines would occur in minutes. Overall, the reactivity of Fe(VI) with Gly and Gly-Gly indicates the significant potential of Fe(VI) to remove amine- and peptide-containing pollutants in water and wastewater., ((c) 2009 Elsevier Ltd. All rights reserved.)