Kinetics of Fe-II-polyaminocarboxylate oxidation by molecular oxygen

摘要

Complexation of iron by naturally-occurring and synthetic organic ligands has a large effect on iron oxidation and reduction rates which in turn affect the aqueous geochemistry of many other chemical constituents. In this study, the kinetics of Fe-II oxidation in the presence of the polyaminocarboxylate synthetic chelating agents ethylene glycol tetraacetic acid (EGTA) and trimethylenediamine-N,N,N',N'-tetraacetic acid (TMDTA) was investigated over the pH range 5.50-8.53. Batch oxidation experiments in the presence of molecular oxygen were conducted using a 2:1 M concentration ratio of polyaminocarboxylate (ligand, L) to Fe-II. The experimental data resembled first order kinetics for the oxidation of Fe-II-L to Fe-III-L and observed rate constants at pH 6.0 were comparable to rate constants for the oxidation of inorganic Fe-II. Similar to other structurally-similar Fe-II-polyaminocarboxylate complexes, oxidation rates of Fe-II-EGTA and Fe-II-TMDTA decrease with increasing pH, which is the opposite trend for the oxidation of Fe-II complexed with inorganic ligands. However, the oxidation rates of Fe-II complexed with EGTA and TMDTA were considerably lower (4-5 orders of magnitude) than Fe-II complexed to ethylenediaminetetraacetic acid (EDTA). The distinguishing feature of the slower-reacting complexes is that they have a longer backbone between diamine functional groups. An analytical equilibrium model was developed to determine the contributions of the species (FeL2)-L-II- and Fe-II(H)L- to the overall oxidation rate of Fe-II-L. Application of this model indicated that the protonated Fe-II(H)L species are more than three orders of magnitude more reactive than (FeL2-)-L-II. These rate constants were used in a coupled kinetic equilibrium numerical model where the ligand to iron ratio (TOTL:TOTFe) and pH were varied to evaluate the effect on the Fe-II oxidation rate. Overall, increasing TOTL:TOTFe for EGTA and TMDTA enhances Fe-II oxidation rates at lower pH and inhibi