Kinetics of the oxidation of ferrous chelates of EDTA and HEDTA in aqueous solution

The kinetics of the reaction of oxygen with ferrous chelates of EDTA and HEDTA was studied in a stirred cell reactor under industrial conditions. The temperature was varied from 20 to 60-degrees-C and the concentration of the ferrous chelate ranged from 0 to 100 mol/m3. The initial pH was 7.5. Under these conditions, the reaction appeared to be first order in oxygen and second order in ferrous chelate. The activation energy of the oxidation reaction is 27.2 +/- 2.3 kJ/mol for the ferrous EDTA complex and 36.0 +/- 1.1 kJ/mol for the HEDTA complex. At 25-degrees-C, the rate constant k21 for Fe(II)HEDTA is 2.8 x 10(-2) and for Fe(II)EDTA 6.5 X 10(-2) m6/(mol2.s). The diffusivity of these chelates (D(B)) could be related to the diffusivity of oxygen (D(A)) by D(B)/D(A) = 0.183. Contrary to expectations, the overall reaction stoichiometry was found to be less than 4 due to side-reactions. Generally, the overall reaction stoichiometry was between 3 and 4. An overall reaction model is presented that explains both the results of this work and earlier results from open literature. A rate equation is derived from this model, see eq 48, with (2 + nu(B)+) being the overall reaction stoichiometry.