In order to understand the speciation of EDTA, the exchange kinetics of Fe(III)-EDTA with other cations (Zn, Ca) are studied under the conditions of natural river water. Measurements in sewage treatment plant effluents indicate that EDTA is partly present as Fe(III)-EDTA. At equilibrium, Fe-EDTA introduced into river water is expected to exchange with Zn(II) and Ca to form Zn-EDTA or Ca-EDTA. The exchange of Fe-EDTA with Zn(II), which was followed by voltammetric measurement of labile Zn, as well as the disappearance of Fe-EDTA, which was measured as a photolabile species, occur on a time scale of days. The results are presented in terms of second-order rate laws, with the average observed rate constant, k = 10.3 M-1 s-1 for the exchange of Zn(II) with Fe-EDTA in river water; the dissociation of Fe-EDTA appears to be the rate-limiting step for this reaction, with a first-order rate constant of 4 × 10-7 s-1. The exchange of Zn(II) with Ca-EDTA is much faster, with k = 1.1 × 103 M-1 s-1. From the first-order rates, a half-time of about 20 days is obtained for the exchange of Fe(III)-EDTA in the river water. If a slowly dissociating complex, such as Fe-EDTA, is initially present, the initial speciation is determining the fate of EDTA in river water; equilibrium speciation of EDTA and trace metals may not be reached within the time scale of the river flow if the inputs occur as Fe-EDTA.
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Xue et al. (1995) studied this question.