Kinetics and mechanism of rhenium-ethylenediaminetetraacetic acid (Re(IV)-EDTA) complex degradation; For 99Tc-EDTA degradation in the natural environment

Mechanism and kinetics of Rhenium complexes as a surrogate of Technetium-99 (99Tc) is worthy of study from radioactive waste safe disposal perspective. Re(IV)-EDTA was synthesized via the reduction of Re(VII) with Sn(II) in the presence of Ethylenediaminetetraacetic acid (EDTA). The Re(IV)-EDTA was then degraded by H2O2 (7%–30%) at pH of 3–11 in ionic strength I = 0–2 M solution. The Re-EDTA was observed to degrade more rapidly at pH of ≤ 3–4 than one of ≥ 10–11 and remained stable at pH = 7–9. The Re-EDTA was degraded in accordance with the H+ addition mechanism in the acidic range and ligand charge transfer in the alkaline region. Complex degradation followed the zero-order rate kinetics for the H+ and Re-EDTA parameters, apart from a pH of 3, for which degradation was a better fit to first order kinetics. A higher Re(IV)-EDTA stability at a pH of 7–9 demonstrated that Re(IV)-EDTA (or 99Tc(IV)-EDTA) tends to be more persistent in natural environments similar to the pH range of 7–9. [Display omitted] •Re-EDTA and [ReO-Cl5]2- complexes were degraded by H2O2.•Re-EDTA degradation followed exponential model in pH ≤ 4.•Re-EDTA degradation followed a linear model in pH ≥ 10.•Re-EDTA was stable at pH 7–7.5 because H2O2 undergoes self-degradation.•99Tc-EDTA complex may follow the same Re-EDTA degradation model.