hide
Free keywords:
Transmetallation; Kinetics; DTPA chelate; Rare earth elements; Cu
DDC:
550 - Earth sciences
Abstract:
The kinetics of transmetallation of Gd3+ in diethylenetriaminpentaacetic acid (Gd-DTPA) by rare earth elements (except Yb), Y3+ and Cu2+ has been studied in test solutions ranging from pH 5 to 6.5 in order to understand the distribution and longevity of their chelates in the hydrosphere. Chelated and non-chelated species were separated by adsorption of the free ions by clay minerals before analysis by ICPMS. This simple method allows determining the distribution of DTPA chelates at concentration levels as low as a few 10 pmol/l. The transmetallation constants at ionic strength of 0.01 mol/l and room temperature are commensurable with those derived from published formation constants of the corresponding DTPA complexes. It could be proved that DTPA chelates of rare earth elements, Y and Cu are stable at least over 6 months. The pattern of the second-order rate constants shows a strong tetrad effect, which is not displayed by the pattern of transmetallation constants. Based on these results, the rate of transmetallation of Gd-DTPA under natural conditions is derived. The resultant process yields 10% turn-over of initial Gd-DTPA within two months, if Cu2+ concentrations are below 1 nmol/l. At higher concentrations, the turn-over increases with elapsed time. Equilibrium among REE and Y is expected only after tens of years. This makes Gd-DTPA a powerful tracer in hydrological studies.
