Transmetallation of Gd-DTPA by Cu, Y and lanthanides and its impact on the hydrosphere

The concurrent exchange of REE3+ and Y3+ (combined to M3+) and Cu2+ for Gd3+ in Gd-DTPA (Gd-diethylenetriaminepentaacetic acid or gadopentetic acid) in the presence of clay is a very slow process if the concentrations of M3+, Cu2+ and Gd-DTPA in solution are in the range of 0.01–22 nmol/L. The kinetics of transmetallation was followed for 1033 h without reaching equilibrium, although the release of metal ions from the clay pool is a fast process. The sum of all newly formed mono-nuclear M-DTPA species is less than the difference [Gd-DTPAo] − [Gd-DTPA] even after 1033 h but the sum of all derived M-DTPA + Cu-DTPA chelates exceeds this difference indicating that within this time span poly-nuclear chelates of Cu also formed. Formation of CuGd-DTPA chelates is the fastest process followed by formation of less stable MGd-DTPA chelates. With progress of formation of CuGd-DTPA the concentration of Gd-DTPA is lowered and consequently MGd-DTPA decomposes. Furthermore Cu2+ reacts with MGd-DTPA to form CuM-DTPA. The observed rate constants vary from species to species, whereas the pseudo-first-order-rate constants kM are nearly the same for all lanthanides. The observed rate constant for kCu exceeds those of kM because Cu concentrations are higher than M. The changes in M speciation under the influence of DTPA are estimated for a typical composition of surface water. Input of Gd-DTPA leaves only La and, to a lesser degree, Ce unaffected by transmetallation. The total concentrations of both Cu and intermediate to heavy REE increase, whereas total Gd decreases because released Gd3+ is adsorbed by clay minerals. Depending on Cu2+ and GdL2− concentrations in natural surface and groundwaters, Gd-DTPA decreases by about 10% within a year. Equilibrium is theoretically reached only after more than 70 a.