The stability of some metal EDTA, DTPA and DOTA complexes: Application as tracers in groundwater studies

Studies of the stability of various metal EDTA, DTPA and DOTA complexes in order to evaluate their applicability as non-sorbing tracers have been perf ormed. In laboratory tests, the stability generally increases for the indiv idual metal ions in the EDTA<DTPA<DOTA order. For most metal ions, the same trend can be observed for the thermodynamic stability constants. In the in situ experiment, various metal EDTA tracers were used in very low concentr ations; YbEDTA(-), for example had a breakthrough and recovery which were v ery similar to the non-sorbing tracers used. According to the extremely low tracers concentrations used, thermodynamic data indicate that all metal ED TA tracers should have been decomplexed as a result of the competition with the naturally occurring cations in the groundwater. This was not found, wh ich indicates that the decomplexation rate and sorption mechanism are impor tant in estimating the applicability of the metal complexes as tracers. The DOTA complexes of elements in the middle of the lanthanide series have ind icated high stability in the laboratory tests and therefore appear to be go od candidates as non-sorbing tracers. However, in contrary to the metal EDT A tracers, the DOTA complexes of La3+ and Lu3+ Seemed to be slightly delaye d in the in situ experiment.