Effect of Mg and Ca on the Stability of the MRI Contrast Agent Gd–DTPA in Seawater

Abstract

Gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) is widely applied as a contrast enhancer in medical MRI. As Gd-DTPA is only minimally captured in wastewater treatment plants (WTPs) or degraded by UV light and other oxidative processes, concentrations in rivers have increased globally by orders of magnitude following its introduction in 1987. The complex also seems impervious to estuarine scavenging and is beginning to emerge in coastal waters, yet it is unknown how its stability is changed by competition for the DTPA ligand from major seawater cations. We performed potentiometric titrations at seawater ionic strength (0.7 M NaClO 4 ) to determine dissociation constants of the five DTPA carboxylic acid groups, as well as stability constants of Mg, Ca, and Gd complexes with the fully deprotonated and single-protonated ligand. These are in general agreement with literature values at low ionic strength and confirm that complexes with Ca are more stable than with Mg. A new finding, that the DTPA complexes of Mg and Ca appear to be hydrolyzed at elevated pH, implies that their coordination in these chelates is less than hexadentate, enabling additional competition with Gd from dinuclear Mg and Ca species. Side-reaction coefficients for trace-metal-free seawater, calculated from our results, suggest that the higher abundance of Mg and Ca may significantly destabilize Gd-DTPA in coastal waters, causing dissociation and release of as much as 15% of the organically complexed Gd from the ligand. This effect could magnify the particle-reactivity and bioavailability of anthropogenic Gd in sensitive estuarine habitats, indicating an urgent need to further study the fate of this contaminant in marine environments.

Rare Earth Element Behavior in a Sandy Subterranean Estuary of the Southern North Sea

“…This suggests that the samples are roughly of the same age, as it has been shown that there is a degradation of the complexes over time (Holzbecher et al, 2005). Several factors have been found to influence the destabilization of Gd-complexes e.g., transmetallation or competition for binding places of the DPTA complexes by Ca, Mg, or K ions (Schijf and Christy, 2018).…”

“…However, the authors suggested that different biogeochemical conditions in natural waters (e.g., different concentrations of organic matter) potentially influence the degradation of Gd complexes (Holzbecher et al, 2005). In estuaries, up to 15% of Gd-DPTA can dissociate due to the competition of Mg or Ca ions for DTPA (Schijf and Christy, 2018). Yet, because of the many different complexes in use, their stability may differ under natural conditions and little is known about degradation and transformation of the complexes under natural conditions (Birka et al, 2016).…”

Compound-specific recording of gadolinium pollution in coastal waters by great scallops

“…The situation is different in the sea, but few data exist on the stability of GBCAs in marine waters. At present, only gadopentetate dimeglumine has been studied in depth 26 . This contrast agent is substantially dissociated upon mixing of river water with seawater in estuaries.…”

Lifecycle of Gadolinium‐Based Contrast Agents

“…Speciation analysis of the different stages of wastewater treatment shows samples after the biological treatment stage and effluent contain unidentifiable Gd species, suggesting that species transformation had occurred 46 . It is understood that degradation of the Gd‐chelate complex is possible from bacterial attack, UV photo‐oxidation, as well as transmetallation 48 . Models 48 demonstrate that the stability of Gd DTPA complexes are about six orders of magnitude lower in saltwater than in freshwater due to substantial transmetallation competition from abundant magnesium and calcium.…”

“…It is understood that degradation of the Gd‐chelate complex is possible from bacterial attack, UV photo‐oxidation, as well as transmetallation 48 . Models 48 demonstrate that the stability of Gd DTPA complexes are about six orders of magnitude lower in saltwater than in freshwater due to substantial transmetallation competition from abundant magnesium and calcium. This suggests that upon mixing of river water with seawater in estuaries, up to 15% Gd can be released from the DTPA ligand due to transmetallation competition from Mg and Ca alone, as modeled with marine water samples from Hatje et al 48 …”