Speciation of gadolinium-based contrast agents in wastewater and seawater: a case study in the northern Adriatic Sea

Coastal environments receiving treated municipal wastewater are increasingly exposed to anthropogenic gadolinium derived from gadolinium-based contrast agents (GBCAs). However, information regarding the occurrence and fate of individual GBCAs in Mediterranean marine waters remains scarce. This study investigated total gadolinium and GBCA speciation in raw and treated wastewater from an advanced wastewater treatment plant and in the receiving waters of the Gulf of Trieste (northern Adriatic Sea). Analysis of raw and treated wastewater revealed low overall Gd removal efficiencies (∼24%), confirming that tertiary treatment processes fail to prevent GBCA discharge. High-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC–ICP-MS) identified the macrocyclic complexes gadoteridol (Gd-HP-DO3A) and gadoterate (Gd-DOTA) as the predominant GBCAs, with Gd-HP-DO3A accounting, on average, for over 60% of total Gd (Gd tot ) in wastewater. An optimised solid-phase extraction (SPE) sample preparation procedure using porous carbon enabled the first determination of individual GBCAs in high-salinity Mediterranean seawater. In the Gulf of Trieste, Gd-HP-DO3A exhibited a widespread spatial and temporal presence. However, GBCAs accounted for only 2% to 37% of Gd tot in seawater samples, indicating that natural riverine geogenic sources and unmonitored macrocyclic GBCAs constitute the remaining Gd fraction. These findings confirm that persistent macrocyclic GBCAs are efficiently transferred from wastewater to Mediterranean coastal ecosystems, establishing anthropogenic Gd (Gd anth ) and specific GBCA species as robust molecular tracers of wastewater effluent dispersion.