EVALUATING THE FATE OF GADOLINIUM THROUGH MUNICIPAL WASTEWATER TREATMENT PLANT OPERATIONS

A primary pathway for natural and synthetic estrogens, and other emerging contaminants, to enter aquatic ecosystems is effluent from municipal wastewater treatment plants (WWTP). Some of these compounds have been shown to disrupt normal endocrine function in aquatic organisms at environmentally relevant concentrations. Evaluating and amending WWTP practices to minimize the release of these compounds is an important area of research, but since these compounds are typically present at trace concentrations and undergo complex chemical partitioning and transformation, it is difficult to quantify the change in estrogen loads through a WWTP. Recent work on the inorganic chemistry of WWTP effluents has shown that gadolinium (Gd), a rare earth element (REE), is enriched in the effluent in many urban areas because of its use as a contrasting agent in magnetic resonance imaging (MRI). The Gd-organic complexes used in MRIs are chemically stable. If the MRI-derived Gd behaves conservatively during routine treatment plant operation, it could be a useful tracer of the more reactive estrogenic compounds during WWTP operations.

To evaluate how Gd behaves throughout the wastewater stream, four full-scale WWTPs operating with various technologies were studied. Samples were collected throughout the solids treatment train, and aqueous samples were collected from key locations. Samples were analyzed for inorganic constituents, estrogenic compounds, and other organic wastewater compounds. The REE patterns of the aqueous samples (WWTP influent and primary and secondary effluents) show that Gd is enriched in the aqueous phase throughout the wastewater stream, Gd/Gd* ranges from 13 to 137, where Gd* is the expected Gd value of a smooth, shale-normalized REE pattern. In contrast, there was no Gd enrichment in the solid phases throughout the wastewater train, Gd/Gd* of ~1. Solid phases included primary sludge and solids produced along the wastewater train such as dewatering, anaerobic digestion, acid-phase digestion, composted, and pelletized sludges. Since Gd is not enriched in any of the solid phases, it appears that the MRI-derived Gd behaves conservatively through routine WWTP operations. Work is underway to quantify the flux of Gd and estrogenic compounds through the wastewater stream to further test the use of Gd as a tracer.