Occurrence of perfluorinated carboxylic acids in Mexico City's wastewater: A monitoring study in the sewerage and a mega wastewater treatment plant

Highlights: • Five PFCA detected in Mexico City's wastewater over a seven-month monitoring study. • Concentrations of short-chain PFCA were higher than those of long-chain compounds. • Primary and secondary treatment systems are unable to remove even PFCA. • The content of the even PFCA were higher at the effluent of biological treatment. • Reuse of wastewater in agricultural irrigation spread PFCA in croplands. An analytical method based on liquid chromatography coupled to mass spectrometry was validated to quantify five perfluorinated carboxylic acids (PFCA) namely, perfluorobutanoic acid (PFBA), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), and perfluoroundecanoic acid (PFUnA), in wastewater produced in a megacity. Sampling was performed on a monthly basis, obtaining samples from the undergrounded sewerage system and the main open-air canal transporting wastewater out the city. Steady levels of the sum of the target PFCA (ƩPFCA) were determined on both sites through the study: 419.4 ± 24.3 ng L⁻¹ in undergrounded sewage and 591.1 ± 39 ng L⁻¹ in the open-air canal. Short-chain PFCA (PFBA, PFHxA, and PFHpA) were abundant, while concentrations of PFOA and PFUnA remained lower in both sampling sites. The open-air canal was transected in four sampling points, which were sampled throughout the monitoring campaign, finding that furtive discharges of municipal and industrial wastewater increased the levels of short-chain PFCA, while those of PFOA and PFUnA were depleted. Relevant concentrations of PFBA (176.9 ± 3.3 ng L⁻¹), PFHxA (133.4 ± 2.5 ng L⁻¹), PFHpA (116.6 ± 3.9 ng L⁻¹), PFOA (133.1 ± 3.5 ng L⁻¹), and PFUnA (23.5 ± 6.5 ng L⁻¹) were found 60 km downstream, where the wastewater transported by the open-air canal is used in irrigation. A fraction of sewage is treated in a conventional wastewater treatment plant. The concentration of short-chain PFCA increased in effluent, adding extra loads of PFBA, PFHxA, and PFHpA to the environment.