Ozone as oxidizing agent for the total oxidizable precursor (TOP) assay and as a preceding step for activated carbon treatments concerning per- and polyfluoroalkyl substance removal

Abstract

Several thousands of highly persistent per- and polyfluoroalkyl substances (PFAS) exist and it is therefore challenging to analytically determine a larger spectrum of these compounds simultaneously in one sample. It is even more difficult to efficiently remove mobile PFAS in wastewater treatment plants (WWTPs) to protect the receiving waters. The total oxidizable precursor (TOP) assay is an approach that enables the detection of the total PFAS content in a sample via oxidation of precursors, followed by subsequent analysis of the perfluoroalkyl acid (PFAA) concentration before and after oxidative processes. Activated carbon combined with a preceding ozonation step is considered a promising tool for the removal of micropollutants but considering PFAS removal efficiencies in effluents for this process combination more information is required. The focus of the study was to implement and assess the TOP assay with ozone as oxidizing agent to estimate the total PFAS content in a WWTP effluent. Additionally, granular activated carbon (GAC) and powdered activated carbon (PAC) with a preceding ozonation step was tested for the removal efficiencies for 22 PFAS. For the TOP assay the obtained accordance in molarity using spiked tap water as quality control was 95.2% (15 mg O3/L) and 99.1% (6 mg O3/L). Applying the TOP assay, an estimated total PFAS content of 840 ng/L was determined in the respective effluent, which was 91.1% higher than obtained by target PFAS analysis, implying the presence of unknown precursors not included in common monitoring. While all treatment techniques that included ozone or a preceding ozonation step solely transformed precursors and long-chain perfluoroalkyl acids (PFAA, i.e., >C9) to shorter congeners, PAC was the only tested water treatment application that was able to remove 19.3% of the total PFAS molarity.