Land-application of biosolids containing per- and polyfluoroalkyl substances (PFAS) can disseminate these pollutants into crops, soils and environmental water. Several US states have already implemented regulations on biosolids application based on PFAS content, necessitating alternative management methods. Recent studies indicate that pyrolysis of the biosolids can significantly reduce the PFAS load in the produced biochar, which can then be used as a safer soil amendment. However, not much is known about the transformation products and underlying degradation mechanisms, and how these are affected by pyrolysis conditions such as temperature and biosolids composition. In this study, biosolids were sampled from multiple water resource recovery facilities across the US to represent different compositions and PFAS concentrations, and were pyrolyzed at 300^oC, 500^oC and 700^oC. The fate of 40 PFAS was quantified using EPA method 1633, and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) was leveraged to substantially expand the number of identifiable transformation products and aid in developing degradation reaction schemes. In addition to the monitoring of PFAS fate in as-received samples, individual compounds – including perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorooctanesulfonamide (FOSA), N-ethyl perfluorooctanesulfonamido ethanol (N-EtFOSE), 5:3 fluorotelomer carboxylic acid (5:3 FTCA), 8:2 fluorotelomer sulfonic acid (8:2 FTSA), and perfluoro-3,6-dioxaheptanoic acid (NFDHA) – were spiked into the biosolids before pyrolysis to elucidate their transformation patterns. Our results show that even at 300^oC, pyrolysis successfully removed >96% of the PFAS mass, which may allow the production of higher-quality biochar at lower energy investment and without compromising environmental safety. Ultimately, the results of this work will promote our understanding of pyrolysis as a viable means for removing PFAS from biosolids and other media, at lower costs and energy requirements compared to current practices.

Gregory Dooley

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