The fate of per- and polyfluoroalkyl substances (PFAS) during pyrolysis and co-pyrolysis of biosolids with alum sludge and wheat straw

The literature suggests that the pyrolysis of biosolids merely volatilizes per- and polyfluoroalkyl substances (PFAS) into gas or liquid phase, requiring a thermal oxidizer operating at 900-1200 degrees C with more than 2 second residence time, to destroy the volatilized PFAS. This study investigates the potential and extent of destruction of PFAS under pyrolysis environment. Pyrolysis and co-pyrolysis experiments were conducted in a bench-scale horizontal fixed-bed reactor. Co-pyrolysis experiments was carried out by blending biosolids with alum sludge or wheat straw at a 1:1 (w/w) mixing ratio at 600 degrees C. Gas and solids residence times in the reactor were approximately 10 seconds and 1 hour, respectively. The study also explored the effects of lime and biochar as catalysts on PFAS destruction. Results showed near complete removal of PFAS from biochar with the total PFAS concentration reducing from 409.9 ng/g in biosolids to 0.31 ng/g in biochar, suggesting significant volatilization or decomposition at 600 degrees C. However, PFAS concentrations in volatile products were also significantly low (5.5 ng/g in bio-oil and 0.15 ng/g in scrubber water), indicating 99.4 % PFAS destruction efficiency. The co- pyrolysis of biosolids further improved PFAS removal, likely due to dilutive and synergistic effects. However, catalysis with lime and biochar did not have a significant effect on the PFAS destruction efficiency. This research highlights the potential for near complete PFAS destruction in pyrolysis and co-pyrolysis under the current experimental conditions but emphasizes the need for further investigations under the operational parameters of large-scale pyrolysis plants.