Stabilisation of PFAS in biosolids by activated carbon and biochar: Implications for leaching control

Biosolids, the major by-products of wastewater treatment plants (WWTPs), are widely applied to agricultural land as cost-effective fertilisers and soil amendments. However, they have emerged as a source of per- and polyfluoroalkyl substances (PFAS) released into the environment. This study investigated PFAS leachability from 22 biosolids collected from various Australian WWTPs before and after addition of two sorbents, granular activated carbon (GAC) and biochar, applied at 1% (w/w). A total of 22 PFAS compounds with different CF₂ chain lengths and functional head groups were detected in the biosolids, with perfluorooctane sulfonate (PFOS) the most abundant (11.3-107 µg kg⁻¹). PFAS leachability from untreated biosolids decreased with increasing CF₂ chain length and varied with the functional head group. The GAC and biochar sorbents effectively stabilised PFAS, with greater efficiency for long-chain compounds. GAC exhibited slightly higher stabilisation efficiency than biochar, likely due to its larger surface area and greater pore accessibility. Despite variations in biosolids properties, immobilisation significantly reduced PFAS leachability, with PFOS immobilisation exceeding 85% in 80% of tested biosolids. Furthermore, the sorbents reduced the leachability of pharmaceuticals present in the biosolids, although the efficiency depended on individual compound chemistry. Overall, this study demonstrates that immobilisation using GAC or biochar can effectively reduce PFAS leachability from biosolids. This approach may help limit PFAS mobility in the environment and biosolids application in agriculture.