Solid digestate-derived biochar: Characterisation and polycyclic aromatic hydrocarbon removal efficiency

The integration of pyrolysis following anaerobic digestion presents advantages in waste-to-resource conversion and maximises energy recovery by transforming solid digestate into valuable products such as biochar. This study provides a comparative analysis of the composition of primary solid digestate (PSD) and secondary solid digestate (SSD) from an on-farm anaerobic digester treating animal manure, assessing the effectiveness of the resulting biochar as an adsorbent for the removal of polycyclic aromatic hydrocarbons. PSD and SSD were characterised by high lignin contents of 39.33 and 42.7 wt%, respectively, with the primary difference being the higher ash content in SSD compared to PSD. Results indicated that PSD biochar (PSD-B500) exhibited superior quality, with a higher specific surface area (249 m 2 ·g –1 ), micropore volume (0.106 cm 3 ·g –1 ), and a greater concentration of acidic functional groups (6.64 mmol·g –1 ) compared to SSD biochar (SSD-B500) (31.9 m 2 ·g –1 , 0.0019 cm 3 ·g –1 , and 5.39 mmol·g –1 , respectively). According to the Langmuir isotherm model, the adsorption capacities of phenanthrene, anthracene, fluoranthene, and pyrene on PSD-B500 were 86%, 411%, 900%, and 83% higher than on SSD-B500, respectively. Overall, using PSD in the pyrolysis process produced high-quality biochar with superior surface area and functional group availability compared with SSD.