Effect of thermochemical treatment of sewage sludge on its phosphorus leaching efficiency: Insights into leaching behavior and mechanism

Thermochemical conversion, including hydrothermal processing, pyrolysis and incineration, has become a promising technology for sewage sludge (SS) treatment and disposal. Furthermore, acid leaching is considered as an effective method to recover phosphorus (P) from SS and its thermochemical treatment products. This study has investigated the potential of P reclamation from SS and its thermochemical derivatives, including hydrochar (HC), biochar (BC), and SS incinerated ash (SA). Comparative analyses of physicochemical properties of these derivatives revealed a decrease in hydroxyl and aromatic groups and an increase in aliphatic and oxygen-containing functional groups in HC and BC. Leaching experiments using 1 M sulfuric acid (H₂SO₄) and 1 M oxalic acid (C₂H₂O₄) suggested that H₂SO₄ slightly outperformed C₂H₂O₄ in terms of P leaching efficiency. HC achieved 79.1 % optimal leaching efficiency in 60 min using H₂SO₄, while BC, SS, and SA required 360 min to achieve comparable efficiency. SS and BC reached optimal leaching efficiency at 74.1 % and 76.2 % in H₂SO₄, while SA achieved 80.9 % in C₂H₂O₄. Importantly, HC and SA are more favorable for P extraction using acid leaching, whereas BC tends to be a potential P carrier. Time-dependent kinetics revealed a two-stage leaching process, i.e., fast and slow reaction stages. Shrinking core model indicates product layer diffusion as the primary rate-limiting step in both stages. Overall, these fundamental insights play an important role in practical P recovery through acid leaching of SS derived residues after thermochemical treatment.