Hydrothermal processing (HTP) is an efficient thermochemical technology to achieve sound treatment and resource recovery of sewage sludge (SS) in hot-compressed subcritical water. However, microplastics (MPs) and heavy metals can be problematic impurities for high-quality nutrients recovery from SS. This study initiated hydrothermal degradation of representative MPs (i.e., polyethylene (PE), polyamide (PA), polypropylene (PP)) under varied temperatures (180-300 °C) to understand the effect of four ubiquitous metal ions (i.e., Fe³⁺, Al³⁺, Cu²⁺, Zn²⁺) on MPs degradation. It was found that weight loss of all MPs in metallic reaction media was almost four times of that in water media, indicating the catalytic role of metal ions in HTP. Especially, PA degradation at 300 °C was promoted by Fe³⁺ and Al³⁺ with remarkable weight loss higher than 95% and 92%, respectively, which was ca. 160 °C lower than that in pyrolysis. Nevertheless, PE and PP were more recalcitrant polymers to be degraded under identical condition. Although higher temperature thermal hydrolysis reaction induced severe chain scission of polymers to reinforce degradation of MPs, Fe³⁺ and Al³⁺ ions demonstrated the most remarkable catalytic depolymerization of MPs via enhanced free radical dissociation rather than hydrolysis. Pyrolysis gas chromatography-mass spectrometry (Py GC-MS) was further complementarily applied with GC-MS to reveal HTP of MPs to secondary MPs and nanoplastics. This fundamental study highlights the crucial role of ubiquitous metal ions in MPs degradation in hot-compressed water. HTP could be an energy-efficient technology for effective treatment of MPs in SS with abundant Fe³⁺ and Al³⁺, which will benefit sustainable recovery of cleaner nutrients in hydrochar and value-added chemicals or monomers from MPs.

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.