With increasing concerns over contamination by heavy metals (HMs) in sewage sludge, treatment technologies integrating detoxification and resource recovery are urgently needed. This study used sewage sludge (SS) and four lignocellulosic biomasses, including pine sawdust (PS), bagasse (BS), sunflower straw (SFS), and water hyacinth (WH), to investigate the migration, transformation, and environmental risks of Cr, Cu, Mn, and Zn during co-hydrothermal carbonization (Co-HTC). Co-HTC reduced the solid-phase concentrations of HMs in hydrochars and promoted their transformation from exchangeable/reducible fractions to oxidizable/residual fractions. The hydrochar derived from SS and BS (H-SS&BS) showed the best stabilization of Cr and Mn, with oxidizable and residual fractions reaching 88.58% and 63.73%, respectively, whereas the hydrochar derived from SS and PS showed superior immobilization of Cu and Zn, with corresponding fractions of 83.30% and 70.93%, respectively. Risk assessment showed that the geoaccumulation index (I_geo) values of Cr, Cu, and Zn in H-SS&BS decreased to 0.65, 1.71, and 3.58, respectively. Life cycle assessment (LCA) indicated that BS and SFS systems had lower overall environmental burdens due to improved fuel quality and energy recovery potential. Overall, Co-HTC provides a promising strategy for the simultaneous stabilization of HMs, risk reduction, and resource recovery from sewage sludge.

Supercritical water gasification (SCWG) of biomass is a promising technology for efficient hydrogen production where the introduction of reactive co-solvents is effective to regulate reaction pathways and enhance gasification performance. In this study, we systematically investigate the effects of co-solvent concentration on SCWG reaction behavior and product characteristics. Corn stover was used as the feedstock with tetrahydrofuran (THF) and N-methyl-2-pyrrolidone (NMP) as co-solvents. The results show that the process exhibits the best synergistic gasification performance at a co-solvent concentration of 10%, where both THF and NMP enhance gas production, while NMP shows a more pronounced effect, achieving a hydrogen volume fraction of 34.45% and a total gas yield increase of 7.98 mmol·g⁻¹. Mechanistic analysis indicates that nitrogen-containing radicals produced from NMP decomposition promote the transformation of aromatic intermediates and suppress condensation reactions, thereby significantly enhancing gasification efficiency, while THF primarily facilitates radical-mediated depolymerization and hydrogen transfer processes, contributing to improved gasification performance. Structural characterization further reveals that the solid residues from NMP system possess higher specific surface area, well-developed mesoporous structures, and abundant nitrogen-doped active sites, and exhibit promising electrochemical energy storage performance. This study elucidates the enhancement of co-solvents in SCWG through regulating radical reaction pathways and solid structural evolution.