Synergistic Stabilization of Heavy Metals during Co-Hydrothermal Carbonization of Sewage Sludge with Lignocellulosic Biomass: Speciation Transformation, Risk Reduction and Sustainability Assessment

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.