Advancing hydrothermal carbonization: Assessing hydrochar's role and challenges in carbon sequestration

The increasing urgency to reduce atmospheric CO₂ emissions has driven research into sustainable carbon sequestration technologies, with hydrochar (HC) emerging as a promising material. HC is derived from hydrothermal carbonization (HTC), a thermochemical process that converts biomass into a carbon-rich solid at moderate temperatures and self-generated pressure in an aqueous environment. Due to its unique reaction pathways, HC differs significantly from biochar (BC) derived from pyrolysis in terms of application, performance, and structural characteristics. Despite HC's potential for long-term carbon storage, critical gaps remain in understanding its sequestration mechanisms, influencing factors, and optimization strategies-hindering its effective application. This review critically evaluates HC's carbon sequestration capacity, focusing on overlooked complexities that influence its performance. Key parameters, including feedstock composition, reaction temperature, pH, and residence time, are systematically examined to elucidate their impact on HC's structural integrity and carbon stability. Special attention is given to the role of lignin in enhancing stability and thermal resilience, as well as the concept of carbon-ash recalcitrance, where mineral embedding enhances carbon stability. To assess HC's long-term sequestration effectiveness, this study analyzes key indicators such as thermal stability, chemical resilience, aromaticity, and dissolved organic carbon (DOC) leaching.Besides, this review explores innovative strategies for improving HC's sequestration performance, including HTC liquid recycling, chemical modification, and salinity control. By integrating expert-driven insights and identifying research gaps, this synthesis advances theoretical understanding while outlining future directions for optimizing HC as a sustainable carbon sink. Ultimately, this work establishes HC as a critical material in global carbon management efforts and climate change mitigation.