In-situ carbonate mineral coupling by hydrothermal carbonization: a strategy for enhanced hydrochar stability and long-term carbon sequestration

Abstract

The development of effective carbon sequestration materials is crucial for mitigating climate change. This study introduces a novel one-step hydrothermal carbonization (HTC) strategy to enhance hydrochar carbon sequestration by in-situ coupling carbonate minerals (CaCO₃ and MgCO₃). Unlike traditional biochar-mineral coupling methods, which suffer from poor interaction and uneven mineral distribution, this approach enables uniform mineral nucleation and crystalline growth within the hydrochar matrix. The result shows that the hydrothermal environment facilitates effective ion dispersion and the formation of a stable organic–inorganic composite, with functional groups like carboxyl and hydroxyl on the hydrochar surface promoting mineral nucleation. Mineral-coupled hydrochars exhibited significant improvements in carbon sequestration capacity compared to pristine hydrochar. Ca coupling resulted in a 69.9 % reduction in carbon loss rate under oxidative conditions, a 41.86 % increase in thermal stability, and a 63.86 % reduction in dissolved organic carbon (DOC). Mg coupling also enhanced stability, reducing carbon loss by 47.8 % and DOC by 54.5 %, although its thermal stability (R₅₀ = 0.53) and mineral integration were less pronounced due to the formation of smaller, less crystalline MgCO₃ particles. These improvements are attributed to the synergistic interaction between the carbon matrix and mineral phase, which reduces O-containing groups, enhances condensation, and forms a crystalline barrier. This work demonstrates that mineral coupling during HTC significantly enhances hydrochar stability and carbon retention, offering a scalable and sustainable solution for long-term carbon storage and negative-emission technologies.