Co-hydrothermal carbonization of biomass: advances, challenges, and future perspectives in soil remediation applications

Biomass co-hydrothermal carbonization (co-HTC) is an environmentally friendly technology that enables the synergistic conversion of diverse biomass feedstocks (e.g., crop residues and livestock manure) into value-added product-hydrochar. This process offers high efficiency, sustainability, and product versatility, making it highly promising for agricultural applications. The solid-phase product, hydrothermal carbon, is characterized by a well-developed porous structure, excellent adsorption capacity, and stable carbon composition, making it valuable for soil improvement, carbon-based fertilizer production, and pollutant removal. In recent years, significant progress has been made in optimizing hydrothermal carbonization (HTC) properties, enhancing soil fertility, mitigating agricultural pollution, and developing multifunctional carbon-based materials. However, challenges remain in the synergistic processing of different biomass types, process optimization, and large-scale implementation. This review summarizes recent advances in co-HTC and its agricultural applications, focusing on (1) the fundamental mechanisms underpinning co-HTC, highlighting synergistic interactions such as Maillard and Mannich reactions; (2) the influence of key process parameters (temperature, solid–liquid ratio, reaction time, feedstock type/ratio) on hydrochar properties and yield; (3) nutrient migration and transformation (N, P, K) and their relevance to hydrochar functionality; and (4) applications in enhancing soil physicochemical properties, microbial richness, heavy metal immobilization, and crop productivity. Finally, the review discusses current challenges—such as synergistic reaction complexity, process optimization, and scale-up limitations—and proposes future research directions to guide biomass valorization and sustainable agricultural development.