High-value valorization of rice husk into N/S co-doped hierarchical porous biochar for burst-free controlled release of urea

The valorization of agricultural and forestry residues into advanced functional materials represents a crucial pathway for promoting a sustainable circular bioeconomy. To unravel the impact of solid-phase carbonization kinetics, nitrogen and sulfur co-doped hierarchical porous biochars (BCM s ) were engineered via the synergistic carbonization and interfacial reconstruction of rice husk with ternary natural biopolymers (lignosulfonate, carboxymethyl cellulose, and chitosan) across a controlled temperature gradient. Tracking the thermally driven evolution of the carbon lattice revealed that carbonization at an optimal temperature of 500℃ maximizes defect proliferation. The optimized biochar-based composite (BCM 500 , specific surface area of 9.802 m 2 /g) exhibited a high equilibrium urea adsorption capacity of 132.273 mg/g alongside robust anti-interference capabilities and a stable capacity retention of 67.9% after five regeneration cycles. Comprehensive thermodynamic and kinetic analyses, integrated with high-resolution X-ray photoelectron spectroscopy, elucidated that interfacial nutrient immobilization is predominantly governed by a spontaneous exothermic chemisorption process. Engineered topological defects, including pyridinic-N, pyrrolic-N, and oxidized sulfur, act as specific active centers to firmly anchor urea molecules via strong intermolecular hydrogen bonds and electrostatic interactions. Furthermore, a practical melt-infiltrated slow-release fertilizer (BCM 500 UF) formulated with this carbonaceous carrier demonstrated a prolonged and stable nutrient supply profile in dynamic aqueous environments. The synergistic intervention of hierarchical pore steric hindrance and high-energy exothermic chemical bonding effectively eliminated the initial burst release phenomenon. Quantitative mathematical modeling confirmed that the half-release time was extended to 3.388 leaching cycles, while the initial release intercept was suppressed to 7.191%. This work elucidates multi-site interfacial exothermic anchoring mechanisms, offering a scalable waste-to-wealth paradigm for next-generation smart agro-materials.