Waste-derived hierarchical biochar with lignosulfonate/melamine-induced mesopore formation and S-assisted N retention for sustained nutrient delivery.

The sustainable valorization of waste biomass into high-value functional materials via advanced thermochemical processes is a critical frontier for environmental nutrient management. Herein, a novel spatial-infilling and interfacial reconstruction strategy was proposed to engineer a rice husk biochar carrier via the synergistic vacuum impregnation and co-pyrolysis of lignin sulfonate and melamine. This process induced mesopore formation, pore-structure reconstruction, and pore-size redistribution, shifting the average pore width from 28.41 to 2.71 nm while increasing the surface area from 5.42 to 120.70 m2/g. Meanwhile, sulfur-assisted nitrogen retention promoted the enrichment of Lewis-basic pyridinic-N and polarizable thiophenic-S sites. Adsorption investigations demonstrated a highly efficient, spontaneous, and exothermic multilayer urea capture capacity (Qm = 236.49 mg/g; ΔH° = -16.97 kJ/mol), driven by internal pore-filling and strong interfacial hydrogen bonding/dipole interactions. Crucially, the 2.71 nm nanoconfinement effect restricted urea crystallization, trapping the guest molecules in an amorphous state that established a robust kinetic barrier against dissolution. Multi-model release kinetics confirmed the profound suppression of the initial burst release, with the first-cycle rapid release fraction plummeting from 87.89 % (pure urea) to 13.79 %. The composite displayed a highly stable, slow-phase diffusion-governed sigmoidal profile (λ = 4.80). Pearson correlation verified that the release longevity was controlled by a dual-effect synergistic mechanism combining physical confinement within tortuous channels with robust chemical anchoring by polar functional groups, rendering this engineered biochar a highly promising nutrient delivery platform for sustainable agriculture.