Controlled-release kinetics of potassium and organic carbon from vinasse-impregnated biochar for integrated valorization of urban and agro-industrial residues

This study presents an integrated strategy for valorizing urban tree-pruning residues and sugarcane vinasse through the production of biochar-based fertilizers with controlled nutrient release potential. Biochar derived from Syzygium cumini pruning residues was produced by pyrolysis, chemically activated with H₃PO₄, and impregnated with raw vinasse, generating raw (BCB), activated (BCA), vinasse-impregnated raw (BCBV), and vinasse-impregnated activated (BCAV) biochars. Chemical activation increased the BET surface area from 23.38 to 137.51 m² g⁻¹, while vinasse impregnation provided high potassium and dissolved organic carbon (DOC) loadings (267250 mg K⁺ kg⁻¹ and 191375 mg C kg⁻¹). Leaching tests showed limited K⁺ and DOC release from non-impregnated biochars, whereas impregnated materials released 59–69% of the loaded K⁺ within 24 h but less than 8% of DOC, indicating strong carbon retention. Kinetic modeling using Weibull, Korsmeyer–Peppas, Higuchi, first-order, and second-order models showed the best fit for the second-order model (R² ≈ 0.9999), although interpreted here as an empirical descriptor. Activated biochar exhibited slower K⁺ release, attributed to enhanced pore development and transport resistance. The results demonstrate the potential of vinasse-impregnated biochars as sustainable nutrient carriers within a circular economy framework.