Design, Formulation, and Kinetic Evaluation of a Slow-Release NPK Fertilizer Derived from Engineered Biochar and Sewage Sludge
DOI:
https://doi.org/10.70882/josrar.2026.v3i5.237Keywords:
Slow-release fertilizer, Biochar, Sewage sludge, Nutrient release kinetics, Fickian diffusion, Waste valorizationAbstract
Conventional mineral fertilizers exhibit poor nutrient use efficiency (<50% for N and P), driving environmental degradation through eutrophication and greenhouse gas emissions. This study reports the design, formulation, and mechanistic evaluation of a biochar-based slow-release fertilizer (BSRF) synthesized from ball-milled groundnut shell biochar and sewage sludge-two abundant waste streams. BSRF exhibited sustained release of N (78 ± 2.8%), P (68 ± 2.5%), and K (72 ± 2.6%) over 30 days, with time to 50% release (T₅₀ = 18.2 ± 1.2 to 18.9 ± 1.1 days) compared to 5.1–5.3 days for conventional NPK fertilizer (p < 0.001). Multi-model kinetic analysis (first-order, Higuchi, and Korsmeyer-Peppas) revealed Fickian diffusion-controlled release (n = 0.40–0.42; R² > 0.98), with first-order rate constants (k₁ = 0.058–0.068 day⁻¹) 3.5- to 4.2-fold lower than conventional fertilizer (p<0.01). Agronomic evaluation demonstrated 19% higher maize shoot biomass (4.82 ± 0.31 vs. 4.05 ± 0.28 g plant⁻¹; p < 0.05). Heavy metal concentrations (Pb: 38 ± 2; Cd: 0.9 ± 0.1; total Cr: 55 ± 4 mg kg⁻¹) complied with EU Regulation 2019/1009 limits. This waste-derived composite demonstrates diffusion-controlled slow-release behavior, offering a scalable, low-cost alternative to polymer-coated formulations while advancing circular economy principles in sustainable agriculture.
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Copyright (c) 2026 Shangkum Yildun Goji, Susinya Habila, Japhet Joel Gongden, Stephen John Dache, John Stephen Gushit, Dauda Kankemwa (Author)
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