Fungal-infused alginate biowaste fertilizer granules: mechanistic analysis of nutrient release from composite hydrogel systems

Fungal-infused alginate biowaste fertilizer granules offer a sustainable route for converting industrial residuesinto controlled-release nutrient delivery systems; however, the transport mechanisms governing nutrientdiffusion within composite hydrogel matrices remain poorly resolved. In this study, calcium–alginate granulesincorporating sewage sludge ash (SSA) and brewer’s spent grain (BSG) were engineered to elucidateformulation–structure–transport relationships controlling nutrient release behavior. A two-factor, three-levelfactorial design was employed to evaluate the combined effects of SSA:BSG ratio (30:70–70:30, w/w) andCaCl₂ concentration (1–10%, w/v) on hydrogel performance. Nutrient release kinetics were well described byHiguchi and Korsmeyer–Peppas models (R² = 0.91–0.98), indicating diffusion-dominated transport witheffective diffusivities ranging from 1.2 × 10⁻⁷ to 6.8 × 10⁻⁶ cm² s⁻¹. Increasing CaCl₂ concentrationreduced cumulative phosphorus release by up to 65% over the test period, consistent with increased crosslinkdensity and reduced polymer mesh size, whereas higher BSG fractions enhanced swelling and increased nutrientflux by approximately 1.5–2.3-fold relative to SSA-rich formulations. Quadratic response surfaces revealedstrong interaction effects between filler composition and crosslinker concentration, defining formulation regionsexhibiting moderated nutrient release profiles. Functional validation through short-term plant bioassaysdemonstrated increases in seed germination (up to 22%), shoot length (18–31%), and root length (25–40%)relative to non-inoculated controls under formulations exhibiting intermediate diffusivity. These findingsprovide quantitative mechanistic insight into diffusion-controlled nutrient release from heterogeneous alginatehydrogel systems and establish transferable design criteria for waste-derived controlled-release materials.