Revealing the Synergistic Effect of Eggshells and Phosphate Regulation on Cothermal Decomposition of Walnut Shells: A Sustainable Low-Temperature Preparation Strategy for an Adjustable Phosphorus Release Biochar Fertilizer

Considering the low utilization efficiency, environmental pollution, and resource depletion of conventional phosphate fertilizers, the development of highly efficient and controlled-release phosphate fertilizers has become an urgent priority. A novel biochar-based slow-release phosphorus fertilizer (EWPF) was synthesized through copyrolysis (400 °C) strategy utilizing walnut shells (WSs), eggshells (ESs), and KH2PO4. The excellent release performance of EWPFs was prepared via in situ phosphorus fixation at low temperature by the chemical reaction between ES-derived CaCO3 and phosphate. The optimized formulation with a mass ratio of 3:7:4 (ES/WS/KH2PO4, termed E3W7P4Fs) exhibited an ideal pore structure and a cumulative phosphorus release of 48.7% over 30 days in static water. Kinetic analysis revealed that the phosphorus release mechanism transitioned from a Higuchi diffusion-controlled model to a Ritger–Peppas diffusion-erosion-coupled model with increasing phosphorus loading. XRD and XPS analyses confirmed the formation of stable Ca–P compounds, which significantly reduced the water-soluble phosphorus (H2O–P) fraction to 14.7% and increased the HCl-extractable phosphorus (HCl-P) fraction to 40.6%. Pot experiments showed that E3W7P4Fs treatment increased stem length, fresh weight, and plant phosphorus content by 39.1, 42.8, and 13.1%, respectively, compared with CF. This work presented a sustainable and green strategy for valorizing waste biomass into high-value slow-release fertilizers, offering a promising solution to enhance phosphorus use efficiency and mitigate environmental risks.