Zn/Fe-Layered Double Hydroxide Composites with Kelp-Derived Biochar for Phosphate Recovery and Reutilization as a Slow-Release Fertilizer

Phosphorus scarcity and inefficient fertilizer utilization highlight the need for sustainable phosphorus recovery and reuse strategies. In this study, a Zn/Fe-layered double hydroxide (Zn/Fe-LDH)-kelp-derived biochar (KBC) composite (Zn/Fe-LDH@0.5KBC) was synthesized via co-precipitation for phosphate capture and subsequent reutilization as a slow-release fertilizer. The incorporation of KBC improved the dispersion of LDH nanosheets and generated a hierarchical porous structure with a specific surface area of 122.13 m2/g. As a result, Zn/Fe-LDH@0.5KBC exhibited a high phosphate adsorption capacity of 132.52 mg P/g and reached adsorption equilibrium within 240 min. Kinetic and isotherm analyses indicated that phosphate adsorption was dominated by chemisorption and was best described by the Sips model. Comprehensive scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses revealed that phosphate removal occurred through synergistic mechanisms, including electrostatic attraction, interlayer anion exchange, surface complexation, and metal phosphate precipitation. The P-loaded composite exhibited diffusion-dominated phosphorus release in soil and significantly enhanced pak choi growth. Compared with the control, labile phosphorus increased from 2.8% to 6.8%, while moderately labile phosphorus increased from 6.3% to 14.1%, indicating improved phosphorus availability. These findings demonstrate an effective strategy for integrating phosphate recovery from wastewater with agricultural reuse and provide insights into the development of multifunctional adsorbent-fertilizer systems for circular phosphorus management.