Sustainable Phosphate Remediation via Hierarchical Mg-Fe Layered Double Hydroxides on Magnetic Biochar from Agricultural Waste

Addressing aquatic phosphate pollution requires advanced materials that combine high selectivity with recyclability. Here, we present a hierarchically structured composite integrating Mg-Fe layered double hydroxides (LDHs) with magnetic biochar derived from mulberry branches—an abundant agricultural byproduct. Through hydrothermal synthesis, the composite achieves a unique architecture combining Fe3O4-enabled magnetic recovery (2.63 emu·g−1 saturation) with LDHs’ anion exchange capacity and biochar’s porous network. Systematic characterization reveals phosphate capture mechanisms dominated by hydrogen bonding through deprotonated carboxyl groups, inner-sphere complexation with metal oxides, and interlayer anion exchange, enabling 99.22% phosphate removal at optimal conditions (pH 6, 25 °C). Crucially, the material demonstrates exceptional selectivity over competing Cl− and NO3− ions while maintaining 87.83% efficiency after three regeneration cycles via alkaline treatment. Kinetic and thermodynamic analyses confirm chemisorption-driven uptake aligned with pseudo-second-order kinetics (R2 > 0.9998) and Langmuir monolayer adsorption (7.72 mg·g−1 capacity). This waste-derived magnetic composite establishes a sustainable paradigm for eutrophication control, merging selective phosphate sequestration with energy-efficient recovery for circular water treatment applications.