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.

Using single metal salts of zirconium, lanthanum, and iron as raw materials and sodium alginate as a cross-linking agent, a new composite adsorbent was prepared via the co-precipitation method and embedding immobilization technology, and its phosphorus adsorption performance in wastewater was evaluated. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used for characterization, and a 0.5 mol·L⁻¹ sodium hydroxide solution was used to regenerate the adsorbent. The experimental results demonstrated that the adsorption rate reached 99.88% when the wastewater volume was 50 mL, the initial concentration of phosphorus-containing wastewater was 5 mg·L⁻¹, the pH was 5, the dosage of composite adsorbent was 0.2 g, and the adsorption time was 200 min. The prepared adsorbent could reduce the initial phosphorus concentration of 5 mg·L⁻¹ to 0.006 mg·L⁻¹ in simulated wastewater, and from 4.17 mg·L⁻¹ in urban sewage to undetected (<0.01 mg·L⁻¹), thus meeting the discharge requirements of the grade A standard of the Urban Sewage Treatment Plant Pollutant Discharge Standard (GB18918-2002). The adsorption process conformed to the Freundlich adsorption isothermal equation and quasi-second-order kinetic equation, and the adsorption reaction was exothermic and spontaneous. More importantly, after three lye regeneration tests, the removal rate of phosphorus in water remained above 68%, that is, the composite adsorbent could be reproducibly fabricated and recycled. The characterization results showed that the surface of the composite adsorbent was rough, with a complex pore structure. After phosphorus removal, the surface morphology of the composite adsorbent showed a similar honeycomb structure, with a P-H, P-O stretching vibration peak and a characteristic P2p peak. At the same time, the proportion of hydroxyl groups (M-OH) on the metal surface decreased after adsorption. Our findings thus demonstrate that the mechanism of phosphorus removal is mainly based on the coordination exchange reaction between phosphate and metal active sites and surface hydroxyl groups, resulting in the formation of granular phosphate deposits.

Modified eggshell adsorbent was prepared with 5% potassium permanganate solution to adsorb phosphorus in simulated wastewater. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to clarify their phosphorus removal mechanisms. Isothermal and kinetic equations were fitted to explain the mechanism of phosphorus adsorption. The results showed that the optimum conditions were as follows: phosphorus concentration of 5 mg·L^-1, wastewater pH of 2, adsorbent dosage of 3.5 g, adsorption time of 350 min, phosphorus adsorption rate of modified eggshell was 90.81%, and the experimental results were relatively stable. According to the characterization, the eggshell adsorbent has good crystallinity, irregular surface structure and abundant pore channels，and the main functional groups on the surface are hydroxyl (-OH) and C-H groups, which enhanced the adsorption effect.