Dual-functional biochar with Mg-Zn-Fe LDH and phosphate groups: A versatile adsorbent for RhB, MO, and OTC in water

Mg-Zn-Fe LDH was anchored on phosphate-modified reed straw biochar. • Mg 1 Zn 1 Fe-LDH@PBC reached a surface area of 394.466 m 2  g −1 . • The adsorbent efficiently removed RhB, MO, and OTC from water. • Maximum RhB adsorption capacity reached 659.2 mg g −1 . • Removal efficiency remained around 70% after five cycles. Sustainable conversion of waste biomass into high-performance adsorbents provides an effective route for organic wastewater remediation. In this study, phosphate-modified reed straw biochar-supported layered double hydroxide composites (LDH@PBC) were synthesized via a facile co-precipitation method. Binary and ternary LDHs containing Mg 2+ , Zn 2+ , and Fe 3+ were constructed on the biochar matrix, and their adsorption performance toward rhodamine B (RhB), methyl orange (MO), and oxytetracycline (OTC) was systematically evaluated. The optimized Mg 1 Zn 1 Fe-LDH@PBC exhibited a well-developed porous structure, abundant surface functional groups, and the highest specific surface area of 394.466 m 2  g −1 . Owing to the synergistic effects of phosphate-modified biochar and Mg-Zn-Fe LDH nanosheets, the composite maintained stable adsorption efficiency over a broad pH range. The optimized Mg 1 Zn 1 Fe-LDH@PBC achieved a maximum experimental RhB adsorption capacity of 659.2 mg g −1 under the optimized dosage condition. In contrast, Langmuir isotherm fitting gave theoretical maximum monolayer adsorption capacities of 944.898, 696.552, and 303.146 mg g −1 for RhB, MO, and OTC at 298 K, respectively. Kinetic and isotherm analyses suggested that the adsorption process may be associated with multiple interactions, including electrostatic attraction, ion exchange, hydrogen bonding, π–π interactions, and possible surface complexation. In addition, the removal efficiency remained around 70% after four adsorption–desorption cycles, indicating favorable reusability. This work highlights a promising biomass-derived LDH/biochar adsorbent for efficient and sustainable removal of multiple organic pollutants from water.