Engineered SrSnFe₂O₄@δ-MnO₂/activated biochar heterocomposite with a porous surface for efficient o-nitrophenol...

Water contamination by phenolic compounds remains a critical environmental challenge, requiring adsorbents that combine high efficiency, structural stability, and facile recovery. In this study, we report a novel heterostructured composite, SrSnFe₂O₄@δ-MnO₂/BC_KOH, synthesized from cotton-branch-derived biochar through sequential KOH activation, MnO₂ growth, and magnetic ferrite incorporation. Unlike conventional MnO₂/biochar systems, the incorporation of SrSnFe₂O₄ induced α-to-δ MnO₂ phase transformation, heterointerface strain, and defect formation, enhancing surface functionality and adsorption reactivity. Comprehensive characterization (SEM, XRD, BET, XPS, VSM) confirmed a highly porous carbon framework, abundant oxygen-containing groups, and strong magnetic properties (M _s = 54.87 emu g^-1), enabling efficient pollutant capture and rapid magnetic separation. The composite exhibited excellent adsorption of o-nitrophenol (o-NP), achieving 99.04% removal within 60 min and a maximum adsorption capacity of 525.51 mg g^-1. Adsorption followed the Freundlich isotherm and pseudo-second-order kinetics, and XPS analysis revealed synergistic interactions between surface functionalities and o-NP molecules. The material maintained good reusability (50.29% after five cycles) and demonstrated effective removal in real wastewater samples. This work presents a scalable strategy for constructing magnetically retrievable δ-MnO₂/biochar heterostructures with superior adsorption performance, highlighting their potential for practical wastewater remediation.