Synthesis of zero valent Fe/Mn anchored Alstonia scholaris flower biochar for wastewater treatment and bactericidal application

Zerovalent iron and manganese-embedded NaOH-activated Alstonia scholaris flower biochar composites (ZMNASB and ZFNASB) were prepared using sodium borohydride-assisted metal reduction and strategic intercalation into the biochar matrix for Eriochrome Black T (EBT) and Cr(VI) removal from aqueous solutions, along with simultaneous antibacterial use. Both materials were characterized by FTIR, SEM-EDX, BET surface area, TEM-SAED, VSM, and PXRD. The zero-point charges were found to be 7.20 and 6.81, respectively. A pseudo-second-order model best described the uptake dynamics. EBT adsorption followed the Freundlich model (R² = 0.999), while Cr(VI) adsorption followed the Langmuir model (R² = 0.999). ZMNASB and ZFNASB displayed maximum uptake capacities of 242.082 and 293.225 mg g⁻¹, respectively, for EBT, and 56.376 and 50.566 mg g⁻¹, respectively, for Cr(VI) at 328 K and pH 7. Both scavenging processes are endothermic (12.496 to 25.709 kJ mol⁻¹) and favorable (−0.109 to −7.419 kJ mol⁻¹). Using 50% methanol and diluted NaOH, the adsorbents were successfully regenerated (70–76%), enabling three cycles of reuse. Their practical applicability was demonstrated by their ability to effectively remove contaminants from field wastewater (∼73.2–94% efficiency). The column adsorption capacity of both composites was 20.046–272.477 mg g⁻¹ for EBT and Cr(VI). To address disposal concerns, spent materials were pyrolyzed to transform into secondary adsorbents with efficiencies of 50–68%. Furthermore, an approximately 3 cm inhibitory zone was observed against both Staphylococcus aureus and Escherichia coli bacteria using the composites at a concentration of 100 mg mL⁻¹. With a surface area of 56.008 and 101.571 m² g⁻¹, the materials exhibit dual advantages and outperform several contemporary materials.