Hydrothermal synthesis of Zr-doped chitosan carbon-shell protected magnetic composites (Zr–Fe3O4@C) for stable removal of Cr(VI) from water: Enhanced adsorption and pH adaptability

In this study, zirconium-doped carbon shell-protected magnetic Fe3O4 composite (Zr–Fe3O4@C) was synthesized by hydrothermal method with chitosan assistance. Chitosan as a stabilizer successfully prevented the aggregation of Fe3O4, resulting in approximately a 30.7% increase in the specific surface area of Zr–Fe3O4@C. TEM analysis revealed that Zr–Fe3O4@C exhibited a relatively uniform square shape with a size of about 120 nm. The synergistic effect between iron and zirconium oxides resulted in a nearly 20% increase in the adsorption capacity of Zr–Fe3O4@C (Fe/Zr molar ratio of 3:1) for Cr(VI) compared to Fe3O4. The introduction of zirconium also significantly improved the acid and base resistance (1–9) and environmental adaptability of Zr–Fe3O4@C composites. With good magnetic properties (53.5 emu/g) and separation recycling properties, Zr–Fe3O4@C showed rapid water separation within 20 s. At a pH of 3 and 30 °C, Zr–Fe3O4@C achieved a maximum adsorption capacity of 132.72 mg/g by the Langmuir isotherm. Furthermore, even after six consecutive treatments, the adsorption performance of Zr–Fe3O4@C for Cr(VI) only decreased by about 9.8%. The stable adsorption performance and regeneration ability suggested that Zr–Fe3O4@C would be considered a reliable candidate for Cr(VI) wastewater remediation.