Fe3O4-loaded phosphoric acid-modified walnut shell biochar for uranium adsorption from simulated groundwater

Abstract

Uranium contamination in groundwater poses a serious threat to human health and ecological security, necessitating the development of efficient and sustainable remediation materials. In this work, a magnetic biochar composite, Fe₃O₄-loaded phosphoric acid-modified walnut shell biochar (Fe₃O₄/P-WSBC), was synthesized and evaluated for U(VI) removal from simulated groundwater. Batch adsorption experiments showed that equilibrium was reached within 60 min, and the kinetic data were well described by the pseudo-second-order model (R² = 0.998). The Langmuir isotherm provided the best fit, yielding a maximum monolayer adsorption capacity of Q_max = 23.05 mg g⁻¹ at 40 °C. Thermodynamic analysis indicated that the adsorption process was spontaneous (ΔG < 0) and endothermic (ΔH > 0). Fe₃O₄/P-WSBC shows good U(VI) selectivity against common coexisting ions, with removal efficiency decreasing by <10% even with Cu²⁺, Mn²⁺, Zn²⁺, K⁺, especially Cu²⁺. After five consecutive adsorption–desorption cycles, the adsorbent still exhibited a good adsorption capability. X-ray photoelectron spectroscopy (XPS) revealed that U(VI) removal involved electrostatic attraction and complexation with surface –OH, C–O, and CO groups, accompanied by partial reduction of U(VI) to U(IV) by Fe²⁺. These quantitative results establish Fe₃O₄/P-WSBC as an efficient and reusable material for the remediation of uranium-contaminated groundwater.