Magnetite-activated carbon nanocomposite derived from steel waste and rice husks for trace removal of Pb(II) and Cd(II) from drinking water: Synthesis, characterization, and adsorption performance

Lead (Pb) and cadmium (Cd) contamination of drinking water at trace concentrations (μg/L) poses severe public health risks, particularly in resource-limited countries. This study reports the synthesis, characterization, and adsorption performance of a magnetite-rice husk activated carbon (Fe 3 O 4 @RHAC) nanocomposite derived from two waste streams — iron oxide waste powder (IOWP) from a steel-rolling mill and rice husk — for simultaneous trace-level Pb(II) and Cd(II) removal from drinking water. Fe 3 O 4 @RHAC was characterized by BET analysis, SEM/TEM, XRD, Raman, and FTIR spectroscopy, and by vibrating sample magnetometry, revealing a surface area of 440.38 m 2 /g, a pH at the point of zero charge of 5.5, and superparamagnetic behaviour. Batch experiments identified optimal conditions of pH 6 and adsorbent dosage of 50 mg/L, yielding removal efficiencies of 95.57% for Pb(II) and 93.93% for Cd(II) at 100 μg/L. Kinetics followed the pseudo-first-order model; equilibrium data conformed to the Sips isotherm with maximum capacities of 11,855 μg/g for Pb(II) and 13,838 μg/g for Cd(II). Thermodynamic analysis confirmed endothermic, spontaneous adsorption driven by entropy gain. Competing ion studies showed divalent cations caused greater interference than monovalent ions, yet distribution coefficients remained high (Kd > 98 L/g) under competitive conditions. Adsorption proceeded via a multi-mechanistic pathway dominated by electrostatic attraction and outer-sphere surface complexation with oxygen-containing functional groups, supplemented by ion exchange. The adsorbent retained over 80% capacity across five consecutive HCl regeneration cycles. Applied to four real drinking water sources in Uganda, the nanocomposite achieved WHO-guideline-compliant Pb in three of four cases and WHO-compliant Cd in all applicable cases. This work establishes a circular-economy pathway for converting steel and agricultural wastes into high-performance, magnetically recoverable drinking water treatment materials.