Removal of Cd(ii) from aqueous solution by ferrate-modified sludge biochar: optimization of preparation conditions, adsorption performance, and mechanism

Abstract

The escalating discharge of highly toxic cadmium (Cd(II)) effluents presents critical threats to environmental security. To simultaneously address this remediation challenge and promote solid waste resource utilization, this study engineered a novel magnetic biochar (Fe@SBC) via the co-pyrolysis of excess municipal sludge and potassium ferrate (K₂FeO₄). Through orthogonal array optimization, specific synthesis parameters (pyrolysis at 600 °C for 2 h with a mass ratio of 2.0) were identified as critical for maximizing Cd(II) remediation. Kinetic and isotherm modeling revealed that the adsorption process aligns with pseudo-second-order and Langmuir models, indicative of monolayer chemisorption. Under optimal conditions (pH 5.0, 25 °C), Fe@SBC attained a peak adsorption capacity of 155.28 mg g⁻¹, nearly doubling the 83.89 mg g⁻¹ capacity of unmodified sludge biochar. Thermodynamic analysis characterized the uptake as spontaneous, endothermic, and entropy-driven. Mechanistically, the superior performance of Fe@SBC was attributed to a synergy of surface complexation with oxygenated groups, inorganic precipitation, electrostatic attraction, and cation–π interactions. Furthermore, the material exhibited robust performance over five regeneration cycles. Given its cost-effectiveness, high efficiency, and magnetic separability, Fe@SBC is a promising candidate for sustainable heavy metal wastewater treatment.