Magnetic Alumina for Chromium Ion Remediation: Isotherm Analysis and Kinetic Modeling

Heavy metal contamination in aquatic systems poses serious environmental and health risks, driving the need for high-performance adsorbents with enhanced surface reactivity. This study presents the novel fabrication of cobalt (Co), iron (Fe), and nickel (Ni)-alumina (Al) composites for efficient chromium (Cr) ion remediation. The role of oxygen vacancies (OV), metal substitution, and surface hydroxylation in tuning adsorption efficiency was systematically investigated using physical mixing (PM) and chemical precipitation (CP) methods. Among the composites, FeAl from PM method exhibited the highest Cr adsorption capacity of 500.0 mg g^-1, achieving 91% removal at pH 7, 40 °C, and an initial concentration of 25 mg L^-1. Zeta potential analysis confirmed enhanced electrostatic interaction, with FeAl displaying the most negative surface charge of -26.3 mV, which shifted to -15.4 mV after Cr adsorption, indicating effective ion binding and charge neutralization. Comprehensive characterization by FTIR, XRD, SEM-EDX, BET, ESR, and XPS confirmed that Fe incorporation promoted meso-macroporous structure (pore diameter = 296.15 nm), improved magnetic properties, and increased surface hydroxyl density, which collectively facilitated effective Cr ion diffusion and uptake. Adsorption followed the Langmuir model (R ² = 0.9899) and pseudo-first-order kinetics, indicating monolayer, predominantly physisorption-driven uptake. Thermodynamic results (ΔH = - 26.6 kJ mol^-1; ΔS = - 0.0994 kJ mol^-1·K^-1) suggested spontaneous, exothermic adsorption. These findings identify FeAl as a scalable adsorbent with high Cr ion removal efficiency and structural stability, offering potential for sustainable heavy metal remediation in industrial wastewater treatment.