Extraction of thorium from aqueous system using citric acid modified corn cob: characterization, kinetics,...

This study investigates the potential of low-cost, eco-friendly, chemically modified corn cob treated with citric acid as an efficient adsorbent for the removal of Th⁴⁺ ions from aqueous solution. The structural, morphological, and surface characteristics of the natural and modified adsorbent were investigated by FTIR, XRD, SEM, EDX, BET, and XPS techniques. Citric acid modification significantly enhanced the surface area, porosity, and the number of oxygen-containing functional groups, particularly carboxyl groups, thereby improving the adsorption performance. The effects of pH, contact time, adsorbent dose, initial Th⁴⁺ concentration, and temperature were examined. The maximum adsorption efficiency reached 90.2% at pH 4, with equilibrium achieved in 25 min at room temperature, and an optimal adsorbent dose of 1 g L^-1. The adsorption equilibrium data were well described by the Langmuir isotherm model, indicating monolayer adsorption with a maximum capacity of 196.08 mg g^-1. The kinetic studies showed that the process followed the pseudo-second-order model. Thermodynamic parameters (ΔG = - 19.08 to - 22.73 kJ mol^-1, ΔH = + 17.23 kJ mol^-1, ΔS = 121.83 J mol^-1K^-1) indicate that the process is spontaneous, endothermic, and accompanied by increased randomness at the solid-liquid interface. Desorption using 0.05 M HNO₃ achieved an efficiency of 94%, which slightly decreased to 88% after five successive cycles.