Fixed-Bed Column Adsorption of Copper Ions Using Activated Orange Charcoal: Performance, Thermodynamic Studies, and SEM-FTIR Characterization

Abstract

This study evaluates the performance of hydrochloric acid (HCl)-activated orange charcoal as a sustainable bio-based adsorbent for the removal of Cu²⁺ ions from aqueous solutions using a fixed-bed column system. The influence of key operational parameters, including initial copper concentration, adsorbent dosage, solution pH, contact time, and temperature, was systematically evaluated to optimise the adsorption process. The physicochemical properties of the prepared adsorbent were characterised using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). SEM analysis revealed a highly porous honeycomb-like morphology with well-preserved vascular channels, providing an extensive surface area and favourable pore architecture for metal ion adsorption. FTIR analysis confirmed the presence of oxygen-containing functional groups, particularly hydroxyl and carboxyl moieties, which serve as the primary active sites responsible for Cu²⁺ binding. The experimental findings demonstrated that adsorption performance was strongly influenced by the investigated operating conditions. The maximum adsorption capacity (Qₑ) of 69 mg/g was achieved at an initial Cu²⁺ concentration of 500 ppm, an adsorbent dosage of 0.2 g, and an optimum solution pH of 6.5, resulting in a copper removal efficiency of approximately 89% after 30 min of contact time. Furthermore, increasing the operating temperature from 298 to 313 K enhanced the adsorption capacity, indicating that the process is thermally favourable. Thermodynamic evaluation yielded positive values of ΔH (1.28 kJ/mol) and ΔS (4.29 J/mol·K), suggesting an endothermic adsorption process accompanied by increased randomness at the solid–liquid interface. The positive Gibbs free energy (ΔG = 5.14 J/mol) indicates that adsorption is non-spontaneous under the investigated standard conditions but may become more favourable at elevated temperatures.The dynamic adsorption behaviour of the fixed-bed column was successfully interpreted using the Adams–Bohart, Thomas, and Yoon–Nelson models. Among these, the Adams–Bohart model exhibited the highest predictive capability, with a coefficient of determination (R²) of 0.91, indicating good agreement with the experimental breakthrough data. Collectively, the morphological, spectroscopic, thermodynamic, and modelling results demonstrate that HCl-activated orange charcoal is an efficient, low-cost, and environmentally sustainable adsorbent with considerable potential for the treatment of copper-contaminated wastewater and other heavy metal remediation applications.