Preparation and characterisation of citric acid-modified activated carbons for the adsorption of Fe(II) ions from aqueous solutions

In this study, two citric acid-modified activated carbons, thermally treated at 450°C (AC450-A) and 600°C (AC600-N), were prepared and investigated for removal of Fe(II) ions from aqueous solutions. The samples were characterized using Fourier-transform infrared spectroscopy (FTIR), elemental analysis (C, H, N and S), Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), thermal analysis and determination of the point zero charge (pH pzc ). The adsorption performance was evaluated as a function of solution pH (2-10), initial Fe(II) concentration (5 - 15 mg/L for kinetic experiments), contact time (0-360 min) and temperature (20-40°C). The results demonstrated that citric acid modification introduced oxygen-containing functional groups on the activated carbon surface, whereas thermal treatment at 600°C promoted carbonization and enhanced structural stability. Fe removal was strongly influenced by solution pH owing to simultaneous changes in surface ionization and iron speciation. The experimental kinetic data were better described by the pseudo-second-order model, while the Langmuir isotherm provided the best fit to the equilibrium data. The Langmuir maximum adsorption capacities ranged from 4.47 to 6.76 mg/g for AC450-A and from 4.62 to 9.27 mg/g for AC600-N over the investigated temperature range. The calculated Gibbs free energy values were negative over the investigated temperature range, indicating thermodynamically favourable Fe retention, while the temperature dependence differed between the two adsorbents. These findings highlight the potential of activated carbons prepared from walnut shells as promising carbonaceous materials for iron removal from aqueous solutions, while simultaneously promoting the valorisation of local agricultural biomass in accordance with the principles of the circular economy.