Aqueous nitrate ion adsorption/desorption by olive solid waste-based carbon activated using ZnCl2

ZnCl₂ activated carbon (ZnCl₂-AC, with specific surface area ~1480 m²/g) effectively adsorbs nitrate from water with uptake capacity (~5.5 mg/g) four-fold that for other commercial activated carbons (CACs). Kinetics show a small activation energy value (~7 kJ/mol) consistent with a diffusion-controlled process. This is further confirmed by intra-particle diffusion study. Negative thermodynamic parameters (ΔH° ?13.859 kJ/mol, ΔS° ?43.8 J/mol K, and ΔG° ?0.808 kJ/mol) indicate spontaneous nitrate adsorption. The high specific surface area rationalizes the high uptake capacity for the ZnCl₂-AC. A Langmuir type adsorption, is observed. ZnCl₂-AC effectively adsorbs the nitrate from water, even at high concentrations, and brings the concentration to allowed maximum values. Despite these advantages, the ZnCl₂-AC exhibits low desorption (~20%) of pre-adsorbed nitrate ions into concentrate solutions. Up to ~40% desorption is observed by grinding the pre-saturated ZnCl₂-AC. Developing other methods to completely recover the adsorbed nitrate and the adsorbent for multiple use is needed.