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

Abstract 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.