Natural clinoptilolite as an exchanger of Ni2+ and NH4+ ions under hydrothermal conditions and high ammonia concentration

Abstract
Natural clinoptilolite from Caimanes deposit (Moa-Cuba) has been used for the nickel removal from aqueous solution by ion exchange. The Ni2+ elution, using ammonia-rich solution, has been also studied in form of a nickel removal–elution ion-exchange cyclic process. The stability of clinoptilolite as an exchanger of Ni2+ and NH4+ cations under hydrothermal conditions and high ammonia concentrations is analyzed. No loss in the clinoptilolite exchange capacity of Ni2+ and NH4+ ions during the Ni2+ removal–elution cycles was found. The treatment of the clinoptilolite with NH4OH concentrated solutions provokes not only the Ni2+ elution by ion exchange, but also a substantial increase of the Ni2+ ions removal capacity of the zeolite. This is explained by OH anions adsorption (or occlusion) on the zeolitic phase particles and the consequent nickel hydroxide precipitation.