Ammonium modified natural clinoptilolite to remove manganese, cobalt and nickel ions from wastewater: Favorable conditions to the modification and selectivity to the cations

- Activation energy of NH4+ ion exchange in clinoptilolite (CLI) is not very high.
- Temperature interval for the efficient production of NH4-CLI is wide.
- NH4-CLI removes Co2+ and Mn2+ selectively at 25 °C, producing a well Ni2+ separation.
- Increasing the temperature enhances the NH4-CLI capacity to remove all metal cations.
- Selectivity order of the metal-CLI forms by NH4+ at 25 °C is Mn-CLI > Co-CLI > Ni-CLI.

The removal of manganese, cobalt and nickel from industrial ammoniacal wastewaters has been studied using natural Cuban clinoptilolite modified to ammonium form. Favorable conditions to obtain the ammonium-clinoptilolite form (NH4-CLI) were determined from detailed kinetic studies of NH4+ ion-exchange in the clinoptilolite (CLI). Obtained NH4-CLI was characterized by IR spectroscopy and X-ray diffraction. NH4-CLI was used to remove Mn2+, Co2+ and Ni2+ from mixed aqueous solutions. The thermodynamic study on ion-exchange equilibrium between NH4+ in solution and these metallic cations from metal-clinoptilolite forms was performed. Increasing the temperature promotes the NH4+ exchange, which is controlled by the intracrystalline diffusion of ions inside the clinoptilolite. However, the activation energy of this process is not very high. This provides a wide temperature interval for the NH4-CLI efficient production. NH4-CLI removes Co2+ and Mn2+ by ion-exchange selectively at room temperature, being poorly selective to Ni2+, producing a well ions separation. Increasing the temperature greatly enhances the NH4-CLI capacity to remove these metal cations, which is substantially higher to Ni2+. The obtained values for the free energy variation and equilibrium constant lead to outline that the selectivity order of the metal-clinoptilolite forms by NH4+ at room temperature is Mn-CLI > Co-CLI > Ni-CLI. Divergences in the behavior to these exchange processes are associated with difference in the interaction of cations with their ligands in solution as well as into zeolite framework, and accessibility at ion-exchange sites inside clinoptilolite.

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