Nickel and cobalt adsorption on hydroxyapatite: a study for the de-metalation of electronic industrial wastewaters

In the present study, the Ni(II) and Co(II) adsorption efficiency and selectivity, as well adsorption mechanisms on a stoichiometric hydroxyapatite (HAP) surface have been investigated. Characterization studies (N2 adsorption/desorption and X-ray powder diffraction (XRPD) analyses) and adsorption tests under various operative conditions provided detailed information about the use of HAP in the de-metalation of wastewaters containing Ni and Co as polluted metal species. The sorption capacity of HAP has been evaluated by static batch adsorption tests varying initial concentration of Ni(II) and Co(II) species (from ca. 0.25 to 4.3 mM), contact time (from 15 min to 24 h), and pH (from 4 to 9) operative parameters. Proposed mechanisms of adsorption of Ni(II) and Co(II) on HAP surface are ion-exchange and surface complexation; a partial contribution of chemical precipitation from bulk solution should be considered at pH 9. In addition, adsorption isotherms of Ni(II) and Co(II) on HAP have been collected at 30 °C and pH 4 and modeled by employing different equations. The maximum sorption capacities have been quantified as 0.317 mmol \({\text{g}}_{{{\text{HAP}}}}^{{ - 1}}\) (18.6 mg \({\text{g}}_{{{\text{HAP}}}}^{{ - 1}}\)) and 0.382 mmol \({\text{g}}_{{{\text{HAP}}}}^{{ - 1}}\) (22.5 mg \({\text{g}}_{{{\text{HAP}}}}^{{ - 1}}\)) for Ni(II) and Co(II), respectively. Selectivity to Co and Ni in the adsorption process on HAP has also been investigated; HAP has higher affinity towards Co than Ni species (Co:Ni = 2.5:1, molar ratio).