High uptake of Cu2+, Zn2+ or Ni2+ on calcined MgAl hydroxides from aqueous solutions: Changing adsorbent structures

Although previously a variety of studies have proposed the removal of anions and organic matters from contaminated water by calcined hydroxides (CHTs), their role in uptaking potentially toxic metal ions from effluents had rarely been investigated. In the present study, simulated wastewater containing Cu2+, Zn2+ or Ni2+ was used to investigate the adsorption performance of CHTs. Langmuir and Freundlich isotherm models were employed to fit the equilibrium experiments, and it was found that the Langmuir model was more appropriate to describe the adsorption isotherm. The maximum adsorption amount was higher than some other adsorbents, specifically, being 6.583, 7.535 and 6.152 mmol/g for Cu2+, Zn2+ or Ni2+ under the proposed conditions. For kinetic data, the pseudo-second-order kinetic model appeared to be the best-fitting model compared to the pseudo-first-order and Elovich models. Thermodynamic analysis revealed that Cu2+, Zn2+ or Ni2+ sorption on,CHTs was spontaneous and endothermic. In the reusability study, the sorption capacity of the adsorbent did not vary remarkably in the initial four sorption/calcination cycles for Cu2+, Zn2+ or Ni2+ removal. By virtue of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron dispersive X-ray analysis (EDX), it was speculated that the adsorption mechanism for Cu2+, Zn2+ or Ni2+ consisted of two steps: First, potentially toxic metal ions formed into hydroxide precipitations and adhered to the surfaces of adsorbents with high alkalinity. Second, the hydroxides participated in the process of adsorbents reconstructed the hydrotalcites structures through isomorphous substitution. (C) 2015 Elsevier B.V. All rights reserved.