Treatment of aqueous solutions containing nickel using crandallite‐type compounds

The removal of nickel from aqueous solutions streams has been investigated using an artificial amorphous crandallite‐type compound, CaAl₃(OH)₆(HPO₄)(PO₄) (Ca‐crandallite), synthesized in our laboratory. Equilibrium ion‐exchange isotherms in an aqueous medium of Ca²⁺/Ni²⁺ at different pH values at 293 K have been determined. The experimental equilibrium data were satisfactorily correlated using a Langmuir‐type empirical equation. At low pH values, the hydrogen ion competes with the heavy metal cation and the percentage removal of metal declines. It was found that the operating capacity of Ca‐crandallite with respect to the metal ion increased with the pH of the solution, in accordance with a second‐degree polynomial equation. However, the pH should not be allowed to rise to levels at which chemical precipitation as nickel hydroxide would occur, with 7.00 the highest value tested. Taking into account the variation of operating capacity with pH, the system exhibited a unique separation factor, namely all the experimental points can be described by a unique isotherm in a dimensionless form. The Ca‐crandallite showed a high capacity, 2.176 meq g^?1, for the exchange of Ni(II) from nickel nitrate solutions and the rate of exchange of metal increases with increasing solution temperature due to the enhancement of effective intraparticle diffusivity. Copyright © 2005 Society of Chemical Industry