Adsorption of Zn2+ ions on Fe(III)-hydroxide

The extent of Zn2+ ion adsorption on Fe(III)-hydroxide prepared by hydrolyzing Fe (NO3)3 was measured as a function of the solution pH(5-7) and the amount of the hydroxide. Acid-base titrations of the hydroxide were also carried out, and its surface area was measured by the BET method. It was found that the concentration of the adsorbed Zn2+ ions is proportional to the concentrations of Zn2+ ions in solution and Fe(III)-hydroxide, but is inversely proportional to the second power of the hydrogen ion concentration: [Zn2+sub(ad)] = K [Zn2+] [Fe(III)]/[H+]2 This behavior is explained by considering the adsorption reaction to be Zn2+ + 2 -FeOH reversible -(FeO)2Zn sub(ad) + 2 H+, were -FeOH is the site at which the Zn2+ ion can adsorb. The -FeOH type site was found to prevail over the -FeOH2+ and -FeO- type sites in the pH region investigated, and the concentration of -FeOH sites was much higher than that of the adsorbed Zn2+ ions. (author)