Factors Affecting the Adsorption of Zn 2+ and Cd 2+ Ions from Aqueous Solution onto Vermiculite

Abstract

Experiments were conducted to investigate the influence of different factors on the adsorption of Zn²⁺ and Cd²⁺ ions from aqueous solution onto vermiculite. The ratio of the initial concentration of ions, C₀, to the adsorbent concentration, W₀, was shown to be the key factor determining the equilibrium ion adsorption density, q_e. Subject to the magnitude of C₀/W₀, the value of q_e could either attain its maximum level, q_m (the maximum adsorption density), when the solid surface was fully saturated, or approach its critical limit, q_c (the C₀/W₀ ratio), when the ions in the bulk solution had been completely removed.

The main mechanism for adsorption in the systems studied was ion-exchange. An increase in Zn²⁺ or Cd²⁺ ion adsorption from the single metal ion solution resulted in an increase in the concentrations of unadsorbed K⁺, Na⁺, Ca²⁺ and Mg²⁺ ions remaining in the bulk solution. Competition between adsorbing ions was observed in the mixed K⁺–Zn²⁺–Cd²⁺ ion system. Although temperature had a positive effect upon the adsorption of Zn²⁺ and Cd²⁺ ions, its effect was statistically negligible over the range 15–45°C. The negative effect of decreasing pH values on the adsorption process was only significant below a pH value of 4.0, indicating that the adsorption of Zn²⁺ and Cd²⁺ ions would be significantly depressed when high amounts of H⁺ ions are present in the system.