Factors Affecting the Adsorption of Zn2+ and Cd2+ Ions from Aqueous Solution onto Vermiculite

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, qe. Subject to the magnitude of C0/W0, the value of qe could either attain its maximum level, qm (the maximum adsorption density), when the solid surface was fully saturated, or approach its critical limit, qc (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.