RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

A study of adsorption of Zn(II) and Cd(II) ions at the α-Fe₂O₃/electrolyte solution interface is presented. For cations, the concentration range was from 1×10⁻⁶ to 1×10⁻³ mol/dm³. Adsorption measurements were combined with determination of the surface charge density and ζ potential. The ionization and complexation constants of the surface hydroxyl groups of α-Fe₂O₃, determined from the potentiometric titration data, were pK_a1=5.99; pK_a2=10.45; pK_ClO4=7.26; pK_Na=9.25. The pH_pzc of α-Fe₂O₃ in NaClO₄ solution was 8. Analysis of the cation adsorption data as a function of pH allowed us to determine characteristic parameters of the adsorption edge as pH_50%+ and ΔpH_10–90%. The parameters for Cd(II) ions (for all initial Cd(II) concentrations) at α-Fe₂O₃/NaClO₄ solution interface were: pH_50%=6.5 and ΔpH_10–90%=0.8. For Zn(II), a shift of pH_50% adsorption was observed from 6 to 7. The value of ΔpH_10–90% was 0.8 and it did not change with concentration increase.