Specific adsorption of Zn(II) and Cd(II) ions at the α-Fe2O3/electrolyte interface structure of electrical double layer

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 pKa1=5.99; pKa2=10.45; pKClO4=7.26; pKNa=9.25. The pHpzc 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 pH50%+ and ΔpH10–90%. The parameters for Cd(II) ions (for all initial Cd(II) concentrations) at α-Fe2O3/NaClO4 solution interface were: pH50%=6.5 and ΔpH10–90%=0.8. For Zn(II), a shift of pH50% adsorption was observed from 6 to 7. The value of ΔpH10–90% was 0.8 and it did not change with concentration increase.