Reload Index (ZRiChK UMCS)

 

 

The MECHANISM of ADSORPTION of Zn(II) and Cd(II) IONS at a-Fe2O3 ELECTROLYTE INTERFACE -ELECTRICAL DOUBLE LAYER STRUCTURE

 

WŁADYSŁAW JANUSZ and   STANISŁAW CHIBOWSKI

DEPARTMENT of RADIOCHEMISTRY and COLLOID CHEMISTRY

 

 

The specific adsorption of cations at the metal oxide/electrolyte interface has been discussed (reported) in theoretical as well as practical point of view in a large number of scientific journals. The adsorption of cations at the metal oxide/electrolyte interface that is important process which regulates of distribution of cation in the environmental systems (soils, sediments and aquifers)[1].

Mechanism of adsorption of Zn(II) and Cd(II) ions at the monodispersed a-Fe2O3/electrolyte interface has been studied by different experimental techniques (potentiometric titration, microelectrophoresis and radio adsorption measurements of zinc and cadmium ions). a-Fe2O3 (hematite) was obtained by hydrolysis of FeCl3 +HCl solution at 100oC by 24 hours  according to Matijevic procedure[2]. The specific surface of the oxide was 31.9 m2/g (BET method); mean diameter particle diameter 133nm; polydispersity= 0.24.

Divalent ions can occupy one or two surface adsorption sites. For hydrolysable metal cations the following reactions can take place[3]:

                                                              (1)

                                               (2)

                                           (3)

Pole tekstowe:  
Fig. 2. Adsorption Cd(II) vs. pH at a Fe2O3/0.001M NaCO4 –1*10-6 M Cd(II)
Pole tekstowe:  
Fig. 1. Adsortpion Zn(II) vs. pH at a-Fe2O3/0.001M NaCO4 –1*10-6 M Zn(II)
The selected characteristic results of adsorption of Zn(I)) i Cd(II) ions at the Fe2O3/electrolyte interface are presented on the Figs 1-4

Pole tekstowe:  
Fig. 4. Surface charge vs. pH at a-Fe2O3/0.001M NaCO4 –CD(II) solution interface.

Pole tekstowe:  
Fig. 3. Surface charge vs. pH at a-Fe2O3/0.001M NaCO4 –Zn(II) solution interface.

Sumary:

·The adsorption of Zn(II) and Cd(II) ions at the a-Fe2O3/electrolyte interface increases with the increase of the pH of the solution. Such behavior suggests the reaction of exchange with H+ ions with surface hydroxyl groups.

·The point zero of charge at the a-Fe2O3/NaClO4 solution interface in the presence of Cd(II) or Zn(II) ions shifts towards lower values of pH and the significant  decrease of surface charge density is observed at pH>6. This effects is characteristic for the adsorption of metal cations on oxides.

·Adsorption of Cd(II) and Zn(II) at the a-Fe2O3/NaClO4 interface runs throught the exchange of H+ ions of one surface hydroxyl groups. Contribution of the reaction of these ions with two surface hydroxyl groups to adsorption is small and it increases with increase of pH.

·The effect of presence of Cd(II) and Zn(II) ions on the z potential in the a-Fe2O3/NaClO4 systems is significant at high concentartion of Me(II) ions, for Zn(II) ions the charge reversal effect was observed. The shift of pHiep tworads higher alkaline region of pH scale in the presence of Cd(II) ions is.

 

References:

[1]    Stumm, W. (1992) Chemistry of the Sold – Water Interface, Willey Interscience Publ, J. Willey & Sons, Inc., New York.

[2]    E. Matijević, Ann. Rev. Mater. Sci., 15 (1985) 483-516.

[3]    P. Schindler, in Adsorption of Inorganics a Solid Liquid Interfaces, M.A. Anderson and A.J.Rubin eds, Ann Arbor Sci., Ann Arbor 1981, p.1