Reload Index (ZRiChK UMCS)

The COMPETITIVE ADSORPTION of Ca2+ and Zn(II) IONS at a-SiO2 ELECTROLYTE INTERFACE -ELECTRICAL DOUBLE LAYER STRUCTURE

Władysław Janusz, Jacek Patkowski and Stanisław Chibowski

DEPARTMENT Of RADIOCHEMISTRY And COLLOID CHEMISTRY

The adsorption of divalent cations at the metal oxide/electrolyte interface causes significant changes of the electrical double layer properties at that interface. These cations may be adsorbed forming outer- or inner- sphere complexes [1]. Adsorbed divalent cations can occupy one or two hydroxyl groups on the metal oxide surface [2].

Cations adsorbed on one hydroxyl group increase concentration of negatively charged groups but compensation of this charge in IHP is two times higher compared to cation adsorbed on two hydroxyl groups. Such a mechanism of adsorption leads, at high adsorption densities, to overcharging the compact part of electrical double layer that the charge reversal effect at the zeta potential as a function pH may be observed.

In the natural or technological systems aqueous solutions usually contain many soluble species e.g. cations. At the interface of these solutions the competition of the adsorption of ions may be observed. The common cation for the natural aqueous system is Ca2+. In this paper the competition of adsorption of Ca2+ and Zn(II) at the SiO2/aqueous solution of NaClO4 is presented.

Mechanism of adsorption of Ca2+ and Zn(II) ions at the fumed a-SiO2/electrolyte interface has been studied by different experimental techniques (potentiometric titration, microelectrophoresis and radio adsorption measurements of zinc and calcium ions). The SiO2 from Aldrich without purification was used. The sample had the specific surface 261.7 m2/g (BET method). The mean diameter particle (by means PCS method) was 174nm and polydispersity = 0.15.

As seen in Figs. 1 and 2 „edge of adsorption” typical for multivalent cations is observed for both Ca2+ and Zn(II) [3]. This edge is observed for Ca2+ in the pH range from 7 to 10 and for Zn(II) in the pH range from 6.2 to 7.5. With the increase of initial cation concentration from 1×10-6 to 1×10-3 mole/dm3 the slope of the edge of adsorption, characterized by DpH10-90% parameter, increases from 1.9 to 3.4 for adsorption of Ca2+ It also changes similarly from 1.2 to 1.6 for Zn(II) adsorption. The Ca2+ adsorption at the SiO2/electrolyte interface result with a small increase of surface charge so that the adsorption of this cation probably takes place at the negatively charged surface sites ºSiO-. The adsorption of Zn(II) ion leads to a significant increase of negatively charged groups.

 



Fig. 1. Adsorption of Ca2+ ions as a function of pH at the SiO2/NaClO4 solution interface.

 


Fig. 2. Adsorption of Zn(II) ions as a function of pH at the SiO2/NaClO4 solution interface.

 


Presence of Zn(II) ions at high concentrations leads to shifting the edge of adsorption of Ca2+ at the SiO2/electrolyte interface at low concentrations. With an increase of a concentration of Ca2+ this effect became smaller. It is characterized by the pH50% parameter, which shows the location of the edge of adsorption (Fig. 3).

Presence of the Ca2+ has a small effect on adsorption of Zn(II) cation at the SiO2/electrolyte interface but the shift of the adsorption edge towards lower pH and increase of the slope of the edge is observed (Fig. 4).

 


Tabela 1. pH50% parameter at the SiO2/10‑3M NaClO4 solution interface for the Ca2+.

CZn(II)

[mol/dm3]

CCa2+

[mol/dm3]

0

5*10-6

1*10-4

1*10-3

1*10-6

7.1

 

 

8.6

1*10-4

7.9

 

7.9

 

1*10-3

9.6

9.6

 

9.4

1*10-2

 

10.0

 

 


Tabela 2. pH50% parameter at the SiO2/10‑3M NaClO4 solution interface for the Zn(II) ions.

CCa2+

[mol/dm3]

CZn(II)

[mol/dm3]

0

1×10-6

1*10-4

1*10-3

0.01

5*10-6

6.3

 

 

6.1

6.2

1*10-4

6.6

 

6.3

 

 

1*10-3

7.5

7.3

 

7.0

 

 

 


[1]    K. F. Hayes, L. E. Katz, Application of X-ray Adsorption Spectroscopy for Surface Complexation Modelling of Metal Ion Sorption in: Physics and Chemistry of Mineral Surfaces (P. V. Brady Ed), CRC Ser. Chem. Phys. Surf. Interfaces, CRC Press New York 1996, p.1147-223.

[2]    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.

[3]    A. P. Robertson, J. O. Leckie, Cation binding predictions of surface complexation models effects of pH, ionic strength, cation loading, surface complex and model fit, J. Colloid Interface Sci., 188,444-427, 1997.