Reload Index (ZRiChK UMCS)

 

 

ADSORPTION OF IONS AT THE MONODISPERSED METAL OXIDE/ELEKTROLYTE-ELECTICAL DOUBLE LAYER STRUCTURE

 

WŁADYSŁAW JANUSZ

DEPT. OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

Fig. 2. Surface charge density vs. pH at the SiO2/ 0.1 mole/dm3 NaCl -Ba2+

 

 

The investigation of the adsorption of divalent cations at the metal oxide/ electrolyte interface is important for description of the distribution of cations between solid and liquid phase and changing of the surface properties of the solid. The adsorption of metal ions at the metal oxide/solution interface is important in many contexts such as: mineral growth or their dissolution or weathering, transport of metal ions in ground waters, processing of ores, treatment of wastes etc[1].

Oxides in water solution are electrically charged because of the ionization and complexation of the surface hydroxyl groups. Adsorption of metal ions at the oxide/electrolyte interface may involve the reaction of them with hydroxyl groups, resulting in the loss of the one or more water molecules from the hydration shell of the adsorbate ion and formation of a relatively strong chemical bond. Divalent ions, like Ca2+, may react with two or one surface hydroksyl groups[2,3]. The Ca2+, Sr2+ and Ba2+ exist as nocomplexed ions in wide range of pH, so neglecting of adsorption of their hydroxyl form is fully justified.

The adsorption of Ca2+, Sr2+ and Ba2+ ions at the SiO2/NaCl solution interface was investigated by means of radiotracer method from 10-6 to 0.001 mole/dm3 of Me2+ ion concentration. Beside the surface charge density, the z potential and mean diameter of particles as a function of pH and ions concentration were measured.

 

Fig. 1. Adsorption density of Ba2+ ions as a function of pH at the SiO2/NaCl solution interface.

Fig. 4. Mean diameter of SiO2 particles in 0.001 mole/dm3 NaCl- Ba2+ solution.


Fig. 3. The z potential vs pH in the SiO2/0.001mole/dm3 NaCl -Ba2+ system.

 

Sumary:

·The adsorption of Ca2+, Sr2+ and Ba2+ ions at the SiO2/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.

·As the adsorption of these cations do not changes surface charge density at the SiO2/electrolyte interface, so it may be result of interaction of cations with negatively charged groups ( or ). The dependence of adsorption from background electrolyte concentration may indicate that  groups play an important role in the adsorption.

·The adsorption of Ca2+, Sr2+ and Ba2+ ions lead to the significant changes of the z potential of SiO2. At low concentrations the adsorption cause the decrease of absolute values of the z potential. In alkaline region of pH high concentrations of cations cause overcharging of the compact layer of the electrical double layer, so the charge reversal is observed.

·The Ca2+, Sr2+ and Ba2+ ions at the concentration of 0.01 mole/dm3 cause the aggregation of SiO2 particles, so the coagulation process is result of the z potential changes.

References:

[1]  G.E. Brown, G.A. Parks, P.A. O’Day, Sorption at mineral-water interfaces: macroscopic and microscopic perspectives, in Mineral Surfaces (D.J., Vaughan and R.A.D. Pattrick eds ) Chapman & Hall, London 1995, p.129.

[2]  P. Schindler, Surface Complexes at Oxide-Water Interface 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 and J.O. Leckie J. Colloid Interface Sci. 188, 444-472, 1997.