Reload Index (ZRiChK UMCS)

 

 

Specific Adsorption of Ca(II) and Fe(III) Ions at the Tin Dioxide /Electrolyte Interface-Structure of the Electrical Double Layer

 

W³adys³aw Janusz

DEPARTMENT Of RADIOCHEMISTRY And COLLOID CHEMISTRY

 

 

The study of the adsorption of the Ca2+ and Fe(III) ions at the SnO2/electrolyte solution interface are presented. The adsorption measurements accompanied the surface charge density and z potential determinations. The adsorption of ions was investigated by radioisotopes method using 45Ca or 55Fe isotopes. The surface charge at the metal oxide electrolyte was determined by means of the potentiometric titration. The z potential was measured using Zetasizer 3000 (Malvern, England).

The adsorption of the di- and trivalent cation data as a function of pH exhibit the dramatic increase in adsorption density from zero to 100% that is observed within a very narrow pH range, usually smaller than two pH units. This sharp increasing course of the adsorption is called “adsorption edge” and it is characterized by two parameters: pH50% is the pH at which 50% of initial concentration of cation is adsorbed and DpH10-90% is the difference of pH where adsorption of cation changes from 90% to 10%.


The edge of the adsorption of Ca2+ ions is above pHpzc (pHpzc=4) of SnO2. The edge of Ca2+ ions adsorption characterised by pH50% shifts from 5,5 to 8.5 with the increase Ca2+ ions concentration from 5*10-8 to 1*10‑2 mol/dm3.

Fig. 1 Adsorption of Ca2+ at the SnO2/0.001 M NaCl interface.

At low initial concentrations of Ca2+ the surface charge density at the SnO2/electrolyte solution interface remains almost unchanged. However at concentration of the order of 0,001mol/dm3 decrease of surface charge is observed as result of the replacing of hydrogen in surface hydroxyl groups by calcium ions. The comparison of decrease of surface charge and the Ca2+ adsorption data, led to the conclusion that Ca2+ ions exchange one H+ ions. The adsorption and surface charge data are consistent with zeta potential measurements. At low concentration z potential is in error range the same, but at initial concentration of Ca 0.001 the above pH=5 the increase of zeta potential and overcharging of compact part of edl are observed.


The “adsorption edge” for for Fe(III) at SnO2/0.1 mol/dm3 NaCl solution interface is below pH=3 Fig.2 . For all investigated initial concentrations of Fe(III) ions adsorption edge is below pHpzc and the adsorption process of Fe(III) ions leads to release of H+ ions.

Fig. 2 Adsorption of Fe(III) at the SnO2/0.001 M NaCl interface.

Adsorption of Ca2+ and Fe(III) at the SnO2/NaCl interface proceeds via the exchange of H+ ions with one surface hydroxyl group and is characterized by the following values of the reaction adsorption constants: pK1s =1.98, pb21s=8.57for Ca2+ and for Fe(III) pK1s =-0.06, pb21s=2.40. The contribution of the reaction of these ions to the adsorption with two surface hydroxyl groups is small but it increases with increasing pH.

The effect of the presence of Ca2+ and Fe(III) ions on the z potential of the SnO2/NaCl systems is significant. For low initial concentration of Ca2+ ions the small changes of zeta potential value are observed at pH >6, and at higher concentration the adsorptions of ions Ca2+ lead to charge reversal. Adsorption of Fe(III) ions shift the pHiep towards higher pH values.

The effect of the presence of Ca2+ and Fe(III) ions on the z potential of the SnO2/NaCl systems is significant. For low initial concentration of Ca2+ ions the small changes of zeta potential value are observed at pH >6, and at higher concentration the adsorptions of ions Ca2+ lead to charge reversal. Adsorption of Fe(III) ions shift the pHiep towards higher pH values.