Reload Index (ZRiChK UMCS)

SPECIFIC ADSORPTION OF Cd(II), Ni(II) AND Pb(II) IONS AT THE Al2O3:SiO2/ NaClO4 SOLUTION INTERFACE-STRUCTURE OF THE ELECTRICAL DOUBLE LAYER

 

W³adys³aw Janusz, ewa Skwarek and marlena mATYSEK

DEPARTMENT Of RADIOCHEMISTRY And COLLOID CHEMISTRY

 

 

Study of the adsorption of the Cd(II), Ni(II) and Pb(II) ions at the mixed oxide (composed of SiO2 and Al2O3 )/NaClO4 solution interface are presented. The cation adsorption measurements are compared with the surface charge density and z potential changes. The adsorption of Cd(II), Ni(II) and Pb(II) ions was investigated by radioisotope method using 115Cd, 63Ni or 210Pb isotopes respectively. The surface charge at the metal oxide/electrolyte interface was determined by means of the potentiometric titration. The z potential was measured using Zetasizer 3000 (Malvern, England). As adsorbents the pirolytic oxides of SiO2, containing 1, 3 and 8 % of Al2O3, denoted as SA1, SA3 and SA8 respectively, were used.

The adsorptions of the divalent cation as a function of pH is characterized by so called „adsorption edge” (sharp increase of the adsorption in a narrow pH range) see Fig 1 and 2. The position of this edge on pH scale is characterized by pH50% that is the pH at which 50% of initial concentration of cation is adsorbed, whereas the slope of the edge is characterized by DpH10-90% that is the difference of pH where adsorption of cation

changes from 10% to 90% [[1]].


Fig. 1 Adsorption of Ct(II) at the SA12/0.001 M NaClO4 interface from initial concentration 1*10-6M Cd(II) ), Ni(II) or Pb(II)


Fig. 2 Adsorption of Ct(II) at the SA1/0.001 M NaClO4 interface from initial concentration 1*10‑3M Cd(II), Ni(II) or Pb(II)


 


The edge of the adsorption of Ct(II) ions shifts towards alkaline region of pH with increase of initial concentration of the cation Fig 3, whereas the slope of the edge decreases.


Fig. 3. Adsorption parameter (pH50%,) of Ct(II) ions at the SA8/NaClO4 solution  interface.

 


The sequence affinity of Cd, Ni and Pb ions at the mixed SiO2:Al2O3 oxides is as follows Pb(II)>Cd(II)> Ni(II), what is seen in Fig 3.

At low initial concentrations of Ct(II) the surface charge density at the oxide series SA/electrolyte solution interface remains almost unchanged, due to low concentration of adsorbed metal cations. However, at concentration of the order of 0,001mol/dm3 a decrease of surface charge density is observed from pH value that adsorption edge starts. This is caused by release of the hydrogen ions from thje surface hydroxyl groups by divalent metal ions.

Electrokinetic’s measurements show that z potential slightly increases in the presence of metal cations at concentration of 0.0001 M above pH=, but when concentration increases to 0.001 M the charge reversal of z potential is observed. Such effect may be a result of surface precipitation or adsorption of divalent cation on one surface hydroxyl group. The last mechanism of the adsorption leads to the overcharging of the surface charge in compact part of the electrical double layer [[2]].

 

References:



[[1]] 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 (1997) 444.

[[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.