Fig. 3 Dynamic force-profile measurements during the adsorption of copper ions by a silica tip for a copper concentration of 1.5 x 10"6 M. Ionic strength = 0.005 M NaCl, pH=5.5, A = 5.43 x 10" 21 J.
Metal ion sorption equilibrium using various metal ion concentrations is shown in Fig. 5. At equilibrium, a strong attractive force was found in all solution conditions, indicating that the surface of the silica particle was fully covered by copper ions. The surface charge was positive in all cases. Therefore at equilibrium, the surface charges of a silica sphere and a glass plate have opposite signs, which lead to an attractive force. The measured forces for all solution conditions demonstrated the same behavior. The zeta potential of the silica particle at equilibrium was estimated to be +65 mV, which matched the calculated result according to the DLVO theory based on Eq. 6. This behavior shows that for the concentration range used in these experiments, the metal ion concentration had no effect on sorption at equilibrium, which can be explained by the full coverage of sorption sites.
Fig. 4 Dynamic force-profile measurements during the adsorption of copper ions by a silica tip for a copper concentration of 7.6 x 10"7 M. Ionic strength = 0.005 M NaCl, pH=5.5, A = 5.43 x 10" 21 J.
Fig. 5 Equilibrium state of adsorption of copper ions by a silica tip. Ionic strength = 0.005 M NaCl, pH = 5.5, A = 5.43 x 10" 21 J.
Fig. 6 Effect of ionic strength on force-profile measurements at equilibrium.
Fig. 7 Dynamic force-profile measurements during the adsorption of copper ions by a silica tip for an ionic strength of 0.05 M NaCl. pH = 5.5, copper concentration = 7.6 x 10"5 M, A = 5.43 x 10" 21 J.
Effect of Ionic Strength
All sorption experiments were conducted at 7.6 x 10" 5M copper concentration and at a pH of 5.5, using different ionic strengths. The force-distance profiles between a silica sphere and a glass plate in a variety of NaCl solutions without copper ions (Fig. 6) indicate that the magnitude of electrostatic repulsion decreases with increasing ionic strength. This behavior can be explained by Eq. 4, in which decay length is reduced as ionic strength is increased because of the compression of the electrical double layer. As the ionic strength is increased, as shown in Eq. 7, this value overcomes the repulsion caused by an increase in the surface charge density.
Fig. 8 Dynamic force-profile measurements during the adsorption of copper ions by a silica tip for an ionic strength of 0.0005 M NaCl. pH=5.5, copper concentration = 7.6 x 10" 5 M, A = 5.43 x 10" 21 J.
Fig. 9 Equilibrium state of adsorption of copper ions by a silica tip. pH = 5.5, copper concentration = 7.6 x 10" 5 M, A = 5.43 x 10" 21 J.
Figs. 7 and 8 present the curves for force-vs.-separation distance for the silica-glass system at a copper concentration of 7.6 x 10"5 M in 0.05 and 0.0005 M NaCl solutions, respectively. These figures show that sorption is a relatively fast process. In the case of high ionic strength, surface charge reversal occurred 2 min after the copper ion solution was introduced into the system. However, at a much lower ionic strength, a repulsive force was still observed after the same period of time. Although the latter system required a longer period of elapsed time before reversal of the surface charge occurred, both systems reached equilibrium within approximately the same time (1 hr).
Equilibrium measurements of the same copper concentration but performed with solutions of different ionic strength are illustrated in Fig. 9. All force curves exhibited a strong attractive force at equilibrium, confirming the findings of Subramaniam et al.[6] that ionic strength has no significant effect on sorption equilibria.
CONCLUSION
Atomic force microscopy was used to investigate the effect of metal concentration and ionic strength on metal ion sorption. Changes in interaction forces as a function of time demonstrated surface charge reversal and the occurrence of sorption. This study also showed that metal concentration has an effect on sorption dynamics (e.g., charge reversal occurs more slowly and the system requires a longer period of time to reach equilibrium at a lower concentration). However, ionic strength has no significant effect on sorption kinetics. Moreover, neither metal concentration nor ionic strength exhibits any effect on sorption equilibria, indicating that surface sites of the silica particle are fully covered by copper ions under the experimental conditions in this study.
