Monovalent-divalent exchange model

Figure 8. Predicted concentrations of major surface and solution species in monovalent-divalent exchange model fits of Figures 7 and 9. The diffuse-layer species represent the excess (K+, Na+, Ca2+) or deficit (Cl ) of ions relative to the bulk solution. Part a log C versus log TOTNa for fit of Na -CVf" exchange data (Figure 7) without consideration of surface complexation. Part b log C versus log TOTNa for fit of Na+-Ca2+ exchange data (Figure 7) with surface complexation of Na and Ca2+. Part c log C versus leg TOTKfor fit of K -C( + exchange data (Figure 9) with surface complexation of K+-Ca2+. Continued on next page.

Ion-exchange models are commonly used to describe radionuclide sorption onto the fixed-charged sites of materials like clays. Ion exchange will be strongly affected by competition with monovalent and divalent ions such as Na" " and Ca, whereas it will be less dependent on pH over the compositional ranges common for natural waters. Many studies of strontium and caesium sorption by aluminosilicates (e.g., Wahlberg and Fishman, 1962 Tamura, 1972) have been carried out within the framework of ion-exchange theory. Early mechanistic smdies... 

When heterovalent exchange (e.g., exchange involving both mono- and bivalent ions) takes place, which is frequently seen in soil-solution systems, the quantitative treatment runs into further difficulties. In this case, the determination of mole fractions also becomes problematic. The Vanselow equation assumes that the monovalent and bivalent ions are equivalent when calculating mole fractions. Other empirical equations, however, simply introduce some factors for the ions with different valencies. For example, according to Krishnamoorthy and Overstreet (1949), this factor is 1 for monovalent ions, 1.5 for divalent ions, and 2 for trivalent ions, which is not in agreement in stoichiometry. Another model... 

A polyelectrolyte can be utilized as a model to investigate the mechanism of ion selectivity of ion exchangers. The ion selectivity exhibited by polyelectrolytes is therefore an important property for practical applications. Our work deals specifically with polycarboxylic polyelectrolytes in salt free aqueous solutions. In the first part, the activities of monovalent and divalent counterions are determined and discussed in connection with the nature of the counterion and polyelectrolyte charge density. Then, experimental results are presented showing clearly the existence of selectivity and affinity sequences. 

To address the theoretical limitation of the Nikolsky-Eisemnan equation, a more general description of the equihbrium responses of hquid membrane ISEs in mixed ion solutions was proposed (41). The model is based on phase boundary potentials under an equilibrium exchange of an analyte and an interfering co-ion at the membrane/sample solution interface. With ionophore-based membranes, the ion-exchange process is followed by complexation of the ions with an ionophore, where free ionophore was assumed to be always present in excess to simplify the model. The charge of the ions was not fixed so that their effect on the potentiometric responses can be addressed by the model. Under equilibrium conditions, the model demonstrated that the Nikolsky-Eisemnan equation is valid only for ions with the same charge (zj = Zj). The selectivity coefficient, however, can still be used in the new model to quantify the potentiometric responses in the mixed ion solution. For example, the potentiometric responses to a monovalent cation in the presence of a divalent cation are given as... 

Fig. 2. Models proposed for CDen-montmorillonite formed by ion exchange of the terminal -Nf group with the interlayer monovalent cation (I) and by coordination of the aminoethylamino group to the interlayer divalent cation (II and III) M = Na, Ca, Zn, and Cu(II).

Swider, K. E. and Rolison, D. R. (2000) Reduced poisoning of platinum fuel-cell electrocatalysts supported on desulfurized carbon. Electrochem. Solid-State Lett. 3,4-6 Tandon, R. and Pintauro, P. N. (1997) Divalent/monovalent cation uptake selectivity in a Nafion cation-exchange membrane experimental and modeling studies. J. Membr. Sci. 136, 207-219 Tan, J., Chao, Y. J., Van Zee, J. W. and Lee, W.-K. (2007) Degradation of elastomeric gasket materials in PEM fuel cells. Mater. Sci. Eng. A, 445-446, 669-675 Tawfik, H., Hung, Y. and Mahajan, D. (2(X)7) Meted bipolar plates for PEM fuel cell - A review. J. Power Sources 163, 755-767... 

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