Vanselow Selectivity Coefficients Research Articles

Binary and Ternary Exchange of Potassium on Calcareous Montmorillonitic Soils

AbstractSimultaneous K exchange with Ca, Mg, and Na occurs in soils, but most of the published information is obtained from binary systems. The theoretical question of whether selectivity coefficients obtained in binary systems are valid for ternary systems has not yet been clarified; however, no significant difference has been found between selectivity coefficients in binary and ternary systems. Potassium exchange with Mg plus Ca was studied in two calcareous soils with variable concentrations of Na, using batch and miscible‐displacement methods. Preference for K over the divalent cations was found in both soils, regardless of Na concentration and the experimental methods. Modified Gapon (kG) and Vanselow (kV) selectivity coefficients were calculated for each cation pair in binary and ternary systems. In both soils, the kv and the kG decreased with increasing K fraction in the solution, but the kG value was less affected than the kv. A simple mathematical solution of the Gapon binary equations, K − (Mg + Ca) and Na − (Mg + Ca), in combination with an assumption of constant cation‐exchange capacity (CEC), was used to predict the amount of exchangeable K as a function of solution composition in a ternary system. The high correlation of predicted exchangeable K with the directly measured values shows that binary selectivity coefficients can be used in multicationic systems on calcareous montmorillonitic soils.

Double layer theory prediction of Al-Ca exchange on clay and soil

The Poisson-Boltzmann equation was solved numerically for the case of a diffuse electric double layer containing mixed divalent and trivalent electrolytes. The predicted Gapon selectivity coefficient was found to range over two orders of magnitude while the Vanselow selectivity coefficient changed by 10% over the range of ionic ratios considered. The formation of tactoids should increase the affinity of the exchanger for cations with higher ionic charge. Ionic strength has little effect on predicted selectivity coefficients. The results of double layer calculations approximated, but systematically deviated from, two sets of published values for Al 3+-Ca 2+ exchange on montmorillonite. Consideration of the speciation of Al 3+, particularly including the dimer Al 2(OH) 2 4+, improved the agreement between predicted values and published results. Predicted cation exchanger composition is in agreement with published base saturation data for mineral soils.

Influence of Electrolyte Concentration on Quaternary Cation Exchange by Silver Hill Illite

AbstractA study of Na‐K‐Ca‐Mg exchange on specimen illite in a perchlorate background of varying ionic strength is reported. Sodium‐saturated Silver Hill illite was reacted at pH 7 with perchlorate solutions of differing Na/Ca or Na/Mg ratio and total electrolyte concentration (9.2, 5, 3.6, 2 and 1 molc m−3). All soluble and exchangeable cations were measured directly. No effect of the total electrolyte concentration on the total adsorbed charge was observed. The exchange isotherms and Vanselow selectivity coefficients based on the composition data showed that Silver Hill illite exhibited more or less equal affinity for Ca and Mg. Both bivalent cations were slightly preferred to Na in the three more concentrated electrolyte backgrounds, but at concentrations of 2 and 1 molc m−3, a slight preference for Na over Ca and Mg was observed. Exchangeable K, which represented 6 to 18% of the total adsorbed charge, decreased with the content of exchangeable Ca or Mg and, at a given bivalent charge fraction, was dependent on the total perchlorate concentration. The clay exhibited a very high preference for K over the three other cations which increased as the content of exchangeable K decreased.

Quaternary Cation Exchange on Silver Hill Illite

AbstractThe simultaneous exchange of Na, K, Ca, and Mg on Silver Hill illite suspended in 5 mol m−3 perchlorate background at pH 7 was investigated systematically. Composition data for both the exchanger and the equilibrium aqueous solution phase were obtained directly. The data showed that the total adsorbed metal charge was independent of exchanger composition provided that all four cations were considered. Analysis of the composition data with exchange isotherms and calculated Vanselow selectivity coefficients showed that: (i) K+ was preferred over Na+, Ca2+, and Mg2+, and the preference decreased as EK decreased (E = charge fraction on the clay mineral); (ii) bivalent cations were preferred only slightly over Na+ for ENa > 0.01, whereas for ENa ≤ 0.01, the reverse was true; (iii) no preference was shown in Ca‐Mg exchange at ENa values between 0 and 0.13.

The Effect of pH on Sodium‐Calcium and Potassium‐Calcium Exchange Selectivity for Cecil Soil

AbstractCation exchange capacity (CEC) and cation‐exchange equilibria involving Na, K, and Ca were measured on the <50µm fraction of Cecil sandy loam soil (Typic Hapludult) at pH values between 4 and 8. This was achieved by equilibrating the soil separate in acetate buffers followed by equilibration in 0.1 M chloride solutions. Exchangeable cations were removed by five sequential extractions with 0.5 M Mg(NO3)2. The CEC increased two‐ to threefold between pH 4 and 8, indicating the presence of a large amount of pH‐dependent charge. CEC measured with Ca exceeded that measured with Na and was presumed to result from a greater amount of exchange between Ca ions in solution and hydrogen on the soil organic fraction. In Na‐Ca and K‐Ca exchange systems, Ca accounted for most of the increase in adsorbed metal cation charge as pH increased from 4.5 to 6.5. Estimates of exchangeable hydrogen were obtained from the relationship between CEC and pH and included in calculations of adsorbed mole fractions. Vanselow selectivity coefficients indicated that the Cecil soil preferred Ca over Na but that selectivity for Na increased sharply at low pH and low Na mole fractions. The selectivity for K relative to Ca also increased with decreasing K mole fraction with K preferred over Ca at low K mole fractions. Potassium added to Na‐Ca systems displaced more Na than Ca from the exchanger, resulting in an apparent increase in selectivity for Ca relative to Na.

Quinoline Sorption on Na-Montmorillonite: Contributions of the Protonated and Neutral Species

AbstractDilute aqueous solutions of quinoline were contacted with Na-montmorillonite to elucidate the sorption process of the neutral and protonated species. Sorption occurs via a combination of ion exchange and molecular adsorption and yields S-type isotherms. Exchange between the quinolinium ion (QH+) and Na can be described by means of Vanselow selectivity coefficients and a thermodynamic exchange constant (Kex). Due to the apparent adsorption of the neutral species at high mole fractions (x) of the solid phase, the thermodynamic standard state was defined as 0.5 mole fraction. The selectivity at pH ~4.95 of the QH+ species over Na (at XQH+ = 0.5) was determined to be Kv = 340. At pH ≥ 5.5 surface mole fractions of 0.5 could not be obtained without adsorption of the neutral species. This study suggests that at dilute solution concentrations quinoline is sorbed preferentially as the cation even at pHs ≫ pKa. A critical surface-solution concentration is apparently necessary for adsorption of the neutral species.

Calcium‐Magnesium Exchange on Illite in the Presence of Adsorbed Sodium

AbstractBinary and ternary cation exchange experiments were performed with Na+, Ca2+, and Mg2+ on Silver Hill illite and on a soil illite suspended in 50 mol m−3 perchlorate at pH 7. The binary experiments indicated no preference for Ca over Mg on the specimen illite, but selectivity for bivalent cations over Na+ and for Ca2+ over Mg2+ on the soil illite. The ternary exchange data showed that, for both illites, there was no effect of adsorbed Na on the Ca‐Mg exchange isotherm at exchangeable sodium percentages from 0 to 30%, either for freshly‐prepared clay suspensions or for those aged up to 270 d in NaClO4. No significant differences in the total adsorbed metal charge (Qo) were observed among the binary and ternary exchange experiments, and Qo was independent of the degree of bivalent cation saturation (ECa or EMg) in the Na → Ca and Na → Mg exchange experiments on the soil illite. The Vanselow selectivity coefficient for Mg → Ca exchange on the soil illite decreased as ECa increased, showing a plateau near ECa ≈ 0.6 and dropping to 1.0 as ECa became equal to 1.0. This behavior was interpreted as evidence for at least two classes of surface complex involving Ca and Mg.

Sodium‐Calcium‐Magnesium Exchange Reactions on a Montmorillonitic Soil: III. Calcium‐Magnesium Exchange Selectivity

AbstractThe Vanselow selectivity coefficient was calculated for Mg → Ca exchange on a montmorillonitic soil separate containing organic matter. Seven sets of cation exchange data were analyzed numerically, encompassing pH values ranging from 5 to 7 in a 50 mol m−3 perchlorate medium and exchangeable sodium percentages ranging from 0 to 25%. The selectivity coefficient for Mg → Ca exchange was observed to decrease from a value near 1.5 at 0% Ca saturation to a value near 1.0 at 100% Ca saturation with no effect of pH on this trend in the range examined. At low Ca saturation of the soil, the selectivity coefficient increased by about 15% as the ESP increased from 0 to 25%. This behavior was shown through a model calculation to be consistent with a mechanism in which Na adsorbs preferentially on the montmorillonitic component of the exchange complex in the soil, leaving the nonmontmorillonitic portion of the exchange complex to adsorb Ca selectively over Mg.

Cation Exchange Equilibria in Soils Expressed as Cation‐Ligand Complex Formation

AbstractA model for representing cation exchange reactions as part of the equilibrium reactions in a soil‐water system is proposed. The exchange reaction in this model is considered to be a composite of two half complexation reactions. These complexation reactions are assumed to occur between the exchanging cations and the negatively charged surfaces considered as hypothetical free ligands. The model was added as a subroutine to the GEOCHEM program which calculates ionic equilibrium in soil water systems. Two formation constants, K′Caf for the Ca‐ligand complex and K′Naf for the Na‐ligand complex were estimated by assuming K′2Naf/K′Caf = KVCa‐Na = 1 in the range 1 > ECa > 0.7. A value of log K′Caf = 2.5 was found to provide the best fit to the relationship between ES = (ENa/(CEC‐ENa)) vs. SAR given by the U.S. Salinity Laboratory Report (1954). The fitted value of K′Caf was used for calculating the K‐ligand formation constant; K′Kt and the Mg‐ligand formation constant, K′Mgr, using the appropriate experimental Vanselow selectivity coefficient. The calculated formation constants corresponding to the cations in a given soil water system were used to predict the distribution of the cations in solution and on the solid phase by means of the GEO‐CHEM program. Good agreement was found for the K‐Ca and Ca‐Mg exchange in soils when comparing experimental data to the ones predicted by the proposed model.

Calcium‐Magnesium Exchange on Wyoming Bentonite in the Presence of Adsorbed Sodium

AbstractThe exchange of magnesium for calcium at 298 K on Wyoming bentonite was investigated in a 50 mol m−3 perchlorate background maintained at pH 7. The charge fraction of Na+ in the aqueous solution phase was controlled at the values of 0.74 or 0.87 during the exchange experiments, resulting in average exchangeable sodium percentages (ESP) of either 16 or 36% on the clay. The Vanselow selectivity coefficient for Ca → Mg exchange was equal to 1.0 and was independent of exchanger composition at both ESP values. This result implied that, under the conditions of the exchange experiments, the clay exhibited equal affinities for Ca2+ and Mg2+ regardless of the quantity of adsorbed sodium present.

EXCHANGE EQUILIBRIA OF POTASSIUM VERSUS CALCIUM AND SODIUM IN SOILS FROM A SEMIARID REGION, INDIA

We studied the potassium-calcium and potassium-sodium exchange equilibria on three soil samples differing in their organic carbon content (OC) and cation exchange capacity (CEC). Homoionic (Ca or Na saturated) soils were equilibrated, with the solutions having a large range of potassium adsorption ratios (PARs) at 50 meq L-1 total electrolyte concentration. We analyzed the data on exchange equilibria, using a thermodynamic approach and various selectivity coefficients. The normalized exchange isotherms for K+-Ca2+ systems suggested higher specificity for K+ to Ca2+ in the first 50 to 75 percent of K+ saturation. In the K+-Na+ system the specificity for K+ to Na+ was, however, higher for the whole range of exchange isotherms. The soil high in OC (Soil 1) had higher preference for K+ than the soils low in OC (Soils 2 and 3). The values of standard free energy change of the exchange reactions (ΔGr°) for K+-Ca2+ and K+-Na+ systems were negative for all the three soils tested. These values were, however, more negative for K+-Na+ than for K+-Ca2+ systems, suggesting higher preference for K+ in the former. A comparison of ΔGr° values suggested that the effect of CEC on the K+ specificity of these soil samples was only marginal, though that of OC was quite spectacular. The values of the Gapon selectivity coefficient (KG) decreased first sharply and then gradually with the increase in exchangeable potassium percentage (EPP). On the basis of (KG), the exchange isotherms were divided into three zones, representing exchange sites with high, medium, and low specificities for K+. The arbitrary limits for the three zones were < 5, 5 to 38, and >38; > 4, 4 to 15, and >15; and > 5, 5 to 15, and > 15 for Soils 1, 2, and 3, respectively. The exchange selectivity coefficient (KN) and AGr° for K+-Na+ systems were different only marginally for Soils 1 and 2. However, for K+-Ca2+ systems, the values of different selectivity coefficients (viz., KG, Vanselow selectivity coefficient, and Krishnamoorthy-Davis-Overstreet selectivity coefficient) and AGr° for Soils 1 were considerably different from those for Soil 2. This suggested a differential role of OC on changing the selectivity of soil for K+ in K+-Ca2+ and K+-Na+ systems.

Thermodynamics of Sodium‐Copper Exchange on Wyoming Bentonite at 298°K

AbstractThe exchange of copper (Cu2+) for sodium (Na+) at 298°K on Wyoming bentonite was investigated in a perchlorate background medium maintained at pH 5.0 to 6.0 and at a total normality of 0.01N. It was found that the exchange process could be well characterized by a Vanselow selectivity coefficient that was equal to 1.32, independent of the exchanger composition, and by a similarly constant apparent adsorbed metal charge equal to 0.92 ± 0.05 eq kg−1. These results are in conflict with previous data obtained at 295°K for Na+‐Cu2+ exchange on montmorillonite in a 0.01N chloride background medium. It was concluded that a principal cause for the differences between the two sets of data could be the presence of CuCl+ complexes in the chloride medium.

Sodium‐Calcium Exchange in Nile Delta Soils: Single Values for Vanselow and Gaines‐Thomas Selectivity Coefficients

AbstractNa‐Ca exchange measurements were carried out on 11 representative soils from the Nile Delta. Experiments were conducted over a wide range of exchanger compositions. Due to their common alluvial origin, all soils showed identical exchange properties.The experimental data appeared to be well described either by a single Vanselow selectivity coefficient or by a single Gaines‐Thomas coefficient. The Gapon coefficient, as originally defined, increased linearly with the exchangeable calcium fraction (or decreased with ESP). After examining the sensitivity of those coefficients to experimental errors, as a function of the exchanger composition, values of 5.97 and 2.30 mol/liter were calculated for the Gaines‐Thomas and Vanselow coefficients, respectively. These correspond to a Gapon coefficient as redefined by Sposito equal to 48 (mol/m3)½.

Sodium/Calcium Exchange in Montmorillonite and Illite Suspensions

AbstractThe effect of salt concentration and exchanger composition on Na/Ca exchange in montmorillonite and illite suspensions (0.02 to 0.08 g clay/g water) was studied, where the equivalent fraction of exchangeable Ca, ECa, ranged from 0.6 to 1. A theoretical evaluation based on double‐layer theory for montmorillonite indicates that the formation of tactoids and internal surfaces at the Ca3+ end of the exchange isotherm can cause the Vanselow selectivity coefficient, KV, to depend on exchangeable cation composition and total electrolyte concentration. Whereas for montmorillonite, KV increased with ECa (0.6 &lt; ECa &lt; 1), as predicted for internal surfaces, the reverse was true for illite, where external surfaces predominate. Likewise, for a given Na adsorption ratio, dilution of the equilibrium solution for illite (0.6 &lt; ECa &lt; 1) and for montmorillonite (ECa ≃ 0.6) decreased KV as predicted for external surfaces.The average Gapon selectivity coefficient for the montmorillonite (0.011) and illite clays (0.023) with ECa ≤ 0.8, suggests that the value for soils, (0.0148) reflects their mixed mineralogy.

Ideal Behavior in Na+-Trace Metal Cation Exchange on Camp Berteau Montmorillonite

AbstractThe conditions under which an exchanger phase will behave as an ideal mixture are established from thermodynamic principles. It is shown that, if a stoichiometric cation-exchange reaction is reversible, the exchanger phase will exhibit ideal behavior if the Vanselow selectivity coefficient is independent of the exchanger composition. This criterion is applied to some recently published data for Na+-trace metal cation exchange on Camp Berteau montmorillonite. An analysis of the data suggests that, so long as the exchange process is reversible, Na+-trace metal cation exchange produces an exchanger phase that behaves as an ideal mixture.

The Gapon and the Vanselow Selectivity Coefficients

AbstractA general thermodynamic relation between the Gapon and the Vanselow cation exchange equilibrium constants is derived and is shown to be violated by the expressions given traditionally in the soil chemistry literature for the Gapon and the Vanselow selectivity coefficients. This result suggests that the selectivity coefficient expressions are mutually in conflict. The traditional expression for the Gapon selectivity coefficient is shown to be in error, fundamentally because the Gapon convention—according to which cations react with an exchanger in equivalents—has no direct molecular interpretation. Therefore, the thermodynamic activity of an exchanger phase cannot be equated numerically with an equivalent fraction, even in some limiting case. The correct expression for the Gapon selectivity coefficient is derived and is discussed briefly for the case of Na‐Ca exchange. It is shown that the traditional expression and the correct one will not be very different numerically if the exchangeable sodium percentage is &lt; 20.