Soil science
Shabnam Jalilian; Faranak Ranjbar
Abstract
IntroductionThe relative preference and the cation exchange capacity of the exchanger are among the important and determining factors in the adsorption and retention of cations. Studies have shown that factors such as valency, the size of the hydrated radius or the relative hydration energy of ions, ...
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IntroductionThe relative preference and the cation exchange capacity of the exchanger are among the important and determining factors in the adsorption and retention of cations. Studies have shown that factors such as valency, the size of the hydrated radius or the relative hydration energy of ions, the type of clay mineral, the concentration of the solution phase, the amount of organic matter, the structural characteristics, and the charge density of the exchanger determine the preferential adsorption of cations in the soil. The aims of this study were: 1) to investigate the effect of contact time, adsorbent dose, and pH on potassium selectivity by bentonite in binary systems including K-Ca, K-Mg, and K-Na based on Gapon, Vanslow, and Gaines-Thomas equations and 2) to investigate the potassium adsorption isotherms by bentonites saturated with calcium, magnesium, and sodium.Materials and MethodsTo saturate bentonite, 1 M solutions of calcium, magnesium, and sodium chloride were separately used. The effects of contact time (10-1440 min), adsorbent dose (0.1-2 g), and pH (3-9) on potassium adsorption and selectivity by bentonites saturated with calcium, magnesium, and sodium in binary systems were investigated. In these experiments, 20 mL of a solution containing 24 meq L-1 of potassium and 6 meq L-1 of the competing cation (Ca, Mg, or Na) were added to the adsorbent. The selectivity coefficients of Gapon, Valselow, and Gaines-Thomas were calculated. Isotherm experiments were also performed to evaluate the effect of different equivalent fractions of potassium (0.1, 0.2, 0.4, 0.6, 0.8, 0.9, and 1) and the competing cation in the solution phase on potassium adsorption. Simple linear, Freundlich, and Temkin equations were fitted to the isotherm data.Results and DiscussionThis study results showed that the adsorption of potassium by Ca-, Mg-, and Na-bentonites increased with increasing contact time and reached its highest value in 24 hours. The pseudo-second-order kinetic equation was better able to describe the process of potassium adsorption by bentonites over time than the pseudo-first-order equation. Potassium adsorption by Mg- and Na-bentonites had a downward trend with increasing the absorbent dose in the range of 0.1-2 g, while Ca-bentonite showed the highest adsorption of potassium in the dose of 0.2 g. With increase in pH, the percentage of potassium adsorbed from the solution phase increased; and reached its maximum value at pH 9. The amount of potassium adsorption by Mg- and Na-bentonite in all pHs was almost the same and at the same time more than Ca-bentonite. The interesting result of this research was that the behavior of Mg-bentonite was more similar to Na-bentonite than to Ca-bentonite. An increase in the negative charge of aluminosilicates with an increase in pH can occur due to the loss of protons by silanol and aluminol groups. The selectivity coefficients of Gapon, Vanslow, and Gaines-Thomas changed under the influence of contact time, adsorbent dose, and pH. Comparing the results of the investigation of the mentioned factors with the results of the selectivity coefficients showed that these coefficients cannot be a definitive criterion for judging the preference of one cation over another cation for adsorption in exchange sites. The isotherm experiment indicated that the amount of potassium adsorption in the solid phase increased with the increase of the potassium equivalent fraction in the solution phase; so the maximum adsorption was observed at the highest initial concentration of potassium (30 meq L-1, which corresponded to the equivalent fraction of 1). The linear adsorption coefficient in the simple linear equation (Kd) showed that potassium adsorption by Na-bentonite was higher than the two others. The highest amount of Kd, 56.0 L kg-1, and the lowest value, 11.9 L kg-1, were obtained for bentonites saturated with sodium and calcium, respectively. The parameter bT, the heat of exchange in the Temkin equation, was estimated to be 4.5, 5.0, and 19.1 (J mol-1) for bentonites saturated with sodium, magnesium, and calcium, respectively. Three simple linear equations, Freundlich, and Temkin were able to describe the adsorption process well. However, based on the highest value of the coefficient of determination (R2) and the lowest value of the standard error (SE), it can be said that the Freundlich equation showed the best fit to the data.ConclusionThe highest adsorption of potassium occurred at a contact time of 24 h, a dose of 0.1 g for Mg- and Na-bentonite and 0.2 g for Ca-bentonite and pH 9. The pseudo-second-order equation described well the kinetics of potassium adsorption by bentonites over time. The results showed that the behavior of Mg-bentonite was more similar to Na-bentonite than Ca-bentonite. The selectivity coefficients of Vanslow, Gaines-Thomas, and Gapon changed under the influence of contact time, adsorbent dose, and pH. The results revealed that it is not possible to definitely determine the preference or non-preference of a cation based only on selectivity coefficients. The isotherm experiment showed that the amount of potassium adsorption increased with the increase of the initial equivalent fraction of potassium in the solution. The highest value of R2 and the lowest value of SE were obtained for simple linear and Freundlich equations, respectively.
