Although previous studies have shown that sulfate can either increase cation leaching or enhance cation adsorption in soil, little is known about the factors behind these phenomena. To learn more about them, calcium adsorption experiments were carried out with kaolinite and gibbsite at initial pH values 4 and 6 and in the presence of 1 or 20 mmolc L−1 of either nitrate or sulfate. The results indicated that limited sulfate-calcium coadsorption occurred on gibbsite when it was in contact with the dilute solution of CaSO4.2H2O at pH ~ 7. Regarding mineral and pH values, calcium adsorption from the concentrated solutions decreased with sulfate possibly because of the presence of ~31% of the CaSO40 ion pair in the concentrated CaSO4.2H2O solutions and the low free calcium activity therein. Calcium adsorption on kaolinite and gibbsite from all concentrated solutions was reduced when the initial pH changed from 4 to 6 suggesting a negative salt effect on that process. In addition to indicating negligible participation of gibbsite in calcium adsorption, our findings also suggest that higher amounts of gypsum applied to lime-amended oxisols reduce the effectiveness of the main oxisol clay-sized mineral capable of adsorbing cations, i.e., kaolinite, to impair calcium leaching. The uptake data were complemented with some zeta-potential measurements, which supported the lack of substantial uptake of calcium even in the presence of sulfate. Some modeling calculations using the only available model covering sulfate and calcium on gibbsite have been done to rationalize the experimental data, but the model is only able to involve pure electrostatic attraction of calcium, which is not sufficient to produce substantial uptake. Finally, the aluminol basal plane that is present on both gibbsite and kaolinite has been additionally studied using second harmonic generation (SHG) down to 4°C, because the ion-pair formation decreases with decreasing temperature. The second harmonic results confirm the patterns observed in the electrokinetic measurements with kaolinite being quite comparable to the sapphire basal plane. Also and quite clearly, the presence of CaSO4 solutions caused temperature dependence different from pure CaCl2 and Na2SO4 solutions. The latter were essentially behaving like pure water. The difference between the calcium chloride and sulfate systems can be explained by sulfate interaction and might be linked to the temperature dependence of the formation of the CaSO4 ion pair. The temperature dependency study could be an important starting point for looking at ice nucleation in the presence of the three different solutions and more strongly link aqueous chemistry to ice nucleation processes.
Part of the book: Advanced Sorption Process Applications