CIadsorption on andisols from CaCl₂and MgCl₂solutions

To evaluate the effects of coexisting cations on Cladsorption in variable-charge soils, we measured the Cladsorption from CaCl₂and MgCl₂solutions on two Andisols as a function of the pH and electrolyte concentration, and compared the results with the adsorption from NH₄Cl and NaCl solutions. For the Choyo B soil in which allophane and imogolite are the dominant exchangers, the Cladsorption from NaCl, NH₄Cl, and MgCl₂solutions was only determined by the pH+pCl values of the bathing solutions. In the CaCl₂solutions, however, the Cladsorption deviated positively from the relationship obtained in using the NaCl, NH₄Cl, and MgCl₂solutions. We considered that this deviation reflected the CaCl⁺ion-pair adsorption on negatively charged sites. In a 50 mmol l^-1CaCl₂solution at pH = 7.1, the estimated CaCl⁺ion-pair adsorption amounted up to about 10% of the adsorbed Ca²⁺and 56% of the adsorbed Clon a mol kg^-1basis. In the lower pH range (< 6.5), however, the CaCl⁺adsorption was negligible. Daikanbo A soil whose exchanger consisted of Al-humus complexes showed a net negative adsorption of Cl. These results suggest that the contribution of MCl⁺(M = Ca or Mg) complexes was not significant. The exclusion volume was relatively constant irrespective of the pH and electrolyte concentration. We attributed these results to the Donnan effect.