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.
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