CL- ADSORPTION ON ANDISOLS FROM CACL2 AND MGCL2 SOLUTIONS

To evaluate the effects of coexisting cations on Cl adsorption in variable-charge soils, we measured the Cl adsorption from CaCl2 and MgCl2 solutions 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 Cl adsorption from NaCl, NH4Cl, and MgCl2 solutions was only determined by the pH + pCl values of the bathing solutions. In the CaCl2 solutions, however, the Cl- adsorption deviated positively from the relationship obtained in using the NaCl, NH4Cl, and MgCl2 solutions. We considered that this deviation reflected the CaCl+ ion-pair adsorption on negatively charged sites. In a 50 mmol L-1, CaCl2 solution at pH = 7.1, the estimated CaCl+ ion-pair adsorption amounted up to about 1000 of the adsorbed Ca2+ and 56% of the adsorbed Cl- on a mol kg-1 basis. 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.