Cl^- Adsorption on Andisols from CaCl_2 and MgCl_2 Solutions

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To evaluate the effect of coexisting cations on Cl^- adsorption in variable-charges soils, we measured the Cl^- adsorption from CaCl_2 and MgCl_2 solutions on two Andisols as a function of the pH and electrolyte concentration, and compared the results with the adsorption from NH_4Cl and NaCl solutions. For the Choyo B soil in which allophane and imogolite are the dominant exchangers, the Cl^- adsorption from NaCl, NH_4Cl, and MgCl_2 solutions was only determined by the pH + pCl values of the bathing solutions. In the CaCl_2 solutions, however, the Cl^- adsorption deviated positively from the relationship obtained in using the NaCl, NH_4Cl, and MgCl_2 solutions. We considered that this deviation reflected the CaCl^+ ion-pair adsorption on negatively charged sites. In a 50 mmol L^<-1> CaCl_2 solution at pH = 7.1, the estimated CaCl^+ ion-pair adsorption amounted up to about 10% of the adsorbed Ca^<2+> 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.
社団法人日本土壌肥料学会の論文