Effect of ionic strength on swelling and the exchange of alkaline earth cations in soil organic matter

Abstract

We studied the equilibrium specificity of 33% neutralized Ca- and Ba-peat (CaP-33 and BaP-33) for Ca2+ and Ba2+ at three different ionic strength levels (I = 0.0040, 0.0082, and 0.0150) by batch equilibration ion exchange experiments. Volume swelling using the pycnometer method and water adsorption of H- and Ba-form peat samples at the three ionic strength levels were also determined. Preference for Ba2+ over Ca2+ in both CaP-33 and BaP-33 decreased with ionic strength until a reversal in preference (Ca2+ over Ba2+) was observed at the highest ionic strength level. The Vanselow selectivity coefficients remained constant with surface composition, and they were similar in both CaP-33 and BaP-33, indicating that the exchange reaction is reversible. The least swollen form was H-peat, and Ca-peat showed slightly less swelling than Ba-peat. A decrease in volume swelling and water adsorption was observed as ionic strength increased, the decrease being greater for CaP-33 than for BaP-33. These phenomena were explained in terms of increasing association of Ca2+ with the carboxylate sites in peat. Increasing association results from a decrease in the average dimension of the exchanger as well as a decreasing dielectric constant as the internal solution becomes more concentrated with increasing ionic strength. © 1988 The Williams and Wilkins.

Source or Periodical Title

Soil Science

ISSN

0038075X

Page

67-72

Document Type

Article

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