HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Griffioen, J. Articles by Appelo, C. A. J. Search for Related Content PubMed Articles by Griffioen, J. Articles by Appelo, C. A. J. Agricola Articles by Griffioen, J. Articles by Appelo, C. A. J.

Adsorption of Calcium and Its Complexes by Two Sediments in Calcium-Hydrogen-Chlorine-Carbon Dioxide Systems

TNO Inst. of Applied Geoscience, P.O. Box 6012, 2600 JA Delft, the Netherlands

C. A. J. Appelo

Inst. for Earth Sciences, Free Univ., de Boelelaan 1085, 1081 HV Amsterdam, the Netherlands

*Corresponding author.

ABSTRACT

The sorption of aqueous complexes as well as free cations greatly influences cation-exchange processes. To clarify whether Ca complexes are adsorbed, isotopic exchange experiments using ⁴⁵Ca, ³⁶Cl, H¹⁴CO₃, and ³H₂O were done to determine exchangeable species of Ca for solutions of different composition including pH. The technique used allowed total exchangeable solutes to be determined for both calcareous and non-calcareous samples. The results show that the CaHCO⁺₃ complex is preferentially adsorbed over free Ca²⁺. We calculated a Gaines-Thomas selectivity coefficient of 26 for CaHCO⁺₃ over Ca²⁺ for a sample in which silicate clay minerals were the main exchangers, and a value of 9.5 for a sample in which organic matter was the main exchanger. Chloride behaves almost nonreactively due to the low aqueous activity of CaCl⁺ to Ca²⁺. However, indications for adsorption of CaCl⁺ were also found. It is hypothesized that anion exclusion of Cl^– cancels out CaCl⁺ adsorption. The inferred difference in selectivity between Ca²⁺ and CaHCO⁺₃ suggests that the selectivity of cation exchangers can be explained by the hard-soft/acid-base principle. The exchanger is a softer Lewis base than H₂O, and prefers the strong CaHCO⁺₃ complex (a soft Lewis acid) to the hard Lewis acid Ca²⁺. If complexes are adsorbed, then the amount of total exchangeable cations is higher than the negative charge surplus of the exchanger. Changes in total salt concentration are a logical consequence of cation exchange between a monovalent Ca complex and divalent Ca, due to the redistribution of Ca complexes in solution into free Ca²⁺ and the monovalent co-ion that occurs.

Received for publication June 13, 1991.