Adsorption of Calcium and its Complexes by two Sediments in Ca-H-Cl-CO2 Systems

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.