SODIUM CALCIUM EXCHANGE WITH ANION EXCLUSION AND WEATHERING CORRECTIONS

In order to accurately model soil reclamation with concentrated electrolytes such as CaCl2 or sea water, it must be determined if there is an effect of salinity on the Na-Ca exchange selectivity. A new procedure for determining Na-Ca selectivity in calcareous and gypsiferous soils was used to study the effects of ionic strength and mineralogy on Na-Ca exchange. Four specimen clay minerals and three whole soils were equilibrated with solutions ranging in concentration from 10 to 1000 mmol(c) L-1 and at Na adsorption ratios from 1 to 50 (mmol L-1)1/2. Exchangeable-cation values were corrected for calcite and gypsum dissolution as well as anion exclusion. The selectivity data were compared with other reported values for Na-Ca exchange in order to determine if there were any trends in selectivity with mineralogy or surface charge density. Generally, Na-Ca selectivity was independent of ionic strength. Vermiculite exhibited near-ideal exchange when compared with the nonpreference isotherm. There was no effect of mineralogy on the Na-Ca selectivity, even though the soils and minerals had various surface charge densities. The Gapon selectivity coefficients (K(G)) for the calcareous Many Farms soil (a mesic Torrifluvent) and the gypsiferous Shiprock soil (a mesic Torriorthent) averaged 0.011 and 0.013 (mmol L-1)-1/2, respectively. Failure to correct for anion exclusion and mineral weathering lowered these average K(G) values to 0.0085 and 0.0064, respectively. The data were compared with the Vanselow (K(v) = 4.3), nonpreference (K(v) = 1.0), Gapon (K(G) = 0.015), and the diffuse double-layer (DDL) models. After the review of the problems associated with the various methods of determining exchangeable cations, we recommend that the alcohol rinse method to remove soluble salts not be used.