Calcite dissolution and Ca/Na ion-exchange reactions in columns with different flow rates through high ESR soil

Abstract

The leaching of a Na⁺-affected calcareous soil with water results in two concurrent recesses: (i) CaCO₃ dissolution, and (ii) replacement of Na⁺ on the cation-exchange complex by Ca²⁺. In the current study, Woodward soil (coarse-silty, mixed, superactive, thermic Typic Haplustept) columns (50 and 100 g), initially equilibrated with 10 mmol Cl L⁻¹ SAR 10 solutions, were eluted with deionized water (approximately 1.2 L; initially equilibrated with atmospheric CO₂) at flow rates of 0.00075 to 0.006 L min⁻¹ under conditions of saturated flow. Column eluate was monitored for pH, carbonate alkalinity, and Na, Ca and Cl concentrations to evaluate the elution of SAR 10 solution, dissolution of CaCO₃ and exchange of Na by Ca on the cation-exchange complex. Following elution with deionized water, the columns were sectioned and analyzed for residual Na. In all cases, the eluate exhibited approximate thermodynamic equilibrium with respect to calcite. Calcite dissolution and Na⁺/Ca²⁺ ion exchange adhered to apparent first-order kinetics, with three approximate zones of stability which corresponded to; (i) elution of the original SAR 10 solution from the column, (ii) elution during the period of Na⁺/Ca²⁺ ion exchange, and (iii) elution following completion of the ion-exchange reaction, respectively. The central zone corresponded to substantial Na⁺ elution, minimum Ca²⁺ elution and maximum alkalinity, due to consumption of Ca²⁺, from CaCO₃ dissolution, by the ion-exchange reaction. The central zone of stability was used to calculate rate constants which were then compared for the influence of flow rate on overall reaction rate. The rate constant approximately doubled as the flow rate was doubled, which implied flow rate control of the reactions, not mechanistic control.