Kinetics of Calcium‐Magnesium Exchange on Soil Using a Stirred‐Flow Reaction Chamber

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Abstract
Ion exchange reaction kinetics may significantly affect effluent concentrations under both laboratory and field conditions. One limitation to the study of exchange reaction kinetics has been the lack of appropriate methods. A stirred‐flow method that uses a constantly stirred reaction chamber with constant, steady inflow of the reactant solution and outflow of reactants and products was tested. The method allows for continuous sampling and accommodates a wide range of soil particle sizes. The basic assumption of a well mixed solution in the chamber has not been tested, however. That assumption was tested over a wide range of flow rates and found to be applicable. The measurable reaction rates using the stirred‐flow method were estimated using a simple empirical equation that allowed the use of the reaction half‐time (t1/2) to describe reaction rates. Values of t1/2 of 3 min and greater were detectable while a 0.3 min t1/2 was not detectable under the experimental conditions. Calcium‐Mg exchange on a Nordya sandy loam (Typic Rhodoxeralf) soil from Israel was investigated. Measured effluent concentrations were compared with values calculated assuming no adsorption preference for Ca or Mg, and assuming local equilibrium between adsorbed and solution phases prevailed. Effluent concentrations were accurately predicted when the reaction direction was from equilibrium with Ca solution to equilibrium with Mg, but not for the reverse. These results suggest that either the local equilibrium or the nonpreference assumption was not appropriate. Flow rate reduction and stoppedflow experiments indicated that local equilibrium prevailed. Equilibrium exchange isotherms obtained using the chamber indicated a preference for Ca over Mg.