Diffusive Transport of Calcium Ion in a Sodium-saturated Porous Cation Exchange System

Abstract

Excess sodium ion (Na+) adversely affects soil physicochemical properties, such as dispersion of clay particles, low hydraulic conductivity, and soil crusting. Therefore, calcium ion (Ca2+) has been frequently chosen to displace excess Na+ from the soil profile, since the affinity for exchange sites is greater for Ca2+ than for Na+. In natural systems where CO2 is ubiquitous, however, Ca2+ undergoes precipitation with CO2 during the diffusion process and the removal of Ca2+ from the solution may cause retardation of Ca2+ transport and reduce displacement efficiency. One-dimensional diffusive Ca2+ transport in a Na+-saturated porous cation-exchange resin model system was investigated to examine the effect of Ca2+ on the removal patterns of Na+ from the model soil. This study presumes that Ca2+ interacts with CO2 in the presence of H2O producing CaCO3 and H+. The production of H+ and CaCO3 was evidenced by a lowering of pH and by the XRD (X-ray diffraction) analysis, respectively. Each acryl column was packed with a mixture of sand and Na+-saturated cation-exchange resin was used to dilute the high cation-exchange capacity (CEC) of the resin with sand to a level (10 cmolc kg-1) of a typical kaolinitic soil. Four rates of Ca treatment including a control were chosen for comparison (×0, ×0.5, ×1 and ×2 CEC). An equivalent amount of CaCl2 for each treatment was applied on the surface of the mixture in each column simulating one-dimensional diffusion from an instantaneous planar source into a semi-finite system. Retardation of Ca2+ transport due to CaCO3 precipitation was characterized by a lowering of pH of the system, and the advancing front of Ca2+ movement in both aqueous and exchanger phases progressed deeper with time. With increasing rates of calcium treatment, diffusion coefficients calculated for Ca2+ increased. Therefore, the efficiency of Na+ removal from the exchange sites was enhanced as the rate of Ca2+ treatment increased. In the soils treated with Ca2+ equivalent to ×1 and ×2 CEC, the removal of Na+ from the exchanger was effective in respect of the effectiveness of excess Na+ removal and the penetration depth of Ca2+ dominance. Considering the subsequent resalinization, the application of Ca2+ at a rate of ×2 CEC was most effective in the presence of CO2. As a result, part of our results suggests that at least the application rate of Ca2+ should exceed the CEC of a target soil to guarantee efficient removal of Na+ from the salt-affected soils