Transport of Zinc and Cadmium in Soils: Experimental Evidence and Modeling Approaches
This study was conducted to assess the suitability of several isotherm equations in describing single solute and binary ion exchange of Zn and Cd in two soils (Windsor and Olivier). Isotherms were determined using batch methods for equilibration times of 1 and 14 d. Ion exchange batch experiments (based on NH4OAc extraction) were carried out at different heavy metal/Ca ratios. Several isotherm equations were capable of describing 1- and 14-d sorption isotherms. However, the general Langmuir-Freundlich was most suitable for describing single solute (Zn and Cd) isotherms, whereas the Rothmund-Kornfeld ion exchange equation well described binary data sets (Zn-Ca and Cd-Ca). Miscible displacement experiments were carried out to study Zn and Cd transport in uniformly packed soil columns (10 cm in length) for different flow velocities. For Windsor soil, the general isotherm equation predicted Zn transport results adequately but not Cd results. Moreover, under conditions of variable ionic strength, the use of constant selectivity (equal affinity) for ion exchange provided consistently superior predictions of Zn and Cd transport than the Rothmund-Kornfeld (variable affinity) approach. Attempts to predict Cd and Zn transport in Olivier soil, using several isotherms and ion exchange approaches, were not successful. Local equilibrium appeared dominant for Windsor soil, whereas nonequilibrium sorption behavior may be rate limiting for Zn and Cd transport in Olivier soil.
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