Transport and Retention-Release of Cadmium in Different Soils: Kinetic Models.

In this study, kinetic batch and miscible displacement column methods were used to assess the reactivity and mobility of cadmium (Cd) in different soils.  Two soils were used a Windsor Loamy sand surface soil from New Hampshire and Bustan sand surface and subsurface soils from Northwestern Egypt.  The respective pH for the three soils was 6.1, 9.2 and 9.4, respectively.  Bustan surface and subsurface soils contained 2.76 and 1.18% CaCO₃, respectively. For all soils, isotherm results indicated strong sorption.  Observed nonlinearities of Cd isotherms were generally described using Freundlich rather than Langmuir model.  Cd release or desorption was strongly time-dependent and was associated with significant Ca release from the high pH Bustan soils.  After 28 d desorption, only small amounts of the Cd already adsorbed were released.  Specifically, based on the amount adsorbed, the amounts of Cd released or desorbed were 36% for Windsor, 3% for Bustan surface soil, and 31% for Bustan subsurface soil.  Moreover, results based on sequential extractions, following 28 d of desorption, indicated that CaCO₃ affected the amounts of Cd strongly sorbed in these soils.  Such results suggest that Cd precipitated as CdCO₃ on carbonate surfaces in Bustan soils. Miscible displacement results from the soil columns were consistent with those from our kinetic batch experiments. A multireaction model (MRM) with nonlinear equilibrium and kinetic sorption with irreversible reactions successfully described time-dependent desorption for all soils.
 Keywords: Cd; Convection dispersion equation; MRTM, sequential extraction