Comparison of Four Equations to Describe the Kinetics of Lead Desorption from Soils

The rate of Pb desorption was investigated from clay (Silty clay, Torrifluvent), CaCO₃-rich (Sandy clay, Calciorthid), and sandy (Sandy loam, Quartzipsamment) soils at two different temperatures. Lead has not been released from CaCO₃-rich soils which suggests irreversible Pb sorption by the soil. The desorption was quite hysteretic from sand and clay soils. The total amount of Pb released from the clay soil exceeded that released from the sandy soil. The lower Pb desorption associated with the sandy soil is probably due to its higher calcium carbonate content relative to the clay soil. The kinetics of Pb desorption were evaluated using the Elovich, modified Freundlich, parabolic diffusion, and first order equations. The first order and parabolic diffusion equations adequately described the kinetics of Pb desorption from clay and sandy soils under isothermal conditions. The choice of first order and parabolic equations among others investigated was based on the goodness of fit and the more scientific theoretical assumptions of the equations. The apparent Pb diffusion rate coefficient (D_d) and desorption rate coefficient (k_d) values from the clay and sandy soils increased with increasing temperature.