Sorption processes in soils as influenced by pore water velocity: 2. experimental results
1983
AKRATANAKUL, S. | Boersma, L. | KLOCK, G. O.
We carried out experiments to measure the rate of adsorption of cadmium for static and dynamic flow conditions. The water content of samples was maintained at —12.6 kPa, corresponding to 87 percent of saturation. Adsorption rates at apparent pore water velocities of 1.3, 2.3, and 2.7 cm/h were measured. Diffusive conductance, Kd, and forward (k1) and backward (k2) rate constants were calculated using the Nelder-Mead Simplex method based on a nonlinear least-square procedure.Values of Kd and of the ratio k1/k2 increased with increasing pore water velocity. The rate of adsorption was higher in a sample with solution flowing through it than in a sample with the same solution under static conditions. The magnitude of Kd ranged from 0.07 X 10] at static conditions to 0.4 x 10] s] with &OV0622; = 2.7 cm/h.Results imply that the thickness of the subsurface layer decreases with &OV0622;, thus increasing the diffusion rate of ions across this layer. When the adsorption process is far from equilibrium, diffusion across the subsurface layer became a rate-limiting step in the adsorption process. Lateral diffusion cannot be ignored in studies of the kinetics of adsorption.The increase in values of k1/k2 with increasing &OV0622; indicates that the surface reaction proceeded at a more rapid rate in the presence of a continuous supply of solution. Flowing solution serves as a continuous source of the introduced ions, as well as a continuous sink for the desorbed ions of other species originally present.The accelerated rate of adsorption induced by flow of solution through the soil may be attributed both to an increase in the rate of diffusion as the thickness of the subsurface layer decreases and to an increase in the rate of the kinetic reactions at the surface as the exchange process proceeds in the presence of the continuous sink.
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