Diffusion and surface reaction processes of absorption on surfaces of soil particles in unsaturated soils

Graduation date: 1979At present the adsorption kinetic of ions by soil particles is considered to be a surface reaction. When the transport of solutes in soils is described using the one-dimensional flow equation, it becomes a serious problem that lateral diffusion of ions from the bulk solution towards the soil surfaces is not considered. The objectives of this study were to re-examine the adsorption process on theoretical and experimental bases in an attempt to develop more appropriate descriptions. We considered the adsorption process to be a three-step mechanism, namely, diffusion of ions from the bulk solution to the "subsurface" layer around soil particles, diffusion across the "sub-surface" layer, and a surface reaction. Diffusion across the subsurface layer is assumed to be at a constant rate and may be the rate limiting process under certain conditions. Ions from the subsurface layer are adsorbed onto the surfaces of soil particles. This process is described by a surface kinetic reaction which is regarded another rate limiting process. A new procedure for measurement of adsorption isotherms was proposed and tested. The new procedure employs a soil sample packed in a short column so that the structure of the soil can be maintained. In our studies solution was added to bring the soil water\ud potential to -0.05 bars. Other water contents could be used. After\ud equilibration, solution was extracted using a pressure membrane\ud apparatus.\ud The resulting isotherms conformed to the Freundlich-type\ud isotherm. For both iodide and cadmium ions, the soil column and the\ud currently used shaking batch procedures produced isotherms that differed\ud significantly from each other, with the magnitude of adsorption\ud being much greater in the soil column.\ud It was hypothesized that the arrangement of soil particles influences\ud the attractive force fields and restricts the transitional freedom\ud of ions to a greater extent than occurs in the shaking batch. It was\ud concluded that the soil column procedure more realistically represents\ud conditions as they exist in soil and therefore is preferable over the\ud shaking batch method.\ud The activation energies for adsorption and desorption were\ud assumed to be logarithmic function of the amounts adsorbed. When the lateral diffusion process is also considered, the equation describing\ud the adsorption kinetics for one-dimensional solute transfer in soil\ud is as/at = [1/1/Kd+1/(k₁S⁻b)] (θ/pb)C - [1/(k₁S⁻²b/k₂)(1/Kd)+1/(k₂Sb)]S, where Kd (sec⁻¹) is the diffusive conductance of the subsurface layer, k₁ and k₂ (sec⁻¹) are surface reaction constants, b is\ud a constant associated with the change of energy of adsorption as a\ud function of surface coverage, (θ/pb)C (μg/g soil) is the amount of\ud ions in the liquid phase and S is the nondimensional representation\ud of S.\ud The kinetic adsorption equation was evaluated for cadmium for\ud four pore water velocities, v, ranging from 0 to 2. 7 cm/hr. The\ud soil water potential was maintained at about -0.126 bars. The calculated\ud values of Kd showed an increase with increasing v impling\ud that the thickness of the subsurface layer decreased with increasing\ud velocity. The magnitudes of Kd of about 0.07 to 0. 4 x 10⁻³ sec⁻¹ are very important. They are sufficiently low to indicate a rate limiting\ud process especially when the system is far from equilibrium. The\ud studies thus confirm our argument that lateral diffusion cannot be\ud ignored in studies of adsorption kinetics. The ratio of the rate constants,\ud k₁/k₂ also increased with v which led us to conclude that the rate of adsorption proceeds faster when there is solute flow in\ud the soil system. This is attributed to the higher rates of lateral diffusion\ud and rapid removal of ions originally present at the adsorption\ud sites