Kinetics of Zn sorption-desorption using a thin disk flow method

In this study we investigated the kinetics of Zn sorption and desorption us ing a short column or thin disk method. The method is based on continuous n ow through a thin soil layer (1 mm) where the effluent was collected using a fraction collector. Two soils were used: a Windsor soil and Mahan soil, B reakthrough results (BTCs) for different now velocities indicated that Zn s orption is instantaneous and equilibrium retention is dominant when a pulse of Zn with a concentration of 2.62 x 10(-5) M is applied. However, based o n now interruption, time-dependent Zn sorption-desorption processes were mo st pronounced when the applied Zn pulse concentration was two orders of mag nitude lower (2.51 x 10(-7) M), This confirms earlier findings of concentra tion-dependent kinetics from batch experiments on Windsor soil (Hinz et al, , 1992), The removal of organic matter and iron oxide, based on peroxide an d peroxide/dithionite-treatments, resulted in doubling and quadrupling Zn r etention, respectively, compared with the untreated Windsor soil. Differenc es between the untreated, peroxide-, and peroxide/dithionite-treated Windso r soils were most pronounced at low input Zn concentrations, suggesting tha t more specific sites became available as a result of the different treatme nts. At high input Zn concentrations, increases of specific sites may not b e significant. For the treated soil, stronger sorption and desorption kinet ic behavior was exhibited compared with the untreated soil. Diffusion into soil minerals or surface-controlled reactions may cause such behavior.