Time-dependent adsorption in near coastal marine sediments: a two-step model

Abstract

Many important physical and chemical processes occur at phase boundaries. The role of surface phenomena is frequently underestimated or overlooked although surfaces play a significant role in many natural science disciplines. Experimental data from the literature indicate that in adsorption from solution, most of the adsorbates move to the adsorbent surface in a relatively short time period. Actual adsorption equilibrium, however, may take longer to establish. In this study time variable parametric experiments were performed with Co and a suspended marine sediment sample. Two different time dependencies were observed: a rapid step that reached equilibrium in 5–10 days, while a slower step continued for more than 100 days. Observed behavior was simulated with a time-dependent model that differentiates the slow and fast steps. The fast step was considered to be due to diffusion of the adsorbate to the external and macropore surface of the adsorbent and exchange at surface sites, while the slow step was considered as diffusion of adsorbate into the adsorbent micropore capillaries where adsorptive binding occurs.