Cobalt(II) interactions with near-coastal marine sediments

Abstract

Chemical speciation of trace metals in aqueous systems is determined by a variety of competitive reactions. Sediments act as sinks for many trace elements, forming surface complexes and otherwise surface-related compounds. Reactions between dissolved trace metals and functional groups at sediment surfaces can be described as coordination reactions similar to those occurring in solution. Factors inherent to both sediment and solution influence the trace metal speciation. These factors include pH, type of mineral, surface area and/or density of reactive surface sites, complexing ligands, and competing ions. The relative importance of some of these factors was studied using an isotope dilution technique. Coastal sediment samples from the Gulf of Mexico were used as the solid matrix. Models developed for pure minerals were used here to (empirically) describe the complex assemblages of minerals in the natural sediment samples. The adsorption experiments showed that cobalt uptake by marine sediment samples followed a two-step time dependency. The initial fast step lasted about five to ten days and the subsequent slow step continued after more than 100 d of equilibration. The pseudoequilibrium reached during the fast step was modeled with an equilibrium model, while the twostep behavior was modeled with a time-dependent model. The two models are related by the reaction stoichiometries. It is believed that the experimental as well as the modeling approach taken in this study, although empirical when used for natural settings, is much more powerful than the use of traditional means to describe trace element interactions with natural solid phases.