Cadmium adsorption and desorption behaviour on goethite at low equilibrium concentrations: effects of pH and index cations

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摘要：

The transport and bioavailability of cadmium is governed mainly by its adsorption–desorption reactions with minerals such as goethite—a common iron oxide mineral in variable charged and highly weathered tropical soils. Soil factors such as pH, temperature, solution Cd concentration, ionic strength and ageing affect Cd adsorption on goethite. The desorption behaviour of Cd from goethite at low concentrations is not fully understood. This study investigates the adsorption–desorption of Cd at low Cd concentrations (Cd adsorbed on goethite from 20 to 300μM Cd solutions) in Na and Ca nitrate solutions of 0.03M nominal ionic strengths.Synthetic goethite prepared by ageing a ferric hydroxide gel at high pH and room temperature was used for Cd adsorption and desorption studies. For desorption experiment 10 successive desorptions were made for the whole range of initial Cd concentrations (20–300μM) in the presence of 0.01M Ca(NO) or 0.03M NaNO solutions.Cadmium adsorption was found to be higher in Na than Ca probably due to the competition of Ca ions with Cd ions for adsorption sites on the surfaces of goethite. The effect of index cation on Cd adsorption diminished with increase in pH from 5.0 to 6.0. Cadmium desorption decreased with increase in pH from 5.0 to 6.0 in both Na and Ca systems. After 10 successive desorptions with 0.03M NaNO at the lowest initially adsorbed Cd approximately 45%, 20% and 7% of the adsorbed Cd was desorbed at pH 5.0, 5.5 and 6.0, respectively. The corresponding desorptions in the presence of 0.01M Ca(NO) were 49%, 22% and 8%, respectively. The Freundlich parameter, , based on each progressive step of desorption at different adsorbed concentration increased with increasing desorption step, which may indicates that a fraction of Cd was resistant to desorption. Low Cd desorbability from goethite may be due to its specific adsorption and/or possibly as a result of Cd entrapment in the cracks or defects in goethite structure.

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cadmium adsorption desorption goethite pH index cations IRON OXYHYDROXIDE IONIC-STRENGTH SOIL SORPTION NEW-ZEALAND CD TIME ZN KINETICS METALS PB