Adsorption of Cd(II) and Pb(II) on bentonite under estuarine and seawater conditions

Abstract

The adsorption of cadmium and lead ions to natural iron bentonite has been studied in seawater of various dilutions. Both ions are well adsorbed under freshwater conditions. However, negligible adsorption of cadmium, as distinguished from lead ions was observed under seawater conditions. The influence of particular seawater constituents on the decrease in cadmium adsorption has been studied. Among the inorganic ions of seawater, magnesium and chloride show the greatest inhibiting effect. The inhibition of cadmium adsorption decreases in the order Mg 2+ > Ca 2+ > Sr 2+. This is at variance with the ion-exchange theory and is probably due to the significant difference in concentration ratios of these elements found in seawater (Mg, 0.053; Ca, 0.010; Sr, 0.00015 mol dm −3). Organic ligands of marine origin, as well as selected sugars, amino sugars and amino acids, decrease cadmium adsorption, while artificial surface active substances (Triton X-100, dodecyl benzene sulphonic acid-sodium salt, dodecyl piridinium chloride), added in concentrations lower than the critical micellar concentration, show no effect. The experiments performed in this work suggest the mechanism of a self-purification ability of river water containing suspended bentonite. However, upon entering the sea, cadmium will be remobilized from the particles of the organically coated bentonite.