Binding of cadmium by montmorillonite humic acid mixtures - Miscible displacement experiments

Abstract

By means of the miscible-displacement approach, investigations have been carried out on cadmium adsorption-desorption by mixtures of freeze-dried montmorillonite and freshly precipitated humic acid. Low Cd surface loadings were employed and acidic (pH 4.5-5) solution conditions were maintained during both adsorption and desorption cycles. Compared with montmorillonite, Cd adsorption by humic acid was much greater so that clay-humic mixtures containing only a few per cent w/w humic acid removed appreciably more Cd from solution than that by clay alone. However, this greater Cd uptake by humic acid was offset by the susceptibility of bound Cd to desorb once the supply of Cd in the column ihput solutions was removed. Thus, both montmorillonite and undissolved humic acid had a limited ability to fix Cd, a result which may have some implication to mechanisms of Cd binding by natural clay-humic mixtures in acidic soil environments. The miscible-displacement approach appears to be well suited to examining solute interactions with poorly soluble, high molecular weight soil humic materials.