Reduced adsorption of caesium on clay minerals caused by various humic substances

The effect of the addition of various humic substances on the adsorption of caesium on two mineral clays has been studied. All measurements were carri ed out in dilute suspension under controlled conditions of temperature and ionic strength. Only a small proportion of the humic substance was adsorbed on the clays (<10%). In general, the affinity of the clay-humic complexes for caesium was less than that of the bare clay. The decrease was greater f or illite than for montmorillonite, and greatest at trace concentrations of caesium and increased with increasing concentration of each humic substanc e. However, no correlation was found between the amount of humic substance adsorbed and the decrease in Cs adsorption when all complexes were consider ed. Neither size nor the origin of the humic substances could explain the e xtent of the adsorption decrease. Since neither steric hindrance nor decrea se in the number of adsorption sites was the driving force behind this phen omenon, it is hypothesized that the affinity of the clay surface is modifie d by the organic macromolecules. The highly selective frayed edge sites of illite are particularly sensitive to the adsorption of polyanions because o f their proximity to anion adsorption sites. The observed decrease in Cs ad sorption may contribute to the unexpectedly high bioavailability of Cs in o rganic soils. (C) 1999 Elsevier Science Ltd. All rights reserved.