SORPTION OF CADMIUM BY COMPLEXES OF KAOLINITE WITH HUMIC-ACID

Levels of cadmium (Cd) in New Zealand pastoral soils have increased due to Cd impurities in applied fertilisers. As there is little information on the interaction of Cd with soil mineral-organic matter complexes, the sorption of Cd by complexes of kaolinite with humic acid has been investigated. Sorption was measured at pH and ionic strength values typically found for solutions of pastoral soils in New Zealand. Sorption increased with the content of humic acid in the complex, and as the pH of the medium was raised from 4.2 to 6.3. Sorption was also influenced by the ionic strength of the ambient solution, notably by the nature of the cation in the added electrolyte. The experimental data were interpreted in terms of the effect of solution pH and ionic composition on the charge characteristics of kaolinite and humic acid. These factors, in turn, influence clay particle association as well as the clay-humic and metal-humic interaction.