SORPTION OF CADMIUM BY COMPLEXES OF KAOLINITE WITH HUMIC-ACID

Levels of cadmium (Cd) in New Zealand pastoral soils have increased du e to Cd impurities in applied fertilisers. As there is little informat ion on the interaction of Cd with soil mineral-organic matter complexe s, the sorption of Cd by complexes of kaolinite with humic acid has be en investigated. Sorption was measured at pH and ionic strength values typically found for solutions of pastoral soils in New Zealand. Sorpt ion increased with the content of humic acid in the complex, and as th e pH of the medium was raised from 4.2 to 6.3. Sorption was also influ enced by the ionic strength of the ambient solution, notably by the na ture of the cation in the added electrolyte. The experimental data wer e interpreted in terms of the effect of solution pH and ionic composit ion on the charge characteristics of kaolinite and humic acid. These f actors, in turn, influence clay particle association as well as the cl ay-humic and metal-humic interaction.