STUDIES ON THE ADSORPTION OF CADMIUM ON HYDROUS IRON(III) OXIDES IN OXIC SEDIMENTS

The influence of freshly formed hydrous iron(III) oxides in the oxic layers in sediments on the exchange processes of cadmium between the interstitial and the overlying water was investigated. By the formation of iron oxides as coatings on clay minerals it is possible to measure the dissolved part of cadmium with differential-pulse anodic stripping voltammetry without disturbances of colloidal particles. The results were compared with theoretical calculations using a surface complexing model. The measurements showed that 0.1 mmol of FeOOH coated on the clay mineral kaolinite (1 g) can adsorb up to 95% of the added cadmium (1 mumol l-1). The measured adsorption agreed well with the estimations of the theoretical model. The investigation of the influence of competitive ions (Ca2+ and PO43-) showed that both ions reduced the proportion of adsorbed cadmium whereas the model postulates higher adsorption with phosphate. The measurements showed that freshly formed hydrous iron(III) oxides may act as a natural barrier to the diffusive flux of toxic trace elements into the overlying water.