Cadmium-99 transfer between water and sediments studied in laboratory experiments and with a computer model.

Abstract

The transfer of Cd between water and sediment is a complex process controlled by factors such as water salinity, pH and content of humic substances as well as sediment composition, redox properties and oxygenation (Luoma 1989, DaviesColley et al. 1984, Lu & Chen 1977). Frequent variations in the above mentioned abiotic parameters occur in estuaries due to tidal changes and fresh water run off. Further, fresh water run off may be polluted with trace metals. A general accumulation of pollutants is believed to occur in estuarine sediments and Cd concentrations as high as 130 mg/kg dry weight sediment have been found in UK estuaries (Bryan & Langston 1992). Several parameters such as water residence time, metal concentration, salinity and bioturbation by sediment dwelling organisms could potentially influence the partitioning of metals between water and sediment (Rasmussen et al. 1998). For instance, in fresh water more than 70% of the total Cd is in the form of Cd. The amount of this ionic form decreases rapidly with increasing salinity due to the formation of Cd-chloro complexes (Kester 1986, Mantoura et al. 1978), probably resulting in decreased adhesion of Cd to sediment particles. When salinity is increasing, the higher water concentrations of Ca and Mg will exchange with sediment bound Cd. Even though sediments usually contain low fractions of organic material, the affinity of this fraction for Cd is high due to the presence of functional groups such as -COOH, -OH and -SH (Stumm 1992). The affinity is believed to be independent of pH and redox processes (Davies-Coley et al. 1984). During oxidising conditions, Cd adheres to Fe and Mn oxides and hydroxides that are part of the inorganic fraction of the sediment. During reducing conditions, Fe and Mn bound to sulphides will exchange with Cd whereby mainly Cd monosulphides of very low solubility is formed (Di Toro et al. 1990). These insoluble complexes are also known as AVS (acid-volatile sulphides) (Pesch et al. 1995).