Sorption of cadmium on suspended matter under estuarine conditions; competition and complexation with major sea-water ions

Sorption of Cd at low concentrations onto river Rhine suspended matter was examined in terms of sorption rate, reversibility and factors such as competition and complexation with major inorganic sea-water ions. More than 90% of the final amount of Cd is sorbed within the first few hours. Desorption experiments show that the process is virtually reversible. In experiments with diluted sea water the sorption of Cd strongly decreases even at low salinity. Sorption isotherms show that the sorption of Cd in NaNO₃ solutions is regulated by the free Cd²⁺ activity. In a Ca(NO₃)₂ environment the Cd sorption decreases with increasing Ca²⁺ concentrations, which implies competition between Ca²⁺ and Cd²⁺ for the different sorption sites. In different electrolyte solutions of similar ionic strength the sorption of Cd decreases in the solution order NaNO₃ > NaCl > NaCl+MgCl₂+CaCl₂ > diluted sea water. Although inorganic speciation calculations show that even at low salinities dissolved Cd is dominated by Cd-chloro complexes, chloride accounts for only about one third of the increased mobility of Cd. As a result of addition of Ca²⁺ and Mg²⁺ the sorption capacity of suspended matter for Cd is further reduced by a factor three.