Relationships between Cd and Zn partitioning and geochemical composition in sediments from Chinese rivers

Adsorption isotherm experiments were conducted to determine the distribution coefficients (Kd) for Cd and Zn between water and sediments from twelve rivers in eastern China. The low metal concentration (<10(-10) M) added into the sediment-water system enabled the direct measurements of linear adsorption isotherms. When other solution parameters, such as ionic strength, were controlled, sediment geochemical characteristics were the major factors affecting metal distribution. The Kd was highly dependent on the total organic carbon contents and Fe-Mn oxide at pH = 7.0, and was affected significantly by the carbonate content in the sediments due to its control of pH. The Kd of the Huanghe sediments, which had the highest carbonate concentration (13.1%), increased sharply for both Cd and Zn when the pH increased. A quantitative model using the experimentally obtained phase distribution coefficients (Kd)i was applied to study the effects of different geochemical phases in the sediment on metal distributions. The (Kd)i of carbonate was correlated significantly with the carbonate content in the sediment, whereas the (Kd)i of total organic carbon (TOC) and Fe-Mn oxide phases were highly pH dependent. Using the (Kd)i for each geochemical phase, the calculation showed that about 91% and 85% of sorbed Cd and Zn, respectively, added to the batch reactors were distributed among the three major geochemical phases: TOC, Fe-Mn oxide, and carbonate. Total organic carbon was the most important binding phase for Cd, and Fe-Mn oxide for Zn. Furthermore, adsorption experiments with simulated sediments show the importance of interactions between different geochemical phases on metal partitioning.