Modeling trace metal partitioning and speciation at the sediment-water interface using a chemical equilibrium model

Abstract

Isotherm adsorption experiments for Pb, Cd and Zn were carried out with coastal sediments from China under oxic sediment-water conditions. The conditional equilibrium constants (Kads) determined from the adsorption isotherms were found to be most strongly correlated with Mn oxides (MnOx) for Pb and Cd, and with particulate organic carbon (POC) for Zn. MnOx and POC were then used as the surrogate metal-binding phases in a sediment-water chemical equilibrium model (SCEM). The apparent equilibrium constants (βsed, Me⁎) of the SCEM were determined to be 109.56±0.41 for Pb (n = 9), 105.25±0.34 for Zn (n = 7), and 106.51±0.29 for Cd (n = 7). The SCEM was then used to predict the free metal ion activities ({Me2+}) in seawaters of Jinzhou Bay, China. The SCEM-predicted {Me2+} compared well with the measured ones for Pb and Zn in seawater. However, the predicted {Cd2+} was higher than measured, which may reflect a detoxification mechanism of marine organisms in Jinzhou Bay seawater. Overall, a model of this kind can be highly useful for predicting {Men+} in the overlying waters based on the surface sediment chemistry, and can contribute to the development of sediment quality guidelines by linking them with the corresponding water quality criteria.