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 (K_ads) determined from the adsorption isotherms were found to be most strongly correlated with Mn oxides (MnO_x) for Pb and Cd, and with particulate organic carbon (POC) for Zn. MnO_x and POC were then used as the surrogate metal-binding phases in a sediment-water chemical equilibrium model (SCEM). The apparent equilibrium constants (β_{sed, Me}^⁎) of the SCEM were determined to be 10^9.56±0.41 for Pb (n = 9), 10^5.25±0.34 for Zn (n = 7), and 10^6.51±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.