A binary aqueous component model for the sediment-water partitioning of trace metals in natural waters

A model defining the overall sediment-water partitioning of a chemical, K(D), and the partitioning of its conservative components, (K(D))i, is presented. With respect to many trace metals in natural waters it is proposed that, through strong and perhaps specific complexation, two independent aqueous components coexist and a binary form of the model is appropriate. For two components of a metal that exhibit unequal partitioning, an inverse relationship between K(D) and particle concentration is predicted. Published experimental measurements of K(D) for metals in river waters, derived under conditions which exclude variable concentrations of preexistent colloidal particles, displayed either an inverse dependence (Cu, Ni, and Pd) or little dependence (Cs) on particle concentration. Regarding the former, iterative fits with the binary model were better than empirical fits based on a third (colloidal) phase model, and suggested the presence of between about 10 and 75% of a particle-reactive component ((K(D))1 approximately 5 x 10(4) to 10(10) mL g(-1)) and 25 and 90% of a less reactive (e.g., strongly complexed) component ((K(D))2 < or = 2.5 x 10(3) mL g(-1)). Regarding Cs, data indicated the presence of a single component whose K(D) was on the order of 10(3) mL g(-1). These observations challenge the conventional means by which sediment-water partitioning is considered and modeled, and imply that a third phase is not always a prerequisite for the particle concentration effect frequently observed in laboratory and field studies.