Adsorption of inorganic pollutants in aquatic systems

Adsorption often dictates the fate of trace inorganic pollutants in aquatic systems. In contrast to the nonspecific sorption mechanism proposed for hydrophobic organic compounds, adsorption of inorganic solutes is viewed as a site-specific process in which ions bind (chemically) at functional groups on particle surfaces. This conceptual model is supported by the finding that adsorption of ions on model adsorbents with relatively well-defined surface groups (e.g., hydrous metal oxides) can be described via mass law equations, a fact which forms the basis for all of the so-called surface complexation models. Surface complexation reactions are distinguished from reactions among monomelic solutes in that the total energy of interaction includes long-range electrostatic effects that vary with surface charge. Electrostatic effects are accounted for by applying a coulombic correction factor (activity coefficient) to intrinsic surface complexation constants. Formulations for the coulombic correction term differ among the various surface complexation models. These models can be used to predict inorganic ion adsorption on hydrous oxides for changing solution conditions on the basis of fewer data than needed with a purely empirical approach.