AN ASSEMBLAGE MODEL FOR CATION BINDING BY NATURAL PARTICULATE MATTER - DESCRIPTION OF PH DPENDENCY, SALT DEPENDENCY, AND CATION-PROTON EXCHANGE

Abstract

SCAMP (Surface Chemistry Assemblage Model for Particles) describes the equilibrium adsorption of protons and metals by natural particulate matter using a combination of submodels for individual binding phases. Interactions with natural organic matter are described with Humic Ion-Binding Model V, and adsorption by oxides with a surface complexation model that allows for site heterogeneity. An idealized cation exchanger is also included. SCAMP uses published parameters for Model V, and parameters for the oxide model are derived from published laboratory data for proton and metal binding by oxides of Al, Si, Mn, and Fe(III). The model is applied to two samples of aquatic suspended particulate matter (SPM), one riverine, the other estuarine, taking into account the proportions of the different binding phases. Simulations suggest that organic matter is the major determinant of surface (proton) charge for the riverine SPM, while fixed-charge clays are dominant in the estuarine material. Predictions of the pH dependence of binding of nine metals (Co, Ni, Cu, Zn, Sr, Cd, Cs, Eu, Pb) by the two SPM samples are in reasonable agreement with observations. The different components of the assemblage provide binding sites with a wide range of apparent equilibrium constants, and log-log binding isotherms therefore have shallow slopes, in the range 0.3-0.5. On a weight-for-weight basis, the metal binding strengths of the different phases increase in the order cation exchanger < silica < aluminium oxide ~ ferric oxide < humic substances < manganese dioxide.