AN ASSEMBLAGE MODEL FOR CATION-BINDING BY NATURAL PARTICULATE MATTER

SCAMP (Surface Chemistry Assemblage Model for Particles) describes the equilibrium adsorption of protons and metals by natural particulate m atter 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 exchange r is also included. SCAMP uses published parameters for Model V, and p arameters for the oxide model are derived from published laboratory da ta 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 m atter (SPM), one riverine, the other estuarine, taking into account th e proportions of the different binding phases. Simulations suggest tha t organic matter is the major determinant of surface (proton) charge f or the riverine SPM, while fixed-charge clays are dominant in the estu arine material. Predictions of the pH dependence of binding of nine me tals (Co, Ni, Cu, Zn, Sr, Cd, Cs, Eu, Pb) by the two SPM samples are i n reasonable agreement with observations. The different components of the assemblage provide binding sites with a wide range of apparent equ ilibrium constants, and log-log binding isotherms therefore have shall ow slopes, in the range 0.3-0.5. On a weight-for-weight basis, the met al binding strengths of the different phases increase in the order cat ion exchanger < silica < aluminium oxide similar to ferric oxide < hum ic substances < manganese dioxide. Copyright (C) 1998 Elsevier Science Ltd.