MODELLING THE SOLID–SOLUTION PARTITIONING OF METALS IN ENVIRONMENTAL SYSTEMS S. LOFTS and E. TIPPING NERC Institute of Freshwater Ecology, Ambleside, England Received 11 June 1999; accepted 23 September 1999 1. Introduction Metal behaviour in environmental systems is strongly dependent upon speciation, the distribution of a metal among different chemical forms (Florence, 1977; Tessier and Turner, 1995). Speciation is dependent upon the reactions of metal ions with dissolved and particulate chemical species. These range in complexity from small ionic species such as the chloride ion, to highly complex molecules such as humic substances, particulate matter such as clays, and living organisms like algae and bacteria. Complex molecules and solid phases may be polyfunctional, possessing a large number of chemical sites that interact with metal ions to different extents. The challenge when modelling metal speciation is to simulate the reactions between metals and these species, and to model the complex mixtures of materials which make up both the dissolved and particulate phases in natural environments. Modelling work at the IFE centres on using laboratory data to simulate natural materials and conditions. Certain components of natural systems, particularly or- ganic matter and metal oxides, are believed to exert the strongest influence on metal speciation (Tipping and Hurley, 1988; Tessier et al., 1985). A large body of data exists on the interactions between metals and pure forms of these materials (i.e. Dzombak and Morel, 1990). Our approach is to combine models for these interactions and to simulate natural materials as mixtures of the pure phases. The work presented here focuses on simulating the distribution of metals between their dissolved and particulate forms. This is a highly important distri- bution; it influences, for example, the cycling of metals within lakes, the fluxes of metals along rivers and across the riverine/estuarine interface, and the mobility of metals in soils. Four examples are presented, two of which relate to laboratory experiments on natural materials, and two that look at metal distributions under field conditions. 2. Model descriptions The models used in this work are the Windermere Humic Aquatic Model (WHAM, Tipping, 1994) and the Surface Chemistry Assemblage Model for Particles (SCAMP, Lofts and Tipping, 1998). WHAM is a model for the speciation of the dissolved phase of natural waters. It includes Humic Ion Binding Model V (Tipping Environmental Geochemistry and Health 21: 299–304, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.