Particle surface site-binding model

Hsi C, Langmuir D (1985) Adsorphon of uranyl onto ferric oxyhydroxides applications of the surface complexation site-binding model. Geochim Cosmochim Acta 49 1931-1941 Ingri J, Widerlund A, Land M, Gustafsson O, Anderson P, Ohlander B (2000) Temporal variation in the fractionation of the rare earth elements in a boreal river the role of colloidal particles. Chem Geol 166 23-45... 

It is surprising that data on natural particles can be fitted over a range of concentrations (representative of those encountered in natural waters) on the basis of a "single-site" surface complex formation model. Apparently similar types of binding groups are predominant and of importance in these particles. 

The approach most often used to treat the surface is the Langmuir model of uniform surfaces. This concept assumes, that all the surface sites are identical and binding energies of the reactants are the same independent on the surface coverage. The interactions between adsorbed particles may be neglected. The ideal adsorbed layer is then considered to be similar to ideal solutions with fast surface diffusion, allowing an application of mass action law with the introduction of surface concentrations and concentrations of free sites into rate expressions of elementary steps. 

Rossmann suggested that the canyons form the binding site for the rhi-novirus receptor on the surface of the host cells. The receptor for the major group of rhinoviruses is an adhesion protein known as lCAM-1. Cryoelectron microscopic studies have since shown that ICAM-1 indeed binds at the canyon site. Such electron micrographs of single virus particles have a low resolution and details are not visible. However, it is possible to model components, whose structure is known to high resolution, into the electron microscope pictures and in this way obtain rather detailed information, an approach pioneered in studies of muscle proteins as described in Chapter 14. 

As an illustration, we discuss a particular model of associative interactions with only one binding site per particle. The adsorbing surface, for simplicity, is a hard wall located at z = 0. 

The issue of bioavailability is further clouded by the physical characteristics of soil and the role of a possible mass transfer limitation. Soil constituents are not simply flat surfaces with free and equal access to all bacterial species. The formation of aggregates from sand-, silt-, and clay-sized particles results in stable structures which control microbial contact with the substrate (Figure 2.7). Discussion of sorption mechanisms and binding affinities must include the possible impact of intra-aggregate transport of the substrate. If the substrate is physically inaccessible to the microorganism then both desorption from soil constituents and diffusion to an accessible site are necessary. The impact of intra-aggregate diffusion on degradation kinetics has been modeled for y-hexachlorocyclohexane (Rijnaarts et al., 1990) and naphthalene (Mihelcic Luthy, 1991). 

Physicochemical models of partitioning at the solid-water interface, such as that used here to model ion exchange, require detailed knowledge about the particles. The surface properties of the mineral phases present, as well as equilibrium constants for ion binding to both fixed and variable charge sites associated with each phase, are required. These data requirements and the uncertainty about modeling sorption in mixtures of minerals (e.g., 48-50) make such models difficult to apply to complex natural systems. This is especially the case for modeling solute transport in soil-water systems, which... 

The kinetic reactions occurring in the sorption of Ni, Cd, and Zn on goethite during a period of 2 hours to 42 days at pH 6 were hypothesized to occur via a three-step mechanism using a Fickian diffusion model (1) sorption of trace elements on external surfaces (2) solid-state diffusion of trace elements from external to internal sites and (3) trace element binding and fixation at positions inside the goethite particle (Bruemmer et al., 1988). 

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