Multiple sorption sites

Except for the competitive Langmuir isotherm (Table 10.7), the simple isotherm models cannot consider competition between multiple sorbates for a single sorption site. They are most applicable to physical adsorption of molecular species where the sorbate is held by much weaker hydrogen bonds or residual bonds. Adsorption of ions and complexed species is generally best studied with models that can consider more independent variables. To accurately predict adsorption of ions that compete for adsorption sites and also form important complexes, may require a model that considers as many as a dozen or more variables. 

A number of experimental techniques have been described for the determination of isotherms based on frontal analysis, frontal analysis by characteristic point, elution by characteristic point, and perturbation methods [12,21,27,169,176-179]. Most authors report single-component isotherm results. Multiple-component isotherm data are more complicated because all components are simultaneously in competition for the sorption sites on the stationary phase. The retention time and peak shapes of any solute is dependent on the concentration and properties of all other solutes in the mixture [12,170,180]. For multicomponent mobile phases in liquid and supercritical fluid chromatography this includes each component of the mobile phase. 

Kinetic models proposed for sorption/desorption mechanisms including first-order, multiple first-order, Langmuir-type second-order, and various diffusion rate laws are shown in Sects. 3.2 and 3.4. All except the diffusion models conceptualize specific sites to or from which molecules may sorb or desorb in a first-order fashion. The following points should be taken into consideration [ 181,198] ... 

There are several ways to determine kinetically that multiple first-order or other reaction order slopes are present, and that they indicate different sites or mechansims for sorption. One could determine rate-limiting steps (Chapter 5), E values could be measured, or materials that affect specific... 

Carey et al. [70] used the BioRedox code to simulate the fate and transport of BTEX and chlorinated ethenes at a contaminated groundwater site at Plattsburgh Air Force Base. Transport of the compounds was modeled using the 3-D advection/dispersion equation, and sorption was assumed to be negligible. While BioRedox is capable of simulating oxidation of multiple electron donors,... 

Time Scales of Metal Sorption Reactions. Metal sorption reactions can occur over time scales ranging from milliseconds to several weeks depending on the sorbate/sorbent system. The type of sorbent can drastically affect the reaction rate. For example, sorption reactions are often more rapid on clay minerals such as kaolinite than on vermiculitic and micaceous minerals. This is in large part due to the availability of sites for sorption. Kaolinite has readily available planar external sites and sorption is often complete in minutes (42). Vermiculite and micas have multiple sites for retention of metals including planar, edge, and interlayer sites, with some of the latter sites being partially to totally collapsed. Consequently, sorption and desorption reactions on these sites can be slow. 

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