Cation exchange, sorption/desorption

Sorption and desorption are usually modeled as one fully reversible process, although hystersis is sometimes observed. Four types of equations are commonly used to describe sorption/desorption processes Langmuir, Freundlich, overall and ion or cation exchange. The Langmuir isotherm model was developed for single layer adsorption and is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate on the surface phase. 

Many reactions on surfaces of soils and their constituents are extremely lapid—occurring on microsecond and millisecond time scales. Examples of these include some cation and anion sorption/desorption reactions, ion-exchange processes, reactions involving hydrolysis of soil minerals, and complexation reactions. 

The C-Fe-pp model exhibited Cation Exchange Capacity, Specific Surface Area, Specific Cationic Surface Charge, Titratable Acidity, and P sorption capacity higher than C-Al-pp. For both model, the amounts of P desorbable by EUF were much lower than the amounts of P initially added. However, the C-Fe-pp model was less able than C-Al-pp in retaining the initially added P. Evidently, distinct mechanisms controlled the interactions of P with C-Al-pp or C-Fe-pp in both sorption and desorption phenomena. In particular, the presence of Al- or... 

On the other hand, sorption/desorption experiments with copper and clay minerals have shown that true cation exchange (with BaCl2) is relatively insignificant and that the triethanolamine reagent (added to raise the pH of the solution) effects the release of additional metal compounds, which may be more appropriately designated as easily extractable phases (41). Extraction of humate phases was performed with O.IIV NaOH the same reagent was used for the determination of the concentrations of humic substances by comparison with standard humic acids (42). 

Sorption on ion exchangers produces marked heat effects when they sorb ions from aqueous solutions. The author used flow microcalorimetry to determine the heat effects caused by the sorption and desorption of ions and discovered that the heats of cation exchange on strongly acidic exchangers were generally very large, but very much dependant on the ion s valency [52]. 

The ion exchange model is most commonly applied in geochemistry to describe the interaction of major cationic species with clay minerals, or the clay mineral fraction of a sediment it has also been applied to zeolites and other minerals, and to ions besides the major cations (e.g., Viani and Bruton, 1992). As the name suggests, the model treats not the sorption and desorption of a species on the surface and in the interlayers of the clay, but the replacement of one ion there by another. 

In a similar study (Comans et al., 1990), the reversibility of Cs+ sorption on illite was studied by examining the hysteresis between adsorption and desorption isotherms and the isotopic exchangeability of sorbed Cs+. Apparent reversibility was found to be influenced by slow sorption kinetics and by the nature of the competing cation. Cs+ migrates slowly to energetically favorable interlayer sites from which it is not easily released. 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

Attempts to model chemical weathering of catchments have used a variety of approaches and were originally designed to understand acidification processes. The BIRKENES code (Christophersen et al., 1982) was one of the first developed to model catchment stream chemistry. It used cation-anion charge balance, a gibbsite equilibrium solubility control for aluminum concentrations, a Gapon ion exchange for metals sorption, and rates for sulfate adsorption/ desorption in a two-reservoir model. The model was calibrated by input mass fluxes and output mass fluxes for the Birkenes catchment in Norway to provide the water flux information and to fit empirical parameters. 

The removal of sodium cations from the parent [Si,Al]MAG by treatment with hydrochloric acid results in the extraction of part of the framework aluminum as indicated by the low amount of ammonia (0.23 mmol/g) evolved in the high-temperature desorption step (Figure 3) from H-[Si,Al]MAG converted to the ammonium form by ion exchange or ammonia sorption prior to TPD. The formation of strong Lewis-acid sites (1462 cm )... 

Gonzalez-Luque and Streat [45] determined the isotherms for the sorption and desorption of uranium from synthetic phosphoric acid solutions in the presence of interfering cations, such as Fe(II) and Ca(II), with these different impregnated resins and one commercial aminophosphinic acid ion-exchange resin. The effect of phosphoric concentration on uranium... 

The cationic forms of Cd, Pb Cr, and Ni were shown to be immobilized in clinoptilolite structure by two mechanisms ion exchange and chemisorption [05M1]. In case of lead and chromium, chemisorption predominates. The contributions of both mechanisms in case of Cd and Ni were equal. The long term kinetics of Cd sorption and desorption by Ca-exchanged clinoptilohte was studied by using isotope exchange technique while maintaining pH at circumneutral values [lOAl]. 

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