Ions, surface 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. 

Differences in the temporal development of release rates for the individual elements are connected with their sorption/desorption behaviour, which is primarily due to pH-effects but may also be influenced by com-plexation, e.g. by elevated concentrations of chloride ions. With respect to the pH-effects, however, there are significant differences in the response of the various solid substrates to the addition of H+-ions, and it may be argued that the pH-values on the solid surfaces -which can be estimated from "pH-titration tests" - are decisive for the behavior of the particular element rather than the pH-values determined in solution. 

Sorption refers to the partitioning of the contaminants from the solution or pore fluid to the solid phase or soil surface. Sorption includes adsorption and ion exchange, and it is dependent on (a) the type of contaminant, (b) the type of soil, and (c) the pore fluid characteristics. Desorption is the reverse process and is responsible for the release of contaminants from the soil surface. Both sorption and desorption are affected by soil pH changes caused by the migration of H" and OH ions, which are produced by the electrolysis reactions. The pH-dependent sorption-desorption behavior is generally determined by performing batch experiments using the soil and contaminant of particular interest. 

As can be seen from Table 3.5 ( 4), Pt nanoparticles formed in HPS are more selective in L-sorbose oxidation than the catalysts based on block copolymers but still the selectivity is not sufficient We believe that low selectivity may be attributed to saturation of the nanoparticle surface with H2 during reduction, which alters the sorption-desorption equilibria of the L-sorbose, oxygen and 2-keto-L-gulonic acid [89]. As was demonstrated in Ref [96], L-sorbose oxidation can be catalyzed not only with Pt(o) but also in the presence of Pt ions. This prompted us to investigate the catalytic properties of HPS-Pt without Pt ion reduction. 

This means that the surface potential is dominated by the ion sorption/desorption at the surface and shows the practical value of the splitting of the electrolyte molar k into the individual ionic values. 

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. 

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). 

In the presence of Pb(II) ions in sulfuric acid, potential oscillations have been observed for galvanostatic oxidation of hydrogen on platinum electrode [129]. This behavior has been attributed to ad-sorption/oxidation/desorption processes of lead on the platinum surface. Lead at high values of coverage is oxidized to insoluble PbS04, which blocks the Pt surface. 

Aquatic sediments are formed in all surface waters by the settling of coarse and fine inorganic and organic particles. They are present in rivers, in lakes and in the oceans, and radionuclides deposited on the surface of the earth will sooner or later come into contact with these sediments. They may enter the sediments by sorption of molecularly-dispersed species (ions, molecules), by precipitation or coprecipitation, by coagulation of colloids (in particular carrier colloids) followed by sedimentation of the particles formed, or by sedimentation of coarse particles (suspended matter). By desorption, the radionuclides may be remobilized and released again into the water. 

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