As sorption

Adsorption and ion exchange share so many common features in regard to apphcation in batch and fixed-bed processes that they can be grouped together as sorption for a unified treatment. These processes involve the transfer and resulting equilibrium distribution of one or more solutes between a fluid phase and particles. The partitioning of a single solute between fluid and sorbed phases or the selectivity of a sorbent towards multiple solutes makes it possible to separate solutes from a bulk fluid phase or from one another. 

Many of the problems arise out of the extrapolation of what happens in solution cultures to soils. Although. solution cultures have served and continue to serve very useful functions in basic research of plant science, they differ from soils in several important ways. The surface area available in soils for processes such as sorption is much greater than in solution cultures, solution cultures are mixed continuously, the microbial ecology differs greatly between the two media, and the status of water and O2 in the two systems is usually quite different. 

Whereas studies have been carried out on the factors (surface coverage, residence time, pH) which influence the desorption of arsenate previously sorbed onto oxides, phyllosilicates and soils (O Reilly et al. 2001 Liu et al. 2001 Arai and Sparks 2002 Violante and Pigna 2002 Pigna et al. 2006), scant information are available on the possible desorption of arsenate coprecipitated with iron or aluminum. In natural environments arsenic may form precipitates or coprecipitates with Al, Fe, Mn and Ca. Coprecipitation of arsenic with iron and aluminum are practical and effective treatment processes for removing arsenic from drinking waters and might be as important as sorption to preformed solids. 

At low temperatures, adsorptive separation becomes important for zeolite membranes as sorption of one species can effectively hinder permeation of other species. 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

On the other hand, Aboul-Kassim [1] assessed the environmental impact of hazardous waste materials in landfills by (1) characterizing the different organic compound fractions present in such wastes and their leachates, (2) determining the toxic effect of each fraction and individual organic compounds, and (3) studying the chemodynamics (i.e., fate and transport) of such leachates by using a battery of laboratory experiments (such as sorption/desorption, photolysis, volatilization, biodegradation). 

The CTRW approach accounts naturally for transport in preferential pathways, with mass transfer to stagnant and slow flow regions the CTRW can account for these physical transport mechanisms, as well as other factors that influence transport of reactive contaminants, such as sorption. 

Such experiments were repeated for eac compound at a variety of pH s and temperatures so that pH-rate constant profiles and activation energies could be obtained. Extraneous experimental complications such as sorption of the compound to container walls, incomplete extraction from aqueous solutions and possible catalysis by metal ions in solution were carefully monitored and accounted for in the final determination of aqueous phase hydrolysis rate constants. Of these possibilities, only sorption to container walls was observed to have a measurable effect on the experimental data. 

Ion exchange is similar to adsorption, since mass transfer from a fluid to a solid phase is common in both processes, i.e. they are basically diffusion processes. Ion exchange is also a sorption process, but ions are the sorbed species in contrast to adsorption, where electrically neutral species are sorbed (Noble and Terry, 2004 Perry and Green, 1999). It is generally accepted that adsorption and ion exchange can be grouped together as sorption for a unified treatment in practical applications. 

Ion exchange shares many characteristics with adsorption, such as mass transfer from the fluid to the solid phase there are, however, some significant differences. Specifically, although both processes can be characterized as sorption processes, the sorbed species are ions in ion exchange, whereas electrically neutral substances are sorbed hi adsorption. Moreover, in ion exchange, the ions removed from the liquid phase are replaced by ions from the solid phase. So, there actually occurs an exchange of ions and not only a removal... 

Sorption of water vapour to or from a food depends on the vapour pressure exerted by the water in the food. If this vapour pressure is lower than that of the atmosphere, absorption occurs until vapour pressure equilibrium is reached. Conversely, desorption of water vapour results if the vapour pressure exerted by water in the food is greater than that of the atmosphere. Adsorption is regarded as sorption of water at a physical interface between a solid and its environment. Absorption is regarded as a process in... 

The process in which chemicals become associated with solid phases is generally referred to as sorption. It is absorption if the molecules attach to a two-dimensional surface, while it is absorption if the molecules penetrate into a three-dimensional matrix. This phase transfer process may involve vapor molecules or dissolved molecules associating with solid phases. 

Figure 11.16 Relationship between sorbed and dissolved amphi-phile concentrations (upper isotherm plot). These different parts of the isotherm reflect changes in the solid surface as sorption proceeds, possibly explainable by the following in portion (I) with low dissolved concentrations, sorption occurs via ion exchange and related mechanisms. At some point, sufficient near-surface concentration enhancement occurs that micelles form there (Ha) and rapid coagulation between oppositely charged micelles and the surface follows (lib). When the surface becomes fLilly coated with such micelles, additional sorption is stopped (III). In portion III, the solid surface charge is converted from one sign to the other, implying sorbates must become physically associated with the particle surface, as opposed to simply being present in the diffuse double layer or the vicinal water layer.

In previous papers, we reported surfactant adsorption onto hydrotalcite-like compounds, i.e. Mg-AI-C03-LDHs [10, 11]. We showed that LDH efficiency in removing anionic surfactant from aqueous solution by adsorption was greater than that of alumina. We also showed that it is possible to use LDH in the calcined form to remove surfactants from aqueous solution, which in turn involves two processes, intercalation by regeneration of the LDH structure and adsorption on the regenerated material. The whole surfactant removal process is designed in this case as sorption. These results emphasised the recyclability of LDHs and their potential use as surfactant removers. 

Chemical reactivity and biological activity can be related to molecular structure and physicochemical properties. QSAR models can be established among hydrophobic-lipophilic, electronic, and steric properties, between quantum-mechanics-related parameters and toxicity and between environmental fate parameters such as sorption and tendency for bioaccumulation. The main objective of a QSAR study is to develop quantitative relationships between given properties of a set of chemicals and their molecular descriptors. To develop a valid QSAR model, the following steps are essential ... 

The active phase of the Deacon catalyst is usually assumed to be a complex melt of copper or chromium and alkaline metal chlorides under reaction conditions, which is distributed within the pore network of an inert carrier [42]. Such supported liquid-phase catalysts (SLPC) are eminently suitable for adsorbing large amounts of the reacting components as sorption takes place in a bulk phase and is not restricted to only a limited number of suitable surface sites. The periodic expansion and contraction of the melt as a result of (de) sorption imposes considerable strains on the carrier structure hence, special mechanically robust support materials are needed to withstand such strains and prevent the catalyst crumbling away and disintegrating after a few cycles. In addition, even when it is immobilized on the carrier, the melt is extremely aggressive and resistant materials must be used for reactor construction. 

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