Release from adsorbent

Gas released from adsorbent bulb into dosing volume and time allowed for equilibration. 

Adsorption is a dynamic process in which some adsorbate molecules are transferring from the fluid phase onto the solid surface, while others are releasing from the surface back into the fluid. When the rate of these two processes becomes equal, adsorption equilibrium has been established. The equilibrium relationship between a speeific adsorbate and adsorbent is usually defined in terms of an adsorption isotherm, which expresses the amount of adsorbate adsorbed as a fimetion of the gas phase coneentration, at a eonstant temperature. 

Figure 11.15 Cation-exchange mia O-LC analysis of a mixture of model proteins (a) the original sample consisting of myoglobin (M), cytochrome C (C) and lysozyme (L) (b) and (c) proteins adsorbed on to and then released from the polyaaylic acid coated fibre with exti ac-tion times of 5 and 240 s, respectively. Reprinted from Journal of Microcolumn Separations, 8, J.-L. Liao et al., Solid phase mia O exti action of biopolymers, exemplified with adsorption of basic proteins onto a fiber coated with polyaaylic acid, pp. 1-4, 1996, with permission from Jolm Wiley Sons, New York.

An analysis of the rate of release of adsorbed atoms from sites with a continuous energy spectrum for the case of an arbitrary distribution function of initial site populations was given by Carter (32). The rate equation for the t th desorption process with x = 1 and negligible readsorption is... 

Figure 3.3 Reaction of a CO molecule released from a CO-terminated tip with an O atom adsorbed on the surface, (a) STM image, taken with a CO-terminated tip, of two O atoms separated by two lattice spacings (2 x 2.89 A) along the [11 0] direction. Grid lines are drawn through the silver surface atoms, (b) Tunneling current during a 1470 mV sample bias pulse with the CO-terminated tip over one of the two O atoms (denoted by Two current rises...

Upon selective absorption of analyte molecules from the ambient environment, the zeolite thin film increases its refractive index. Correspondingly, release of adsorbed molecules from the zeolite pore results in the decrease of its refractive index. The absorption/desorption of molecules depends on the molecule concentration in the environment to be monitored. Therefore, monitoring of the refractive index change induced phase shift in the interference spectrum can detect the presence and amount of the target analyte existing in the environment. 

Desorb The reverse of adsorb to be released from a surface. 

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural... 

The properties of both organic matter and clay minerals may affect the release of contaminants from adsorbed surfaces. Zhang et al. (1990) report that desorption (in aqueous solution) of acetonitrille solvent from homoionic montmorillonite clays is reversible, and hysteresis appears to exist except for K+-montmorillonite. This behavior suggests that desorption may be affected by the fundamental difference in the swelling of the various homoionic montmorillonites, when acetonitrile is present in the water solution. During adsorption, it was observed that the presence of acetonitrile affects the swelling of different homoionic clays. At a concentration of 0.5 M acetonitrile in solution, the layers of K+-montmorillonite do not expand as they would in pure water, while the layers of Ca +- and Mg +-montmorillonite expand beyond a partially collapsed state. The behaviors of K+-, Ca +-, and Mg +-montmorillonite are different from the behavior of the these clays in pure water. Na+-montmorillonite is not affected by acetonitrile presence in an aqueous solution. 

When water flows over a contaminated land surface, pollutants released from higher elevations are transported, as dissolved solute or adsorbed on suspended particles, and accumulate at lower elevations. This behavior is reflected in the spatial variability of contaminant concentration, which affects contaminant redistribution with depth following leaching. If a sorbed contaminant is not of uniform concentration across all soil-size ranges but is higher in the fine sediment fraction, the deposition of this soil fraction controls contaminant redistribution in the subsurface. 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

Such processes lead to the formation of adsorbable halogenated organic compounds (AOX) in high concentrations. Typical concentrations found in a continuous antifelt treatment are shown in Table 4. The high dissolved organic carbon (DOC) determined in the baths is one of the sources for the formation of high concentrations of chlorinated compounds. The formation of chlorinated products is the result of chemical reactions directly with the fiber, with organic compounds released from the fibers, and with added auxiliaries. 

The practical significance of such competition evolves from the experience that silicate and (anionic) humics can increase the efficiency of phosphate fertiliser because these compounds occupy sites suitable for phosphate adsorption (Kingston et ak, 1968 Schwertmann, 1995). Hydroxyl is another anion that competes effectively with adsorbing anions, owing to its location in the inner Helmholz layer. The release of adsorbed phosphate after liming a soil or after inflow of acidic surface soil into weakly alkaline surface waters due to erosion, can be considered as the result of competition between OH and phosphate ions. 

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