Adsorption—desorption

Abstract Adsorption-desorption studies addressing the influence of surfaces and gas molecules associated to energy concepts and different mechanisms and models based on ideal and non-uniform surface energy on metals, metal oxides and semiconductors. 

Keywords Adsorption Desorption Mechanisms Models Potential energy 

Sorption and desorption are chemical reactions by which certain metals (e.g., Fe, Cu, Zn, and Mn) and anions (e.g., phosphate and sulfate) form/break chemical bonds within the coordination shell of atoms comprising the mineral structure. Sorption includes both adsorption and absorption. Physical adsorption refers to the attraction caused by the surface tension of a solid that causes molecules to be held at the surface of the solid. This type can also be reversible. Chemical adsorption (not reversible) involves actual chemical bonding at the solid s surface. Absorption is a process in which the molecules or atoms of one phase penetrate those of another phase. 

The sorption process (abiotic retention) is controlled by the concentration of phosphate in soil pore water and the ability of the solid phase to replenish phosphate into pore water. This can be described in terms of two regulating factors The intensity factor of the sorption process is controlled by the concentration of phosphate in soil pore water, and the capacity factor refers to the ability of the solid phase to replenish phosphate to the soil pore water. 

What happens if this system is loaded with additional soluble inorganic phosphorus  

Biogeochemistry of Wetlands Science and Applications Solid phase 

FIGURE 9.21 Schematic showing phosphorus exchange between solid phase and solution phase. (Modified from Froelich, 1988.) 

---

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

The projection of a domain plot onto its base makes a convenient two-dimensional graphical representation for describing adsorption-desorption operations. Here, the domain region that is filled can be indicated by shading the appropriate portion of the 45° base triangle. Indicate the appropriate shading for (a) adsorption up to Xa - 0.8 (b) such adsorption followed by desorption to Xd - 0.5 and (c) followed by readsorption from Xd = 0.5 to Xa = 0.7. 

XVIII-11 (the paradox of desorption heat exceeding adsorption heat is explainable in terms of a partial irreversibility of the adsorption-desorption process). 

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

The SHG and SFG teclmiques are also suitable for studying dynamical processes occurring on slower time scales. Indeed, many valuable studies of adsorption, desorption, difhision and other surface processes have been perfomied on time scales of milliseconds to seconds. 

Figure Bl.26.8. Adsorption/desorption peaks for nitrogen obtained with the continuous flow metiiod (Nelsen F M and Eggertsen F T 1958 Anal. Chem. 30 1387-90).

Abstract. A model of the conformational transitions of the nucleic acid molecule during the water adsorption-desorption cycle is proposed. The nucleic acid-water system is considered as an open system. The model describes the transitions between three main conformations of wet nucleic acid samples A-, B- and unordered forms. The analysis of kinetic equations shows the non-trivial bifurcation behaviour of the system which leads to the multistability. This fact allows one to explain the hysteresis phenomena observed experimentally in the nucleic acid-water system. The problem of self-organization in the nucleic acid-water system is of great importance for revealing physical mechanisms of the functioning of nucleic acids and for many specific practical fields. 

The adsorption-desorption hysteresis does not disappear or decrease during at least a week of exposure of the NA sample to a r.h. of 56%, this value being chosen because the adsorption hysteresis is the greatest at this r.h. The hysteresis lifetime is great enough to consider the hysteresis as a permanent phenomenon for the processes of the cellular regulation. 

F. Ricca, Suppl. Nuovo Cimento 1, 5, 339 F. Ricca, C. Pisani and E. Garrone, Adsorption-Desorption Phenomena, Proc. 2nd. Intern. Conf. 1971, p. Ill, Academic Press, London and New York (1972) C, Pisani, F. Ricca and C. Roetti, J. Phys. Chem. 77, 657 (1973). 

Fig. 2.29 Comparison of nitrogen adsorption at 78 K on a carbon black (Sterling FT) before and after graphitization (a) The amount adsorbed on the ungraphitized sample plotted against the amount x, adsorbed on the graphitized sample, at the same pressure, b) The corresponding isotherms O, adsorption, , desorption on the ungraphitized sample (4 runs) A. adsorption A desorption, on the graphitized sample (4 runs).

Fig. 230 Adsorption of nitrogen at 77 K on a silica powder a) adsorption isotherms b) /-plot. Broken line, uncompacted powder continuous line, power compacted at 2-00 x 10 N m (130 ton in ). (—>—) adsorption (—<-) desorption. / is the ratio of the amount adsorbed on the powder to the amount adsorbed on the compact at the same relative...

Fig. 5.18 Adsorption isotherm of water vapour at 25°C on microporous silica gel E outgassed at 25°C. O. Adsorption,. desorption.

Fig. 5.19 Adsorption of water vapour on a silica gel which had been heated at 900°C. (The water content, calculated from the loss in weight at 1000°C, was 0-2%.) First run O, adsorption ( ), desorption. Second run , adsorption desorption.

This principle is illustrated in Figure 10 (45). Water adsorption at low pressures is markedly reduced on a poly(vinyhdene chloride)-based activated carbon after removal of surface oxygenated groups by degassing at 1000°C. Following this treatment, water adsorption is dominated by capillary condensation in mesopores, and the si2e of the adsorption-desorption hysteresis loop increases, because the pore volume previously occupied by water at the lower pressures now remains empty until the water pressure reaches pressures 0.3 to 0.4 times the vapor pressure) at which capillary condensation can occur. 

Other Interaction Processes. The selectivity of flotation reagents in a pulp and their functions depend on their interactions with the mineral phases to be separated, but other physicochemical and hydrodynamic processes also play roles. AH adsorption—desorption phenomena occur at the sohd—hquid interfacial region. Surface processes that influence such adsorptions include activation and depression. Activators and depressants are auxiUary reagents. 

For adsorbed hydrocarbons, the adsorption—desorption process can be thought of as a reaction and the adsorption isotherm as a description of the reaction at equihbtium. For the Langmuir isotherm,... 

During Stages II and III the average concentration of radicals within the particle determines the rate of polymerization. To solve for n, the fate of a given radical was balanced across the possible adsorption, desorption, and termination events. Initially a solution was provided for three physically limiting cases. Subsequentiy, n was solved for expHcitiy without limitation using a generating function to solve the Smith-Ewart recursion formula (29). This analysis for the case of very slow rates of radical desorption was improved on (30), and later radical readsorption was accounted for and the Smith-Ewart recursion formula solved via the method of continuous fractions (31). 

Surface Area. Surface area is measured by determining the quantity of nitrogen gas that adsorbs on the particle/crystal surfaces of a dry sample. Determination of surface area by measuring adsorption at gas—soHd interfaces is covered extensively in the Hterature (84). Instmments such as the FlowSorb 2300 are used to control the adsorption/desorption within specific conditions of temperature and pressure. 

There are seven commercial processes in operation six operate in the vapor phase. The Universal Oil Products process operates in the Hquid phase and is unique in the simulation of a moving bed. The adsorption unit consists of one vessel segmented into sections with multiple inlet and oudet ports. Flow to the various segments is accompHshed by means of a rotary valve which allows each bed segment to proceed sequentially through all the adsorption/desorption steps. 

Under Irradiation. Under illumination, the surface characteristics can undergo dramatic changes, altering the nature of the adsorption sites. Thus, dark adsorption—desorption events are altered, and additional events arising from photoadsorption—photodesorption equiUbria take place. 

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

The above stm study also discovered a facile transport of surface gold atoms in the presence of the Hquid phase, suggesting that the two-step mechanism does not provide a complete picture of the surface reactions, and that adsorption/desorption processes may have an important role in the formation of the final equiHbrium stmcture of the monolayer. Support for the importance of a desorption process comes from atomic absorption studies showing the existence of gold in the alkanethiol solution. The stm studies suggest that this gold comes from terraces, where single-a tomic deep pits are formed (281—283). 

Separation Techniques. Current methods for separating fatty acids are by solvent crystaUi2ation or by the hydrophili2ation process. Other methods that have been used in the past, or perhaps could be used in the future, are panning and pressing, solvent extraction, supercritical fluid extraction, the use of metal salts in assisting in separation, separations using urea complexes, and adsorption/desorption. 

Rapid Adsorption-Desorption Cycles For rapid cycles with particle diffusion controlling, when the cycle time is much smaller than the time constant for intraparticle transport, the LDF approximation becomes inaccurate. The generalized expression... 

It is seen, from equation (5), that a graph relating the reciprocal of the corrected retention volume to the concentration of the moderator can provide values for the adsorption/desorption coefficient and the surface area of the stationary phase. Scott and Simpson [1] used this technique to measure the surface area of a reversed phase and the curves relating the reciprocal of the corrected retention volume to moderator concentration are those shown in Figure 2. 

Normal paraffins in the C,o - C,5 range are recovered from petroleum fractions by adsorption-desorption using molecular sieves. Ammonia can be used to desorb the n-paraffins. By employing two beds of sieves, one on adsorption and one on desorption at all times, a continuous flow of the feed and ammonia is maintained. 

Contaminant concentrations Dispersal of airborne contaminants such as odors, fumes, smoke, VOCs, etc. transported by these airflows and transformed by a variety of processes including chemical and radiochemical transformation, adsorption, desorption to building materials, filtration, and deposition to surfaces evolution of contaminant concentrations in the individual zones air quality checks in terms of CO2 levels cross-contamination evaluation of zones air quality evaluations in relation to perception as well as health. Methods ate also applicable to smoke control design. 

---