Kinetics of adsorption processes

A Varian Cary 100 model UWis spectrophotometer was used for optical absorbance measurements. The absorbance measrrrements were conducted in situ for the study of the kinetics of adsorption process. In all experiments, the size and mass of ACC was kept as constant as possible (about 18.0 0.1 mg). Weighed ACC pieces were pre-wetted by leaving in water for 24 h before use. The idea of using pre-wetted ACC originates from previons findings that pre-wetting enhances the adsorption process [9, 11],... 

Dielectric measurements of gas adsorption systems can be performed fairly quickly, typically within a few seconds [6.3]. Hence the kinetics of adsorption processes being slow on this time scale can be observed. Indeed these processes are sometimes invisible to purely manometric or even gravimetric measurements. As examples we mention internal diffusion, reorientation or catalytically induced chemical reaction processes of admolecules within a sorbent material. The mass of the adsorbed phase normally is constant during processes of this type, whereas the dipole moment of the admolecules and hence their polarization changes, cp. Sect. 3.2. 

The kinetics of adsorption processes is much less well understood and much further work needs to be done before the models currently available will be of immediate practical use. 

In general, it seems more reasonable to suppose that in chemisorption specific sites are involved and that therefore definite potential barriers to lateral motion should be present. The adsorption should therefore obey the statistical thermodynamics of a localized state. On the other hand, the kinetics of adsorption and of catalytic processes will depend greatly on the frequency and nature of such surface jumps as do occur. A film can be fairly mobile in this kinetic sense and yet not be expected to show any significant deviation from the configurational entropy of a localized state. 

Wlien a surface is exposed to a gas, the molecules can adsorb, or stick, to the surface. Adsorption is an extremely important process, as it is the first step in any surface chemical reaction. Some of die aspects of adsorption that surface science is concerned with include the mechanisms and kinetics of adsorption, the atomic bonding sites of adsorbates and the chemical reactions that occur with adsorbed molecules. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

Figure 26 shows the redox potential of 40 monolayers of cytochrome P450scc on ITO glass plate in 0.1 KCl containing 10 mM phosphate buffer. It can be seen that when the cholesterol dissolved in X-triton 100 was added 50 pi at a time, the redox peaks were well distinguishable, and the cathodic peak at -90 mV was developed in addition to the anodic peak at 16 mV. When the potential was scanned from 400 to 400 mV, there could have been reaction of cholesterol. It is possible that the electrochemical process donated electrons to the cytochrome P450scc that reacted with the cholesterol. The kinetics of adsorption and the reduction process could have been the ion-diffusion-controlled process. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

H. A.-Kozlowska, J. Klinger, and B. E. Conway, /. Electroanal. Chem. 75, 45 (1911). Fukuda and A. Aramata, The kinetic study of adsorption processes of the phosphate species on platinum)111) in aqueous acidic solutions, J. Electroanal. Chem., in press (1997). 

Abstract Removal of catechol and resorcinol from aqueous solutions by adsorption onto high area activated carbon cloth (ACC) was investigated. Kinetics of adsorption was followed by in-situ uv-spectroscopy and the data were treated according to pseudo-first-order, pseudo-second-order and intraparticle drfiusion models. It was fotmd that the adsorption process of these compotmds onto ACC follows pseudo-second-order model. Furthermore, intraparticle drfiusion is efiective in rate of adsorption processes of these compoimds. Adsorption isotherms were derived at 25°C on the basis of batch analysis. Isotherm data were treated according to Langmuir and Freundhch models. The fits of experimental data to these equations were examined. 

Three kinetic models were applied to adsorption kinetic data in order to investigate the behavior of adsorption process of adsorbates catechol and resorcinol onto ACC. These models are the pseudo-first-order, the pseudo-second-order and the intraparticle diffusion models. Linear form of pseudo-first-order model can be formulated as... 

Besides determination of n. A, or D, follow-up kinetics are accessible from the influence on the position of the peaks, and in particular, the intensity of the reverse peak. Formation of products that are electroactive within the potenhal window scanned causes the appearance of additional peaks. Furthermore, the shape of the peaks allows conclusions to be drawn about the involvement of adsorption processes. 

Finkelstein, N. P., 1999. Addendum to The activation of sulphide minerals for flotation a review. Inter. J. Miner. Process, 55(4) 283 - 286 Fomasiero, D., Montalti, M., Ralston, J., 1995. Kinetics of adsorption of ethyl xanthate on pyrrhotite in situ UV and infiared spectroscopic studies. Langmuir, 11 467 - 478 Forssberg, K. S. E., Antti, B. M., Palsson, B., 1984. Computer-assisted calculations of thermodynamic equilibria in the chalcopyrite-ethyl xanthate system. In M. J. Jones and R. Oblatt (eds.). Reagents in the Minerals Industry. IMM, Rome, Italy, 251 - 264 Fuerstenau, M. C., Kuhn, M. C., Elgillani, D. A., 1968. The role of dixanthogen in xaomthate flotation ofpyrite. Trans. AIME, 241 437 Fuerstenau, M. C. and Sabacky, B. J., 1981. Inter. J. Miner. Process, 8 79 - 84 Fuerstenau, M. C., Misra, M., Palmer, B. R., Xanthate adsorption on selected sulphides in the presence of oxygen. Inter. J. Miner. Process... 

Porosity refers to the volume of pores in a solid. It contributes to the internal surface area of the sample and can influence the kinetics of adsorption. Diffusion into and out of pores is often considered responsible for slow adsorption and desorption processes. Pores vary in size and shape. They have been classified according to their average widths as micropores which are of the order of molecular dimensions (<2 nm), meso- or transitional pores which are between 2-50 nm and macropores which are larger than 50 nm (Sing et al., 1985). The sum of all the pores is called the pore volume (porosity). 

Because polymer adsorption is effectively irreversible, and because adsorption and floe growth occur simultaneously, flocculation is a non-equilibrium process. As a result, performance is largely determined by the kinetics of adsorption and aggregation. Both of these can be regarded as collision processes involving solid particles and polymer molecules. In each case, collisions can arise due to either Brownian motion or agitation of the suspension. The collision frequency v between particles and polymer molecules can be estimated from °... 

A dsorption is normally thought of as the process by which a molecule or atom in a fluid is attached to a solid surface, and it is implied that the molecule (or atom) is in the same location as the site. Kinetics of such processes is concerned with force fields between sites and molecules and forms an important area of surface chemistry. However, in this paper both a wider and more restricted view will be taken of adsorption kinetics in that emphasis will be put on the so-called physical processes that must accompany adsorption, if the overall process is to continue. In particular the kind of kinetics discussed will be that necessary to explain the performance of, or to design an apparatus for, separating or removing components in a fluid stream. 

The intent of this paper is to point out that physical or space processes, which usually influence and frequently control kinetics of adsorption in aqueous systems, can be represented effectively by quantitative models. The rate coefficients in such models are more meaningful than those associated with schemes which do not recognize space processes. Published reports have frequently analyzed data by a chemical model, but in such instances the reaction rate constants are found to... 

By contrast, electrolyte states are much more limited in their distribution than metal conduction band states so that in many cases electron transfer through surface states may be the dominant process in semiconductor-electrolyte junctions. On the other hand, in contrast to vacuum and insulators, liquid electrolytes allow substantial interaction at the interface. Ionic currents flow, adsorption and desorption take place, solvent molecules fluctuate around ions and reactants and products diffuse to and from the surface. The reactions and kinetics of these processes must be considered in analyzing the behavior of surface states at the semiconductor-electrolyte junction. Thus, at the semiconductor-electrolyte junction, surface states can interact strongly with the electrolyte but from the point of view of the semiconductor the reaction of surface states with the semiconductor carriers should still be describable by equations 1 and 2. 

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