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Kinetics of adsorption

The adsorption rate of a certain substance on a surface of a solid state is described by equation of the type [Pg.20]

Here m is the mass of absorbed particles v is the frequency of oscillation of absorbed particles S is the surface area occupied by a single absorbed particles R = Kq exp / kT) is the adhesion coefficient  [Pg.21]

T v exp +Eq / kT) is the average time of contact of adsorption particle with the surface. Integrating equation (1.10) one gets [Pg.21]

Q being the adsorption heat. For small times (f expression [Pg.21]

We should point out that the Langmuir kinetics given by expressions (1.11) and (1.12) which is often observed in experiment is as often violated. In numerous cases the data on adsorption kinetics follows the well-known Roginsky-Zeldovich-Elovich kinetics isotherm [47, 48] [Pg.21]

In conclusion, the different shapes of isotherms describing equilibrium distributions of a contaminant, between geosorbents and aqueous or gaseous phases, depend on the sorption mechanism involved and the associated sorption energy. At low contaminant concentration, all models reduce to essentially linear correlation. At higher contaminant concentration, when sorption isotherms deviate from linearity, an appropriate isotherm model should be used to describe the retention process. [Pg.101]

Adsorption kinetics involve a time-dependent process that describes the rate of adsorption of chemical contaminants on the solid phase. The standard chemical meaning of kinetics usually covers the study of the rate of reactions and molecular processes when transport is not a limiting factor however, this definition is not [Pg.101]

Understanding the kinetics of contaminant adsorption on the subsurface solid phase requires knowledge of both the differential rate law, explaining the reaction system, and the apparent rate law, which includes both chemical kinetics and transport-controlled processes. By studying the rates of chemical processes in the subsurface, we can predict the time necessary to reach equilibrium or quasi-state equilibrium and understand the reaction mechanism. The interested reader can find detailed explanations of subsurface kinetic processes in Sparks (1989) and Pignatello (1989). [Pg.102]

The mechanistic rate law is not applicable to processes in the subsurface, if we assume only that chemically-controlled kinetics occur and neglect the transport kinetics. Instead, apparent rate laws, which comprise both chemical and transport-controlled processes, are the proper tool to describe reaction kinetics on subsurface soil constituents. Apparent rate laws indicate that diffusion and other microscopic transport phenomena, as well as the structure of the subsurface and the flow rate, affect the kinetic behavior. [Pg.102]

Based on these rate laws, various equations have been developed to describe kinetics of soil chemical processes. As a function of the adsorbent and adsorbate properties, the equations describe mainly first-order, second-order, or zero-order reactions. For example. Sparks and Jardine (1984) studied the kinetics of potassium adsorption on kaolinite, montmorillonite (a smectite mineral), and vermiculite (Fig. 5.3), finding that a single-order reaction describes the data for kaolinite and smectite, while two first-order reactions describe adsorption on vermiculite. [Pg.102]

For practical purposes, it is not only important to know how much of a fluorinated surfactant is needed to reduce surface tension to a desired value. The time required to decrease surface tension is also highly significant. Many industrial processes do not allow sufficient time for a surfactant to attain equilibrium and depend on the kinetics of surfactant adsorption. [Pg.133]

The time required for surface tension reduction depends on diffusion processes involved in surfactant adsorption. Kinetic models for surfactant adsorption divide the adsorption process into two steps [67]. The first step is the transport of the surfactant to the subsurface, driven by a concentration gradient or hydrody- [Pg.133]

The kinetics of surfactant adsorption depend on the surfactant structure. Fluorination of the hydrophobe increases the rate (dyldt) of surface tension decrease, but the time needed to attain equilibrium may not be affected considerably. The surface tensions of sodium perfluorooctanesulfonate and its hydrocarbon- [Pg.134]

Dikhtievskaya and Makarevich [75] examined perpendicular and lateral interactions of fluorinated surfactants, /i-C3F70[CF(CF3)CF20], —CF= C(CF3)C00NH4, at the air-water boundary. The time required to attain equilibrium surface tension increased with increasing fluoroalkyl chain length or decreasing surfactant concentration. [Pg.135]

Equilibration is more rapid above cmc than below cmc [61], [Pg.135]


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. [Pg.709]

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. [Pg.293]

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]. [Pg.1289]

Detailed derivations of the isothemi can be found in many textbooks and exploit either statistical themio-dynaniic methods [1] or independently consider the kinetics of adsorption and desorption in each layer and set these equal to define the equilibrium coverage as a function of pressure [14]. The most conmion fomi of BET isothemi is written as a linear equation and given by ... [Pg.1874]

Theoretical Approaches to the Kinetics of Adsorption, Desorption, and Reactions at Surfaces... [Pg.439]

A. N. Semenov, J. F. Joanny. Kinetics of adsorption of linear homopolymers onto flat surfaces Rouse dynamics. J Physique II 5 859, 1995. [Pg.625]

Kinetics of Adsorption and Desorption and the Elovich Equation C. Aharoni and F. C. Tompkins... [Pg.426]

Two general models can describe the kinetics of adsorption. The first involves fast adsorption with mass transport control, while the other involves kinetic control of die system. Under the latter (and Langmuirian) conditions, the surface coverage of tlie adsorbate at time t, Tt, is given by. [Pg.39]

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. [Pg.173]

Figure 11.1 Kinetics of adsorption of CO at a Pt catalyst from a 0.01 M methanol solution at different potentials, (a) Pt black catalyst, with Pt loading 0.8 mg cm . (b) Pto.sRuo.s black catalyst, with catalyst loading 0.8 mg cm (0.1 M H2SO4, T = 298K). Figure 11.1 Kinetics of adsorption of CO at a Pt catalyst from a 0.01 M methanol solution at different potentials, (a) Pt black catalyst, with Pt loading 0.8 mg cm . (b) Pto.sRuo.s black catalyst, with catalyst loading 0.8 mg cm (0.1 M H2SO4, T = 298K).
While very limited data ate presented here, the kinetics of adsorption/decomposition of formic acid molecules [Rice et al., 2002] have been measured by BB-SFG, as shown in Fig. 12.14. A Pt(lll) electrode and a 0.1 M H2SO4 electrolyte containing 0.1 M formic acid were used. The families of spectra at 0.200, 0.025, and 0.225... [Pg.392]

Eeulner P, Menzel D. 1985. The adsorption of hydrogen on Ru(OOOl) Adsorption states, dipole moments and kinetics of adsorption and desorption. Surf Sci 154 465. [Pg.500]

The kinetic of adsorption charging of the surface of semiconductor under relaxation of biographic surfacing charge... [Pg.45]


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