Adsorption-desorption reactions

These equations represent the adsorption-desorption reactions and the surface reaction E, P, and B are, respectively, ethene, propene, and 2-butene, and s represents an active site. If the surface reaction is rate deter-... 

Major problems are associated with using the linear distribution coefficient for describing adsorption-desorption reactions in groundwater systems. Some of these problems include the... 

It has been proposed that the precursor state [81, 82] for the adsorption-desorption reaction consists of weakly physisorbed CO. This can be CO sitting on an occupied site (COad-CO) or on an sterically unfavorable Pt site. According to Ertl [81], the desorption process occurs through a trapping mechanism on such sites if the surface is saturated by chemisorbed CO the desorption channel involves either a COad-CO potential well or a Pt-CO attractive well which is sterically weakened by the presence of pre-absorbed CO . 

Dang et al. (1994) observed that the experimentally determined solubility lines for Zn2+ in 14 soil solutions from southern Queensland with soil pH from 7.45-8.98 and 0.08-2.07% CaC03 were not undersaturated with respect to the solubility of any known mineral form of Zn. Therefore, they suggested that Zn2+ activity was mainly controlled by adsorption-desorption reactions in these soils. Similar observation on solubility of Cr(VI) in arid soils was reported by Rai et al. (1989). In the absence of a solubility controlling solid phase, Cr(VI) aqueous concentrations under slightly alkaline conditions may be primarily controlled by adsorption/desorption reactions (Rai et al., 1989). Chromuim(VI) is adsorbed by iron and aluminum oxides, and kaolinite and its adsorption decreases with increasing pH. 

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. 

Having chosen a particular model for the electrical properties of the interface, e.g., the TIM, it is necessary to incorporate the same model into the kinetic analysis. Just as electrical double layer (EDL) properties influence equilibrium partitioning between solid and liquid phases, they can also be expected to affect the rates of elementary reaction steps. An illustration of the effect of the EDL on adsorption/desorption reaction steps is shown schematically in Figure 7. In the case of lead ion adsorption onto a positively charged surface, the rate of adsorption is diminished and the rate of desorption enhanced relative to the case where there are no EDL effects. 

What remains is to relate the surface potential to activation potentials for the adsorption/desorption reaction steps. Defining the activation potentials as iji, ijjf for the activation required to overcome the EDL potential for the adsorption, desorption steps, respectively, allows the intrinsic rate constants to be directly related to the rate constants k, k (4), i.e.,... 

A mechanism is determined from these data by choosing one which is consistent with the overall equilibrium behavior and which correctly matches the rate relationships derived for the postulated mechanism e.g., assuming the bimolecular adsorption/desorption reaction mechanism, as given in Equation 1, and using the kinetic model described above, the following relationship between xp and reactant and product concentrations can be derived (see Appendix C) ... 

If this mechanism is consistent with the experimental relaxation data, then a plot of xp versus the expression in the brackets of Equation 35 will give a straight line with a slope of kjnt and an intercept at the origin. As shown in Figure 11, the data fit this proposed mechanism quite well. Values for i i0, reactant and product concentrations, and K nt input into Equation 35 are from the equilibrium modeling results calculated at each pH value for which kinetic runs were made. Normally a variety of different mechanisms are tested against the experimental data. Several other more complex mechanisms were tested, including those postulated for metal ion adsorption onto y-A O (7) however, only the above mechanism was consistent with the experimental data. Hence it was concluded that the bimolecular adsorption/desorption reaction was the most plausible mechanism for Pb2+ ion adsorption onto a-FeOOH. 

In an interesting analysis of the effects of reduction of dimensionality on rates of adsorption/desorption reactions (26), the bimolecular rate of 10 M- s- has been reported as the lower limit of diffusion control. Based on this value, the rates given in Table III indicate the desorption step is chemical-reaction-controlled, likely controlled by the chemical activation energy of breaking the surface complex bond. On the other hand, the coupled adsorption step is probably diffusion controlled. 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

The fast reactions of ions between aqueous and mineral phases have been studied extensively in a variety of fields including colloidal chemistry, geochemistry, environmental engineering, soil science, and catalysis (1-6). Various experimental approaches and techniques have been utilized to address the questions of interest in any given field as this volume exemplifies. Recently, chemical relaxation techniques have been applied to study the kinetics of interaction of ions with minerals in aqueous suspension (2). These methods allow mechanistic information to be obtained for elementary processes which occur rapidly, e.g., for processes which occur within seconds to as fast as nanoseconds (j0. Many important phenomena can be studied including adsorption/desorption reactions of ions at electri fied interfaces and intercalation/deintercalation of ions with minerals having unique interlayer structure. 

Suppose (X 2, (X2)2+, and (X)1 are the organic cations of interest which are involved in the adsorption/desorption reactions experiment. 

Adsorption/desorption Reactions that involve solute becoming chemically bonded to the surface of a solid. The reverse process releases solutes from the surface of a solid... 

Examples with the linear surface adsorption-desorption reactions... 

In the case of the full 2D problem with linear surface adsorption-desorption reactions (1), (2), (131) and (132), we present two tests. 

Linear surface adsorption-desorption reactions. Case A2 with the times of flow t — 100, 211 and 350 s... 

Figure 3 Comparison between the volume concentrations (1/H) c dz and for the linear surface adsorption-desorption reactions, Case A2, obtained using our effective problem (eff), average of the section of the concentration from the original problem (pbreeB) and the concentration coming from the simple average (moy) at time, t = 100 s.

Although Eh (redox potential) can affect both the adsorption/desorption reactions that can occur as well as the secondary phases that can form during CCB weathering, Theis Richter (1979) stated that their studies of ash disposal ponds showed that oxidizing conditions rather consistently prevail in the active leaching zone (i.e., the unsaturated zone above the water table). 

Fig. 10. Evaluation of kinetic parameters for the DOC model—HC adsorption/desorption (reaction R7 in Table II). Comparison of the measured and simulated outlet Ci0H22 concentrations in the course of the adsorption/desorption experiment. Synthetic gas mixture, other gases 6% C02, 6% H20, N2 balance, SV = 30,000 h 1 (Kryl et al., 2005). Reprinted with permission from Ind. Eng. Chem. Res. 44, 9524, 2005 American Chemical Society.

In this Chapter we introduce a stochastic ansatz which can be used to model systems with surface reactions. These systems may include mono-and bimolecular steps, like particle adsorption, desorption, reaction and diffusion. We take advantage of the Markovian behaviour of these systems using master equations for their description. The resulting infinite set of equations is truncated at a certain level in a small lattice region we solve the exact lattice equations and connect their solution to continuous functions which represent the behaviour of the system for large distances from a reference point. The stochastic ansatz is used to model different surface reaction systems, such as the oxidation of CO molecules on a metal (Pt) surface, or the formation of NH3. 

Let us study now a stochastic model for the particular a+ib2 -> 0 reaction with energetic interactions between the particles. The system includes adsorption, desorption, reaction and diffusion steps which depend on energetic interactions. The temporal evolution of the system is described by master equations using the Markovian behaviour of the system. We study the system behaviour at different values for the energetic parameters and at varying diffusion and desorption rates. The location and the character of the phase transition points will be discussed in detail. 

Sedlak and Andren (1991b) modeled hydroxyl radical reaction kinetics in the presence of particulate. They found that the reaction kinetics for PCB oxidation in the presence of particulate resulted from the complex interplay between solution-phase OH reactions and reversible adsorption-desorption reactions. A model predicting the reaction kinetics can be described by the following equation ... 

Theoretical Considerations. Hachiya et al. (1979), who studied adorp-tion—desorption kinetics of Pb2+ on a y—AI2O3, proposed the following adsorption-desorption reaction scheme ... 

Mechanism f Langmuir Type Adsorption-Desorption Reaction. Consider the reaction... 

Mechanism II Adsorption-Desorption Reaction Involving Ion Exchange with Two Protons. This mechanism was investigated similarly to mechanism I but did not appear to be operational. 

Mechanism IV Adsorption-Desorption Reaction in the Presence of a Proton Catalyst. The mechanism in which H+ acts as a catalyst is... 

Mechanism V Two-Step Adsorption-Desorption Reactions Involving Ion Exchange with a Proton. Hachiya etal. (1979) felt that the existence of two relaxations in Fig. 4.12 suggested that the Pb2+ adsorption-desorption reactions consisted of two processes, one fast and the other slow. Mechanism III could thus be further divided into two elementary steps ... 

Table 9.7 Examples of the adsorption-desorption reaction in equation (9.2) ...

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