Rapid kinetics, sorption process

For a first assessment of the performance of the different materials, batch experiments were carried out. The kinetics of the sorption processes of arsenic onto the different materials should give an indication of their efficiency. Figure 1 shows the results for the measured As(V) concentrations in dependence on time. The activated carbon gives poor results, as expected. However, the Zr loaded activated carbon shows a rapid reaction. The zirconyl ions at the surface of the activated carbon are a highly efficient phase for the sorption of arsenate. The half-life of this sorption reaction was < 10 min. 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

The kinetics of sorption can be considered as the sum of two processes 1) rapid sorption by labile sites which are in equilibrium with solutes dissolved in bulk solution, and 2) hindered sorption by sites which are accessible only by slow diffusion. Alternatively, sorption kinetics can be modeled by a radial diffu-sional process into spherical sorbents. The slow sorption process prevents complete equilibration within one day, the time used in typical batch experiments. Because the apparent rate of diffusion decreases with increasing hydrophobicity, time to equilibrium is longer for highly hydrophobic compounds. 

Reaction kinetics. The time-development of sorption processes often has been studied in connection with models of adsorption despite the well-known injunction that kinetics data, like thermodynamic data, cannot be used to infer molecular mechanisms (19). Experience with both cationic and anionic adsorptives has shown that sorption reactions typically are rapid initially, operating on time scales of minutes or hours, then diminish in rate gradually, on time scales of days or weeks (16,20-25). This decline in rate usually is not interpreted to be homogeneous The rapid stage of sorption kinetics is described by one rate law (e.g., the Elovich equation), whereas the slow stage is described by another (e.g., an expression of first order in the adsorptive concentration). There is, however, no profound significance to be attached to this observation, since a consensus does not exist as to which rate laws should be used to model either fast or slow sorption processes (16,21,22,24). If a sorption process is initiated from a state of supersaturation with respect to one or more possible solid phases involving an adsorptive, or if the... 

When the kinetics of a sorption process do appear to separate according to very small and very large time scales, the almost universal inference made is that pure adsorption is reflected by the rapid kinetics (16,21,22,26). The slow kinetics are interpreted either in terms of surface precipitation (20) or diffusion of the adsorbate into the adsorbent (16,24). With respect to metal cation sorption, "rapid kinetics" refers to time scales of minutes (16,26), whereas for anion sorption it refers to time scales up to hours TT, 21). The interpretation of these time scales as characteristic of adsorption rests almost entirely on the premise that surface phenomena involve little in the way of molecular rearrangement and steric hindrance effects (16,21). 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

It is this combination that scientists are currently searching for, and utilizing, to advance studies of sorption—desorption processes in natural soil systems, where rapid kinetics have often gone overlooked. As stated above, the acquisition of time-resolved data is critical for elucidating the pathways that govern kinetically controlled processes. Next, we review several of the most novel techniques currently being used to study sorption-desorption phenomena either in situ or in real time, or both. Some of these techniques are so new that they have not yet found widespread or even minimal use in the Earth and environmental sciences. Where appropriate, we illustrate examples from the environmental sciences however, because some of these techniques represent the future of this type of investigation, examples have been taken from more disparate literature. 

Weakly Interactive liquids. Such solvents do not provide sufficient free volume for polymer chain mobility characteristic of the rubbery state ( S). The PET/methanol system Illustrates this amorphous PET imbibed with methanol at s 25 C remains noncrystalline Indefinitely (20), indicating severely restricted chain mobility even in the swollen state. For such systems, the kinetic restrictions to chain rearrangement prevent the rapid achievement of equilibrium in surface layers ( ). As a result, the solvent surface concentration Increases slowly during the sorption process. Several authors (12, 17,28-30) have suggested specific relationships governing the time dependence of the surface concentration in such cases we have employed a simple exponential Increase, viz. ... 

On the other hand, a classification of a sorption process on the basis of kinetics data must be conditioned by other chemical properties of the phosphate-soil mixture. For example, if the soil solution is supersaturated initially with respect to some phosphate solid, precipitation is likely to influence the sorption reaction from the beginning. If the soil minerals have a low degree of crystallinity and/or a high degree of hydration, precipitation may be the dominant sorption mechanism even in the rapid stage.In general, low phosphate concentrations and well-crystallized, relatively unhydrated soil minerals tend to favor adsorption as the phosphate reaction mechanism. Other chemical properties, such as the pH value of the soil solution and the kinds of metals in soil clay minerals, exert a quantitative influence on the rapid stage of phosphate sorption, as do such physical properties as temperature. ... 

In contrast, at temperatures below Tg, we have the so-called Case II and Super Case II transport, the other extreme, in which diffusion is very rapid compared with simultaneous relaxation processes. Sorption processes may be complicated by a strong dependence on swelling kinetics. Finally we have anomalous diffusion, which occurs when the diffusion and relaxation rates are comparable. 

Ionization, sorption, volatilization, and entrainment with fluid and particle motions are important to the fate of synthetic chemicals. Transport and transfer processes encompass a wide variety of time scales. Ionizations are rapid and, thus, usually are treated as equilibria in fate models. In many cases, sorption also can be treated as an equilibrium, although somtimes a kinetic approach is warranted (.2). Transport processes must be treated as time-dependent phenomena, except in simple screening models (.3..4) ... 

The main reasons for investigating the rates of solid phase sorption/desorption processes are to (1) determine how rapidly reactions attain equilibrium, and (2) infer information on sorption/desorption reaction mechanisms. One of the important aspects of chemical kinetics is the establishment of a rate law. By definition, a rate law is a differential equation [108] as shown in Eq. (32) ... 

Though this system is perhaps an extreme example of slow sorption kinetics, it illustrates that the assumption of rapid equilibrium between the sediment and aqueous phases is questionable. The importance of such an observation to the investigation of hydrolysis kinetics in sediment/water systems must be emphasized. Certainly, any model of hydrolysis kinetics in sediment/water systems must include explicit expressions for the kinetics of the sorption/desorption process. 

Finally, we note that the x and solubility parameters of the O-butylated extract are noticeably absent in Table VI. We have attempted to analyze the sorption kinetics according to the Berens-Hopfenberg model in order to correct for adsorption effects, but the treatment yielded unreasonable x parameters. The reason for this is not clear, but we believe it may be due to the fact that di sion into the extract is so rapid. Hole-filling and swelling may have comparable rates so that a separation of the two processes is not possible. 

Karickhoff (1980) and Karickhoff et al. (1979) have studied sorption and desorption kinetics of hydrophobic pollutants on sediments. Sorption kinetics of pyrene, phenanthrene, and naphthalene on sediments showed an initial rapid increase in sorption with time (5-15 min) followed by a slow approach to equilibrium (Fig. 6.7). This same type of behavior was observed for pesticide sorption on soils and soil constituents and suggests rapid sorption on readily available sites followed by tortuous diffusion-controlled reactions. Karickhoff et al. (1979) modeled sorption of the hydrophobic aromatic hydrocarbons on the sediments using a two-stage kinetic process. The chemicals were fractionated into a labile state (equilibrium occurring in 1 h) and a nonlabile state. 

A number of kinetically based models have been used to study soil-pesticide reactions. In many cases, sorption of pesticides has been treated as a rapid-equilibrium, single-valued, reversible process. Some of these models are briefly outlined below. 

Figure 7. Adsorption properties of ferro-carbon particles. (A) Absorption spectra of supernatant fluid after mixing methylene blue solution with the adsorbents and magnetic sedimentation of them. FC-2,3,4 adsorb methylene blue more efficiently, than Cefesorb. (B) Adsorption isotherm of methylene blue on FCM. (C) Kinetics of doxorubicin sorption on FC-4. The adsorption process is rapid, and it takes about 1 h to reach saturation. (D) Kinetics of doxorubicin desorption from FCA. The drug is slowly released in about 14—15 days.

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

Considerable sorption occurs before the first measurement can be made, particularly if batch and flow techniques are employed where the fastest that a measurement can be made is about 15 seconds. For such rapid reactions, chemical relaxation techniques, and preferably real-time molecular-scale techniques, can be used. The latter are discussed later in the chapter. One might ask why it is important to measure such reactions if they are so rapid. Since the reactions are occurring so far from equilibrium, back reactions are insignificant and one can determine chemical reaction rates, devoid of mass transfer processes. Therefore, chemical kinetic measurements are being made, and details about molecular processes and mechanisms can be ascertained. 

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