Slow kinetics, sorption process

The mobilization of arsenic from the tailings material seems to be a slow and continuos process attributed to reduction of iron phases. The seepage water of the middle source contains arsenite as well as arsenate in high concentrations and seems to be the only water source in contact with the tailings material. The concentrations of arsenic downstream are still high and the immobilization process by precipitation of iron hydroxide and coprecipitation or sorption of arsenic is incomplete. A reason for this may be the slow kinetics of the oxidation process and the transport of fine grained hydroxide particles. These particles are mobile and can bind the arsenic (mainly as arsenate) too. 

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). 

For Am there is no evident particle size dependence. The kinetics are considerably slower than for Cs (and Sr). Possibly the sorption process is a volume dependent adsorption with contributions from ion exchange. The sorption and also the diffusion into the particles should be governed by the complicated hydrolysis reactions to be expected at environmental pH. Other tri- and tetra-valent elements would be expected to show a similar slow non-surface related sorption behaviour. 

Selim et al. (1976b) developed a simplified two-site model to simulate sorption-desorption of reactive solutes applied to soil undergoing steady water flow. The sorption sites were assumed to support either instantaneous (equilibrium sites) or slow (kinetic sites) first-order reactions. As pore-water velocity increased, the residence time of the solute decreased and less time was allowed for kinetic sorption sites to interact (Selim et al., 1976b). The sorption-desorption process was dominated by the equilib-... 

Investigations of redox processes in natural water systems have emphasized the disequilibrium behavior of many couples (e.g., 37). The degree of coupling of redox reactions with widely varying rates, and its effect on radionuclide transport in an NWRB needs to be considered. Because of the generally slow kinetics of autoxidation reactions, the potential surface catalyzed reduction of a radionuclide at low temperatures in the presence of trace levels of DO may explain certain sorption data (e.g., 38). 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

Not only thermodynamic factors, but also the slow kinetics ofhydrogen sorption are a barrier to practical application for many materials. In this section we will briefly discuss the expected impact of the particle size on kinetics. First, it is important to realize which step in the hydrogen sorption process is rate limiting, which depends not only on the type of material, but also on the specific experimental conditions. Taking the absorption ofhydrogen (which is usually slower than the desorption at a given temperature) as an example, the following steps can be discerned ... 

Essentially this is an alternative explanation for the large difference in uptake for the various gases where processes seem to be dominated by dipolar and London forces, as noted in related studies (5). This phenomenon has been attributed to activation energy barriers for diffusion of sorbate into the coal matrix (18). We have satisfied ourselves that there are no very slow processes in play and if this activation energy barrier exists, it must be essentially infinite in magnitude. We have analyzed the kinetics of the sorption processes in terms of diffusional mechanisms (19) with very little success. Alternatively, we have gone back to analyze our data in terms of mass action kinetics (20). All our data were acquired at constant (controlled) pressure conditions (21). If, indeed, the sorption... 

The concentration of the initially present slow-reacting Ni (Cj) was assumed to be the sum of extraction steps 3 and 5 (Zeien and Briimmer, 1989) measured in the bulk soil. They correspond to the specifically adsorbed ions as well as to the fraction bound by organic matter. For estimating the value of the buffer capacity b2, it was assumed that the underlying kinetically controlled and reversible sorption process was at equilibrium in the bulk soil, and the quotient... 

In Figure 5 is represented the normal sorption hysteresis of the hazardous coal sample. It is an open hysteresis function measured by Sartorius 4112 Type micro balance applicable to 12 MPa pressure fl.3]. The open hysteresis indicates that at very low equilibrium pressure range a great amount of methane remains in the inside structure of coal. This phenomenon cannot only be explained by very slow kinetic process of desorption because the remained amount of methane is independent of time required to formation of desorption equilibrium pressure. The remained amount of methane can better be emphasized by data shown in Figure 6. 

Modem microbalances do allow one to take weight data every tenth of a second. If the kinetics of a pure gas sorption process is slow compared to this time, it can be easily recorded. Examples have been given in the foregoing Sects. 2.3, 4.4. Here we only want to mention that curves depicting the mass of a sorbent / sorbate sample as function of time do not always show a simple exponential approach to an equilibrium state but may be much more complicated. An example of practical importance is chemisorption of SO2 gas on activated carbon (AC). Due to catalytic properties of the AC, SO2 can be converted via (SO4) to sulfuric acid (H2SO4) which at near ambient conditions periodically falls down in droplets from the carbon sample leading thus to saw-tooth like curves in the balance s recordings. 

Peak diffuseness may be a result of the kinetics of the sorption-desorption process (i.e., slow mass transfer or exchange at sorbent surfaces). Peak diffusion in this case is usually nonsymmetric because the rates of sorption and desorption are not the same. Band spreading due to the final rate of mass exchange is closely related to the diffusion phenomenon. Physical adsorption, for all practical purposes, is instantaneous. The overall process of sorption, however, consists of several parts (a) the movement of sorbate molecules toward the sorbent surface, resulting fi om intergrain diffusion (outer diffusion), (b) movement of sorbate molecules to the inside of pores (i.e., internal diffusion of the sorbate molecules in the pores and surface diffusion in the pores), and (c) the sorption process in general. 

The First-Order Kinetic Model. Karickhoff (1, 68) has proposed a two-compartment equilibrium-kinetic model for describing the solute uptake or release by a sediment. This model is based on the assumption that two types of sorption sites exist labile sites, S, which are in equilibrium with bulk aqueous solution, and hindered sites, Sjj, which are controlled by a slow first-order rate process. Conceptually, sorption according to this model can be considered either as a two-stage process ... 

The main objectives of this chapter are to (1) review the different modeling techniques used for sorption/desorption processes of organic pollutants with various solid phases, (2) discuss the kinetics of such processes with some insight into the interpretation of kinetic data, (3) describe the different sorption/ desorption experimental techniques, with estimates of the transport parameters from the data of laboratory tests, (4) discuss a recently reported issue regarding slow sorption/desorption behavior of organic pollutants, and finally (5) present a case study about the environmental impact of solid waste materials/complex... 

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. 

It can be proposed that superimposed upon the intrinsic random walk of molecules in the NFI channel network are processes of non-diffusional molecular re-orientation leading to an optimal sorbate arrangement. These processes are slow for the relatively "stiff" 2-butyne molecule (due to its triple bond) but fast for the "flexible" n-butane, i.e. the additional regime of sorption kinetics becomes observable if the time constant of diffusion (k /D) is small compared to the time constant of re-orientation. Since the latter process should be Independent of crystal size, size variation will give further evidence for appropriate systems. 

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