Sorption processes surface diffusion

The rates and mechanisms of sorption reactions at the mineral/water interface are critical in determining the mobility, speciation, and bioavailability of metals in aqueous and terrestrial environments. This chapter discusses nonequilibrium aspects of metal sorption at the mineral/water interface, with emphasis on confirmation of slow sorption mechanisms using molecular approaches. It is shown that there is often a continuum between sorption processes, viz, diffusion, sites of varying energy states, and nucleation of secondary phases. For example, recent molecular level in-situ studies have shown that metal adsorption and surface precipitation can occur simultaneously. 

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). 

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). 

Sorption relates to a compound sticking to the surface of a particle. Adsorption relates to the process of compound attachment to a particle surface, and desorption relates to the process of detachment. Example 2.2 was on a soluble, nonsorptive spiU that occurred into the ground and eventually entered the groundwater. We will now review sorption processes because there are many compounds that are sorptive and subject to spills. Then, we can examine the solutions of the diffusion equation as they apply to highly sorptive compounds. 

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. 

Quantification of rate constants for this multistep process hinges on the assumed rate-controlling step. Depending on the steps that are assumed to dictate the rate, reaction rates or diffusion constants are calculated from the net kinetics of reaction or sorption. Various studies have assumed that either of two steps are rate controlling either the surface diffusion or the actual spillover from the source. All analyses have assumed a first-order dependence of the concentrations of atomic hydrogen for each step in the sequence. 

The main problem in the determination of association rates at the gas-liquid interface is the interplay of the mass transport effects and the biospecific sorption process. The experimental studies show that both effects are involved in the binding of antigen to the antibody attached to a surface. The variations of the value of the apparent adsorption rate constant with various experimental conditions reveal the importance of the nonideal effects in such experiments. To determine the effective rate of interaction, it is important both to minimize the diffusion resistances and to estimate this contribution by increasing the amount of information. Studies with varying flow rates, particle sizes, ligand densities. 

To properly understand the fate of trace elements in soils, and particularly to comprehend their mobility with time, kinetic investigations are necessary (Sparks, 1995). Their sorption by soils is often observed to be a multistep process involving an initial fast sorption followed by slow sorption, probably by diffusion into pores of inner soil surfaces (Kinniburgh and Jackson, 1981), due to the presence of surface sites of different reactivity and site preferences (Ainsworth et al., 1994). 

The second stage features the moisture sorption of fibers, which is relatively slow and takes a few minutes to a few hours to complete. In this period, water sorption into the fibers takes place as the water vapor diffuses into the fabric, which increases the relative humidity at the surfaces of fibers. After liquid water diffuses into the fabric, the surfaces of the fibers are saturated due to the film of water on them, which again will enhance the sorption process. During these two transient stages, heat transfer is coupled with the four different forms of liquid transfer due to the heat released or absorbed during sorption/desorption and evaporation/condensation. Sorption/ desorption and evaporation/condensation, in turn, are affected by the efficiency of the heat transfer. For instance, sorption and evaporation in thick cotton fabric take a longer time to reach steady states than in thin cotton fabrics. 

Dzombak, D.A., and Hudson, R.J.M. 1995. The contributions of diffuse layer sorption and surface complexation, in C.P.H. Huang, C.R. O Melia, and J.J. Morgan, eds., Aquatic Chemistry. Interfacial and Interspecies Processes, Advances in Chemistry Ser., vol. 244, Washington D.C., American Chemical Society, Chap. 3, pp. 59-93. 

If the surface is first saturated with a monolayer of protein exposed to steady-state concentration cQ, and then is exposed to a second treatment at concentration 2c0, a second front emerges. The second profile represents the situation where no net protein is adsorbed and thus, in principle, is representative of the diffusion-shifted flow pattern of the nonadsorbed protein. Figure 7 shows both the initial (cQ) and second (2c0) fronts and the subtraction curve which is very close to the ideal step function. If the data are interpreted as solution-borne molecules passing over an inert surface, then (a) adsorption must be essentially instantaneous and (b) the surface must become covered by exhausting the concentration of solute at the front as it moves down the column. The slope of the difference profile should represent the rate of uptake of material on the column, and that is essentially infinite on the time scale of the experiment. The point of inflection of the subtracted front indicates the slowing of the sorption process due to filling of sites on the surface. 

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