Sorption processes types

The fourth type of mechanism is exclusion although perhaps inclusion would be a better description. Strictly, it is not a true sorption process as the separating solutes remain in the mobile phase throughout. Separations occur because of variations in the extent to which the solute molecules can diffuse through an inert but porous stationary phase. This is normally a gel structure which has a small pore size and into which small molecules up to a certain critical size can diffuse. Molecules larger than the critical size are excluded from the gel and move unhindered through the column or layer whilst smaller ones are retarded to an extent dependent on molecular size. 

Similarly to N, most S pools are found in organic form in forest floor and soil humus. However, unlike nitrogen, there are important abiotic processes, especially sulfate sorption processes, which play a critical role in regulating sulfate dynamics in forest ecosystems. An example of this type of exposure pathway was shown in the Habbard Brook whole-tree harvesting experiment, where the decrease in sulfate output from the watershed was attributed to sulfate adsorption, which was enhanced by soil acidification from nitrification (see above). 

We begin with a discussion of the most common minerals present in Earth s crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is (1) a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH (2) the nature of the electrical double layer and how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid and (3) dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. We finish with a discussion of some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces. 

The type of clay present in a soil influences triazine sorption (Brown and White, 1969). Furthermore, variations in surface properties among different samples of the same clay type greatly influence sorption. For instance, sorption of atrazine on 13 clay samples, of which smectite was the dominant mineral, ranged from 0% to 100% of added atrazine (Figure 21.7), and was inversely correlated to the surface charge density of the smectites (Laird et al., 1992). Such data illustrate the complexity of sorption processes and the reason why simple predictive models relying on % OC, % clay, or surface area normalizations may fail to predict accurately the sorption of triazine by a particular soil. 

Very thorough investigations of Eq. (1) subject to conditions (4) and (5) have been made by Crank and others for various assumed forms of D (cj), of which an excellent summary has been given by Crank (1956). The information we need here is not the detailed mathematical expressions for such solutions of Eq. (1) but the characteristic features of sorption processes predicted from this set of equations. Customarily, the sorption processes in which D is a function of ct only and the initial and boundary conditions are given by Eqs. (4) and (5) are referred to as of the Fickian type. Moreover, it is often said that such processes are controlled by the Fickian diffusion mechanism1. 

In section 2.3 we will summarize some representative features of this type of sorption process. 

Basic features of sorption processes of the Fickian type have been clarified by Crank and coworkers through extensive mathematical studies of Eq. (1). The following gives a summary, of the features of particular importance. 

To obtain quantitative information about sorption processes controlled by time-dependent diffusion Crank (1953) solved numerically Eq. (1), coupled with Eqs. (4), (5) and (14), for some assumed forms of >i(Cj), De(Cl) and a(cj. His results agreed reasonably with many typical non-Fickian features known at that time (Park (1953)]. However, when a new type of non-Fickian behavior, now generally called the "two-stage type, was discovered in 1953 by Long and his coworkers, it soon became evident that the concept of time-dependent diffusion was too simple to explain every non-Fickian behavior. This situation remains unaltered at present, and so we shall not go further into this subject. 

Equilibrium between solution and adsorbed or sorbed phases is a condition commonly used to evaluate adsorption or sorption processes in soils or soil-clay minerals. As previously stated, equilibrium is defined as the point at which the rate of the forward reaction equals the rate of the reverse reaction. Two major techniques commonly used to model soil adsorption or sorption equilibrium processes are (1) the Freundlich approach and (2) the Langmuir approach. Both involve adsorption or sorption isotherms. A sorption isotherm describes the relationship between the dissolved concentration of a given chemical species (adsorbate) in units of micrograms per liter (pg L 1), milligrams per liter (mg L-1), microequivalents per liter (pequiv L-1), or millimoles per liter (mmol L-1), and the sorbed quantity of the same species by the solid phase (adsorbent) in units of adsorbate per unit mass of adsorbent (solid) (e.g., pg kg-1, mg kg-1, peq kg-1, or mmol kg 1) at equilibrium under constant pressure and temperature. Sorption isotherms have been classified into four types, depending on their general shape (Fig. 4.13) ... 

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 ... 

Warshawsky, A. in Ion Exchange arui Sorption Processes in Hydrometallurgy (M. Streat and D. Naden, eds.), Wiley, New York, 1987, p. 208. Bondarenko. S. S., Popov, V. M. and Strepetov, V. P. Main Types of Resources and Seals of Processing Hydromineral Raw Materials in Developed and Developing Countries, Review VIEMS, Moscow, 1986. (Russian) Armstrong, E. F. and Miall, L. M. Raw Materials from tbe Sea, Chemical Publ., Brooklyn, NY, 1946. 

The results of sorption and desorption of uranium are shown in Table 2. As shown in Table 2, the desorbed percentages of uranium from the granite sur ces are somewhat greater than sorbed percentages of uranium on granite surfaces. Thus, the results show that the sorption process of U(V1) is a little irreversible for the two types of granite surfaces depending on pH and surface type. This may be due to the frict that small amount of a mineral such as chlorite mainly contribute to the sorption of uranium and the uranium sorbed on this mineral is hard to be desorbed from the mineral sur ce. 

Fig. 1 A quantitative measure of various interactions in ion-exchange type sorption processes.

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