Sorption processes reversibility

Desorption is the reverse of the sorption process. If the pesticide is removed from solution that is in equdibrium with the sorbed pesticide, pesticide desorbs from the sod surface to reestabUsh the initial equdibrium. Desorption replenishes pesticide in the sod solution as it dissipates by degradation or transport processes. Sorption/desorption therefore is the process that controls the overall fate of a pesticide in the environment. It accomplishes this by controlling the amount of pesticide in solution at any one time that is avadable for plant uptake, degradation or decomposition, volatilization, and leaching. A number of reviews are avadable that describe in detad the sorption process (31—33) desorption, however, has been much less studied. 

These are defined as anionic dyes with substantivity for cellulosic fibres applied from an aqueous dyebath containing an electrolyte. The forces that operate between a direct dye and cellulose include hydrogen bonding, dipolar forces and non-specific hydrophobic interaction, depending on the chemical structure and polarity of the dye. Apparently multiple attachments are important, since linearity and coplanarity of molecular structure seem to be desirable features (section 3.2.1). The sorption process is reversible and numerous attempts have been made to minimise desorption by suitable aftertreatments (section 10.9.5). The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper. 

The presence of suspended solid materials increases the extent of LAS biodegradation [13,28], but the rate of the process remains invariable. The influence of the particulate material is due specifically to the increased density of the microbiota associated with sediments. However, suspended solids may also reduce the bioavailability of IAS as a result of its sorption onto preferential sites (e.g. clays, humic acids), although this is a secondary effect due to the reversibility of the sorption process. Salinity does not affect IAS degradation directly, but could also reduce LAS bioavailability by reducing the solubility of this molecule [5], Another relevant factor to be taken into account is that biodegradation processes in the marine environment could be limited by the concentration of nutrients, especially of phosphorus and nitrogen [34],... 

The need to remove mercury from natural gas has been known for many years. UOP is active as the only supplier to date to offer truly regenerable mercury adsorbents. The removal of Fig by UOP s HgSIV is actually a weak reversible chemi-sorption process. 

In order to assess the feasibility of any nuclear waste disposal concept, mathematical models of radionuclide sorption processes are required. In a later section kinetic descriptions of the three common sorption isotherms (3) are compared with experimental data from the mixing-cell tests. For a radionuclide of concentration C in the groundwater and concentration S on the surface of the granite, the net rate of sorption, by a first-order reversible reaction, is given by... 

Appropriate SPE sorbent selection is critical to obtaining efficient SPE recovery of semivolatile organics from liquids. Henry [58] notes that an SPE sorbent must be able to sorb rapidly and reproducibly, defined quantities of sample components of interest. Fritz [73] states that successful SPE has two major requirements (1) a high, reproducible percentage of the analytical solutes must be taken up by the solid extractant and (2) the solutes must then be easily and completely eluted from the solid particles. The sorption process must be reversible. In addition to reversible sorption, SPE sorbents should be porous with large surface areas, be free of leachable impurities, exhibit stability toward the sample matrix and the elution solvents, and have good surface contact with the sample solution [68,73],... 

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) ... 

Several polymer properties are important in determining the ability to sorb vapors. The glass transition temperature, Tg, is the temperature at which a polymer changes from glassy to rubbery, as described in Chapter 4. Above Tg, (in the rubbery state), permeability is governed entirely by diffusional forces and sorption proceeds rapidly and reversibly. The sorption process is very much like absorption into a liquid and, as discussed later in the context of sorption mod-... 

Solute retention in reversed-phase HPLC is dependent on the different distribution coefficients established between a polar mobile and a nonpolar stationary phase by the peptidic components of a mixture. Although there are many similarities between reversed-phase HPLC separations of peptides and the classical liquid-liquid partition chromatographic methods, it is debatable whether the sorption process in reversed-phase HPLC arises due to partition or adsorption events, i.e., whether the nonpolar stationary phase functions as a bulk liquid or as an adsorptive monolayer. These aspects and the theoretical models for reversed-phase HPLC are discussed in a subsequent section. 

It was evident in the earliest profiles obtained for dissolved °Th that this prediction does not hold, in that concentrations of dissolved °Th were observed to increase with depth in a nearly linear fashion (Bacon and Anderson, 1982 Nozaki et al., 1981). These results were interpreted to indicate that sorption processes are a reversible reaction in which particulate thorium may be released back into solution, as represented by k-i in Figure 1 (Bacon and Anderson, 1982 Nozaki et al., 1981). Neglecting advection and diffusion, mass balance equations for dissolved and particulate thorium can be written, respectively, as... 

Figure 3. Domain-complexion diagrams (at left) and phase distribution (at right, condensate in black, vapour in blank) within the pores (sites circles, bonds cylinders) on planes of 3D porous networks for actual states of diverse sorption processes, a) Boundary ascending (BA) curve on network la, b) boundary descending (BD) curve on network 2a, c) primary ascending (PA) curve on network 3a and d) primary descending (PD) curve on network 4a. Rc is the critical radius of curvature at the present state of the sorption process and Rc is the critical radius of curvature at the point of reversal for scanning curves. Shaded areas (pores filled with condensate) delimited by full lines in the complexion diagrams represent current states of the sorption systems, broken lines delimit states at the points of reversal.

The elution of analytes from reversed-phase sorbents is a rather simple process and consists of choosing a nonpolar solvent to disrupt the van der Waals forces that retain the analyte. Because the sorption process is a partitioning process, it is usually only necessary to allow the eluting solvent to have intimate contact with the bonded phase (e.g., C-18) in order to elute the analytes from the sorbent. Because the bonded phases consist of a silica matrix, they have an increased polarity compared to the original hydrophobicity of the C-18 alkane. Thus, the elution solvent must be capable of mutual solubility with the silica surface, as well as with the C-18 or other bonded phase. 

It is to this topic of solute preferential sorption in reverse osmosis that this paper is dedicated. Specifically, this discussion will involve a description of solute preferential sorption, an overview of the literature in the area, and finally a presentation of some recent work on the removal of aromatic hydrocarbons from water. The significance of this work is at least two-fold. From a practical point of view the classes of solutes which demonstrate preferential attraction to the membrane material tend to be organic compounds and the removal and recovery of these solutes from water is environmentally and economically important. From a theoretical point of view an understanding of the phenomena involved is essential to the achievement of a fundamental description of the RO process. Although this paper deals solely with aqueous solutions and cellulose acetate membranes, it Is important to recognize that the concepts discussed can be extended to Include other membrane materials and non-aqueous systems. 

Sorption processes are very effective and include adsorption/desorption (reversible binding at the solid-water interface), absorption (diffusion of pollutants into the solid matrix), precipitation and coprecipitation (incorporation into a freshly formed solid), and occlusion (sequestration of adsorbed pollutants during mineral growth). The most important factors for retention processes are pollutant concentration, the composition of the solid matrix, solution composition (e.g., complexing agents) and E/pH conditions (Brady and Boms 1997). 

Adsorption of vapors on test chamber walls has been previously described by means of models including two or three rate constants for adsorption/desorption processes in the ease of dynamic experiments (Dunn et al., 1988 Colombo et al., 1993) and with three adsorption/desorption constants in the case of static experiments (Colombo et al., 1993). Two rate constants describe a reversible sink whereas three rate constants describe a reversible and an irreversible (i.e. leak type) sink. However, these models did not adequately describe the sorption process(es), especially in the case of long-term tests, as resulted from two observations (Colombo et al., 1993) (a) the model with three sorption rate constants (reversible + irreversible sink) provided a better description of the experimental data than the one-sink model and (b) desorption experiments following adsorption gave strong indications that the irreversible sink was in fact slowly rever-... 

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... 

For the sorption process linearity and reversibility are assumed, i.e. the concentration of the pollutant adsorbed in the soil material is at every instant proportional to its concentration in the soil water ... 

Sorption by zeolites offers a powerful means for the development of methods for super purification (super desulphurization with simultaneous drying) of gases with low-level H2S content (lean gases) and which, at the same time, may have high water (H2O) concentration, up to saturation of the gas by H2O vapour. This principle, if it uses hydrophilic zeolites, is characteristic of reversibility of the sorption process, high separation performance and long lifetime of the sorbent itself. At the same time resulting waste products may not represent environmental... 

In this context, Di Toro et al. [1] made batch-experiments on the equilibrium of sorption of hexachloro-biphenyl on montmorillonite and constructed a sorption model. They additionally considered the partial reversibility of this sorption process. A. M. Rodrigo and... 

---