Solid phase adsorbents

The first is pollutant transport across the boundary layer or surface film to the exterior surface of the adsorbent solid phase particle (i.e.,soils/sediments and their components). 

The second is pollutant transport within the pores of the adsorbent solid phase particle, from the exterior of the particle to the interior surfaces of the particle. Similarly, a pollutant may be transported along surfaces of pore walls. 

If the adsorbent-adsorbate solid phase at equilibrium comprises only the two species, SR C(s) and SR(s), the straightforward methods of chemical thermodynamics can be applied to derive an experimentally accessible relationship between either /sr c or /SR and the conditional equilibrium constant, KMlsc (Sposito, 1983). 

The principles of solid-phase extraction (SPE) or liquid-solid extraction (LSE) are similar to that of liquid-liquid extraction, involving a partitioning of compounds between two phases [22]. In SPE, the compounds to be extracted are partitioned between a solid and a liquid. The interactions responsible for the separation between the liquid and solid phase are non-covalent (ionic, van der Waals, hydro-phobic) and can be modulated by changing the physical properties of the eluent (liquid phase) and the adsorbent (solid phase). 

In ion pair separations with a hydrophobic solid stationary phase, most often alkyl-bonded silica, the binding capacity is limited. The principles of retention in such a system can be expressed as follows (1) the retention of an ionic solute, HA, is accompanied by the displacement of a system ion of the same charge or by the binding of a counterion, X and (2) the solute and the components in the mobile phase may compete for the binding capacity of the adsorbing solid phase. 

In Table 19.1, various kinds of adsorbents (solid phases) used in column chromatography are listed. The choice of adsorbent often depends on the types of compounds to be separated. Cellulose, starch, and sugars are used for polyfunctional plant and animal materials (natural products) that are very sensihve to acid-base interactions. Magnesium silicate is often used for separating acetylated sugars, steroids, and essential oils. Silica gel and Florisil are relatively mild toward most compounds and... 

The determination of R (see Eq. 7.25) by a measurement of the partition coefficient k in the laboratory, by some type of sorption experiment for the determination of adsorbed solid-phase concentration versus solution-phase concentration in equilibrium (so-called isopleths or isotherm), however, supplies values of R which are not comparable with the actual in field retardation factors in a mineral liner, because the specific surface susceptible to the pollutant is very different in both cases. Therefore, based on diffusion coefBcients in free aqueous solutions, the effective diffusion coefficients of organic substances for water-saturated mineral liners were estimated by using a tortuosity factor F= 1/3 (Muller et al. 1997a). Table 7.6 shows the results for various soil materials. 

Adsorbent Solid phase with external and internal surfaces exposed to the molecules of a gas or liquid phase. 

These concluding chapters deal with various aspects of a very important type of situation, namely, that in which some adsorbate species is distributed between a solid phase and a gaseous one. From the phenomenological point of view, one observes, on mechanically separating the solid and gas phases, that there is a certain distribution of the adsorbate between them. This may be expressed, for example, as ria, the moles adsorbed per gram of solid versus the pressure P. The distribution, in general, is temperature dependent, so the complete empirical description would be in terms of an adsorption function ria = f(P, T). 

It is noted in Sections XVII-10 and 11 that phase transformations may occur, especially in the case of simple gases on uniform surfaces. Such transformations show up in q plots, as illustrated in Fig. XVU-22 for Kr adsorbed on a graphitized carbon black. The two plots are obtained from data just below and just above the limit of stability of a solid phase that is in registry with the graphite lattice [131]. 

Solid-Phase Extractions In a solid-phase extraction the sample is passed through a cartridge containing solid particulates that serve as the adsorbent material. For liquid samples the solid adsorbent is isolated in either a disk cartridge or a column (Figure 7.17). The choice of adsorbent is determined by the properties of the species being retained and the matrix in which it is found. Representative solid adsorbents... 

In gas-solid extractions the sample is passed through a container packed with a solid adsorbent. One example of the application of gas-solid extraction is in the analysis of organic compounds for carbon and hydrogen. The sample is combusted in a flowing stream of O2, and the gaseous combustion products are passed through a series of solid-phase adsorbents that remove the CO2 and 1T20. 

Selected Adsorbents for Solid-Phase Extraction of Liquid Samples... 

In a solid-phase extraction the analytes are first extracted from their solution matrix into a solid adsorbent. After washing to remove impurities, the analytes are removed from the adsorbent with a suitable solvent. Alternatively, the extraction can be carried out using a Soxhlet extractor. 

A solid-phase extraction in which the solid adsorbent is coated on a fused-silica fiber held within a syringe needle. 

Spectroscopy is basically an experimental subject and is concerned with the absorption, emission or scattering of electromagnetic radiation by atoms or molecules. As we shall see in Chapter 3, electromagnetic radiation covers a wide wavelength range, from radio waves to y-rays, and the atoms or molecules may be in the gas, liquid or solid phase or, of great importance in surface chemistry, adsorbed on a solid surface. 

Combined Pore and Solid Diffusion In porous adsorbents and ion-exchange resins, intraparticle transport can occur with pore and solid diffusion in parallel. The dominant transport process is the faster one, and this depends on the relative diffusivities and concentrations in the pore fluid and in the adsorbed phase. Often, equilibrium between the pore fluid and the solid phase can be assumed to exist locally at each point within a particle. In this case, the mass-transfer flux is expressed by ... 

Solid phase concentration of adsorbate i, mols i / g solid... 

Hydroxyandrosta-4,6-dien-3-one. A suspension of 42 g of crude androsta-4,6-diene-3j ,17j -diol in 2000 ml of chloroform is treated with 250 g of activated, manganese dioxide. The mixture is then shaken vigorously for 15 min in a stoppered flask. The mixture is filtered and the manganese dioxide washed well with chloroform in order to elute material which initially remains adsorbed on the solid phase. The filtrate is concentrated to a pale yellow, crystalline residue. Recrystallization from acetonitrile gives 38 g (90%) of 17/ -hydroxyandrosta-4,6-dien-3-one as plates mp 211-214°. 

Lateral density fluctuations are mostly confined to the adsorbed water layer. The lateral density distributions are conveniently characterized by scatter plots of oxygen coordinates in the surface plane. Fig. 6 shows such scatter plots of water molecules in the first (left) and second layer (right) near the Hg(l 11) surface. Here, a dot is plotted at the oxygen atom position at intervals of 0.1 ps. In the first layer, the oxygen distribution clearly shows the structure of the substrate lattice. In the second layer, the distribution is almost isotropic. In the first layer, the oxygen motion is predominantly oscillatory rather than diffusive. The self-diffusion coefficient in the adsorbate layer is strongly reduced compared to the second or third layer [127]. The data in Fig. 6 are qualitatively similar to those obtained in the group of Berkowitz and coworkers [62,128-130]. These authors compared the structure near Pt(lOO) and Pt(lll) in detail and also noted that the motion of water in the first layer is oscillatory about equilibrium positions and thus characteristic of a solid phase, while the motion in the second layer has more... 

Figure 11.15 Cation-exchange mia O-LC analysis of a mixture of model proteins (a) the original sample consisting of myoglobin (M), cytochrome C (C) and lysozyme (L) (b) and (c) proteins adsorbed on to and then released from the polyaaylic acid coated fibre with exti ac-tion times of 5 and 240 s, respectively. Reprinted from Journal of Microcolumn Separations, 8, J.-L. Liao et al., Solid phase mia O exti action of biopolymers, exemplified with adsorption of basic proteins onto a fiber coated with polyaaylic acid, pp. 1-4, 1996, with permission from Jolm Wiley Sons, New York.

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

Equation (15) is derived under the assumption that the amount of adsorbed component transferred by flow or diffusion of the solid phase may be neglected. This assumption is clearly justified in cases of fixed-bed operation, and it is believed to be permissible in many cases of slurries or fluidized beds, since the absolute amount of adsorbed component will probably be quite low due to its low diffusivity in the interior of the catalyst pellet. The assumption can, however, be waived by including in Eq. (15) the appropriate diffusive and convective terms. 

The solid phase extraction cartridge (SPEC) is another somewhat vainglorious name given to a short inert plastic tube packed with an adsorbent, usually a reversed phase or an ion exchange resin. The particle size of the packing is often significantly larger than that used... 

The whole sample can be extracted where this is possible or, again, the two phases separated and both extracted by different techniques. As stated before, adsorption of the substances to be determined on the solid phase can become difficult when they are present at very low concentrations and the adsorbed material is a significant proportion of the total sample. 

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