Solvent solid adsorbents

SOLVENT Choice. Solvent extraction is limited to water immiscible solvents. Solid adsorbents do not have this limitation, so miscible solvents, desirable for subsequent analytical or bioassay purposes, can be used. For example, DMSO is preferred for mutagenicity screening and has been used to elute the adsorbed organic material (211-213, 216, 235, 328). For analytical purposes, acid, base, and neutral eluents can be employed for on-column fractionation of the adsorbed organic solutes (78, 80,196). 

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. 

Adsorption The design of gas-adsorption equipment is in many ways analogous to the design of gas-absorption equipment, with a solid adsorbent replacing the liqiiid solvent (see Secs. 16 and 19). Similarity is evident in the material- and energy-balance equations as well as in the methods employed to determine the column height. The final choice, as one would expect, rests with the overall process economics. 

Adsorption, which utilizes the ability of a solid adsorbent to adsorb specific components from a gaseous or a liquid solution onto its surface. Examples of adsorption include the use of granular activated carbon for the removal of ben-zene/toluene/xylene mixtures from underground water, the separation of ketones from aqueous wastes of an oil refinery, aad the recovery of organic solvents from the exhaust gases of polymer manufacturing facilities. Other examples include the use of activated alumina to adsorb fluorides and arsenic from metal-finishing emissions. 

Separation can also be accomplished by solvent extraction, adsorption, and crystallization. Solvent extraction is accomplished by selectively dissolving certain hydrocarbon components. Adsorption is similar to solvent extraction but uses a solid to separate out various components selectively based on their tendency to adhere to the surface of the solid adsorbent. Crystallization uses the differing melting points of the components during cooling, which causes some of its compounds to solidify or crystallize, and separate out of the liquid. 

I first approached my theoretition friend and co-worker of many years, R. Simha, for statistical-mechanical assistance, and we obtained further the cooperation of H. Frisch, then just completing his Ph.D. at the Polytechnic Institute. The model we evolved was that of a macromolecule in solution colliding first with one of its segnents with a solvent-solid interface, becoming adsorbed when a complicated set of energetics becomes negative. 

Process modihcations use tetraethylene glycol as the extraction solvent and a mixture of light aliphatics and benzene as the wash solvent to the main extractor. Water condensate from the steam distillation is used to extract residual solvent from the raffinate and extract streams, so distillation for drying the extraction solvent is eliminated. Solids are removed from the recycled extraction solvent by filtration, while acids and high molecular weight fractions are removed by a solid adsorbent bed. Obviously, these processes are expensive and complicated. Most of the effort focuses on the recovery and not on the extraction. 

Comparison to Solvent Extraction. The methodology for using solid adsorbents is described in detail in a later section. Briefly, it involves adsorption and desorption processes. In the adsorption process, the liquid matrix is water containing very small amounts of organic... 

Additional Background. Two other theoretical considerations provide background for a better understanding of the use of solid adsorbents for analytical and bioassay purposes. These considerations are irreversible adsorption and concentration plus solvent transfer. 

Concentration Plus Solvent Transfer. Concentration of the organic solutes is essential to the determination of many organic contaminants present in water at very trace levels. The solvent transfer is needed for implementation of the separation and detection schemes that do not tolerate the water matrix. For bioassay work, concentration and solvent transfer are also needed because the amounts are too low for direct testing of the water solutions, and dimethyl sulfoxide. (DMSO) is the preferred solvent. In bioassay studies that involve animal exposure, the concentration scheme must accommodate very large volumes of water. Theoretically and practically, these elements of the analytical and bioassay methodologies can be achieved by using solid adsorbents, especially synthetic polymers. 

Solvents Used for Desorbing (Eluting) Organic Compounds from Solid Adsorbents and Corresponding References... 

By analogy to solvent extraction, the column containing the solid adsorbent corresponds to the separatory funnel containing the immiscible organic solvent. The transfer of the solute to the solid adsorbent occurs in an unattended operation requiring no manual effort or additional equipment such as the shakers used in solvent extraction or the distillation apparatus used in some of the automatic extraction devices. This simplicity allows for facile automation either off-line or on-line with the separation and detection procedure (495, 512, 536). 

EMULSION Free. The solvent extraction of many water solutions, particularly waste waters and biofluids, is a prolonged and often frustrating procedure because of the formation of emulsions. This problem is obviated when solid adsorbents are used because phase separation is inherent to the procedure. 

A fourth conclusion, based on the advantages of the use of solid adsorbents, is the gradual replacement of solvent extractions with solid phase extractions. The movement toward this replacement is already evidenced by the commercial availability of several different cartridges of bonded phases and high-surface-area synthetic polymers. 

One problem associated with using solid adsorbents to dry solvents is disposal of the used adsorbent, especially if large quantities of solvent are dried. Usually the adsorbent can be reactivated by careful heating in a stream of inert gas or under vacuum. (If the adsorbent was used to dry olefins or other peroxide-containing solvents, any peroxides must first be destroyed see Section 4.I.D.) Reactivating large quantities of adsorbent, however, can be just as difficult as proper disposal. 

If a liquid is used as die mobile phase, the technique used is liquid chromatography (LC). The solid adsorbent is constrained in a tube or column through which the liquid mobile phase flows. Any number of solvents, buffer solutions, or supercritical fluids can be used as liquid mobile phases. High-pressure liquid chromatography (HPLC) is used if pressure is needed to force die liquid phase through the tube. If the liquid phase moves over a thin adsorbent surface propelled by capillary action, die technique used is thin-layer chromatography (TLC). In general, two types of surfaces are used as the solid phase. 

For the samples that will be subjected to other (so-called interactive) LC techniques, the next question involves the nature of the solvent in which the sample has been or can be dissolved. If this is a non-polar solvent, such as n-hexane, then the sample solution is compatible with Normal Phase LC (NPLC), in which mobile phases with a relatively low polarity are used in combination with more polar stationary phases (see section 3.2.3). In this form of chromatography solid adsorbents (such as silica or alumina) may be used as stationary phases (LSC). Alternatively, polar chemically bonded stationary phases may be used (see section 3.2.2). 

In table 2.2 values are given for a variety of materials, including both typical solvents and typical stationary phases. The inclusion of the latter involves some rigorous assumptions, because the simple definition above (eqn.2.1) bears no relevance for solid adsorbents. Nevertheless, by looking at table 2.2 the usefulness of the solubility parameter as a quantity to describe polarity in quantitative terms becomes instantly apparent. The... 

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