Sorption step

In the amination of pyridine, only a very small amount of 4-aminopyridine is obtained (62HC(14,3)1). Acridine, with no available a-carbon atom, undergoes amination in the 9-position with extreme difficulty in aprotic solvents (72CHE1518). 

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Figure 7. Sorption steps and selective binding in case of simple polymers sorption processes take place comparable to effects occurring in separation columns. The different steps are demonstrated.

The qualitative analysis consists e.g. in stimulating pure sorption steps by using a step inlet disturbance in the fluid concentration of the corresponding species. The responses are different depending on whether the step is kinetically controlled or not (see Figure 2, 3). 

Figure 2. Transient responses when stimulating pure sorption step that is kineti-cally controlled (5). Key left, loading and right, unloading the storage.

Recent experiments have determined that the isotopic fractionation between adsorbed and dissolved Cr(VI) is very small (Ellis et al. submitted). In batch experiments, dissolved Cr(VI) was equilibrated with finely ground alumina or goethite. The results of these experiments should be appficable to natural settings, as oxides chemically similar to alumina and goethite provide the main adsorption sites for Cr(VI) anions. Because the analytical precision of the measurements was 0.2%o, it was necessary to amplify any isotopic shifts so that effects smaller than 0.1 %o could be detected. This was accompfished by repeating the sorption step ten or more times, so that a fractionation between dissolved and adsorbed Cr(VI) of 0.04%o was detectable at the 95% confidence level. 

Many investigators use pulse techniques in which a catalyst reacts with hydrocarbons, oxygen etc. separately in time. This can provide an insight into the nature and significance of the individual reaction and sorption steps, but it should be emphasized that selectivities and other data may be unrepresentative for conditions in a flow reactor. In particular, selectivities may be considerably lower under steady state conditions. If the selectivity differences between pulse and flow experiments are very large, a cyclic mode of operation may be attractive for the practical application of the catalyst concerned. Oxidation and reduction are then separated. 

Desorption kinetic studies were conducted with freshly contaminated soils and aged soils (i.e., soils that were allowed to have extended contact time of 3 months and 5 months during the sorption step). Three levels of contamination were used. To illustrate here, soil desorption kinetics data for 1,3-DCB with silty soil from the PPI site was plotted with the fraction of the contaminant released as a function of time in Fig. 5. Again, the results for other chemicals are not discussed here as the findings are very similar. As shown in Fig. 5, a substantial portion of the contaminant is released within the first 20-30 h, followed by a very slow release over a very long period. This slow release was observed over the entire duration of the experiment (100-450 h). Approximately 60% of 1,3-DCB was desorbed in the first 24 h for freshly contaminated soils. 

The sulfuric acid formed in the sorption step is reduced to sulfur on the carbon by a reaction sequence using hydrogen. This sulfur is then stripped from the carbon. The overall chemistry of the sulfur recovery is ... 

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

Fig. 11 shows a conceptualized process schematic where a composite membrane strip is continuously run through the contaminated sludge (sorption step) and an add bath (desorption step). Such a cyclic process configuration is relatively simple and can be implemented by using the composite membrane as a slow-moving belt. For a sludge containing heavy-metal hydroxide, say Me(OH)2, the process works in two steps ... 

This single-step process could be thought of as a single plate or single sorption step (Fig. 4.3). The word plate comes from the early theory used in chromatography to describe sorption and refers to distillation columns that contain plates. 

If we imagined that the same sorbent in the beaker in Figure 4.3 were placed into a column and the solute passed through the column then a series of sorption steps would occur. Rather than the general equilibration between all of the sorbent and the analyte at once, a series of equilibrations would occur as mobile phase gradually moved the analytes down through the column, and analyte is continually coming into contact with new layers or plates of sorbent deeper in the column (Fig. 4.4). 

The theory for sorption onto the fibers has been described in a study by Louch and co-workers (1992). According to the authors, SPME does not exhaustively extract the solute onto the fiber in most cases because there is only one theoretical plate or one sorption step. Rather, an equilibrium is developed between the aqueous concentration and the sorbed concentration. The number of moles of analyte on the fiber, n, is linearly related to the concentration in the aqueous phase by the following equation ... 

The sorbent bed volume. This value includes the portion of the bed which is not completely saturated when the sorption step is stopped as well as the void space in the bed. The mass transfer rates (transport processes) for sorption have a large influence on the bed size and dimensions. Some design approaches are based on sizing the unused portion of the sorbent bed, as discussed in Section 7.8. 

This phenomenon has been studied using a series of sorption and desorption steps.For example, in Figure 3.20a, a sequence of four sorption steps were carried... 

Figure 3.20 Sorption and desorption of phenanthrene from sediment, (a) Concentration in sediment as a function of equilibrium concentration in solution after 4 sorption steps followed by 49 repetitive desorption steps, (b) Concentration in solution after desorption. Numbers denote equilibrium time (days) for desorption steps. [Reproduced with permission from A. T. Kan, G. Fu, M. Hunter, W. Chen, C. H. Ward, and M. B. Thomson, Environ Sci. Technol. 32, 892 (1998). Copyright 1998, American Chemical Society.]...

It has been concluded from these sorption-desorption smdies that the irreversible compartment can be filled stepwise in a series of sorption-desorption steps. Once the maximum capacity of the irreversible compartment is attained subsequent sorption steps are completely reversible. It has been suggested that the irreversibility results from some conformational change or physical rearrangement of the organic matter during sorption that changes the environment of the sorbed compound and restricts its desorption. However, the namre of these changes are not known. A sorption isotherm has been proposed to account for both the reversible and... 

Solid-phase extraction has been called digital chromatography because its chief use is to retain and then remove analytes of interest in an on-off fashion. Outside of conducting breakthrough studies, the purely frontal chromatographic form has little practical value in TEQA. The elution step, in which an eluent whose polarity is near to that of the analyte, removes all analytes and can be viewed in a manner similar to that done for the sorption step ... 

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

The slow metal sorption step on many minerals and soils occurs over time scales of days and longer. This slow sorption has been ascribed to several mechanisms including interparticle or intraparticle diffusion in pores and solids, sites of low energy or reactivity, and surface precipitation/nucleation (49-51). 

Similar observations on sorption of divalent metal ions were made by Coughlin and Stone (55). They hypothesized that the slow sorption and desorption on goethite was a result of slow pore diffusion. Axe and Anderson (57) also found that sorption of Cd and Sr could be characterized by a model which included two steps a rapid reversible sorption step followed by a slow, rate-limiting process involving the diffusion of the cations through small pores existing along the surface. 

From simulated concentration curves the S/So-values can be extracted. The combination of sorption steps with chemical reaction leads to a larger decrease in selectivity at the same transport limitation level compared to a homogeneous first order reaction as it is indicated in Fig. 3 The relative adsorbabilities (eq.8)... 

Extraction of pesticide residues from liquid samples can be performed using a solid sorbent material. Currently available sorbent extraction techniques include (1) solid-phase extraction (SPE), (2) solid-phase microextraction (SPME), and (3) stir-bar sorptive extraction (SBSE). In the case of solid samples, a liquid extraction of pesticide residues (transfer into a solution) usually precedes the sorption step thus, it should be considered rather as a clean-up than an extraction. Matrix solid-phase dispersion (MSPD) represents a unique SPE approach that combines extraction and clean-up of solid or semisolid food samples in one step. In MSPD, the sample is mixed with a sorbent (Florisil, Cig, Cg) that serves as a solid support in sample disruption and dispersion. The resulting mix is packed into a column from which the analytes are eluted while separated matrix components are retained by the sorbent. The main drawbacks of this approach comprise rather small sample sizes ( 0.5g) and a relatively high consumption of expensive sorbents. 

After the sorption step, the column was washed with 8 M HCl (containing a little concentrated HNOg) until the effluent was practically free from -activity. 

In order to solve the humidity problem of most gas sensors the ICB Muenster developed and instrument called Air-Cbeck in which a short but efficient pre-sampling on Tenax material takes place. With this the sensitivity could be increased as needed, water no longer interferes and the selectivity could also be increased by analyzing the time course of the rapid thermal de.sorption step. The whole adsorption-desorption cycle can be performed automatically within less than 1 min ... 

Most organic compounds occur at extremely low concentrations in seawater. Their determination involves an extraction/sorption step to increase their concentration levels to the sensitivity range of instrumental analytical methods. Compared with other groups of analytes, the risk of adsorptive losses is most important for trace organic compounds. Hierefore, techniques are preferable which involve direct extraction within the sampler or in situ enrichment onto solid adsorbents. Using very large samplers with a comparably high ratio between volume and internal sampler surface, adsorption losses can be minimized but never excluded entirely. 

For the characterization of the sample with optical and X-ray scattering methods a 1 nun thick slice was cut from the aerogel with a diamond saw. The adsorption was performed by placing the cuvette for the optical measurement holding the sample next to a reservoir of 2-propanol for several hours. The amount of 2-propanol adsorbed was determined by weighing. After each sorption step the sample was sealed in the cuvette. 

CO2 adsorption at 29SK in carbon aerogels pyrolized at 1050 °C takes place via two sorption steps ... 

From Eq. (18), we may define the kinetic matrix element C, (surface chemical reactions and sorption steps)... 

In order to measure the solubility and diffiisivity of low molecular weight vapours in PTMSP, sequences of dififerential sorption experiments were performed. The mass uptake as a nction of time for each sorption step was measured and recorded using a Cahn microbalance. Details of the experimental equipment are discussed in a previous publication (15). Prof G.Costa of the CNR-IMAG kindly provided the polymer, which was synthesised using TaCU as catalyst, and had an intrinsic viscosity of 4.5 dl/g in toluene solution at 30 C. 

The vapour plmse pressure was controlled to a fixed value for each sorption step and measured with an accuracy of 0.1 mbar for n-pentane, ethanol and methanol. The use of a more precise manometer was needed for n-hexane and n-heptane sorption in order to measure the vapour phase pressure with an accuracy of 0.01 mbar. 

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