Adsorption of aromatics

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

McBride MB, Pinnavaia TJ, Mortland MM (1997) Adsorption of aromatic molecules by clays in aqueous suspension. In Suffet IH (ed) Fate of pollutants in the air and water environments. Wiley, New York, pp 145-154... 

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

The influence of the metal on electrosorption was also considered in the adsorption of aromatic compounds. Extensive studies of adsorption of different aliphatic and aromatic compounds on a polycrystalline Pt electrode in O.OIMHCI solution were done by Bockris etal. Using different techniques (radiotracer, FTIR, and ellipsometry), they were found that aromatic molecules are adsorbed parallely to the electrode surface and that the potential dependence of adsorption is symmetrical around the pzc. It was concluded that the dependence of adsorption on the potential is mostly determined by the interaction of water with the Pt surface. 

Ward and Getzen (1970) investigated the adsorption of aromatic acids on activated carbon under acidic, neutral, and alkaline conditions. The amount of 2,4-D (10 M) adsorbed by carbon at pH values of 3.0, 7.0, and 11.0 were 60.1,18.8, and 14.3%, respectively. 

E. Blomgren, J. O M Bockris, and C. Jesch, Adsorption of Aromatic Amines at the Interface of Mercury-Aqueous Solutions. J. Phys. Chem. 65 2000 (1961). 

A. Dark Adsorption of Aromatic Contaminants onto TiO2... 

Lewandowski and Ollis have proposed a simple kinetic model describing the transient photocatalytic oxidation of aromatic contaminants [50]. The model considered three chemical species an aromatic contaminant, preadsorbed onto the catalyst in the dark and refreshed continuously from the gas phase a strongly bound, recalcitrant reaction intermediate and final reaction products (CO or CO2), assumed for simplicity to be strictly gas-phase species. The model also assumed that two types of catalyst site were present on the photocatalyst surface, with the first suitable for the adsorption of aromatic contaminants, as well as reaction intermediates, and the second type considered to be more polar in nature, suitable only for adsorption of partially oxidized reaction intermediates. 

In the interpretation of the kinetics, it was concluded that a mechanism involving adsorption equilibrium between methylcyclohexane in the gas phase and methylcyclohexane molecules adsorbed on platinum sites was not very likely. If Eq. (1) were interpreted on such a basis, then b would be an adsorption equilibrium constant. From the temperature dependence of b, one would calculate a heat of adsorption of 30 kcal./mole, which seems too high for adsorption of methylcyclohexane molecules as such. Furthermore, the small inhibition by aromatics casts doubt on such a picture, since the extent of adsorption of aromatics would be expected to be considerably greater than that of methylcyclohexane molecules at equilibrium, in view of the unsaturated nature of aromatics. 

In view of the above considerations, it was proposed that adsorption equilibria are not established and that the small inhibiting effect of aromatics indicates that the rate of adsorption of aromatics is lower than that of methylcyclohexane. A simple kinetic scheme was proposed to account for the observed kinetics ... 

HZSM-5 Adsorption of aromatic compounds, temperatures up to 573 K TR Melsheimer and Ziegler (1992)... 

Equilibrium adsorption of light hydrocarbons and aromatics in zeolites has been investigated by many groups. Examples of adsorption investigations are given by Rebo et al. (12) and Zhu et al. (13-18). Adsorption of aromatics on ZSM-5 was investigated at low temperatures (< 473 K), and the adsorption isotherms were well approximated by the Langmuir model (12) ... 

Ti olecular sieves have been used thus far to separate distinct classes of organic compounds. The separation of n-para Bns from branched paraffins with 5A molecular sieves is well known. Selective adsorption of aromatics from mixed streams with lOX, 13X, and type Y sieves is perhaps less widely known. The use of lOX molecular sieves to separate mixtures of aromatics has been disclosed 1,2,3). These patents disclose separations of mixtures of monocyclic aromatics 1, 2) and dicyclic aromatics (3). These were the first indications that molecular sieves could separate compounds within a single chemical class. 

The EPR studies are carried out in a special cell allowing the adsorption of aromatics and screening the catalyst from the action of moisture or oxygen. The spectra are recorded continuously before and after anthracene adsorption. After equilibrium, the spin concentration is measured by comparing the radical cation EPR spectrum with that of calibrated DPPH solution. 

The brief review of the vast literature on the phenomenological aspects of adsorption of aromatic solutes has highlighted studies that provide clues, either explicitly or implicitly, to the optimization of carbon surface chemistry for removal of specific pollutants from aqueous streams. Here we make an attempt to synthesize the available information. In Section V we then offer suggestions regarding a comprehensive model of adsorption of organic (and inorganic) solutes. 

VI. ROLE OF DISPERSION INTERACTIONS IN THE ADSORPTION OF AROMATICS A CASE STUDY IN CITATION ANALYSIS... 

The complex role of surface chemistry in the adsorption of aromatics has been highlighted and (we hope) clarified in Section IV.B. The seed for the explanations offered there had been planted as early as 1968-1971 in a series of landmark papers by Coughlin and coworkers [450,331,332] and Mattson and coworkers [24,6]. In hindsight, however, it is obvious that the arguments presented in these two series of papers cannot both be correct. Here we quote the critical statements in the key papers and follow their fate with the help of the Science Citation Index. The statistics of their citations are summarized in Table 39. 

Kamegawa, K., Nishikubo, K., Kodama, M., et al. (2005). Aqueous-phase adsorption of aromatic compounds on water-soluble nanographite. Colloids Surf., 254, 31-5. 

Typical examples of the adsorption procedure are the irreversible attachement of metal-N4 complexes on HOPG and the adsorption of aromatic molecules for anchoring complex species. 

When the adsorption of aromatic weak electrolytes is governed by nonelec-trostatic interactions, such as tt-tt dispersion or hydrophobic interactions, the area of the adsorbent occupied by the adsorbate depends on the porosity of the former and the molecular size of the latter. Thus, adsorption from diluted aqueous solution and immersion calorimetry measurements [39] showed that phenol and m-chlorophenol are adsorbed as monolayers by both porous and nonporous carbons with basic surface properties, provided that the adsorptive is undissociated at the solution pH. This did not apply where molecular sieve effects reduced the accessibility of the micropore system. 

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