Adsorption of phenols on activated carbon

Arafat et al. [4] studied the effect of the concentration of inorganic electrolyte on adsorption of benzene, toluene and phenol from aqueous solution at pH 11.6 on one commercial and two modified activated carbons and obtained very different results for these three adsorbates. The uptake of benzene was rather insensitive to the ionic strength. The uptake of toluene systematically decreased when the ionic strength increased. Finally the uptake of phenol was enhanced on addition of 0.5 mol dm KCl, but further addition of salt depressed the uptake and with 0.8 mol dm"" KCl the uptake dropped below that observed at low ionic strength. Adsorption of phenol on activated carbons was recently studied by other research groups [12,13], but without emphasis on the possible effects of pH dependent surface charging. 

FIGURE 6.3 Adsorption isotherms of o-cresol. (a) F400, (b) ACC-10, and (c) ACC-15. (Reprinted from Chemosphere, 55, Lu, Q. and Sorial, G.A., Adsorption of phenolics on activated carbon—Impact of pore size and molecular oxygen, 671-679, Copyright 2002, with permission from Elsevier.)... 

Lu, Q. L. and G. A. Sorial. 2004. Adsorption of phenolics on activated carbon—Impact of pore size and molecular oxygen. Chemosphere 55(5) 671-679. 

Figure 25-3 Adsorption isotherms of phenol on activated carbons with different degrees of oxidation. (Adapted from Ref. [19].)...

Adsorption of phenol and its derivatives from aqueous solutions on active carbons and carbon blacks has been the subject matter of a large number of investigations. Jaroniec and coworkers,Enrique et al. Worch and Zakke, and Magne and Walker studied the adsorption of several phenols from aqueous solutions and found that the adsorption was partly physical and partly chemical in character. Aytekin, ChapUn, and Kiselev and Krasilinkov observed that the adsorption isotherms of phenol from aqueous solutions were step-wise, suggesting the possibility of rearrangement of phenol molecules in the adsorbed phase and their interaction with active sites on the carbon surface. Morris and Weber, however, found that the adsorption isotherms of phenols on active carbons show two plateaus, even... 

Abuzaid and Nakhla, and Vidic et al. found that the adsorption of phenol by activated carbons from aqueous solutions in the presence of molecular oxygen in the test environment resulted in a threefold increase in the adsorption capacity of the carbon. This has been attributed to the oxygen induced polymerization reactions on the surface of the carbon. Juang et al. studied liquid-phase adsorption of eight phenohc compounds on a PAN-based activated carbon Fiber in the concentration range of 40 to 500 mg/L and observed that the chlorinated phenols showed better adsorption than methyl substituted phenols. Moreno-CastiUa et al. studied the adsorption of several phenols from aqueous solutions on activated carbons prepared from original and deminerahzed bituminous coal and found that the adsorption capacity depended upon the surface area and the porosity of the carbon, the solubility of the phenolic compound, and the hydrophobicity of the substituent. The adsorption was attributed to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol. 

The adsorbent bed at breakthrough actually comprises an equilibrium zone and a mass-transfer zone. The system is shown schematically in Fig. 15.21 in which a stable breakthrough curve is depicted moving at uniform velocity through an adsorber and the stoichiometric transfer front superimposed on the actual transfer front. The results for the adsorption of phenol onto activated carbon are shown in Fig. 15.22. The typical S-shaped breakthrough curves are shown at four bed heights. Q = 400 X 10 kg phenol/kg solution and F = 1.67 X 10 mVs. 

Vidic, R. D., M. T. Suidan, and R. C. Brenner. 1993. Oxidative coupling of phenols on activated carbon— Impact on adsorption equilibrium. Environmental Science Technology 27(10) 2079-2085. 

Fig 9 3 Adsorption isotherm of phenol on activated carbon from alcohol-water mixtures of different ratios... 

Caturla, F., Martin-Martinez, J.M., Molina-Sabio, M., Rodriguez-Reinoso, F., and Torregrosa, R. Adsorption of substituted phenols on activated carbon, / Colloid Interface Sci, 124(2) 528-534, 1988. 

Moreno-Castilla et al. [69] have shown that the adsorption of substituted phenols on activated carbons depends on solution pH. Thus at acidic pH the amount ad.sorbed remained practically con.stant or increa.sed slightly with increasing pH. When the pH increased further, there was a decrease in the amount adsorbed the pH at which this decrease took place depended on the difference between the external and internal surface charge density as measured by electrophoretic and titration measurements, respectively. A sharp turn toward a more substantive discussion of coupled pH and surface chemistry effects thus occurred in the mid-1990s, and these publications are analyzed below. The seeds for such a discussion were planted much earlier, however, and we analyze first how and why it took decades for them to flourish. 

Thus, the adsorption of substituted phenols on activated carbon depends on the solution pH [36, 37]. Results found [37] (Table 25.3) showed the uptake to be maximal at acidic pH because the phenols are undissociated and the dispersion interactions predominate. At basic pH, however, the uptake is lower because of electrostatic repulsions between the negative surface charge and the phenolate anions and between phenolate—phenolate anions in the solution. The pH at which the uptake decreases depends on the adsorptive pK and the difference between the pHp c and the pHjgp. 

Moreno-Castilla, C., Rivera-Utrilla, J., L6pez-Ram6n, M.V., et al. (1995). Adsorption of some substituted phenols on activated carbons from a bituminous coal. Carbon, 33, 845-51. 

For the adsorption of micro-organics (p-nitrophenol, benzoic acid) in fixed-bed columns, the half breakthrough time increases proportionally with increasing bed depth but decreases inversely proportionally with increasing water flow rate [54,55]. By studying the adsorption of chloroethylenes on activated carhon fibers, Sakoda et al. [56] determined a linear relationship between the overall mass transfer coefficient (Kfu) and the flow rate Uo. The influence of temperature on file dynamic adsorption of phenol on fibrous activated carbon has also been demonstrated [57]. 

FIGURE 7.13 Adsorption isotherms of different phenols on activated carbon. (From Singer, P.C. and Yen, C. Yu, in Activated Carbon Adsorption, I.H. Suffet and M.J. McGuire, Eds., Ann Arbor Science Publishers, Ann Arbor MI, 1980, Vol. I, p. 167. With permission.)... 

Arafat studied the effect of adding KCl to aqueous solutions of phenol, benzene, and toluene on their adsorption by activated carbons associated with varying amounts of surface oxides and observed that the salt effect was different in different cases. The observed influences in different cases were interpreted in terms of the charge neutralization between the carbon surface and the adsorbate molecules and on the basis of the adsorption of water. The adsorption of water on the carbon surface was found to be CTucial, especially in those carbons, which contained larger amounts of surface oxygen groups. 

In addition, the solubility of phenolics in water is generally regarded as an important factor for their adsorption. It is believed that the lower the solubility of phenolics, the easier the attachment to the activated carbon surface, and as a result, a higher adsorptive capacity can be achieved. However, the adsorptive capacity of phenolics did not follow the order of their water solubility except 2-nitrophenol. They found this phenomenon can be explained by the significant different molecular structures of phenolics. 2-Methylphenol and 2-ethylphenol are three dimensional while 2-chlorophenol and phenol are two dimensional. The adsorbate dimensions played a more important role in the adsorption of phenolics on ACFs due to the narrow PSD of ACFs. As shown in Figure 6.2, the differences of adsorptive capacity on ACC-10 were small as compared to F400. 

Mangun, C. L., Daley, M. A., Braatz, R. D. and Economy, J., Effect of pore size on adsorption of hydrocarbons in phenolic-based activated carbon fibers. Carbon, 1998,36(12), 123 129. 

Diffusion-type models have been used for the adsorption of lead, copper, p-nitrophenol, phenol, p-bromophenol, p-toluene sulfonate and dodecyl benzene sulfonate on activated carbon (Hashimoto etal., 1977 Xiu and Li, 2000 Chen and Wang, 2004 Crittenden and Weber, 1978), and ion exchange of ammonia, lead, and other heavy metals on clinoptilolite (Inglezakis and Grigoropoulou, 2003 Cincotti et al, 2001 Semmens et al, 1978 Cooney et al, 1999). 

It is apparent from the Xm values listed in Table V that capacities for adsorption of the organic pesticides on active carbon are quite large, greater on a molar basis than phenol and sulfonated 2-dodecylbenzene, and greater on a weight basis than any of the three other compounds. On the other hand, the fo"1 values, except for parathion, indicate approach to saturation adsorption only at relatively high residual concentrations. 

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