Adsorption of organic solutes

In this review we concentrate on the studies that attempt to elucidate the importance of carbon surface properties in controlling the equilibrium uptakes of aromatic and aliphatic adsorbates. Rather than comparing model parameters, such as Langmuir or Freundlich constants, we examine the uptakes at comparable equilibrium concentrations and attempt to rationalize the differences observed under different conditions and on different adsorbents. 

nm% concluded that the resulting coverage (380 mVg) is lower than the BET (Ni) area but still so high as to suggest contribution [of pore filling] from pores. Similar comments and (re)calculations were made by Mattson et al. [330J. 

More specifically, we highlight the issues that should contain the answers to the following questions  

What is the maximum uptake of a given adsorbate on a high-surface-area carbon  

What fraction of the carbon surface is covered by the adsorbate at the maximum uptake  

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The mechanisms of adsorption of organic solutes—including hydro-phobic, polar, and ionic species—onto surfaces have been summarized previously (2 5). Assuming that the various adsorptive mechanisms act independently, the free energy of adsorption (AGa[Pg.192]

The studies reported in this paper have focused on more complete elucidation of the nature of the interaction between the hydronium ion and active carbon. Both rate and extent of reaction have been studied as a function of several variables to obtain data which ultimately should contribute to a meaningful interpretation of pH effects on adsorption of organic solutes by active carbon. 

Solid-Phase Extractions Using XAD Resins. The Amberlite XAD series (Rohm and Haas Co., Philadelphia, PA, USA) have been most often used for isolation of marine DOM by SPE. XAD resins are nonionic macroporous copolymers that differ in pore size, surface area, and polarity. Their generally large specific surface areas and more-or-less reversible adsorption of organic solutes from aqueous solution have made them well-suited for isolation of selected fractions of DOM from natural waters. Even though XAD resins have been used far more often to... 

Equilibrium capacity for adsorption of organic solutes on carbon can be predicted to increase with decreasing temperature since adsorption reactions are exothermic. The differential heat of adsorption, AH, is defined as the total amount of heat evolved in the adsorption of a definite quantity of solute on an adsorbent. Heats of vapor phase adsorption... 

Derylo-Marczewska and Jaroniec [28] have reviewed the adsorption of organic solutes from dilute solutions and have provided a useful compilation of published experimental data for both single- and multisolute adsorption isotherms on carbonaceous adsorbents. They also presented a survey of theoretical approaches used to describe the solute adsorption equilibria, including the Polanyi adsorption model, the solvophobic interaction model, the Langmuir adsorption theory, the vacancy solution model, as well as considerations based on the energetic heterogeneity of the adsorbent. In particular, these authors emphasize the... 

Based on extensive experimental evidence regarding the importance of pH and surface chemistry, it is obvious that the same arguments discussed in Section III.B in the context of adsorption of inorganic solutes are applicable also-—and indeed are required—for understanding the adsorption of organic solutes, many of which are weak electrolytes. What does need careful consideration is the answer to the following two questions ... 

Based on such comparisons and taking into account typical uptakes of aromatic solutes (of the order of 0.005 mol/g under most favorable conditions), it is tempting to postulate that the dispersion interactions—which allow a much more effective utilization of the carbon surface—may be the dominant driving force for the adsorption of organic solutes. In particular, aromatic solutes have a natural affinity for the graphene layers on the carbon surface (see Fig. 3) because of the possibility of 7i-7t overlap. Indeed, 7C-7t interactions and 7t-cation complexation are currently very powerful concepts and popular re.search topics [75-77,714,88,715-717,89,90,718,719]. (Thus, for example, the landmark paper by Hunter and Sanders [74] currently has close to 800 citations in the Science Citation Index.)... 

Adsorption of Organic Solutes from Dilute Aqueous Solutions... 

The study of a particular adsorption process requires the knowledge of equilibrium data and adsorption kinetics [4]. Equilibrium data are obtained firom adsorption isotherms and are used to evaluate the capacity of activated carbons to adsorb a particular molecule. They constitute the first experimental information that is generally used as a tool to discriminate among different activated carbons and thereby choose the most appropriate one for a particular application. Statistically, adsorption from dilute solutions is simple because the solvent can be interpreted as primitive, that is to say as a structureless continuum [3]. Therefore, all equations derived firom monolayer gas adsorption remain vafid. Some of these equations, such as the Langmuir and Dubinin—Astakhov, are widely used to determine the adsorption capacity of activated carbons. Batch equilibrium tests are often complemented by kinetics studies, to determine the external mass transfer resistance and the effective diffusion coefficient, and by dynamic column studies. These column studies are used to determine system size requirements, contact time, and carbon usage rates. These parameters can be obtained from the breakthrough curves. In this chapter, I shall deal mainly with equilibrium data in the adsorption of organic solutes. 

The irreversible adsorption of organic solutes, which is of great importance in the regeneration of the adsorbents, is due to stronger interactions than dispersion or hydrophobic interactions. In the case of aromatic compounds such as phenol, it could involve a charge-transfer mechanism between the carbon surface and the adsorbate and/or its polymerization under certain experimental conditions. Therefore, further research is warranted in this area. 

Finally, Chapters 24—27 deal with the environmental apphcations of carbons as adsorbents for the removal of pollutants from aqueous solutions. These four chapters are highly complementary. Thus, Chapter 24, which addresses the problems associated with the removal of inorganic species, finds its alter ego in Chapter 25, which deals with the adsorption of organic solutes from dilute aqueous solutions. Both chapters provide insights into the fundamental reasons for the performance exhibited by a carbonaceous adsorbent. The global topic of water purification using carbons as adsorbents is addressed in Chapter 26, which... 

Adsorption of organic solutes at the surface of suspended particles, that is, the mineral water interface, can be also characterized by specific coordinalive... 

Adsorption of organic solutes, control, 260 Adsorption of surfactants cation enhancements, 261 conceptual models, 261 Adsorption option in SOLMINEQ.88, description, 124—125 Adsorptive additivity, description, 272 Al, thermodynamic properties, 415 Aluminate ion... 

Here the adsorption of the carbamate insecticides and 1-naphthol on kaolinite and bentonite was studied. Adsorption isotherms for Sevin, 1-naphthol, baygon, pyrolan, and dimetilan on bentinite and kaolinite (hydrogen forms) were determined. The adsorption isotherms for the bentonite system are found in Figure 6. These isotherms are similar to those usually encountered in the majority of cases of adsorption of organic solutes from dilute solutions. These types of isotherms are represented according to the classical Freundlich equation ... 

In the adsorption of inorganic solutes, the main fundamental challenge remains how to "activate" the entire surface to achieve maximum removal efficiencies. In the adsorption of organic solutes, the influence of carbon surface chemistry is decidedly more complex. Both electrostatic and dispersive interactions can influence or control the equilibrium uptake of a weak aromatic electrolyte. 

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