Adsorption of Inorganic Solutes

This is a topic of great practical interest because of water treatment and metal recovery applications. Its fundamental aspects are also important for the preparation of carbon-supported catalysts [22], where the catalyst precursor is typically dissolved in water prior to its loading onto the porous support. 

Although essential for life s metabolic processes, chromium in high concentrations can cause serious illnesses. In particular, Cr(Vl) accumulated in waste waters from steelwork, electroplating, leather tanning and chemical manufacturing plants can be carcinogenic. A substantial literature thus exists on the removal of chromium by adsorption on carbonaceous adsorbents [126,127,97,128-130.33, 131-139.124,140-143,125,144-156], 

Adsorption of both Cr(III) and Cr(Vl) species is known to be pH-dependent. From Huang s review not many general conclusions can be drawn except that Cr(Vl) can be readily reduced to Cr(III) at acidic conditions and that intermediate pH favors the removal of both Cr(lll) and Cr(VI) [97]. No specific reference 

Moreno-Castilla and coworkers [139,140] did clarify the relationship between carbon surface chemistry and chromium removal. Table 3 summarizes some of the key results. Upon oxidation of carbon M in nitric acid (sample MO), the surface has become much more hydrophilic and more acidic, and the uptakes increased despite a decrease in total surface area. The enhancement in Cr(III) uptake was attributed to electrostatic attraction between the cations and the negatively charged surface. The enhancement in Cr(VI) uptake (at both levels of salt concentration) was attributed to its partial reduction on the surface of carbon MO (perhaps due to the presence of phenolic or hydroquinone groups), which is favored by the lower pH. The increase in uptake on carbon MO with increasing NaCl concentration is consistent with this explanation, from a straightforward analysis of the Debye-Hvickel and Nemst equations the decrease in uptake on carbon M was attributed to the competition of specifically adsorbed Cl and CrOj- ions on the positively charged surface. 

The interest in adsorption of molybdenum stems primarily from its use as a carbon-supported catalyst [22]. The use of activated carbon for the removal of Mo-99 (used in nuclear medicine) has also been reported ] 158]. Only hexavalent Mo is stable under a wide pH range and in the absence of other complexing agents. At pH 8, the dominant species is M0O4-, while at very low pH there is precipitation of the hydrated oxide between these two extremes polymeric anions are formed [123,159]. 

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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 ... 

Surface Chemistry of Activated Carbons Adsorption of Inorganic Solutes... 

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. 

Adsorption of Inorganic Solutes from Aqueous Solution... 

Equilibrium uptakes of metallic cations, dyes, and even gold and anionic adsorbates are largely governed by electrostatic attraction or repulsion. It is suggested that further studies of the role of the carbon surface are needed to show whether this is indeed the case. If not, only then does the role of specific interactions (e.g. complex formation), or even nonspecific van der Waals interactions, become a reasonable alternative or complementary argument. Indeed, in the adsorption of inorganic solutes, the main fundamental challenge remains how to activate the entire carbon surface in order to achieve maximum removal efficiencies. The solution conditions, at which maximum uptake can be achieved, depend on the surface chemistry of the adsorbent. 

Schindler, P. W. in "Adsorption of Inorganics at Solid/Solution Interfaces" Anderson, M. Rubin, A., Ed. Ann Arbor Science ... 

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. 

Rubin, A. J., and D. L. Mi rci-r. 1981. Adsorption of free and complexed metals from solution by activated carbon. In Adsorption of inorganics at solid-liquid interfaces, ed M. A. Anderson and A. J. Rubin, pp. 295-325. Ann Arbor, Ml Ann Arbor Science. 

Adsorption of Inorganic Species from Aqueous Solutions... 

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... 

Shiao, S.Y. and Meyer, R.E., Adsorption of inorganic ions on alumina from salt solutions Correlations of distribution coefficients with uptake of salt, J. Inorg. Nucl. C/rem., 43, 3301, 1981. 

It is not my intention to present an exhaustive review here. A comprehensive and critical review is provided elsewhere [4]. Boehm has also presented a brief but authoritative review [5]. Instead I summarize here the issues that are now known to be essential for predicting adsorption uptakes, especially in aqueous solutions. The main features of carbon surface chemistry are presented first, and the consequent acid-base behavior of carbons is briefly discussed to illustrate their unique amphoteric character. (These same features should be important in the adsorption of gases and vapors, but a discussion of the relevant evidence, beyond that presented in conjunction with Fig. 1, is not within the scope of the present work.) In Sec. Ill it is shown that these phenomena govern the adsorption of inorganic compounds. In Sec. IV I argue that they are sometimes dominant in the adsorption of organic compounds, but more often they are only a part of the "whole story" ... 

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