Inorganic solutes, adsorption

Whether studying DOM at the level of elemental composition or at the level of functional groups and structural subunits, it is clearly advantageous to be able to isolate DOM from water and to remove any inorganic solutes that were present in the water. A variety of methods have been used for this purpose, among which the most common are methods in which DOM is extracted from water by adsorption on XAD resins and methods in which water is removed from DOM using UF, NF, or RO. 

Adsorption of both organic and inorganic solutes from the aqueous phase has been a very important application of activated carbons. With current increasing emphasis on the more thorough removal of pollutants from potable and waste waters, the use of carbons and the demands placed on their performance are expected to increase. Many buyers of activated carbon will not be able to afford its underutilization or inefficient use. A similar situation, greatly underutilized carbon adsorbents, exists in liquid chromatography applications, and it has been... 

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. 

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 ... 

The simple theoretical description of the adsorption from solutions on solids can be useful for characterizing sorption properties of inorganic sorbents. Such properties as the energetical and structural heterogeneities, surface phase capacity, specific surface area, pore size distribution curves and others are very important with regard to wide application of inorganic adsorbents on laboratory and industrial scales. 

The interaction between mobile phase and stationary phase should not be overlooked. For example, many of the very widely used HPLC columns consist of silica particles coated with chemically bonded hydrocarbons to give a nonpolar reverse phase stationary phase. However, there are generally a number of free silanol (-Si-OH) groups that can play a significant role in the adsorption of solutes. The addition of various mobile phase modifiers (e.g., inorganic buffer ions) can lead to interaction with these groups and alter the nature of the solutes adsorption. This is particularly important in the elution of... 

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 from solution has already been mentioned in Chapter 4 relating to the specific surface area of colloidal silica. These and other methods have also been applied to silica powders and gels. Adsorbates include organic compounds from organic and aqueous solutions as well as organic and inorganic cations and the OH ion from water. Polar compounds in nonpolar (hydrocarbon) solvents are adsorbed on the polar SiOH surface. Adsorption on the dehydrated siloxane surface is not as well understood. However, there is evidence that in water, the siloxane surface is hydrophobic. It adsorbs only hydrophobic materials or groups. 

Adsorption of Inorganic Solutes from Aqueous Solution... 

In wastewater treatments, special attention is given to removal of dangerous inorganic material such as the heavy metals, including mercury, chromium, molyMenum, cobalt, nickel, copper, cadmium, lead, uranium, gold, arsenic, barium, iron and vanadium. Whereas the forces of adsorption from the gas phase are well understood, adsorption from solution of inorganic species is not so well understood. So, it is most pertinent to enquire about the sites which are responsible for adsorption of inorganic species. 

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. 

Adsorption of organic solutes is just as important as inorganic solutes, from the point of view of environmental control. Here, adsorption of organic compounds and of aromatics in particular, is a complex interplay of electrostatic and dispersive interactions. This is... 

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