Activated carbon basic groups

Adsorption by activated carbon is commonly employed for the removal of TNT from aq waste streams, eg, pink water formed in shell-loading operations. Low efficiency in regeneration of the carbon for reuse has led to a study of the factors involved (Ref 99), with conclusions as follows. The TNT is adsorbed at many of the numerous high-energy sites on the surface of the carbon. Basic materials, introduced during activation of the carbon by combustion and oxidation and also present at these sites, then induce oxidation-reduction reactions of the methyl with the nitro groups in the TNT. This is... 

Until more concrete structural information is obtained, the discussion on the catalytic mechanism of carbonic anhydrase must remain rather speculative. The experimental evidence requires the presence in the active site of a basic group being in some manner linked to the metal ion. This group is generally thought to play a critical role either as a nucleophile in a direct reaction with the substrate, or through general base catlysis. Several schemes for the function of carbonic anhydrase have been proposed (16, 41, 50, 78, 79) ... 

Nitriles (RCN) get hydrolysed to carboxylic acids (RC02H) in acidic or basic aqueous solutions. The mechanism of the acid-catalysed hydrolysis (Following fig.) involves initial protonation of the nitrile s nitrogen atom. This activates the nitrile group towards nucleophilic attack by water at the electrophilic carbon. One of the nitrile n bonds breaks simultaneously and both the n electrons move onto the nitrogen yielding a hydroxyl imine. This rapidly isomerises to a primary amide which is hydrolysed under the reaction conditions to form the carboxylic acid and ammonia. 

All of these features considerably improve the sorption capacity of carbons, and their capacity to remove contaminants and pollutants interacting with the surface of carbons in a dispersive way [177], In addition, active carbon contains heteroatoms such as oxygen, and, to a smaller degree, nitrogen and sulfur. These atoms are bound to the activated carbon surface in the form of functional groups, which are acidic or basic, giving the activated carbon surface an acidic or basic character, respectively [173,178], It is as well necessary to state that the chemical heterogeneity of the carbon surface is mostly the result of the presence of heteroatoms [175],... 

It has been known for many years that activated carbons appear to exhibit both acidic and basic properties (Boehm, 1966 Puri, 1970 Bansal et al, 1978a,b). The acidic properties have been attributed to the presence of phenolic and carboxylic acid groups (and possibly surface lactones), whereas the basic character has been more difficult to explain (see Bansal et al., 1988, p. 102). 

Oxidation and incorporation of nitrogen usually affect the surface chemistry of carbons [12, 20, 24]. The types and numbers of basic and acidic groups evaluated using Boehm titration [3, 4]. are collected in Table 2. The data reported indicate that the initial carbons differ in their acidity as a result of the activation method and the type of an organic precursor [1]. BAX is more acidic than BPL. This is expected for a phosphoric acid activated carbon [25]. After modification with urea and heat treatment at 723 and 1223 K, the overall surface chemistry... 

In this work, the efTect of anodic oxidation treatments on activated carbon fibets (ACFs) was studied in the context of Cr(Vl), Cu( II), and Ni( II) ion adsorption behaviors. Ten wt% phosphoric acid and sodium hydroxide were used for acidic and basic electrolytes, respectively. Surf properties of the ACFs were determined by XPS. The specific surface area and the pore stnicture were evaluated from nitrogen adsorption data at 77 K. The heavy metal adsorption rates of ACFs were measured by using a UV spectrometer and 1C P. As a result, the anodic treatments led to an increase in the amount of total acidity by an increase of acidic functional groups such as carboxyl, lactone, and phenol, in spite of a decrease in specific surface area, due to the pore blocking by increased acidic functional groups. 

The selectivity of activated carbons for adsorption and catalysis is dependent upon their surface chemistry, as well as upon their pore size distribution. Normally, the adsorptive surface of activated carbons is approximately neutral, such that polar and ionic species are less readily adsorbed than organic molecules. For many applications it would be advantageous to be able to tailor the surface chemistry of activated carbons in order to improve their effectiveness. The approaches that have been taken to modify the type and distribution of surface functional groups have mostly involved the posttreatment of activated carbons or modification of the precursor composition, although the synthesis route and conditions can also be employed to control the properties of the end product. Posttreatment methods include heating in a controlled atmosphere and chemical reaction in the liquid or vapor phase. It has been shown that through appropriate chemical reaction, the surface can be rendered more acidic, basic, polar, or completely neutral [11]. However, chemical treatment can add considerably to the product cost. The chemical composition of the precursor also influences the surface chemistry and offers a potentially lower cost method for adjusting the properties of activated... 

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