Activated carbon adsorption behavior

Adsorption — An important physico-chemical phenomenon used in treatment of hazardous wastes or in predicting the behavior of hazardous materials in natural systems is adsorption. Adsorption is the concentration or accumulation of substances at a surface or interface between media. Hazardous materials are often removed from water or air by adsorption onto activated carbon. Adsorption of organic hazardous materials onto soils or sediments is an important factor affecting their mobility in the environment. Adsorption may be predicted by use of a number of equations most commonly relating the concentration of a chemical at the surface or interface to the concentration in air or in solution, at equilibrium. These equations may be solved graphically using laboratory data to plot "isotherms." The most common application of adsorption is for the removal of organic compounds from water by activated carbon. 

The steady interest in the effects of the chemistry and physics of the carbon surface on pollutant removal from waters has been ignited by the U.S. Clean Water Act (enacted in 1972, amended as the Water Quality Act in 1987). The most recent interest stems from the Safe Drinking Water Act Amendment of 1996. Activated carbon adsorption has been cited by the U.S. Environmental Protection Agency (www.epa.gov) as one of the best available control technologies. Furthermore, the most recent efforts to understand the adsorption of the same pollutants by soils [7,8] can benefit from comparisons of similarities and differences with respect to the behavior of activated carbons. 

It is well known that both the adsorbate and the adsorbent properties play a very important role in activated carbon adsorption. Adsorption is a manifestation of complicated interactions among the three components involved, that is, the adsorbent, the adsorbate, and the solvent. Normally, the affinity between the adsorbent and the adsorbate is the main interaction force controlling adsorption. However, the affinity between the adsorbate and the solvent (i.e., solubility) can also play a major role in adsorption. Hydrophobic componnds have low solubility and tend to be pushed to the adsorbent surface and hence are more adsorbable than hydrophilic compounds. Meanwhile, we know that phenolic compounds with different fnnctional groups can lead to different solubility, which may lead to different oligomerization extent. Therefore, the adsorption behavior of phenolic componnds with different fnnctional gronps has to be nnderstood. As illustration, we consider the interpretation of experimental isotherms by Ln and Serial (2007) for the adsorptive capacity of five different phenolics on GAC F400 and two ACFs, ACC-10 and ACC-15, under both anoxic and oxic conditions (Fignre 6.2). 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

Other composite photocatalysts were prepared by mounting immobilized anatase particles on mesoporous silica and silica beads [189-191], The behavior of anatase-mounted activated carbons was also studied in detail [192-194], It was even suggested that carbon-coated anatase exhibits better performance in photocatalysis than anatase itself, demonstrating high adsorptivity, inhibition of interaction with organic binders, etc. [195,196],... 

Representative rate data for 2,4,5-T and parathion for the experiments on adsorption of pesticides on active carbon are presented in Figures 1 and 2. The (C0 — C)/m values in these plots represent the amount of solute, both in micromoles and milligrams, removed from solution per gram of carbon. Good linearization of the data is observed for the experiments, in accord with expected behavior for intraparticle-transport rate control. Similar linearization was obtained also for data for the other pesticides. The linear traces facilitate comparison of relative rates of adsorption of pesticides, and such comparison is made in column 1 of Table III, using the square f the slope pf each plot as the relative rate constant for the experiment. 

The porous textural characterization of activated carbons is a very important subject due to the growing interest in the preparation of materials with well-defined pore structures and high adsorption capacities. Porosity characterization is an essential task to foresee their behavior in a given use and requires a combination of different techniques. Gas adsorption techniques constitute the most common approach to the characterization of the pore structure of porous materials. However these techniques have some limitations. 

First results on n-complexation sorbents for desulfurization with Ag-Y and Cu(I)-Y zeolites have been reported recently [3,4]. In this work, we included the known commercial sorbents such as Na-Y, Na-ZSMS, H-USY, activated carbon and activated alumina (Alcoa Selexsorb) and made a direct comparison with Cu(l)-Y and Ag-Y which were the sorbents with n-complexation capability. Thiophene and benzene vapors were used as the model system for desulfurization. Although most of these studies can be applied directly to liquid phase problems, Cu-Y (auto-reduced) and Ag-Y zeolites were also used to separate liquid mixtures of thiophene/benzene, thiophene/n-octane, and thiophene/benzene/n-octane at room temperature and atmospheric pressure using fixed-bed adsorption/breakthrough techniques. These mixtures were chosen to understand the adsorption behavior of sulfur compounds present in hydrocarbon liquid mixtures and to study the performance of the adsorbents in the desulfurization of transportation fuels. Moreover, a technique for regeneration of the adsorbents was developed in this study [4]. 

Recently, rotor-type adsorption systems attract much attention, because the systems can effectively control humidity and air pollutants such as volatile organic compounds (VOCs), nitrogen oxide (NOx) and sulfiuic oxide (SOx). Gas remediation efficiency is principally concerned to the gas capturing ability of the adsorbents impregnated into the ceramic honeycomb rotor such as zeolite, silica, activated carbon, etc. Therefore, it is one of the most important works to develop the adsorbent with good absorption-desorption behaviors. 

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