Activated carbon adsorption properties

Properties of Activated Carbon Adsorption Isotherm Models Design Consideration of PAC Systems Regeneration... 

Most experimental results reported in literature were obtained using commercially available activated carbons. A few other studies were carried out with self synthesized activated carbons derived from different natural organic materials. The preparation of activated carbons consists of two main stages termed carbonization and activation, and the choice of conditions under which these processes are carried out, gives practically infinite number of combinations. The temperature, time and atmosphere (the nature of the gas or gas mixture, and its pressure or flow rate) are the main variables. Moreover, different additives can be added before or between these processes to modify the final product. Wood and coal are the most common precursors, but sorption properties of activated carbons derived from other materials, e.g. coconut shells or plum kernels can be also found in literature. Although the precursor material certainly has some effect on the adsorption properties of activated carbon, these properties are chiefly defined by the conditions of thermal treatment and purification vide infra). Therefore statements like, "activated carbon from coconut shells has higher affinity to certain adsorbate than activated carbon from wood" are misleading and they should be avoided. 

Perrich, J. R., 1981. Activated Carbon Adsorption for Wastewater Treatment, Boca Raton, FL CRC Press. Reid, R. C., J. M. Prausnitz and B. E. Poling, 1987. The Properties of Gases and Liquids. New York McGraw Hill. 

Activated carbons are versatile adsorbents. Their adsorptive properties are due to their high surface area, a microporous structure, and a high degree of surface reactivity. They are, used, therefore, to purify, decolorize, deodorize, dechlorinate, separate, and concentrate in order to permit recovery and to filter, remove, or modify the harmful constituents from gases and liquid solutions. Consequently, activated carbon adsorption is of interest to many economic sectors and concern areas as diverse as food, pharmaceutical, chemical, petroleum, nuclear, automobile, and vacuum industries as well as for the treatment of drinking water, industrial and urban waste water, and industrial flue gases. 

Bardina, I. A., et al., Adsorption properties and structure of some active carbons, Adsorpt. Sci, Technol., 10, 211-220(1994). 

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). 

Activated carbons contain chemisorbed oxygen in varying amounts unless special cate is taken to eliminate it. Desired adsorption properties often depend upon the amount and type of chemisorbed oxygen species on the surface. Therefore, the adsorption properties of an activated carbon adsorbent depend on its prior temperature and oxygen-exposure history. In contrast, molecular sieve 2eohtes and other oxide adsorbents are not affected by oxidi2ing or reducing conditions. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

The term activation refers to the development of the adsorption properties of carbon. Raw materials such as coal and charcoal do have some adsorption capacity, but this is greatly enhanced by the activation process. There are three main forms of activated carbon. 

Modification techniques for activated carhon were used to increase the removal capacity by surface adsorption and to improve the selectivity to volatile organic compounds (VOCs). Modified activated carbons (MACs) were prepared by modifying the purified activated carbon with various acids or bases. The effects of adsorption capacity and modified contents on the textural properties of the MACs were investigated. Furthermore, VOC adsorption and desorption experiments were carried out to determine the relationship between the adsorption capacity and the chemical properties of the adsorbents. High adsorption capacity for the selected VOCs was obtained over lwt%-H3P04/AC (lwt%-PA/AC). As a result, MAC was found to be very effective for VOC removal by adsorption with the potential for repeated use through desorption by simple heat treatment. 

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

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