Adsorption of phlitrophenol from Dilute Aqueous Solution An Experiment in Physical Chemistry with an Environmental Application Mary M. Lynam, James E. Kilduff, and Walter J. Weber, Jr. Environmental and Water Resources Engineering,The University of Michigan, Ann Arbor, MI 48109 Surface processes are essential to daily life, moreover to life itself (13). The adsorption of organic molecules by granular activated carbon (GAC) constitutes a surface process. Adsorption involves the accumulation or concentration of a substance, the adsorbate, at a surface, the adsorbent, and it can occur at the interface between any two phases, for example, liquid-liquid, gas-liquid, gas-solid, or liquid-solid (4). History and Use of Adsorbents The use of charcoal to purify water is a practice dating back more than 30 centuries. Popular belief once held that charcoal purified water by destroying its harmful and unaesthetic properties. It was not until the latter part of the nineteenth century that it became apparent that some solids, such as charcoal, purify gases and solutions by removing, rather than destroying, impurities from these phases and by concentrating them in another phase. Yet more time elapsed until theories emerged to explain this phenomenon. In 1888 Kayser first used the term adsorption to distinguish surface accumulation from the intermolecular phenomenon that characterize the absorption process. Activated carbons were developed in the fint quarter of the 20th century and used mainly for gas-phase applications. The development of synthetic chemicals and the concomitant need for separation and purification procedures spumed research and development into new technologies for adsorption from solution. In particular, the application of adsorption technology in the treatment of water and wastewater bas evolved only within the last 30 years. A concise history of the development of adsorbents including activated carbon and their use in gas- and solution-phase applications is provided elsewhere (5). Previous papers in thrs Journal have focused on removal of dyes by sand and activated carbon (6).This paper
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demonstrates the use of GAC in the removal of p-nitrophenol (PNP) (Fig. 1) from dilute solution illustrates the use of the Langmuir and Freundlich equations in interpreting the expe"menta1 results
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Journal of Chemical Education
Activated Carbon Manufacture This is the term used to refer to a microporous adsorbent of carbonaceous origin manufactured by the following steps. Carbonization The carbon is heated to very high temperatures, causing burnoff of noncarbon impurities. It is usually conducted in the absence of air and results in the formation of a char with relatively little internal surface area. Oxidation This process imparts activity to the carbon. It involves the treatment of the char with oxidizing gases such as steam, air, or carbon dioxide at high temperatures. Treatment with these oxidizing gases causes erosion of the surface and the development of a porous structure with an extensive internal structure. The pore surfaces contain
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Phenols are toxic to humans. Thus, when present in water they pose a significant environmental hazard and NO ---2 must be removed. Phenolic compounds are known to be discharged in Figure pNitmphewastewaters from industries such as no1 molecule. refineries and manufacturers of pharmaceuticals and pesticides. They are also present in pesticide runoff from agricultural land (7).Carbon adsorption is commonly used to remove nonbiodegradable organic substances that are dissolved in water. This is a suitable experiment to perform in advanced physical or analytical chemistry laboratories for a variety of reasons. PNP is a relatively small molecule and thus should demonstrate rapid equilibrium. PNP is easily ana-
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lyzed using W spectrophotometry, so this experiment can be carried out during the time allotted for laboratory work. A similar experiment on adsorption from solution is dommented elsewhere (8).Much research has been conducted in the past using PNP, and results from internal reflectance infrared spectroscopy suggest the formation of a donor-acceptor complex between the carbon surface and the PNP molecule. It was postulated that the carbonyl p u p s present on the carbon surface donate electron density into the PNP ring. When all the carbonyl surface sites are exhausted adsorption continues by complexation of the PNP molecule with the carbon rings of the basal planes (9).The presence of the nitro substituent, which is a strong electron-withdrawing group, facilitates a reduction in electron density in the pi-system of the ring.
Figure 2. Structure of activated carbon.
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