Carbon Materials as Adsorbents for the Removal of Pollutants from the Aqueous Phase
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Carbon Materials
as Adsorbents for the Removal of Pollutants from the Aqueous Phase
Carlos Moreno-Castilla and José Rivera-Utrilla Introduction Activated carbons are the most important carbon materials used in water treatment.1 Their known world production is around 500,000 tons per year, of which about 80% is used for liquid-phase applications. These solids are manufactured in powder or granular form from a large variety of raw materials and are unique and versatile adsorbents due to their highly developed porosity, their large surface area (which in some cases can be up to 3000 m2/g), and their variable surface chemistry.2,3 The removal of pollutants from the aqueous phase by activated carbons occurs by means of an adsorption process. The surface of the adsorbent can bind molecules dissolved in the aqueous phase because of van der Waals’ dispersion forces and electrostatic interactions, resulting in a higher concentration of the adsorbate at the solid–liquid interface than in the bulk aqueous phase. Once the activated carbons have exhausted their adsorption capacity, they can be regenerated for reuse. Here we present a short overview of the preparation, surface characteristics, and application of activated carbons to water treatment, emphasizing the role of the surface characteristics that determine the removal of pollutants from the aqueous phase.
demonstrate low degradation by aging. In practice, they are of vegetable or fossil origin: wood, fruit stones or pits, nutshells, coals, petroleum residues, and so on. These materials are activated either physically or chemically to obtain the activated carbons. The physical activation methods involve two steps at high temperature (700–800C): carbonization in an inert atmosphere and subsequent activation in carbon dioxide or steam. During the carbonization step, the volatile matter and noncarbon species are eliminated, leaving behind a fixed carbon mass with a rudimentary pore texture. The gases used in the activation step partially gasify the carbonized material, thereby opening and developing its porosity.2,3 Chemical activation methods involve the carbonization, between 500C and 800C, of the precursor impregnated with a chemical agent such as potassium hydroxide, phosphoric acid, or zinc chloride. The chemical agent produces dehydration and degradation of the material, which results in the creation of the porous structure.2,3 The surface properties of the final product, such as porosity, surface area, and surface chemistry, depend on the raw material and activation process used.
Preparation
Activated carbons are nongraphitic carbons with a structure made up of small, imperfect graphene sheets and strips, often bent, that are randomly distributed to give a tridimensional structure. The variably sized gaps between the sheets constitute the porosity, which is created during the
Activated carbons can be prepared as a powder or in granular form from a variety of raw materials. These materials have to be abundant and cheap, with a high carbon content and low inor
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