In adsorption, the molecules of a dissolved contaminant become attached to the surface of a solid adsorbent. The most widely used adsorbent is granular activated carbon (GAC) because its porous structure provides a relatively large surface area per unit volume (1000-2000 m2/g). Collection of the molecules on the surface of the adsorbent is due to chemical or physical forces. Chemical adsorption is due to actual chemical bonding at the solid's surface. Physical adsorption is due to van der Waals' forces, which are weak bonds compared to chemical adsorption. However, because of the weak nature of these bonds, adsorbed molecules can be easily removed with a change in the solute concentration or the addition of enough energy (regeneration) to overcome the bonds. This capacity to remove certain molecules adsorbed on carbon and, thus, the possibility for repeated carbon reuse is what allows activated carbon adsorption to be a cost-effective technology.
Environmental engineers commonly use carbon adsorption to remove organic contaminants from water or air; however, they also use it to remove a limited number of inorganic contaminants as shown in Table 9.17.1. The effectiveness of GAC depends on the molecular weight, structure, and solubility of the contaminant as well as the properties of the carbon, the water temperature, and the presence of impurities such as iron and manganese. The influence of each of these parameters on the absorbability of organic contaminants is shown in Table 9.17.2. As shown in this table, carbon adsorption is suitable for high molecular weight and low solubility and polarity compounds (U.S. EPA 1988), such as chlorinated hydrocarbons, organic phosphorous, carbonates, PCBs, phenols, and benzenes. GAC can also be used in conjunction with other treatment technologies. For example, GAC can be used to treat the effluent water or offgas from an air stripper (Crittenden 1988).
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