Adsorption is a physicochemical process that generally occurs at the interface of fluid-solid phases and is sometimes used to remove certain species that cannot effectively be removed from the wastewater stream by other conventional technologies. Although it is quite possible that liquid-liquid or gas-liquid interaction results in adsorption, it is more common in waste-water treatment to observe interactions of two fluid phases as absorption. Adsorption and absorption, which belong to the same category of physic-ochemical processes called sorption, are two different phenomena. The main difference of these two lies in the equilibrium phase they eventually form: If it is homogeneous, it is absorption; otherwise, it is adsorption. Adsorption can be either physical (such as binding caused by van der

Waals forces) or chemical/activated (such as covalent bonds). It is quite possible that both styles of adsorption exist in wastewater treatment employing adsorption because a variety of substances could be in the stream.

In general, the materials being adsorbed are called adsorbates; the materials adsorbing adsorbates are termed adsorbents. The majority of the adsorbents in wastewater treatment are solids. These solids can be roughly divided into three categories: carbons, inorganic materials, and synthetic polymers. Carbon-based adsorbents such as activated carbons are the most common adsorbents in use in wastewater treatment plants. They are effective and relatively inexpensive; they even can be made from agricultural wastes. For example, peanut shells from the deep south of the U.S. are currently tested for use as adsorbents for heavy metal and other toxic material removals. Other sources of raw materials for carbon-based adsorbent manufacturing include petroleum coke, wood, and coconut shells.

Inorganic materials used for adsorption are classes of activated alumina and zeolites. These materials are expensive and more size- or/and species-specific. They are more likely to be found in fine chemical manufacturing, pharmaceutical, and other high-profit margin industries.

Synthetic polymers can be used as adsorbents because they are easily functionalized on the surfaces. Ion exchange polymers target ions in the solution, so they are good candidates for removing ions in water purification or wastewater treatment to remove certain heavy metal or other toxic ions from wastewater. Polymers that do not rely on an ion exchange mechanism utilize the hydrophobic or hydrophilic interactions (a stronger form of molecular force than van der Waals forces) to adsorb the adsorbates.

In order to describe the capability of an adsorbent to adsorb certain adsorbate species, we often examine an isotherm of the adsorbent— an adsorption data plot of amount of adsorbed per unit mass of the adsorbent versus concentration of adsorbates in the solution. There are four common isotherms (curves) typical of all adsorptions: linear, Freundlich, Langmuir, and "unfavorable," as shown in Fig. 3.10. The last one has an upward curvature on adsorption versus concentration plot and receives no favor, as its name suggests; therefore, it is so termed but never tended. The isotherms reveal not only the capacity of the adsorbent but also the adsorption patterns and its equilibrium state. The linear isotherm is a simplistic mathematical gimmick, straightforward but unrealistic (Equation 3.10):

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