Ion exchange and carbon adsorption are unrelated technologies, and often have different objectives. They are however oftentimes used in compliment to achieve
Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications. An organic ion exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile ions for ions of similar charge from the surrounding medium. Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin. The industry application most familiar with ion exchange technology is metal plating. Most plating process water is used to cleanse the surface of the parts after each process bath. To maintain quality standards, the level of dissolved solids in the rinse water must be regulated. Fresh water added to the rinse tank accomplishes this purpose, and the overflow water is treated to remove pollutants and then discharged. As the metal salts, acids, and bases used in metal finishing are primarily inorganic compounds, they are ionized in water and could be removed by contact with ion exchange resins. In a water deionization process, the resins exchange hydrogen ions (H+) for the positively charged ions (such as nickel, copper, and sodium), and hydroxyl ions (OH-) for negatively charged sulfates, chromates. and chlorides. Because the quantity of H+ and OH ions is balanced, the result of the ion exchange treatment is relatively pure, neutral water. Ion exchange technology is applied in many other industry sectors, including the petroleum and chemical industries, as well as general wastewater treatment applications. The technology is most often compared to reverse osmosis, since both technologies are often aimed at similar objectives. In this regard, in addition to discussing ion exchange as a technology, we will also review some of the operational tradeoffs and
The history of carbon adsoprtion in the pruification of water dates back to ancient times. Adsorption on porous carbons was described as early as 1550 B.C. in an ancicnt Egyptian papyrus and later by Hippocrates and Pliny the Elder, mainly for medicinal purposes. In the 18th century, carbons made from blood, wood and animals were used for the purification of liquids. All of these materials, which can be considered as precursors of activated carbons, were only available as powders. The typical technology of application was the so-called batch contact treatment, where a measured quantity of carbon and the liquid to be treated were mixed and, after a certain contact time, separated by filtration or sedimentation. At the beginning of the 19th century the decolourisation power of bone char was detected and used in the sugar industry in England. Bone char was available as a granular material which allowed the use of percolation technology, where the liquid to be treated was continuously passed through a column. Bone char, however, consists mainly of calcium phosphate and a small percentage of carbon; this material, therefore was only used for sugar purification. At the beginning of the 20th century the first processes were developed to produce activated carbons with defined properties on an industrial scale. However, the steam activation and chemical activation processes could only produce powder activated carbon. During the First World War, steam activation of coconut char was developed in the United States for use in gas masks. This activated carbon type contains mainly fine adsorption pore structures suited for gas phase applications.
After World War II technology advances were made in developing coal based granular activated carbons with a substantial content of transport pore structure and good mechanical hardness. This combination allowed the use of activated carbon in continuous decolourisation processes resulting superior performance. In addition optimization of granular carbon reactivation was achieved. Today many users are switching from the traditional use of powdered activated carbon as a disposable chemical to continuous adsorption processes using granular activated carbon combined with reactivation. By this change they are following the modem tendency towards recycling and waste minimization, thereby reducing the use of the world's resources. In this chapter we will explore the use of activated carbon in standard water treatment applications.
This overview will familiarize with the technology, which can be used as standalone, or in conjunction with other technologies such as RO, ion exchange and others. Although not discussed per se, this technology has found historical use in more recent times in many groundwater remediation projects. It is most often thought of as the workhorse in groundwater pump-and-treat applications.
Remember to refer to the Glossary at the end of the book if you run across any terms that are unfamiliar to you.
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