Ion Exchange - Physical Chemical Treatment Ion Exchange...

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i Physical Chemical Treatment Ion Exchange Introduction Ion Exchange processes are used for the removal of color, organics, nitrates and hardness. Both column tests and isotherms can be used to determine the capacity of ion exchange resins for a given application. Ion Exchange is similar to adsorption processes and are common to water and wastewater treatment. Theory Ion exchange is defined as a mechanism or process for mass transfer of ions, usually between two phases. The ion exchange process consists of a chemical reaction between ions in a liquid phase and ions in a solid phase (Reynolds, 1982). This process or mechanism of mass transfer is frequently used in water treatment for the purpose of removing specific ions from drinking water. Typical applications of ion exchange in drinking water treatment are for the removal of hardness (calcium and magnesium), the removal of nitrates, and demineralization. Generally, ion exchange is considered as a simple stoichiometric reaction as shown in Equation 1 for a cation exchanger. This reaction describes the transfer or removal of A n+ from a liquid phase to a solid phase, (R - ) n A n+ . Hence the removal of A n+ from solution. An R B n B R A n n n +− + + + + + ( ) ( ) (1) where: A = cation A B = cation B R = cation exchange resin n = charge According to the law of mass action, the equilibrium relationship for the reaction described in Equation 1 has the form (Weber, 1972): K a B a a A B A n n = R n A ( )( ) ( ) ( ) a RB (2) where: a A = activity of ion A a B = activity of ion B If the activity coefficients are ignored, as is common in dilute aqueous environments, molar concentrations can be substituted for activity. Concentrations are used in practice because they are measured more easily than activities (Clifford, 1990). The equilibrium expression for the reaction can be written as shown in Equation 3. Ion exchange resins exhibit some degree of selectivity or preference for certain ions during exchange (Baier, 1989). This equation is significant because it provides a means of
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ii measuring the selectivity or capacity of a specific resin for a given ion. Note that K B A includes activity coefficient terms that are functions of ionic strength and thus the measure of selectivity, or K B A , is not actually constant, but varies with water quality and is somewhat site specific. However, general engineering calculations can be made without having selectivity coefficients for all aqueous environments. K BA AB B A nn n n = + + + + [] [ ] [ ] (3) where: K B A = resin's selectivity of A over B A = cation A concentration in solution (meq/L) B = cation B concentration in solution (meq/L) A = cation A concentration in resin (meq/L) B = cation B concentration in resin (meq/L) Based on the above equation, the resin's selectivity of cation A over cation B can be determined if the cation concentrations in the solution and resin are known. The greater the selectivity coefficient, K , the greater the preference for the ion by the resin.
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Ion Exchange - Physical Chemical Treatment Ion Exchange...

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