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Letting the molar concentration of Csq be [Csq] gmol/L, the corresponding concentration in geqlL is

Note: From q[Csq], the units of q is equivalents per mole.

Therefore, the total concentration in gram equivalents per liter of removable cations in solution, [CatT]eq, is the sum of all the cations. Let there be a total of i cations. Thus,

As [CatT]eq of cations is removed from solution, a corresponding number of equivalent concentrations of anions pair with the H+ ions displaced from the cation bed.

Let [AnionT]eq and [HT]eq be the total anions and the hydrogen ions displaced, respectively. Since the number of equivalents of one substance in a reaction is equal to the number of equivalents of all the other substances participating in the reaction,

Let the [AnionT ]eq from the effluent of the cation exchanger be introduced into an anion exchanger. For the anion exchanger operating under the OH cycle, the total equivalents of OH- released from the anion bed is equal to that of the anions, [AnionT]eq, removed from solution. Let [OHT ]eq be this total OH-. Since [AnionT ]eq is equal to [HT]eq, [OHT ]eq must be equal to [Ht ]eq. This means that all the acids produced in the cation exchanger are neutralized in the anion exchanger, and all ions in the water have been removed by using the combination of cation exchanger followed by anion exchanger.

On the surface, the combination of cation exchanger and anion exchanger would mean that pure water is produced. As shown in Equations (16.1) and (16.2), however, the unit process of ion exchange is governed by equilibrium constants. The values of these constants depend upon how tightly the removed ions from solution are bound to the bed exchanger sites. In general, however, by the nature of equilibrium constants, the concentrations of the affected solutes in solution are extremely small. Practically, then, we may say that "pure water" has been produced.

By analogy with Equation (16.5), i=m

As with qi, the units of tt are equivalents per mole.

Example 16.1 A wastewater contains the following ions: CrO4 = 120 mg/L, Cu2+ = 30 mg/L, Zn2+ = 15 mg/L, and Ni2+ = 20 mg/L . Calculate the total equiv-

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