Ion Exchange Reaction

Exchange sites exhibit an affinity for certain ions over others. This phenomenon is helpful in removing objectionable

FIG. 7.36.1 The ion-exchange reaction.

FIG. 7.36.1 The ion-exchange reaction.

ionic materials from process streams. Environmental engineers have studied the affinity relationships and have identified certain simple rules. First, ions with multiple charges are held more strongly than those of lower charges. Ions with the same charge are held according to their atomic weight with heavier elements held more strongly. The affinity relationships for cation and anion exchangers are as follows:

Cation Exchangers:

• Divalent—Ra > Ba > Sr > Ca > Mg >> Na

Anion Exchangers:

• Monovalent—I > Br > NO3 > Cl > HCO3 > F > OH

The affinity relationship can also be expressed with the following equilibrium (selectivity) equations based on the reversibility of ion-exchange reactions and the law of mass action:

The following equations are for the divalent-monovalent reactions:

KCa = [RCa] [Na+]2 KNa [RNa]2 [Ca++] KQ = y(1 - x)2

In these equations, the brackets represent the ion concentration in the resin and the liquid phase. The y, x notation expresses the reactions as equivalent ratios. For the divalent-monovalent reaction, Q is the resin capacity, and Co is the total concentration of the electrolyte in solution. Plotting these equations, usually as y versus x plots, shows the exchange processes that occur in the exchange zone or a batch contactor.

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