Page 333 - Water and wastewater engineering

8-2 WATER AND WASTEWATER ENGINEERING

8-1 INTRODUCTION

Ion exchange * is a reversible reaction in which a charged ion in solution is exchanged for a simi-
larly charged ion electrostatically attached to an immobile solid particle. The largest application of
ion exchange in water treatment is for softening, where calcium, magnesium and other polyvalent
cations are exchanged for sodium (Clifford, 1999). It is used both in individual homes [called
point-of-entry (POE) or point of use (POU)] and in municipal systems. Ion exchange is also used
to remove specific contaminants such as arsenic, barium, nitrate, and radium.
In common practice the raw water is passed through a bed of resin. The resin is the made by
polymerization of organic compounds into a porous matrix. Commercially available resins are
selected for the bed. Typically, in water softening, sodium is exchanged for cations in solution.
When the bed becomes saturated with the exchanged ion, it is shut down and regenerated by
passing a concentrated solution of sodium back through the bed.
Because of its large application in softening water, the focus of this chapter is on this
application.

8-2 FUNDAMENTAL CONCEPTS OF ION EXCHANGE

Ion Exchange Resins and Reactions
Ion Exchange Resins. The most common polymeric resin matrix is a cross-linked polystyrene
to which charged functional groups are attached by covalent bonding. Divinylbenzene (DVB)
is used as a cross-linking agent with the styrene. A higher DVB cross-linkage provides a more
stable resin but will result in slower ion exchange kinetics. The common functional groups are

in four categories: strongly acidic (for example, sulfonate, SO 3 ); weakly acidic (for example,

carboxylate, COO ); strongly basic (for example, quaternary amine, N (CH 3 ) 3 ), and weakly
basic (for example, tertiary amine, N(CH 3 ) 2 ).

Figure 8-1 is a schematic representation of a resin bead and two typical functional groups.

Cation exchange resins contain mobile positive ions, such as hydrogen (H ) or sodium (Na ),

that are attached to immobile functional acid groups, such as sulfonic ( SO 3 ) and carboxylic

( COO ) groups. The functional groups are fixed to the resin matrix or backbone. These are the
cation ion exchange sites. The number of sites is finite, and when they all have been exchanged
the ion exchange resin will no longer soften the water.
Strong Cation Exchange Reactions. The word “strong” in strong cation exchange does not
refer to the physical strength of the resin but rather to the Arrhenius theory of electrolyte strength
in which the functional group of the resin is dissociated completely in its ionic form at any pH.
Equation 8-1 represents the exchange of sodium for calcium in the form of carbonate hardness
and Equation 8-2 represents the exchange of sodium for noncarbonate hardness
á
ź
Ca(HCO ) 2{ ź SO Na } { ź RSO } Ca 2á 2NaHCO 3 (8-1)
3
3 2
3 2

á
ź
CaCl 2{ ź SO Na } { ź SO } Ca 2á 2NaCl (8-2)
2
3
3 2

*Often noted in the literature as IX.