Ion exchange - Principles of Chemical Separations with Environmental Applications - page 222

c RT = 1.0 to 1.4 eq / L, and the resins can degrade, particularly at temperatures above

60 ◦ C [6].
These resins work well over all pH ranges, and readily remove all anions. The preference
series for the most common anions is [8]:

SO 2 4 NO 3 CrO 2 4 I − > Br − > Cl − > OH − . > > >

Like the strong-acid exchangers, they can split neutral salts into their corresponding bases
via the hydroxide cycle, and they are also often used in a chloride cycle to remove nitrates
and sulfates from municipal water supplies. The hydroxide cycle is regenerated with a
strong base like NaOH, while the chloride cycle is regenerated with NaCl [4].

NaCl

R + OH − → R + Cl −

+ NaOH +

Hydroxide cycle:

R + Cl − → R + OH − NaOH + NaCl +

NO 3

R + Cl − → R + NO 3 Cl − + +

Chloride cycle:

R + NO 3 R + Cl − +

+ →

NaNO 3 . NaCl

Even weakly ionized substances, like silica and CO 2 , can be removed with strong-base
exchangers. Sometimes these exchangers are used after a cation exchanger for complete
water demineralization [4].

Weak-base exchangers

A common type of weak-base exchanger uses the same polystyrene-DVB polymer but
contains tertiary amine groups (Figure 8.5) [6].
The weak-base resins are fully ionized at low pHs and not ionized at all at high pH [5].
Like the weak-acid resins, they can suffer from oxidation and organic fouling.
Weak-base exchangers are able to remove anions from strong acids like Cl − ,SO 2 4 ,
and NO 3 . They do not remove anions from weak acids very well (silicic, HCO 3 ,

CH 2 CH 2

CH 2

N

CH 3 CH 3

Figure 8.5 Weak-base ion-exchange monomer (tertiary amine on polystyrene)
[6]. Reproduced with kind permission of Kluwer Academic Publishers.