Environmental Engineering Reference
In-Depth Information
they can split only alkaline salts (e.g., NaHCO
3
but not NaCl or Na
2
SO
4
). The preference
series for common ions is similar to the strong-acid exchangers, but the H
+
position is
moved to the left, sometimes as far as Ag
+
(and therefore, weak-acid resins are easier
to regenerate than strong-acid resins) [7]. Carboxylic functional groups have such a high
affinity for H
+
that they can use up to 90% of the acid (HCl or H
2
SO
4
) regenerant, even
with low acid concentrations [4].
Weak-acid exchangers do not require as high a concentration driving force as strong-
acid exchangers do. They do require an alkaline species to react (carbonate, bicarbonate,
or hydroxyl ion):
2(
R
−
H
+
)
(2
R
−
)Ca
2
+
+
Ca(HCO
3
)
2
+
→
2(H
2
CO
3
)
.
(8.5)
The regeneration step can be performed with HCl or H
2
SO
4
. These ion-exchange resins
are often used for simultaneous softening and dealkalization in water treatment, and they
are favored when the untreated water is high in Ca
2
+
and Mg
2
+
but low in dissolved CO
2
and Na. Sometimes a weak-acid exchange process is followed with a strong-acid exchange
polishing step to minimize the higher cost of the strong-acid exchange process [4].
Strong-base exchangers
The two most common strong-base resins are also based on polystyrene-DVB polymers
[6]. They have fixed reactive sites that are derived from quaternary ammonium groups
(Figure 8.4). Type I has a greater chemical stability, but Type II has a higher regeneration
efficiency and higher capacity. Both of these resins are fully ionized and are essentially
equivalent to sodium hydroxide [4]. Typical wet-exchange capacities are in the range of
CH
CH
2
CH
CH
2
CH
2
CH
2
CH
3
N
CH
3
CH
3
CH
3
N
CH
3
CH
2
CH
2
OH
Figure 8.4
Strong-base ion-exchange monomer (quaternary ammonium struc-
tures). Type I is shown on the left and Type II is on the right [6]. Reproduced with
kind permission of Kluwer Academic Publishers.
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