Environmental Engineering Reference
In-Depth Information
In order for the resin to maintain charge neutrality, the fixed
R
−
group will attract
counterions (H
+
and Na
+
in the previous example reactions) and it is these groups which
will exchange with another ion in solution. The resin tends to prefer ions of higher valence,
and also ions that are smaller and can more easily enter the resin pore structure. The
preference series for the most common cations is [7]:
Ba
2
+
>
Pb
2
+
>
Sr
2
+
>
Ca
2
+
>
Ni
2
+
>
Cd
2
+
>
Cu
2
+
>
Co
2
+
>
Zn
2
+
>
Mg
2
+
Ag
+
>
Cs
+
>
K
+
>
NH
4
Na
+
>
H
+
.
>
>
Since H
+
is the least preferred, the second step of the hydrogen cycle will require a large
excess of acid before the concentration driving force is sufficient to favor regeneration;
60-75% of the regenerant can often go unused [4]. As stated earlier, this is an important
cost and disposal consideration when using this technology.
Weak-acid exchangers
Weak-acid cation-exchange resins have weak fixed reactive sites such as the carboxylic
group (
-
COOH). They are usually copolymers of DVB and acrylic or methacrylic acid [6].
The polyacrylic acid is shown in Figure 8.3, and properties are listed in the previous
Table 8.1.
The weak-acid resins have a large number of acid groups that will only be partially
ionized. Unlike strong-acid resins, these resins are not useful at low pHs (4 or 5) since
the functional groups are not ionized and their effective capacity is zero under these
conditions. Weak-acid resins also swell much more than strong-acid resins (sometimes
as much as a 90% increase in volume when H
+
is replaced by the much larger Na
+
ion)
which can cause excessive pressure drop, resin rupture, and equipment breakage. They
are chemically stable, but may break from repeated swelling and shrinking cycles, and
they are also “tight” compared to the strong-acid exchangers, so that the mass transfer
resistances inside the resin are high and the resulting mass transfer zone in the bed is
long [6].
Weak-acid resins can quickly remove cations from weak bases (like Ca
2
+
and Mg
2
+
),
but they are not efficient at removing cations from strong bases (like Na
+
and K
+
), and
CH
2
CH
CH
2
CH
C
HO
O
CH
CH
2
n
m
Figure 8.3
Weak-acid ion-exchange monomer (polyacrylic acid with DVB
crosslink) [6]. Reproduced with kind permission of Kluwer Academic Publishers.
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