Page 284 - Adsorbents fundamentals and applications

POLYMERIC RESINS 269

dealkalization, desilicazation, and adsorption of ionic constituents from dilute
solutions. For ion exchange, the selectivity increases with increasing valence of
+
the ion, for example, with sulfonic acid group: Th 4+ > Al 3+ > Ca 2+ > Na ,and
+
+
with increasing atomic number at constant ionic valence: Cs > Rb > K >
+
+
2+
Na > Li and Ba 2+ > Sr 2+ > Ca 2+ > Mg .
+
Among the ion exchange resins, those based on weak bases and weak acids
exhibit higher ion-exchange capacities than those based on strong bases and
strong acids. However, regeneration of the strong-acid or strong-base resins is
easier than that for the weak-acid or weak-base resins. For example, weak-acid
resins require a two-step regeneration, first with a strong mineral acid, such as
HCl, which exchanges H + for hardness, followed by NaOH neutralization to
+
displace H with Na .
+
Demineralization or deionization removes dissolved ionic impurities from
water by a two-step process with cation- and anion-exchange resins. The former
exchanges hydrogen for cations in solution. The acidic effluent passes through a
column of anion-exchange resin that replaces the anions in solution with hydrox-
+
ide. The H from the cation-exchange resin are then neutralized by OH from
−
the anion resin.
Since it was first proposed in the classic paper of Hollis (1966), the polymeric
resins have been widely used as column packings for gas chromatography. For
example, nonpolar styrene/DVB is used under the trade name Porapak Q, and its
very polar derivative is used as Porapak T. These columns are used for analysis of
various gas mixtures, including wet mixtures, based on differences of interactions
with different gas molecules.

9.3.2. Comparisons of Resins and Activated Carbon
Many comparative studies have been performed on isotherms and column break-
through performance between resins and activated carbon. The work of Weber
and co-workers has been summarized elsewhere (Weber and van Vliet, 1981a
and 1981b; Faust and Aly, 1987). Weber et al. have compared 10 different sor-
bents, including resins, activated carbons, and carbonaceous polymers. Data from
selected sorbents, in terms of the two parameters in the D–R equation (see Chap-
ter 3), are summarized in Table 9.7:

2
W = W 0 exp[−(RT ln(C s /C)) /B] (9.6)
where W 0 is the maximum capacity, C s is the saturated concentration, and B is
the affinity coefficient.
The D–R isotherms of a number of compounds on activated carbon and resin
are shown in Figure 9.26. The Calgon Filtrasorb 400 is an activated carbon
for aqueous phase applications. XAD-2 and XAD-4, as mentioned earlier, are
styrene/DVB resins. XAD-8 is cross-linked polyacrylic ester resin. XE-347 is a
partially pyrolyzed resin.
The affinity coefficient, B, is an indication of the strength of the sorbent–sor-
bate interactions. From the isotherm data, two general facts become clear: