Page 561 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

516 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

The glauoconites are: SiO 2 , 50%; iron oxides and alumina, a few angstroms but for one that is not strongly cross-linked
29%; CaO, 2%; MgO, 3%; K 2 O, 8%; and water, 8% (Collins, the mesh size may be 100 Å after swelling. The cross-linking
1937). Grains of natural glauconite can be broken with the also serves to make the polymer framework insoluble in water
fingernail; thus they are processed to harden. The processing (Millar, 1983b, p. 23); without the cross-linking, the resin
of glauconite from the raw material involves the following: would be soluble.

1. Washing to remove mud, fine grains, quartz sand 16.1.4.4.2 Framework of Resins
(amount of material removed may be 50% of ori- The most commonly synthesized ion-exchange resins are
ginal) giving a product size of 0.2–6.2 mm made of styrene monomers, which are polymerized, and
2. Scouring with caustic to dissolve less stable grains cross-linked with di-vinyl benzene (DVD). Fixed ionic groups
and to increase exchangeable sodium are added, which contain the charge sites. Figure 16.3 delin-
3. Sodium silicate and weak-acid treatment to provide a eates the building of such an ion-exchange resin (adapted
hard silica envelope from Dow Chemical, 1964, p. 3).
4. Marketing under a trade name Figure 16.3a shows the benzene molecule, i.e., the
‘‘benzene-ring,’’ which is one of the building blocks of a
16.1.4.3.7 Synthetic Zeolites styrene monomer. Figure 16.3b shows the benzene molecule
The synthetic zeolites are either precipitated gel types or combined with a vinyl group, i.e., H 2 C¼¼CH 2 , to give a
fusion types. The former are obtained by precipitating a styrene monomer (vinyl benzene). Figure 16.3c illustrates
reaction from solutions of sodium silicate and sodium sulfate the styrene monomers combined to form a styrene polymer.
or sodium aluminate. The dried gel is broken into sizes The styrene polymer is a linear, two-dimensional structure. The
needed for softening. The fusion types are obtained by heating cross-linking is by di-vinyl benzene (DVD), shown shaded in
mixtures of minerals, such as sodium carbonate, kaolin, and Figure 16.3d. The next step involves combining the DVB
feldspar, to the melting point. After cooling, the material is (shaded) with polystyrene, illustrated in Figure 16.3e. The
crushed and sized (Collins, 1937). Commercial zeolites are copolymerization of the linear polymer with the DVD, giving
usually in the form of pellets, granules or beads, and powders. the cross-linking, ties the linear chains together, and yields an
Glues and binders are used to form these shapes. insoluble three-dimensional structure. As noted, the DVD pro-
portion can be varied. Figure 16.3f shows a portion of the final
16.1.4.4 Synthetic Resins molecule, with a sulfonic acid group, which provides the
A resin is a solid or liquid organic polymer (Apple Dictionary, exchange site when attached to a styrene molecule. The H
2009). A model of a synthetic ion-exchange resin suggested by attached to the SO 3 is really H and is the exchangeable ion.

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Robert Kunin (1983, p. 46) is that of a plate of congealed Figure 16.4 illustrates the more complete cross-linked
spaghetti. The strands of pasta may represent the polymer chains polystyrene showing SO 3 functional groups attached to the

and the points of contact between strands are the cross-linked styrene molecules and with DVB linkages between the styr-
points. The ion-active groups are located along the strands, i.e., enes that have no functional groups. As seen, the system may
polymer chains. The mass as whole is a solid, made so by the be extended indefinitely. With each DVB cross-linkage, many
strands adhering to one another, but with a porous character. additional styrene=functional groups may be added, to which
Microscopically, the mass is heterogeneous, i.e., the cross-link- yet other DVD groups may be added. As indicated, the DVB
ing is not uniform, nor are the ion-active groups spaced uni- has the role of cross-linking the styrene molecules, which is
formly. Externally, i.e., macroscopically, the resin bead appears the basis for building a complex macromolecule of high
homogeneous. By contrast, a zeolite has an ordered structure, molecular weight.
i.e., a crystal, with regular pore sizes and spacing. A variety of functional groups may be attached to the
styrene molecule. Sulfonate is the most common for strong-
16.1.4.4.1 Cross-Linking acid ion-exchangers and is used for the illustrations in both
The cross-linked polymer network is the framework for the Figures 16.3 and 16.4, with H as the counterion.
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ion-exchanger. The degree of cross-linking can be adjusted by
controlling the DVB (di-vinyl benzene) proportion of the 16.1.4.4.3 Functional Ionic Groups
reactants, i.e., the molar ratio of DVB to styrene. The degree After the desired cross-linked matrix is formed, the remaining
of cross-linking is expressed quantitatively as the ‘‘nominal task is to add the desired fixed ionic groups. This may be done
DVB content’’ and is defined as ‘‘the mole percent of di-vinyl by substitution of these groups in the benzene ring during or
benzene in the polymerization mixture.’’ For example, 10% after polymerization, or by starting with monomers that carry
DVB means one DVB molecule per nine styrene molecules. ionic groups. The ionic groups are attached only to the ‘‘para’’
General purpose ion-exchanger resins contain between 8% position on the benzene ring of the styrene and not to the
and 12% DVB but resins with as little as 0.25% DVB and DVB. The point is illustrated in Figure 16.3f. The ion-
as much as 25% DVB have been prepared (Helfferrich, 1962). exchange behavior of a resin is determined by the particular
The degree of cross-linking determines the pore size of the species of fixed ionic group attached to the benzene ring.
network and the swelling propensity of the resin. For a highly Table 16.3 shows ionic groups commonly used in commercial
cross-linked resin the mesh size (within the polymers) is only ion-exchange resins. The number of groups per 100 benzene