Page 558 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Ion-Exchange 513

synthetic zeolites. The first zeolite plant was completed in
1924, with 10 plants in 1930, 45 in 1935, 110 in 1938, 199 TABLE 16.1
in 1941, and in 1945 there were 238 plants serving 4.4 million Minerals Having Ion-Exchange Properties
persons in the United States. By 1950 about 150 municipal
Class Name Chemical Formula
systems had been installed and about 30,000 ion-exchange
systems were used by the industry (AWWA, 1951). Propri- Zeolites Analeite Na[Si 2 AlO 6 ] 2 HOH
Chabezite (Ca,Na) [Si 2 AlO 6 ] 2 6HOH
etary soft water service got underway in 1937 at Rockford,
Harmotome (K,Ba) [Si 5 Al 2 O 14 ] 5HOH
Illinois and then in Wheaton, Illinois, followed by Hagers-
Heulandite Ca[Si 3 AlO 8 ] 5HOH
town, Maryland in 1938. By 1944, there were 200 central
Natrolite Na 2 [Si 3 Al 2 O 10 ] 2HOH
service regenerating plants serving 310 communities.
Sadalite Na 8 Al 6 Si 6 O 24 C 12
Siliceous zeolite xNa 20 Al 2 O 3 zSiO 2 aHOH
16.1.3.2 Removals of Specific Ions
Clays Montmorilonite Al 2 [Si 4 O 10 (OH) 2 ] yHOH
The list of specific ions that may be removed by ion-exchange Illite (K 2 H 3 O) (Al,Mg,Fe) 2 (Si,Al) 4 O 10 [(OH) 2 ,
includes: ammonia, various heavy metals, radioactive ions, (H 2 O)]
boron, nitrates, fluoride. Nitrate pollution is not uncommon in Kaolinite Al 2 O 3 2SiO 2 2H 2 O
shallow aquifers and heavy metals are a concern at hazardous Vermiculite (Mg,Fe ,Al) 3 (Al 2 Si) 4 O 10 (OH) 2 4H 2 O
2þ
wastes sites. Excess fluoride occurs in some drinking water
sources. Boron may be a residual in seawater deionization.
The perfectly uniform pore size of a given zeolite aids its ion-exchangers are granular, i.e., irregular, while resins are
selectivity for certain ions, e.g., Cs ,Sr , and heavy metals beads, with sizes generally about 0.5–l mm.
2þ
2þ
such as lead, i.e., Pb . Clinoptilolite, a mineral in the zeolite
2þ
family, is useful in concentrating radioactive ions or certain 16.1.4.1 Mineral Ion-Exchangers
heavy metals for containment and controlled disposal Most natural ion-exchange minerals are crystalline alumino-
(Vaughan, 1988, p. 27). Ammonia removal from sewage silicates with cation-exchange properties (Helferrich, 1962).
effluents is another application of clinoptilolite (Vaughan, Table 16.1 lists seven minerals of the zeolite group and three
1988, p. 27). The attenuation of ammonia peaks in treated clays. The zeolites have a lattice structure which is open with
sewage effluent, e.g., channels connecting. The lattice has a negative charge for
each aluminum atom. The charge is balanced by cations
C column influent, NH 4 Þ 16 mg=L
þ
which are free to move within the lattice. The clays in Table
ð
to 16.1, on the other hand, have a loose layer structure and can
swell, increasing the interlayer distance.
C column effluent, NH 4 Þ 1 mg=L,
þ
ð
16.1.4.2 Clays
was demonstrated in laboratory studies using clinoptilolite
packed-bed columns (Beler Baykal et al., 1996, 1997). A clay is a mineral particle characterized by its small size, i.e.,
1 mm. A variety of mineral types comprise the clays; com-
16.1.3.3 Deionization mon ones include montmorillonite and kaolinite. Another,
Deionization has many industrial applications, e.g., boiler also with ion-exchange properties, is vermiculite; cadmium
feed water, microchip production, analytical laboratories, uptake was shown to follow the Freundlich isotherm
2þ
and process water for various purposes. The process involves for 0.2 < C(Cd) 2mgCd=L, i.e., X*(Cd ) ¼ 1.82
2þ 0.948
a cation-exchanger charged with H and an anion-exchanger C(Cd ) , in which X*(Cd) ¼ (mg Cd =g vermiculite)
2þ
þ
2þ
2þ
charged with OH and may be done sequentially through and C(Cd ) ¼ (mg Cd =L solution) (Das and Bandyopad-
each bed or through a ‘‘mixed-bed’’; the cations and anions hyay, 1993, p. 2).
are exchanged for H and OH , respectively. 16.1.4.3 Zeolites

þ
For reference, the specific electric conductance (see gloss-
By the early twentieth century, chemists were trying to syn-
ary) of deionized water is typically about 0.05 mS=cm, which
thesize zeolite crystals and invented structures not found in
is about the same as distilled water. Typical values for ambi-
nature (Kerr, 1989, p. 100). Some 40 natural zeolites and over
ent waters are: Lake Tahoe, 97 mS=cm; Lake Mead, 850
mS=cm; Atlantic Ocean, 43,000 mS=cm. 70 synthetic zeolites have been discovered (Kato, 1995, p. 7).
These numbers have increased, however, since research on
zeolites has been continuous (see INZA, 2010; IZA, 2010).
16.1.4 MEDIA
For example, 194 zeolite ‘‘Framework Types,’’ each desig-
An ‘‘ion-exchanger’’ is a solid substance having an open nated by a code of three capital letters, e.g., ABW, ACO,
framework, which may be either crystalline or macromolecu- AEI, . . . , YUG, ZON, are seen within a matrix format (IZA,
lar, depending upon the ion-exchanger material. Because the 2010). Each code within the matrix is linked to 3D wire-frame
framework is open the exchange sites on it are accessible by drawing and other depictions that describe its characteristic
diffusion to any ions in a surrounding aqueous solution that ‘‘framework.’’ Within a given framework, numerous ‘‘mater-
are smaller than the pores within the solid. As a rule, mineral ials’’ may be included. For example, the Framework Type