Page 580 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Ion-Exchange 535

To convert to hardness expressed as CaCO 3 , access to the interior of the beads (Helfferich, 1964,
MW(CaCO 3 ) ¼ 100.087 g=mol or 50.044 p. 61). The term was coined by Robert Kunin and his
associates at Rohm and Haas (Kunin, 1983, p. 48) to
g=equivalent. Therefore, 5.708 meq hardness=L
50.044 mg CaCO 3 =meq ¼ 285 mg hardness as distinguish the foregoing pore structure (network)
CaCO 3 =L, which is a ‘‘hard’’ water. (As a note, from the gel-type (intertwined polymer strands with
expressing hardness in terms of CaCO 3 is probably cross-linking to provide adhesions between strands).
used because of tradition and because it has been The macro-reticular structure results in pore sizes
used so extensively that the relative concentrations in 50–1,000,000 Å, which, therefore, provide for
terms of CaCO 3 are understood with respect to rela- large molecules to gain access to the interior sites.
2
tive impacts. Hardness as equivalents per liter would The surface areas are 7–600 m =g dry resin (Kunin,
be a more direct and a more rational expression.) 1979, p. 2).
Heavy metal: A metal that has a ‘‘higher’’ molecular weight Microporous: Zeolite is classed as a microporous solid
such as lead, cadmium, mercury, etc.; usually these because of its regular repeating array of pores
metals are considered toxic at low concentrations, within crystal units connected by angstrom-sized
e.g., mg=L. channels (see Van Tassel et al., 1994, p. 925). As
Ion-active group: See functional group. related to a resin, the term microporous refers to
Ion-exchange: (1) The process of exchanging an ion in solu- pores 20 A; gel-type resins are in this category
tion for one of the same charge that was attached to a (Kunin, 1979, p. 1).
‘‘site’’ within an ion-exchanger. Changing Ca 2þ in Milliequivalents: The number of equivalent weights divided
solution for Na in the ion-exchanger is an example. by 1000. Usually, the expression is applied to the
þ
The foregoing is a traditional softening application. expression of the concentration of a given ion in
In years since the 1980s, USEPA drinking water meq=L.
regulations have been increasingly important, and Mixed-bed: A mixed-bed contains a cation-exchanger
engineers have sought ion-exchangers that have charged with H þ and an anion-exchanger charged
selective properties, e.g., to remove arsenic, fluoride, with OH ; both are synthetic resins, in bead form.

boron, nitrate. (2) A separation process which util- In the removal cycle, the various cations, e.g., Na ,
þ
izes functional or active sites on so-called inert Ca ,Mg , exchange with hydrogen and the
2þ
2þ
matrices (Calmon, 1985, p. 2). various anions, e.g., Cl , HCO 3 ,SO 4 ,NO 3 ,
2

Ion-exchanger: An ion-exchanger can be any solid substance exchange with OH , with water being the product.

having an array of ion-exchange sites. Usually the Monomer: A unit of a polymer. Styrene is a monomer that
substances-providing sites are either crystalline or comprises polystyrene.
macromolecules. A material, e.g., a zeolite or other pH: Defined as the negative log of the hydrogen ion concen-
14
crystal structure, a synthetic resin, or other material, tration. For water, [H ][OH ] ¼ 10 and pH ¼

þ
that may have exchange sites within its structure. 14-pOH .

The sites are accessible by diffusion from an external pK: The negative log of the equilibrium constant for an
solution. Ions that are smaller than the pore size may acid-base reaction. To illustrate, consider the disso-
be excluded from the space. ciation of acetic acid: HA ¼ > H þ A . The equi-

þ
Isotherm: The relationship between the solid phase concen- librium statement is, K a ¼ [H ][A ]=[HA]. Take
þ

tration of a given ion and its aqueous phase logs of both sides, log K a ¼ log[H ] þ log[A ]

þ
concentration at a specified temperature. log[HA]. Multiply both sides by ( 1) to get, log
Leakage: The appearance in the effluent of an ion-exchange K a ¼ log[H ] log[A ] þ log[HA]. Then, pK a ¼
þ

column of ions which should be removed. Leakage pH þ p[A ] p[HA], which defines pK a for the dis-

may be due to unfavorable equilibria or incomplete sociation of acetic acid. For acetic acid, pK a (acetic
regeneration (Dow, 1964, p. 73). acid) ¼ 4.7.
Macro-porous: A generic term that refers to resins with pores Polymer: A repeating monomer unit, e.g., polystyrene.
200 Å (Kunin, 1979, p. 1). On the other hand, the Poly-electrolyte: A repeating monomer unit, e.g., polystyr-
macro-porous resins may also refer generically to ene that has ion-functional groups, e.g., sulfonic
those resins that have large pore sizes, which are, at acid, attached to a monomer.
the same time, of the macro-reticular type with pore Pore (for an ion-exchanger resin): Spaces of distances
sizes 50–1,000,000 Å (Kunin, 1979, pp. 2–3). between polymer chains and cross-links. The sizes
Macro-reticular ion-exchange resin: A highly porous resin depend on the degree of cross-linking (Kunin, 1983,
prepared by adding an organic solvent for the resin p. 48).
monomers, but a poor solvent for the polymer, to the Quaternary: Etymology: Middle English, from Latin quater-
polymerization mixture. As polymerization pro- narius, adjective, consisting of four each, from qua-
gresses, the solvent is squeezed out by the growing terni four each (from quater four times) þ -arius –ary.
copolymer regions. Spherical beads obtained with 1 a : a group of four. (http:==unabridged.merriam-
pores several hundred angstrom units provide easy webster.com)