Page 274 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Coagulation 229

Boltzman equation: A gas occupying a given volume has an Chelate: Complex involving multidentate ligand. See ligand.
overall average energy per mole; the gas molecules . Monodentate ligand—Ligands that attach at only
actually have a distribution of energies as defined by one point, such as H 2 O, OH ,Cl , and CN .

the Boltzman equation, that is, N=N o ¼ e (E=kT) . . Multidentate ligand—Ligands that attach at two
Bridging: Generally, the term applies to the condition in or more sites.
which single polymer molecules attach to two or Chelate: Polydentate ligands can produce a chelate (Greek
more suspended particles. for claw), a complex in which a ligand forms a ring
Briggs cell: A cell of specified dimensions that provides for a that includes the metal ion (Shriver and Atkins,
specified flow through with a specified voltage 1999, p. 220).
between plates (see Black and Smith, 1962 who Chelating agent: A multidentate ligand.
describe an operational cell as referenced in a 1940 Clay: Soil fraction characterized by small size, for example,
article by D.R. Briggs). 2 mm, negative charge, and plasticity when mixed
Bronsted acid: A substance that may contribute a proton to with water (Mitchell, 1993, pp. 18–30). Some alu-
solution or cause a proton to be contributed to solu- minosilicate clays listed are as follows: kaolinite,
tion. A water ligand bound to a central metal ion may illite, vermiculite, smectite (group includes mont-
contribute a proton to solution. The complex is thus morillonite), and chlorite (Hemingway and Sposito,
acting as a Bronsted acid. 1996, p. 87). Clays may have cations on the layered
Brownian motion: Random motion of molecules in a gas due surfaces that may be exchanged easily with other
to thermal energy, measured by the parameter, kT in cations in solution and are called ‘‘exchangeable’’
gases (in which k is the Boltzman constant and T is the cations. The amount of exchangeable cations
absolute temperature). Note that k ¼ R=N o , where R is expressed in milleequivalents per 100 g of dry clay
the universal gas constant and N o is Avogadro’s num- is called the ‘‘cation exchange capacity’’ (CEC),
ber, that is, the number of molecules in a mole. sometimes called the ‘‘base exchange capacity.’’
Carboxyl group: A chemical group, COOH, which may Montmorillonite has a CEC 70 meq=100 g.
be attached to a larger carbon-based molecule, Also, montmorillonite is characterized by interlayer
for example, a polymer. A characteristic of the group swelling. Illites, on the other hand, are distinguished
is its ionization, which may leave a charged site, that by the lack of interlayer swelling. For kaolinite,
is, at pH > 4, COOH þ H 2 O ! COO þ H 3 O . 1 < CEC < 10 meq=100 g (van Olphen, 1977,

þ
Cartridge filter: Fiber-wound filter about 30 cm long pp. 57–76).
and 12 cm diameter and hollow in center, used Coagulant: (1) A substance, usually a trivalent cation, which
commonly since the 1980s for evaluation of filter may combine directly with colloids to form ‘‘micro-
performance or assessment of organisms in natural flocs;’’ more likely however, the hydrolysis products
water bodies. The results are expressed as numbers of Al 3þ (or Fe ) reacting with water combine with
3þ
of organisms of different species and particles suspended colloids and charged particles. (2) Any
retained per liter of water that has passed through chemical that destabilizes a sol suspension (adapted
the filter. The sampling=analysis procedure and from O’Melia, 1978, p. 241).
results is designated, ‘microscopic particulate analy- Coagulant aid: A substance that may be added to rapid-mix
sis’ (MPA). Figure 13.2 shows two cartridge filters to improve the results of coagulation. The most
as used in the evaluation of particle removal for a common is a polymer. In cases of low turbidity
slow sand filter. The cartridge is placed in a pressur- water, a clay, such as bentonite has been used to
ized holder designed to permit water flow from the aid coagulation.
outside to the hollow center. Recommended flow Coagulation: (1) Coagulation comes from the Latin word,
through the cartridge is 4 L=min (1 gpm), albeit coagulare—to be driven together and is brought
flows of perhaps 40 L=min (10 gpm) have been about by a reduction of the repulsive potential of
used. After removal, the cartridge is placed in a the electrical double layer in accordance with the
1 gal size Zip-loct plastic bag and placed on ice DLVO theory causing particle destabilization
for transport to a laboratory for MPA analysis. See (Black, 1967, p. 277; Stumm and O’Melia, 1968).
microscopic particulate analysis. (2) The chemical process of reducing the zeta poten-
Cation: Ion with positive charge, for example, Na ,Ca , tial and destabilization of particles (Moffett, 1968,
þ
2þ
Al ,Fe ,. . . . p. 1256). (3) The chemical reduction of repulsive
3þ
3þ
Cationic polymer: Polymer with attached ionic groups that forces between particles such that the van der
are positive. Waals forces become dominant so that particles
Central ion: Usually a metal ion that coordinates with a will stick when they collide and form aggregates
ligand. called ‘‘microflocs.’’ The process remains an ‘‘art’’
Charge neutralize: Attachment of a positive charged poly- (Gregory, 1975, p. 61). (4) Concerning kinetics, the
meric metal ion species or cationic polymer to a coagulation reactions are nearly instantaneous and
negatively charged colloid. the only time required for their completion is what