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Coagulation 197

Coagulation theory, involving colloids, color, coagulants, and colloid n þ Al (OH) (H 2 O) n mþ
n
the coagulation process, evolved over decades. Two of the ! colloid [Al (OH) n (H 2 O) n ] (m n)þ (9:3)
key players in the formative decades of coagulation chemis-
try, A.P. Black and Wilfred Langelier, were academic chem-
where
ists in the water treatment industry and are a part of the lore of
colloid n is the negatively charged colloid with n negative
the field. Many others who contributed to modern theory are
charges
cited (and many others were not cited due to limitations in the
colloid [Al (OH) n (H 2 O) n ] (m n)þ is the colloid and metal
scope of this text).
ion complex
9.3.2 COAGULATION REACTIONS The positively charged complexes, on the left side in Equation
9.3, become incorporated within the diffuse ‘‘double layer’’ of
The reactions of metal ions, for example, Al 3þ and Fe , with
3þ
a negatively charged particle, which results in the reaction
water result in a variety of products, with the species depen-
product, that is, a colloid–metal ion complex, on the right
dent on pH, dosage, ionic strength, alkalinity, and perhaps
side, called here a ‘‘microfloc.’’ The van der Waals attractive
other factors. The reactions are complex and are only sum-
forces between the resulting ‘‘microflocs’’ then dominate,
marized here.
permitting them to attach to one another, forming a precipi-
tate. The job of rapid-mix is to induce collisions between the
9.3.2.1 Metal Ion Reactions with Water
reactants on the left side of Equation 9.3 to form such micro-
At low pH, for example, 4 < pH < 6, when Al 3þ or Fe 3þ react
flocs. Since the life of the complexes is very short, that is, in
with water, the reaction products are ‘‘complexes’’ with water.
terms of milliseconds, a very large fraction of the collisions
At higher pH levels, for example, 6 < pH < 10, and especially
between reactants must occur before leaving the ‘‘rapid-mix’’
at higher dosages, metal hydroxide is the major product. The
(Chapter 10).
two categories of reactions are
9.3.2.2.2 Sweep Floc (pH > 6)
1. Complexes,
The ‘‘sweep-floc’’ reaction (Amirtharajah and Mills, 1982) is
between aluminum hydroxide precipitate, Equation 9.2, and
Al 2 (SO 4 ) 3 þ nH 2 O
colloids in suspension and occurs at pH > 6,
! 2Al (OH) (H 2 O) n nþ þ H þ 3SO 4 2 (9:1)
þ
n
colloid þ Al(OH) ! colloid [Al(OH) ] (9:4)

3
3
2. Metal ion precipitate,
In sweep-floc, the positively charged Al(OH) 3 precipitate
þ 2 (9:2)
Al 2 (SO 4 ) 3 þ6H 2 O!2Al(OH) 3 þ6H þ3SO 4 contacts the negatively charged colloids through random con-
tacts, for example, through turbulence in ‘‘rapid-mix,’’ result-
where ing in particle attachment and enmeshment. The aluminum
Al 2 (SO 4 ) 3 is the aluminum sulfate, that is, alum hydroxide floc is amorphous in nature with large surface
nþ 2
Al (OH) n (H 2 O) n is the hydrated aluminum area, for example, 159–234 m =g for ferric floc (Randtke,
complex with water (variable charge and vari- 1988, p. 41), which also facilitates particle enmeshment
able waters of hydration) (Matijevic, 1967, p. 337). The removal of the suspended
Al(OH) 3 is the aluminum hydroxide precipitate microscopic particles is, in general, proportional to the floc
(waters of hydration not shown) surface area.
Figure 9.3 shows two photomicrograph examples of alum
9.3.2.2 Two Coagulation Mechanisms floc enmeshing diatoms. Both figures (Figure 9.3a and b)
The two coagulation zones in Al 3þ and Fe 3þ coagulation are illustrate the amorphous but differing character of an alumi-
as follows: (1) charge neutralization for pH < 6; and (2) sweep num hydroxide floc particle enmeshing diatoms. The particles
floc for pH > 6. The reaction products for the two zones are are not free to disengage, that is, they appear to involve
‘‘hydrolysis-products’’ and Al(OH) 3 precipitate, respectively bonding between the particles and the alum floc.
(Amirtharajah and Mills, 1982).
9.3.2.3 NOM Removal by Metal Coagulatants
9.3.2.2.1 Charge-Neutralization (pH < 6) Independent variables that affect NOM removal include pH
In Equation 9.1, the Al product is actually an array of ‘‘poly- and coagulant dosage, with dosage being proportional to the
nuclear’’ Al cations, also called aluminum ‘‘hydrolysis’’ prod- humic concentration, that is, it is stoichiometric (Black and
ucts, or ‘‘complexes,’’ and occur in the pH range, pH 6. Willems, 1961, p. 592; O’Melia et al., 1979, p. 594; Dempsey
The complexes are positively charged, have only a few milli- et al., 1984). The ‘‘zones’’ of coagulation for NOM were
seconds of life, and react, in turn, with negatively charged delineated by Edwards and Amirtharajah (1985), which
microscopic particles. The reaction is termed ‘‘charge-neutral- extended work done for turbidity removal by Amirtharajah
ization,’’ and is depicted as and Mills (1984).