Page 255 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

H transformed into an amorphous aluminum hydroxide precipi-
H
O tate with neutral charge. Therefore, the aluminum hydroxide
coagulant must be dispersed uniformly, by effective rapid
H H
mix, within the first second of introduction. The second view
H O O H
Al is that positive-charged polynuclear species (see Section
9.5.3.6) such as Al13 are also formed, and are effective in
O O O
H Al Al H coagulation; but such species are more stable and have a
O longer life and so the rapid dispersal of alum is not so critical.
H O O H
They emphasize, however, that mixing is important whatever
O O Al O O the alum chemistry. As a note, the amorphous aluminum
H Al Al H hydroxide precipitate, if allowed to age, ends up eventually
O O O as gibbsite and bayerite, the most stable forms (Marshall,
H O H
Al Al 1964, p. 148). A conclusion is that alum chemistry is complex.
O O O O
O O O A comprehensive review of alum chemistry is provided by
Al Al H
H Gregory (2006, pp. 121–153), who also gives assessments of
O O O the state of knowledge on the various facets of the topic.
O O
H H H H 9.6 SYNTHETIC ALUMINUM POLYMERS
About 1985, a new commercial product, synthetic poly-
FIGURE 9.12 Ball-and-stick representation of AlO 4 Al 12 (OH) 24
aluminum chloride (PACl), appeared on the market; a similar
(H 2 O) 12 . (Adapted from Bertsch, P.M. and Parker, D.R., Aqueous
7þ
product, poly-aluminum sulfate (PAS), appeared soon after.
polynuclear aluminum species, in The Environmental Chemistry of
Aluminum, 2nd edn., Sposito, G., Ed., CRC-Lewis Publishers, Boca
Raton, FL, 1996, Chap. 4, Figure 2, p. 124.) 9.6.1 CHARACTERISTICS OF PACl
The utility of PACl is that the synthesis process produces
form may be modified to moles per liter by the conversion, preformed charged polymers, for example, Al 13 O 4 (OH) 24 ,
7þ
[mol=L] ¼ 10 pC . Table CD9.7 is the corresponding ferric 3þ 2þ
as well as Al , Al(OH) , and Al(OH) 4 (Pernitsky and
iron coagulation diagram. Edzwald, 2000). Such species retain their identity over time
and over a broad pH range and are effective over the range of
9.5.3.6 Polynuclear Species water treatment temperatures (Edzwald, 1993, p. 27). Both
Figure 9.12 depicts a ‘‘ball-and-stick’’ model of an Al-hydrated PACl and PAS are available in liquid form and are easy to
complex (Bertsch and Parker, 1996, p. 124) and is indicative of use, for example, can be metered in liquid form.
the wide variety of such complexes. As seen, the water mol-
9.6.1.1 Description of PACl
ecules are shown attached to Al central atoms with hydrogen
atoms vulnerable to detachment. Of the variety of aluminum The manufactured PACl product is a pale yellow liquid, with
hydrolysis species, with a few given in Table 9.5, those that have properties: 1.12 < SG 1.28, 4.3 viscosity 5.2 cp; 2.0
convincing experimental support include: Al 2 (OH) 2 , pH 2.6 (PPG Industries, c. 2000). The product is available
4þ
Al 2 (OH) 5 ,Al 3 (OH) 8 ,Al 3 (OH) 4 ,Al 8 (OH) 20 (H 2 O) 5 , in tank trucks, 2000 lb plastic containers, and 55 gal plastic
4þ
5þ
þ
þ
drums. The liquid is slightly corrosive and should be stored in
6þ 18þ
Al 6 (OH) 12 (H 2 O) 12 ,Al 54 (OH) 144 (H 2 O) 36 ,andAl 13 O 4
7þ (Bertsch and Parker, 1996, p. 122). tanks lined with materials such as epoxy, rubber, PVC, FRP,
(OH) 24 (H 2 O) 12
etc. The feed pump should be of material that will withstand
9.5.3.7 Summary of Alum Speciation acids The product is stable for several months if the tempera-
The aqueous equilibrium chemistry of aluminum may be ture is maintained from 108Cto 408C (158F 968F).
explained in terms of the following species (Dempsey et al.,
1984): 9.6.1.2 Electrophoretic Mobility: Comparing Alum
and PACl
5þ,
4þ
. Three polymeric species: Al 2 (OH) 2 ,Al 3 (OH) 4 Figure 9.13a and b show EM of alum and PACl, respectively, as
7þ
and Al 13 O 4 (OH) 24 a function of pH in deionized water, that is, near zero particle
. Five monomers: Al , AlOH , Al(OH) 2 , concentration and negligible ion concentrations. In both plots,
2þ
3þ
þ
EM decreases as pH increases. Comparing, the two plots, that is,
Al(OH) 3 , and Al(OH) 4
. A solid precipitate, Al(OH) 3 (s) Figure 9.13a and b, the overall higher EM values for PACl is
evident. Figure 9.13a shows that for alum EM is affected by
Regarding the alum chemistry of sweep-floc coagulation, two temperature, with values lower at 258Cthan at 48C. Figure 9.13b
views are given by Bache et al. (1999, p. 210). The first is that shows that for PACl the temperature has no discernible effect.
the positive-charged Al monomers (see preceding paragraph), The qualities indicated (high EM at all pH, no discernable
the effective species, form within 1 s before being temperature effect, and positive EM values even in deionized