Page 248 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Coagulation 203

TABLE 9.4
Molecular Weights of Alum and Ferric Forms
Atomic Weights Aluminum Forms Ferric Forms
Element Atomic Weight Compound MW Compound MW
Al 26.982 Al 2 (SO 4 ) 3 14.3H 2 O a 599.536 Fe 2 (SO 4 ) 3 4.5H 2 O b 480.872
Fe 55.845 Al 2 (SO 4 ) 3 14H 2 O 594.136 Fe 2 (SO 4 ) 3 b 399.876
S 32.066 Al 2 (SO 4 ) 3 18H 2 O 666.132 Fe 2 O 3 159.687
O 15.999 Al 2 (SO 4 ) 3 342.15 Fe 2 119.690
c
H 1.000 Al 2 O 3 101.961 FeCl 3 162.203
O 15.999 Al 2 53.964 FeCl 3 6H 2 O c 270.197
Cl 35.4527
a
The number of waters of hydration for aluminum sulfate is sometimes given as 14.3; the number recommended by C. Lind
(General Chemical) was 14, that is, without the decimal.
b
The number of waters of hydration for ferric sulfate is 4.5, which is an approximate value (AWWA Standard B406-97,
AWWA, 1997). The hydrated form dissolves readily; the anhydrous form does not. The liquid form contains about 50% of
the dry form by weight.
c
Ferric chloride is available in liquid form; waters of hydration are not included in the molecular weight (AWWA Standard
B407-98). The AWWA Standard is for the liquid form only. The solid form is available in two forms: hexahydrate and
anhydrous.

species depends on the pH and concentration (that is, ionic Table 9.4 lists chemical formulae for compounds of each
strength) of the solution. of the two trivalent ions, that is, Al , and Fe , respectively.
3þ
3þ
The associated molecular weights are listed which permits
conversion from one form to another. The atomic weights
9.5.1 ALUMINUM AND FERRIC IONS
are listed on the left for convenience.
The trivalent ions, aluminum, Al , and ferric iron, Fe ,
3þ
3þ
have similar reactions with water. The difference between Example 9.1 Conversion of Concentration
the two is mostly in the equilibrium constants, that is, not in Expressions
the kind of complexes formed.
Problem
9.5.1.1 Waters of Hydration
Suppose that 100 mg=L alum as Al 2 (SO 4 ) 3 14H 2 Oisa
Knowledge of waters of hydration is needed to calculate the required coagulant dosage. Determine equivalent expres-
molecular weight of a given coagulant and to designate the sions in terms of mg Al 2 (SO 4 ) 3 =L and mg Al =L. Table 9.4
3þ
coagulant used. In manufacturing the aluminum salt is provides ready reference for atomic weights and molecu-
hydrated as Al 2 (SO 4 ) 3 14H 2 O, which is commercial alum. lar weights, respectively.
The reagent grade alum is hydrated as Al 2 (SO 4 ) 3 18 H 2 O.
Solution
Ferric sulfate is hydrated as Fe 2 (SO 4 ) 3 4.5 H 2 O (AWWA
The ratio of the molecular weights is the basis for conver-
Standard B406-97, AWWA, 1997). Solid ferric chloride sions. Thus,
may be hydrated as Fe 2 Cl 3 6H 2 O, but may be anhydrous;
in liquid form it is not hydrated (AWWA Standard B407-98, 1. To convert from Al 2 (SO 4 ) 3 14H 2 Oas mg=Lto
AWWA, 1998a). mol=L,

9.5.1.2 Expressing Concentrations 100mg Al (SO 4 ) 3 14H 2 O Al 2 (SO 4 ) 3 14H 2 O mol
2
The concentration of aluminum or ferric ion may be expressed L solution 594,136mg Al (SO 4 ) 3 14H 2 O
2
in a variety of ways, for example, Al ,Al 2 (SO 4 ) 3 , 0:0001683 molAl 2 (SO 4 ) 3 14H 2 O
3þ
Al 2 (SO 4 ) 3 14H 2 O, Al 2 (SO 4 ) 3 18H 2 O. For alum, the expres- ¼ L solution
sion used most frequently is Al 2 (SO 4 ) 3 14H 2 O, and is under-
stood as a rule. Reagent grade alum, on the other hand, which is 2. Regardless of the form of expression number of
moles is the same. Therefore, in terms of Al 2 (SO 4 ) 3 ,
used often in research, is always expressed as Al 2 (SO 4 ) 3
18H 2 O. In research, concentrations are often expressed as
‘‘mol=L,’’ along with the mass concentration in ‘‘mg salt 0:0001683 mol Al 2 (SO 4 ) 3 342,150 mg Al (SO 4 ) 3
2
L solution
species=L.’’ The important thing is to qualify the concentration mol Al 2 (SO 4 ) 3
in the terms of the expression used. Example 9.1 illustrates a 57:58 mg Al (SO 4 ) 3
2
calculation protocol for conversion between forms. ¼ L solution