Page 714 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Precipitation 669

and with residual Ca 2þ and Mg 2þ concentrations measured as a ‘‘complex’’ ion (or a ‘‘chelate’’). Manganese, as another
the dependent variables. The jar test results are plotted with example, has several oxidation states, for example, Mn(s),
Ca 2þ and Mg 2þ concentrations plotted on the ordinate against which reacts with H þ in the aqueous phase to form
the lime or lime=soda dosages on the abscissa. Mn (aq) and H 2 (g), E 8 ¼ 1.1.8 V (Silberberg, 1996,
2þ
pp. 982–984). Also, as stated by Silberberg, like chromium,
21.2.2.5 Lime-Soda Process manganese can use all its valence electrons in its compounds,
The portion of hardness that has no alkalinity counterions exhibiting every possible oxidation state, with the þ2, þ4,
associated with it is ‘‘permanent’’ hardness. For example, if and þ7 states being most common. As the oxidation state of
manganese increases, its ‘‘valence state electronegativity’’
the counterions are Cl ,SO 4 ,NO 3 , etc., the carbonate,
2

that is, CO 3 , must be supplied from an external source; the also increases and its oxides change from basic to acidic.
2
most common is soda, that is, Na 2 CO 3 . Looking at the net Mn(II) is basic, and manganese (III) oxide, Mn 2 O 3 , is ampho-
reaction only, the associated reaction is teric. Manganese (IV) oxide, MnO 2 , is insoluble and thus
shows no acid–base properties. Manganese (VII) oxide,
Mn 2 O 7 , forms by reaction of Mn with pure O 2 and reacts
Ca 2þ þ CO 3 2 ! CaCO 3 (21:7)
with water to form permanganic acid, HMnO 4 , a strong oxi-
dizing agent. All manganese species with oxidation states
The soda ash demand is the stoichiometric amount needed to
>þ2 act as oxidizing agents, but the permanganate ion is
satisfy the requirement for permanent hardness.
particularly powerful. As with chromium in its highest oxida-
tion state, MnO 2 is a much stronger oxidant in acidic solu-

21.2.3 CHEMISTRY OF METALS tion than in a basic solution, that is,
The chemistry of metal ions may be rather complex because of
MnO 4 (aq) þ 4H (aq) þ 3e
þ

the different valence states and complexes that may form. For
example, arsenic is stable in four oxidation states, that is, þ5, ! MnO 2 (s) þ 2H 2 O E ¼ 1:68 V (21:8)

þ3, 0, and –3, depending on the redox potential (with values
MnO 4 (aq) þ 2H 2 O(aq) þ 3e

high to low, respectively). The aqueous forms are mostly
arsenite, As(III), and arsenate, As(V); and in an oxidizing ! MnO 2 (s) þ 4OH E ¼ 0:59 V (21:9)

environment the arsenate predominates, and under moderately
reducing conditions arsenite is the major species. Because of Table 21.4 summarizes the foregoing discussion, showing
slow kinetics, however, both arsenite and arsenate may exist in oxidation states of manganese, with examples on the line
either redox environment. Some forms of arsenic are removed below, and the acid–base level associated with the respective
more easily than others, with oxidation state being a major oxidation state. The top line depicts the expected appearances
factor. To illustrate further the complexity of removal, the of the respective oxidation states, with the second line
reaction between arsenate and iron (Fe ) forms an insoluble describing color.
3þ
precipitate, ferric arsenate, FeAsO 4 ; on the other hand, ferric As related to natural cycles of manganese, Mn , the
2þ
arsenite, FeAsO 3 , is soluble in water and does not precipitate. reduced form, occurs in a reducing atmosphere, that is,
Many of the other metals, for example, Al, Cd, Cr, Cu, Fe, anoxic. When withdrawn for drinking water, the Mn 2þ will
Hg, Mn, Pb, also have multiple oxidation states and=or may oxidize at some point in the system, with MnO 2 being a
serve as the ‘‘central’’ atom with associated ‘‘ligands’’ to form product; MnO 2 causes the black water noted by customers
TABLE 21.4
Examples of Oxidation States of Manganese

Depiction
Color Pale pink a Black Br-blake a Green a Purple a
Oxidation state Mn(II) b Mn(III) Mn(IV) b Mn(VI) Mn(VII) b
Example Mn 2+ Mn 2 O 3 MnO 2 MnO 4 2– MnO 4 –
Oxide acidity Basic Acidic

Source: Adapted from Silberberg, M., Chemistry—The Molecular Nature of Matter and Change,
Mosby-Year Book, Inc., St. Louis, MO, 1996.
a
Silberberg (1996, p. 984; Casale et al. 2002, p. 109).
b
Most common species.