Page 539 - Materials Chemistry, Second Edition

CAT3525_C16.qxd 1/27/2005 12:42 PM Page 510
510 Waste Management Practices: Municipal, Hazardous, and Industrial
Converting mg/L of Cu to mol/L of Cu,
2
[Cu ] (0.50 mg/L)/[(63.54 g/mol)(1000 mg/g)] 7.87 10 6 mol/L
The solubility product equation above is rearranged to solve for the hydroxide concentration.
2
6
[OH ] (2.0 10 19 )/(7.87 10 ) 2.54 10 14

)
[OH ] (2.54 10 14 1/2
1.59 10 7
7
pOH log (1.59 10 )
6.80
pH 14 – pOH
14 – 6.8
7.20
Thus, the pH should be set to 7.20 to remove the soluble copper to 0.5 mg/L.
The neutralization of alkaline waste is achieved by reaction with an adequate quantity of an acid
to bring the solution pH to within the desired range. Methods of neutralizing alkaline wastes include
(Haas and Vamos, 1995):

● Adding appropriate amounts of strong or weak acid to the waste
● Adding compressed carbon dioxide gas to the waste
● Blowing acidic flue gas (e.g., from coal combustion or municipal solid waste [MSW]
incineration) through the waste
● Mixing the alkaline waste with an acidic waste
Alkaline wastes are most commonly neutralized by reaction with mineral acids, typically sulfu-
ric and hydrochloric acid. Sulfuric acid is popular by virtue of its relatively low cost. Neutralization
residence times of 15 to 30 min. are recommended with sulfuric acid (Wilk et al., 1988; Haas and
Vamos, 1995). A disadvantage of using sulfuric acid is that it will form sludges when reacted with
calcium-containing alkaline wastes.
Hydrochloric acid (HCl) is more expensive than sulfuric acid; however, HCl neutralization will
not produce sludges when neutralizing calcium-containing alkaline wastes. Neutralization reaction
residence times of 5 to 20 min. are recommended with HCl (Wilk et al., 1988; Haas and Vamos,
1995). A disadvantage of using HCl is that it can form a corrosive and irritating acid mist upon reac-
tion. Nitric acid (HNO ) may also be used for neutralization; however, it is dangerous because it is
3
a powerful oxidizing agent.
16.3.2 CHEMICAL PRECIPITATION

During the chemical precipitation of hazardous waste streams, a soluble hazardous species is
removed from the solution by the addition of a precipitating reagent; an insoluble compound sub-
sequently forms that contains the hazardous constituents (Equation 16.2). The precipitate is
removed from the solution using a physical separation technique such as sedimentation or filtration.
Coagulants or flocculants may be added to the mixture to enhance the separation of the precipitate
from the soluble phase. Examples of common inorganic coagulants are aluminum sulfate (alum),
(Al (SO ) ·18H O) and ferric sulfate (Fe (SO ) )
4 3
2
4 3
2
2
A schematic of a typical precipitation process is provided in Figure 16.3.
Precipitation processes are typically geared toward the removal of dissolved inorganic ions, par-
ticularly metals:

Cd 2 (aq) HS (aq) → CdS(s) H (aq) (16.2)