Page 542 - Materials Chemistry, Second Edition

CAT3525_C16.qxd 1/27/2005 12:42 PM Page 513
Hazardous Waste Treatment 513
of effluents and wastes including wastewater and sludges containing oxidizable constituents
(Manahan, 1994). Ozone is a strong oxidant, and decomposes in a short time; it is therefore gener-
ated on-site by an electrical discharge through dry air or oxygen. Ozone must be used with caution
as it is a nonselective and a rapid oxidant. Similarly, H O and Cl are nonselective and highly reac-
2
2
2
tive depending on their initial concentration.
Cyanide-bearing wastewater generated by the metal-finishing industry is often oxidized with
alkaline chlorine or hypochlorite solutions. In these reactions the chlorine is correspondingly
reduced. In the process, the cyanide contaminant is initially oxidized to a less toxic cyanate and then
to carbon dioxide and nitrogen in the following reactions (U.S. EPA, 2000a):
2NaOH Cl → NaOCl NaCl H O (16.7)
2
2
NaCN Cl → CNCl NaCl (16.8)
2
CNCl 2NaOH → NaCNO NaCl H O (16.9)
2
2NaCNO 3Cl 4NaOH → 2CO N 6NaCl 2H O (16.10)
2
2
2
2
Oxidation of cyanide may also be accomplished with hydrogen peroxide, ozone, and electrolysis
(Dawson and Mercer 1986; Blackman, 2001).
EXAMPLE 16.2
A metal processing industry produces 95,000 L per day of a waste stream containing 325 mg/L
cyanide as NaCN. Calculate the stoichiometric quantity of Cl required daily to destroy the cyanide.
2
SOLUTION

Combining reactions 16.7 to 16.10, we obtain
2NaCN 5Cl 12 NaOH → N Na CO 10NaCl 6H O (16.11)
2 2 2 3 2
From this reaction, we can see that a total of 2.5 mol of Cl are required to react completely with 1
2
mol of NaCN.
The total mass of NaCN generated daily is
6
(95,000 L/day) (1 kg / L) (325 parts /10 parts) 30.88 kg NaCN/day
The kg-mol of NaCN generated per day is
(30.88 kg/day) / (49 kg/kg-mol) 0.630 kg-mol NaCN/day
The amount of Cl required daily is
2
2.5 (0.63) 1.576 kg-mol Cl
2
(1.576) (70.9 kg/kg-mol)
111.7 kg/day Cl required
2
Note. Due to reaction with other contaminants, more than the stoichiometric amount of Cl will
2
be required to complete this process.
EXAMPLE 16.3

For the waste stream in Example 16.2, determine the stoichiometric amount of NaOH required to

oxidize the cyanide to N . Refer to Equations 16.8 to 16.10 for the conversion of CN into N .
2 2

The molecular weight of CN 26, Cl 70.9, and NaOH 40.
2