Page 541 - Materials Chemistry, Second Edition

CAT3525_C16.qxd 1/27/2005 12:42 PM Page 512
512 Waste Management Practices: Municipal, Hazardous, and Industrial
Metals can also be precipitated from solution by the action of a reducing agent such as sodium
borohydride (Manahan, 1990):

4Cu 2 NaBH 2H O → 4Cu NaBO 8H (16.6)
4
2
2
Precipitation reactions result in the formation of the treated effluent and a sludge containing most
of the contaminants originally present in the solution. After precipitation, the volume of the sludge
should be substantially less than the volume of the original solution. Therefore, the precipitation
process is considered a volume-reduction process and not a destruction process. The sludge may
contain mostly water and must be dewatered before disposal. Dewatered sludges are often charac-
teristic hazardous wastes based upon Toxicity Characteristic Leaching Procedure testing or are
listed hazardous wastes based on the industrial process from which the wastes were generated. Such
sludges may require further treatment prior to landfilling as required by the LDR.
The formation and settling of inorganic precipitates (commonly termed flocculation and sedi-
mentation) can also entrap both dissolved and colloidal organic contaminants via physical and
chemical mechanisms. The removal of organic contaminants by precipitation is viewed as a bene-
ficial side-reaction of the process (Haas and Vamos, 1995).
Industries producing wastewaters amenable to precipitation include metal plating and finishing,
steel and nonferrous, inorganic pigments, mining, and electronics. Landfill leachate and contami-
nated groundwater can also contain hazardous species that are suitable for chemical precipitation.

16.3.3 OXIDATION AND REDUCTION
Oxidation and reduction reactions are applied for the treatment, removal, and destruction of a vari-
ety of inorganic and organic wastes. By definition, an oxidation reaction increases the valence of
an ion due to the removal of electrons; conversely, a reduction reaction decreases an ion’s valence
because electrons are added to its shell.
For oxidation to proceed, an oxidizing agent is reacted with the waste in question. A number of
useful and common oxidizing agents are listed in Table 16.3. Some of the more common ones
include ozone (O ), hydrogen peroxide (H O ), and chlorine gas (Cl ). Ozone, employed as an
2
2
3
2
oxidant gas at levels of 1 to 2 wt% in oxygen and 2 to 5 wt% in air, has been used to treat a variety
TABLE 16.3
Commonly Used Oxidizing and Reducing Agents
Oxidizing Agents
F
2
O 2
O
3
Cl
2

Permanganate (MnO 4 )
2 )
Chromate (CrO 4
Dichromate (Cr O 2 )
2 7
Nitric acid (HNO )
3
)
Perchloric acid (HClO 4
Sulfuric acid (H SO )
2 4
Reducing Agents
Sodium
Magnesium
Aluminum
Zinc
Metal hydrides: NaH, CaH , LiAlH
2 4