Applicability Limitations

This technology is used to treat aqueous wastes containing metals. Limitations include the fact that metals have different optimum pH levels for precipitation. Chelating and complexing agents can interfere with the process. Organics are not removed except through adsorptive carryover. The resulting sludge may be hazardous by definition, but often may be taken off the list by special petition.

Precipitation has many useful applications to hazardous waste treatment, but laboratory jar tests should be made to verify the treatment. The jar test is used to select the appropriate chemical; determine dosage rates; assess mixing, flocculation and settling characteristics; and estimate sludge production and handling requirements.

one compound is raised, while the oxidation state of another compound is reduced. Oxidation and reduction change the chemical form of a hazardous material: rendering it less toxic; changing its solubility, stability, or separability; or otherwise changing it for handling or disposal purposes. In the reaction, the compound supplying oxygen, chlorine or another negative ion, is called the oxidizing agent while the compound supplying the positive ion and accepting the oxygen is called the reducing agent. The reaction can be enhanced by catalysis, electrolysis or irradiation.

Reduction lowers the oxidation state of a compound. Reducing agents include: iron, aluminum, zinc, and sodium compounds. For efficient reduction, waste pH should be adjusted to an appropriate level. After this is accomplished, the reducing agent is added and the resulting solution is mixed until the reaction is complete. This treatment can be applied to chemicals such as hexavalent chromium, mercury, and lead. Other treatment processes may be used in conjunction with chemical reduction.

Cyanide-bearing wastewater generated by the metal-finishing industry, is typically oxidized with alkaline chlorine or hypochlorite solutions. In this process, the cyanide is initially oxidized to a less toxic cyanate and then to carbon dioxide and and nitrogen in the following reactions:

In the first step, the pH is maintained at above 10, then the reaction proceeds in a matter of minutes. In this step great care must be taken to maintain relatively high pH values, because at lower pHs there is a potential for the evolution of highly toxic hydrogen cyanide gas. The second reaction step proceeds most rapidly around a pH of 8, but not as rapidly as the first step. Higher pH values may be selected for the second step to reduce chemical consumption in the following precipitation steps. However, cyanide complexes of metals, particularly iron and to some extent nickel, cannot be decomposed easily by the cyanide oxidation method.

Cyanide oxidation can also be accomplished with hydrogen peroxide, ozone, and electrolysis.

Status. Commercially available.

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