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Experimentally determined solubilities of metal hydroxides.

FIG. 11.15.9 Solubilities of metal hydroxides at various pH's. (Reprinted, with permission from Graver Water.)

Experimentally determined solubilities of metal hydroxides.

FIG. 11.15.9 Solubilities of metal hydroxides at various pH's. (Reprinted, with permission from Graver Water.)

Soluble sulfides, such as hydrogen or sodium sulfide, and insoluble sulfides, such as ferrous sulfide, are used for precipitation of heavy metals. Sodium bisulfide is commonly used for precipitating chromium out of solution. Sulfates, including zinc sulfate or ferrous sulfate, are used for precipitation of cyanide complexes. Carbonates, especially calcium carbonate, are used directly for precipitation of metals. In addition, hydroxides can be converted into carbonates with carbon dioxide, and easily filtered out.

Hydroxide precipitation with lime is most common; however, sodium sulfide is sometimes used to achieve lower effluent metal concentrations. Solid separation is effected by standard flocculation/coagulation techniques. The residuals are metal sludge and treated effluent with an elevated pH and, in the case of sulfide precipitation, excess sulfide.

The metal's valence state is important in the process of precipitation. For example, ferrous iron is considerably more soluble than ferric iron, making oxidizing agent treatment to convert ferrous iron to ferric iron an essential part of the iron-removal process. Another example is hexava-lent chromium, Cr+6, which is more soluble than the less hazardous trivalent form. Chromates must be reduced before removal of trivalent chromium in a precipitation process. Also, the engineer must consider the possibility of complex ion formation when dealing with waste water containing ammonia, fluoride, cyanide, or heavy metals. For example, an iron complex may be the ferrocyanide ion, which is soluble, and remains in solution unless the complex is broken by chemical treatment.

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