Sulfide precipitation is based on low solubility of metal sulfides. Metal sulfide solubilities are much lower than those of hydroxides. Therefore, very efficient metal removal can be accomplished by the use of sulfide precipitation. Metal sulfide solubilities are also dependent on pH. Figure 9 presents the solubility of metal sulfides depending on pH . The process is
conducted at alkaline pH values to obtain minimum solubilities, but more importantly to avoid toxic H2S gas evolution. Sulfide can be added as soluble sulfide or insoluble sulfide. In the first method, soluble sulfides such as sodium sulfide, calcium polysulfide, or sodium hydrosulfide, are added while pH is maintained above 8.0. In the second method slightly soluble ferrous sulfide is added as a slurry. This slurry can be obtained by mixing FeSO4 and sodium hydrosulfide. Most heavy metal sulfides have lower solubilities than FeS, therefore heavy metal sulfides precipitate as FeS dissolves. The reaction is performed at pH values above 8 and iron precipitates as hydroxide [3,36]. An important advantage of the process is the reduction of chromium by sulfide, which is oxidized to the elemental state. Chromium precipitates as hydroxide. The reaction for the case of FeS addition is:
The process has other advantages. One of the most important advantages is the ability to precipitate the complexed metals without pretreatment and at normal operating conditions. The process does not require fine pH adjustment nor does it necessitate a two-stage treatment for different metals.
Sulfide precipitation has some disadvantages as well as strict operating rules. Sulfide precipitates have smaller particle sizes and poor settling characteristics. Thus, the use of coagulants is essential. The amount of sludge produced is greater than that produced by hydroxide precipitation. Handling and disposal of the sludge is difficult due to its hazardous nature. Excess sulfide is present in the effluent, which has H2S evolution potential and must be removed by air oxidation in an additional tank.
The operation of sulfide precipitation begins with the adjustment of wastewater pH to alkaline values. Lime or sodium hydroxide can be used to increase the wastewater pH. After neutralization, hydroxide sludges form and can be separately removed. Sulfide is added in a controlled manner. Excess sulfide addition causes sulfide odors, and may also cause formation of colloidal precipitates, which are quite difficult to precipitate. Overdosing also increases the need for sulfide oxidation after precipitation. Sulfide level is controlled by oxidation reduction potential (ORP) measurement, or by using sulfide-specific electrodes.
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