Waterwright
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Removal
of Cadmium from Wastewater
Cadmium is found in aerospace plating shop wastewater. Cadmium is used almost exclusively on aerospace hardware such as landing gear components. It can also be found in very low concentrations in rinse waters from solder cleaning operations where it is a contaminant in the solder.
Cadmium is usually regulated to concentrations of less than 0.1 mg/l. Cadmium cannot be precipitated to the regulatory levels by pH adjustment alone. However, the precipitates of cadmium carbonate, cadmium phosphate and cadmium sulfide are all very insoluble. It can also be removed very effectively in chelating ion exchange resins. However, it cannot be removed by reduction, since its oxidation potential is too high.
In plating rinse water, the cadmium is usually complexed with cyanide and the cyanide must be oxidized before the cadmium will be available for treatment.
The equipment used for precipitation includes batch treaters for concentrated waste, continuous precipitation for dilute concentrations and ion exchange. When precipitation is used, the two step process must be used. Precipitation of cadmium carbonate, phosphate or sulfide is followed by the precipitation of the carbonate, phosphate or sulfide by the addition of calcium or iron.
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