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When strong complexes are present in the effluent to be treated, hydroxides precipitation may not be fully satisfactory.
In these situations proceeding with a sulfide process may become necessary.
The sulfide process makes the best of the reaction among polluting metals which are contained in the wastewater and the sulfide ions, which become available thanks to the reagent sodium sulfide.
Consequently we see a double exchange reaction with formation of insoluble cupric sulfide (solubility product Kps=8.5x10-45).
This very low product of solubility makes the residual metal concentrations extremely low, even when strong chelating agents may be found.
Iron, introduced as electrolyte, takes part of the reaction with the formation of intermediate compounds, less stabilized than the polluting metals, and contributes to the creation of mud in the form of
ferric hydroxide (solubility product Kps=1.1x10-36).
Through the proportion of powdered activated carbon in lime solution, it is possible to reduce the concentration of the organic substances and surfactants, if necessary.
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