Which combination of chemicals is most effective for creating sludge solids when treating chromium?

The combination of ferrous sulfate and lime is effective in treating chromium and creating sludge solids due to the chemical reactions that occur between these substances and chromium species in wastewater. When ferrous sulfate is introduced into the treatment process, it acts as a reducing agent, converting hexavalent chromium (Cr(VI)), which is highly toxic, into trivalent chromium (Cr(III)), which is much less harmful and can precipitate out of solution.

The addition of lime, on the other hand, increases the pH of the wastewater, leading to the precipitation of chromium hydroxide when it reacts with the trivalent chromium ions. This precipitation transforms the chromium into a solid form that can be more easily settled and separated from the liquid. As a result, this combination not only helps in reducing the toxicity of chromium in effluent but also effectively forms sludge solids that can be removed from the treatment system.

In contrast, the other combinations mentioned do not demonstrate the same effectiveness in creating sludge solids for chromium treatment due to their different chemical properties and mechanisms of action.