Question 10 of 18: Removing Metals From Industrial Wastewater

Question 10: Removing Metals From Industrial Wastewater (4 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The standard approach is chemical precipitation followed by solid–liquid separation. Most heavy metals form highly insoluble hydroxides at an elevated pH, so the wastewater is dosed with an alkali (lime, Ca(OH)₂, or caustic soda, NaOH) to raise the pH into the metal's minimum-solubility range, converting the dissolved metal ion into an insoluble hydroxide precipitate; a polymer coagulant/flocculant is then added to aggregate the fine precipitate into a settleable floc, which is removed by clarification (gravity settling) and, for a final polish, filtration. Example: chromium removal from an electroplating rinse water. Hexavalent chromium (Cr⁶⁺, as chromate/dichromate) does not precipitate directly as a hydroxide, so it is first chemically reduced to the trivalent form (Cr³⁺) using sulfur dioxide or sodium metabisulfite under acidic conditions ($\text{pH}\approx2$–3): $$2\,CrO_4^{2-} + 3\,SO_2 + 4\,H^+ \rightarrow Cr_2(SO_4)_3\text{-type products} + \dots$$ the pH is then raised to about 8–9 with lime, precipitating Cr(OH)₃, which settles out as a hydroxide sludge that is dewatered and disposed of as a hazardous (or, where permitted and economical, metal-recovery) waste. Other common metals (Cu, Zn, Ni, Cd) are removed by the direct hydroxide-precipitation step without the reduction stage, each at its own optimum precipitation pH; sulfide precipitation and ion exchange are alternative technologies used where a lower residual metal concentration or metal recovery is specifically required.