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Difficulty: HardWater Pollution, Industrial Effluents, and Water Treatment

An industrial electroplating plant produces wastewater contaminated with dilute tetraoxosulfate(VI) acid (H2SO4H_2SO_4) and toxic copper(II) ions (Cu2+Cu^{2+}). Prior to releasing this effluent into municipal drainage, which of the following chemical treatments is most suitable to simultaneously neutralize the acidity and precipitate the heavy metal ions as an insoluble sludge?

  1. Treatment with calcium hydroxide, Ca(OH)2Ca(OH)_2, to form insoluble calcium sulfate and copper(II) hydroxideAnswer
  2. B
    Addition of concentrated trioxonitrate(V) acid, HNO3HNO_3, to convert the copper ions into soluble copper(II) trioxonitrate(V)
  3. C
    Boiling the wastewater to remove the permanent hardness caused by dissolved copper(II) tetraoxosulfate(VI)
  4. D
    Chlorination of the wastewater to oxidize synthetic non-biodegradable polymers and dissolved heavy metals into harmless gases

Answer

Treatment with calcium hydroxide, Ca(OH)2Ca(OH)_2, to form insoluble calcium sulfate and copper(II) hydroxide
Treatment with calcium hydroxide (slaked lime) provides hydroxide ions (OH−OH^-) that neutralize free H+H^+ ions from the acidic effluent and react with dissolved copper(II) ions (Cu2+Cu^{2+}) to form insoluble copper(II) hydroxide, Cu(OH)2Cu(OH)_2, which precipitates out of solution as removable sludge.

Step-by-Step Solution

1
Analyze the chemical nature of the industrial effluent contaminants.
The effluent contains free hydrogen ions (H+H^+) from H2SO4H_2SO_4 (causing low pH) and soluble toxic Cu2+Cu^{2+} ions.
Effective treatment must address both acidity and heavy metal toxicity.
2
Evaluate the chemical reaction between slaked lime, Ca(OH)2Ca(OH)_2, and the effluent components.
Acid neutralization: H2SO4(aq)+Ca(OH)2(s)→CaSO4(s/aq)+2H2O(l)H_2SO_4(aq) + Ca(OH)_2(s) \rightarrow CaSO_4(s/aq) + 2H_2O(l). Heavy metal precipitation: Cu2+(aq)+2OH−(aq)→Cu(OH)2(s)Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s).
Calcium hydroxide neutralizes H+H^+ ions and precipitates Cu2+Cu^{2+} as insoluble Cu(OH)2Cu(OH)_2 sludge.
3
Confirm the physical removal mechanism of the precipitate.
The solid precipitate settles out as sludge during sedimentation and can be separated by filtration.
Precipitation converts dissolved pollutants into solids that can be easily removed prior to discharge.

Key Concept

Industrial Effluent Remediation and Heavy Metal Precipitation
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Topics

Water Pollution, Industrial Effluents, and Water Treatment

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