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Difficulty: Very hardSoil Pollution, Pesticides, Heavy Metals, and Waste Management

Agricultural soil adjacent to a battery recycling plant became severely contaminated with toxic lead compounds. Environmental engineers applied slaked lime, Ca(OH)2\text{Ca(OH)}_2, to immobilize the heavy metal by precipitating it as insoluble lead(II) hydroxide, Pb(OH)2\text{Pb(OH)}_2. However, excessive liming caused the soil pH to rise above 10.510.5, triggering a secondary leaching event where lead concentrations in groundwater rapidly increased. Which chemical reaction accounts for the unexpected re-mobilization of lead under strongly alkaline soil conditions?

  1. Formation of soluble tetrahydroxoplumbate(II) complex ions, [Pb(OH)4]2−[\text{Pb(OH)}_4]^{2-}, due to the amphoteric behavior of lead(II) compounds in excess baseAnswer
  2. B
    Precipitation of lead(II) hydroxide behaving as a purely basic compound that undergoes complete salt hydrolysis to yield free soluble lead cations
  3. C
    Oxidation of lead(II) ions by stratospheric ozone trapped in soil pores, converting insoluble lead compounds into highly soluble lead(IV) oxide
  4. D
    Condensation of lead ions with soil humic acids to form synthetic addition polymers that undergo rapid microbial degradation

Answer

The re-mobilization of lead in strongly alkaline soil is caused by the formation of soluble tetrahydroxoplumbate(II) complex ions, [Pb(OH)4]2−[\text{Pb(OH)}_4]^{2-}, owing to the amphoteric character of lead(II) compounds.
Lead(II) compounds exhibit amphoterism. While moderate liming precipitates lead as insoluble Pb(OH)2\text{Pb(OH)}_2, over-liming creates a high concentration of OH−\text{OH}^- ions (pH > 10.5). This causes the precipitate to dissolve by forming the soluble complex anion [Pb(OH)4]2−[\text{Pb(OH)}_4]^{2-} (tetrahydroxoplumbate(II)), leading to groundwater pollution.

Step-by-Step Solution

1
Analyze the chemical behavior of lead(II) hydroxide upon initial neutralization.
Adding moderate amounts of base causes Pb2+\text{Pb}^{2+} ions to precipitate as insoluble lead(II) hydroxide: Pb(aq)2++2OH(aq)−→Pb(OH)2(s)\text{Pb}^{2+}_{(\text{aq})} + 2\text{OH}^-_{(\text{aq})} \rightarrow \text{Pb(OH)}_{2(\text{s})}.
This temporarily immobilizes heavy metal contamination in the soil matrix.
2
Determine the effect of excess hydroxide ions (pH > 10.5) on the precipitate.
Because lead is an amphoteric metal (similar to aluminium and zinc), its oxide and hydroxide react with excess alkali: Pb(OH)2(s)+2OH(aq)−→[Pb(OH)4](aq)2−\text{Pb(OH)}_{2(\text{s})} + 2\text{OH}^-_{(\text{aq})} \rightarrow [\text{Pb(OH)}_4]^{2-}_{(\text{aq})}.
The formation of the complex tetrahydroxoplumbate(II) anion brings lead back into aqueous solution, increasing heavy metal leaching into groundwater.

Key Concept

Amphoteric behavior of heavy metals in soil remediation and waste management
Estimated Time:2m 0s
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Topics

Soil Pollution, Pesticides, Heavy Metals, and Waste Management

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