An agricultural soil contaminated by acid mine drainage displays an abnormally low pH of 4… — JAMB UTME

An agricultural soil contaminated by acid mine drainage displays an abnormally low pH of $4.0$ , causing heavy metal ions such as $\text{Pb}^{2+}$ and $\text{Cd}^{2+}$ to remain highly soluble and toxic to crops. Which chemical substance is most appropriate to treat the soil to reduce acidity and precipitate these toxic metal ions?

Calcium carbonate ( $\text{CaCO}_3$ )Answer
B
Ammonium tetraoxosulfate(VI) (( $\text{NH}_4)_2\text{SO}_4$ )
C
Sodium chloride ( $\text{NaCl}$ )
D
Trioxonitrate(V) acid ( $\text{HNO}_3$ )

Answer

Calcium carbonate (

\text{CaCO}_3

) is the most suitable chemical agent for treating acidic, heavy-metal-polluted soil.

Calcium carbonate (

\text{CaCO}_3

) acts as a basic liming material. When applied to acidic soil, it reacts with hydrogen ions to raise the soil pH toward neutral levels. As the pH increases, heavy metal cations such as

\text{Pb}^{2+}

and

\text{Cd}^{2+}

form insoluble hydroxides and carbonates, immobilizing them and preventing plant absorption or groundwater leaching.

Step-by-Step Solution

Identify the cause of heavy metal mobility in soil.

Low soil pH (acidic conditions) keeps heavy metal ions in soluble, bioavailable forms like

\text{Pb}^{2+}

and

\text{Cd}^{2+}

Solubility of heavy metal cations increases significantly under acidic conditions.

Determine the required chemical treatment to remediate soil acidity.

Adding a basic neutralizing agent (liming) increases soil pH.

Neutralization lowers

[\text{H}^+]

concentration in the soil solution.

Evaluate the effect of raising soil pH on dissolved metal cations.

Heavy metal ions react with carbonate and hydroxide ions to form insoluble precipitates such as

\text{Pb(OH)}_2

and

\text{CdCO}_3

Raising pH decreases the solubility of heavy metals, immobilizing them in soil matrix.

Key Concept

Soil Liming and Heavy Metal Immobilization

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