Why is there a concern about copper smelting plants?
1. They may release lethal quantities of carbon monoxide into environment.
2. The copper slag can cause the leaching of some heavy metals into environment.
3. They may release sulphur dioxide as a pollutant.
Select the correct answer using the code given below.

The correct answer is 2 and 3 only.

Let us analyze each of the statements to understand the environmental concerns associated with copper smelting plants:

1. They may release lethal quantities of carbon monoxide into environment.
This statement is incorrect. While industrial operations involve combustion, copper smelting is not a major source of lethal carbon monoxide ($\mathrm{CO}$) emissions into the atmosphere. The main gas of concern emitted during the roasting and smelting of sulfide copper ores is sulfur dioxide (${\mathrm{SO}}_2$).

2. The copper slag can cause the leaching of some heavy metals into environment.
This statement is correct. Copper slag is a byproduct generated during the smelting process. It often contains heavy metals such as arsenic, cadmium, lead, and zinc. When slag is dumped or used inappropriately, rainwater and weathering can cause these heavy metals to leach into the surrounding soil and groundwater, posing serious environmental and health hazards.

3. They may release sulphur dioxide as a pollutant.
This statement is correct. Most copper is extracted from copper sulfide ores (such as chalcopyrite, ${\mathrm{CuFeS}}_2$). Roasting and smelting these sulfide ores releases large amounts of sulfur dioxide (${\mathrm{SO}}_2$) gas. Sulfur dioxide is a major air pollutant that contributes to acid rain and respiratory problems.

Therefore, statements 2 and 3 are correct, making 2 and 3 only the correct choice.