Treatment of acid mine drainage (coal mines) Acidic drainage from coal mines forms when groundwater percolates through mined voids. What is the appropriate treatment before discharge?

Introduction / Context:
Acid mine drainage (AMD) occurs when sulfide minerals oxidize, creating acidic, metal-laden water. Discharge without treatment damages streams, kills fish, and mobilizes heavy metals. Engineering controls aim to neutralize acidity and precipitate metals.

Given Data / Assumptions:

• Coal mine drainage is acidic.
• Receiving waters require protection from low pH and dissolved metals.
• Standard, practicable remedies are considered.

Concept / Approach:
Neutralization raises pH to near neutral, converting dissolved metals to insoluble hydroxides. Common reagents include lime (Ca(OH)2), limestone (CaCO3), soda ash (Na2CO3), or caustic soda (NaOH). This is more reliable and ethical than dilution and far superior to inaction.

Step-by-Step Solution:

Verification / Alternative check:
Field practice and regulations worldwide require neutralization; passive systems (limestone drains, wetlands) and active lime dosing are widely used.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming high flow rivers make dilution acceptable; compliance requires treatment to meet limits regardless of stream flow.

Final Answer:
neutralised by alkali treatment.