Process selection and sustainability: For producing caustic soda via the mercury electrolytic cell, what is the single most adverse factor challenging the continued use of this process?

Introduction / Context:
Multiple technologies exist for chlor-alkali production: diaphragm, membrane, and mercury cells. Technology choice today is driven not only by economics but by environmental impact and regulatory compliance.

Given Data / Assumptions:

• Mercury cells employ a flowing mercury cathode and produce ~50 % NaOH.
• Concerns include capital cost, energy use, and environmental emissions.
• Global trends favor membrane cells due to sustainability.

Concept / Approach:
The decisive drawback of mercury cells is the risk and reality of mercury emissions and contamination of water bodies and ecosystems. Even with best practices, historical releases and decommissioning risks are substantial, leading to strict regulation and phase-out in many regions. Cost or density challenges are secondary to environmental harm.

Step-by-Step Solution:
List possible disadvantages → cost, handling, purity, pollution.Identify the dominant, process-ending issue → environmental mercury pollution.Select option (d).

Verification / Alternative check:
Industry transitions and policy frameworks (e.g., Minamata Convention) document phase-out/retirement of mercury technologies due to environmental impact.

Why Other Options Are Wrong:

• (a), (b), (c): operational/economic issues but not the principal barrier.
• (e): mercury cells actually give high-strength caustic; efficiency issues are not the dominant cause of phase-out.

Common Pitfalls:
Overlooking environmental compliance costs and liabilities when comparing legacy and modern cell technologies.

Final Answer:
Mercury pollution of water streams and the environment