Industrial pollution — the common toxic pollutant associated with the chlor-alkali industry and also with certain battery manufacturing operations is which of the following?

Introduction:
Process industries may release characteristic pollutants depending on technology. Historically, the chlor-alkali industry used mercury cell technology, and mercury compounds have also been used in certain battery types. The question asks for the common pollutant linked to both sectors.

Given Data / Assumptions:

• Industry 1: chlor-alkali (chlorine and caustic soda production).
• Industry 2: battery manufacture (not limited to modern lithium-ion chemistry).
• We are focusing on historically significant pollutants.

Concept / Approach:
Mercury cell electrolyzers (now phased out in many places) used liquid mercury as a cathode, causing mercury emissions when controls were inadequate. Mercury batteries (e.g., mercuric oxide) were once common for small devices.

Step-by-Step Solution:

Verification / Alternative check:
Regulatory transitions away from mercury cells and restrictions on mercury in batteries reinforce this association.

Why Other Options Are Wrong:

• Brine: a process feed, not a pollutant per se.
• Phosphate: associated with detergents or fertilizers, not these specific industries.
• None of these: incorrect because mercury is well-documented.

Common Pitfalls:
Assuming contemporary membrane cell plants imply no historical pollutant; the question references the known legacy issue.

Final Answer:
Mercury is the characteristic pollutant associated with both sectors.