Thermal pollution and toxicity amplification A 10°C rise in receiving water temperature can double the toxic effects of which contaminant commonly discussed in environmental toxicology?

Introduction / Context:
Thermal pollution occurs when heated effluents elevate the temperature of natural waters. Temperature influences chemical speciation, volatilisation, metabolic rates of aquatic organisms, and toxicity of dissolved pollutants. Engineers must anticipate how a heat load can magnify toxic impacts even when concentrations remain unchanged.

Given Data / Assumptions:

• Receiving water temperature rises by roughly 10°C due to a discharge.
• Toxic effects are evaluated at comparable pH and oxygen conditions.
• Focus is on the relative change in toxicity with temperature, not absolute lethal doses.

Concept / Approach:
For weak acids/bases and certain inorganic species, temperature shifts alter dissociation equilibria and biological uptake. Cyanides, especially potassium cyanide in water, can form undissociated hydrogen cyanide (HCN). As temperature rises, the fraction of HCN in equilibrium increases and its volatility and membrane permeability increase, enhancing toxicity to aquatic life. Empirical observations and classical environmental texts note that a 10°C increase can roughly double the toxic effects of cyanides under comparable conditions.

Step-by-Step Solution:

Verification / Alternative check:
Acute LC50 values for fish and invertebrates show stronger toxicity at elevated temperatures for cyanide-bearing waters, consistent with increased HCN fraction.

Why Other Options Are Wrong:

• Coal ash: Suspended solids and trace metals are temperature-influenced, but not typically described by a “doubling per 10°C.”
• Ortho-xylene: Organic solvent toxicity is less governed by a simple 10°C doubling rule.
• None of these: Incorrect because cyanide is the classic example.
• Chromium(VI) salts: Highly toxic but not characterised by this specific temperature doubling heuristic.

Common Pitfalls:
Assuming all toxicants respond similarly to temperature; ignoring speciation and bioavailability changes unique to cyanide chemistry.

Final Answer:
Potassium cyanide