Environmental Chemistry and Toxicology
What this domain tests
This domain covers pollutant fate, transport, transformation, bioavailability, biomagnification, dose-response and risk assessment. The exam tests heavy metals, pesticides, persistent organic pollutants, pH and solubility, and acute versus chronic toxicity.
Core rules and concepts
- Persistence, mobility and lipid solubility determine whether a chemical spreads and accumulates.
- Biomagnification is increasing concentration along a food chain; bioconcentration is uptake from water into an organism.
- Dose-response relationships show that toxicity depends on dose, exposure route, duration and organism sensitivity.
- LD50 is the dose killing 50% of test organisms; LC50 is the lethal concentration in water or air. Lower values mean higher acute toxicity.
- Essential elements such as copper and zinc become toxic above a threshold; non-essential metals such as lead and mercury have no known safe biological requirement.
Worked example
Water contains 0.0005 mg L⁻¹ of a persistent pesticide. Fish contain 0.8 mg kg⁻¹. The fish-to-water concentration ratio is 0.8 ÷ 0.0005 = 1,600. If a bird feeding on fish contains 8 mg kg⁻¹, the bird-to-fish ratio is 10. Reason: the pesticide is lipid-soluble and poorly excreted, so concentration rises at higher trophic levels. The units differ, so the ratio is a biomagnification indicator, not a strict thermodynamic constant.
Traps that cost marks
Do not say all synthetic chemicals are toxic or all natural ones are safe. Acute toxicity is not the same as chronic toxicity. A low LC50 means high toxicity. Bioaccumulation depends on uptake and elimination, not concentration alone. Remember that pH changes can alter metal solubility and toxicity.