CHEM3120 · Environmental and Analytical Chemistry
Persistent Organic Pollutants
Lecture 5 covers persistent organic pollutants (POPs) in CHEM3120: PCBs, DDT and dioxins such as 2,3,7,8-TCDD — why they persist, how they bioaccumulate up food chains, DDT's mode of action on nerves, and how contaminated sites are remediated. These are conceptual short-answer topics, often paired with the analysis of POPs by gas chromatography from the techniques block.
What this chapter covers
- 01POPs: PCBs, DDT, dioxins — lipophilic, chemically stable, long environmental half-lives
- 02Bioaccumulation and biomagnification: fat-soluble pollutants concentrate up the food chain
- 03DDT as an insecticide: persistent, low vapour pressure, fat-soluble, slow-reacting
- 04DDT mode of action: holds open Na+ nerve channels, causing continuous nerve impulses
- 05Dioxin formation: heating a 2,4,5-trichlorophenylate salt on silica (280 °C) gives 2,3,7,8-TCDD
- 06Analysis of POPs by gas chromatography (links to the techniques block)
- 07Remediation: nanofiltration, dredging, sediment washing, phytoremediation
Why DDT is both an effective insecticide and an environmental problem
- +1(a) Three properties: it is chemically persistent (a long environmental half-life), it has a low vapour pressure so it evaporates only slowly and stays where applied, and it is fat-soluble (lipophilic) and reacts slowly. [any three]
- +1(b) Mode of action: DDT holds open the voltage-gated Na+ channels in nerve membranes, so the neuron keeps firing continuous impulses — the insect suffers uncontrolled nerve activity and dies.
- +1(c) The same persistence and fat-solubility mean DDT is not broken down and instead dissolves into fatty tissue, so it bioaccumulates and biomagnifies up the food chain to top predators — the property that makes it effective is exactly what makes it an environmental hazard.
- +1One remediation method: nanofiltration from water (also acceptable: dredging, sediment washing or phytoremediation of contaminated sites).
Key terms
- Persistent organic pollutant (POP)
- An organic compound that resists environmental breakdown, is fat-soluble and toxic, and therefore persists and bioaccumulates — e.g. PCBs, DDT and dioxins.
- PCBs
- Polychlorinated biphenyls: stable, lipophilic industrial compounds that persist and bioaccumulate; often analysed by gas chromatography.
- DDT
- An organochlorine insecticide that is persistent, low-volatility and fat-soluble; it acts by holding open Na+ nerve channels, and its persistence drives bioaccumulation.
- 2,3,7,8-TCDD (dioxin)
- 2,3,7,8-tetrachlorodibenzo-p-dioxin, a highly toxic dioxin; forms when a 2,4,5-trichlorophenylate salt is heated on silica (about 280 °C).
- Bioaccumulation / biomagnification
- The build-up of a fat-soluble pollutant within an organism (bioaccumulation) and its increasing concentration at higher trophic levels of a food chain (biomagnification).
- Phytoremediation
- Using plants to take up, contain or degrade pollutants in soil or water; one of several remediation options for POP-contaminated sites (with nanofiltration, dredging and sediment washing).
Persistent Organic Pollutants FAQ
What makes a pollutant persistent?
Chemical stability plus low reactivity and low volatility. A persistent organic pollutant resists photochemical and biological breakdown, so it has a long environmental half-life, and because it is fat-soluble it partitions into living tissue rather than being flushed away in water. Those same features let it travel and accumulate, which is why PCBs, DDT and dioxins remain in the environment long after use.
Why does DDT bioaccumulate up the food chain?
DDT is lipophilic (fat-soluble) and chemically persistent, so instead of being metabolised and excreted it dissolves into fatty tissue and stays there. Each organism absorbs the DDT stored in everything it eats, so concentrations rise at each trophic level — biomagnification — leaving top predators with the highest burdens. The persistence that made DDT a durable insecticide is exactly what makes it an ecological hazard.
How does a dioxin like 2,3,7,8-TCDD form?
In this unit the taught route is thermal: heating a salt of the 2,4,5-trichlorophenylate ion on silica at around 280 °C produces 2,3,7,8-tetrachlorodibenzo-p-dioxin. It is an unwanted by-product chemistry rather than a deliberate synthesis, which is why dioxins turn up as trace contaminants from high-temperature processes involving chlorinated precursors.
Can AI help me revise persistent organic pollutants for CHEM3120?
Yes. Sia can help you connect each POP property to its consequence (persistence and fat-solubility → bioaccumulation), rehearse DDT's Na+-channel mode of action, and organise the remediation options, then quiz you in the short-answer style this topic uses. It explains and checks your reasoning; it does not write graded answers for you, and University of Sydney academic-integrity rules apply.
Exam move
POPs are conceptual marks, so learn them as linked property-consequence pairs rather than isolated facts: persistence and low volatility → stays in the environment; fat-solubility → bioaccumulation and biomagnification; DDT → Na+ channels held open. Be ready to name the dioxin 2,3,7,8-TCDD and its thermal formation route, and to list remediation options (nanofiltration, dredging, sediment washing, phytoremediation). Tie this block to the techniques block, since POPs like PCBs are analysed by gas chromatography — a favourite cross-topic question. This material suits weekly quizzes and Part A short-answer, so keep it warm across the semester and confirm the exam format on Canvas.
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