MEV 018: Unit 04 – Dispersion of Toxic Substances

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 UNIT 4: DISPERSION OF TOXIC SUBSTANCES

4.0 Introduction

Toxic substances released into the environment rarely stay confined to their original location. Through complex physical, chemical, and biological processes, these substances can be transported across long distances, affecting ecosystems and human health globally. This unit focuses on how toxic substances disperse through air, water, soil, and food chains. It also discusses their transformation processes, pathways of human exposure, and key concepts such as bioaccumulation, biomagnification, and biosorption.

4.1 Objectives

After studying this unit, you will be able to:

  • Understand the global dispersion of toxic substances and the mechanisms involved.
  • Differentiate between direct and indirect exposure pathways.
  • Distinguish between degradable and non-degradable toxicants in food chains.
  • Explain how ecosystems influence the fate and movement of toxicants in different media.
  • Describe transformation processes, exposure routes, and the concepts of biosorption, bioaccumulation, and biomagnification.

4.2 Global Dispersion of Toxic Substances

Toxicants released locally can become global pollutants due to atmospheric and oceanic circulation systems. Substances like persistent organic pollutants (POPs) and heavy metals are capable of long-range transport (LRT), reaching even remote ecosystems such as the Arctic.

Mechanisms for global dispersion include:

  • Atmospheric circulation: Volatile chemicals can evaporate, travel with winds, and re-deposit via rain or dry fallout.
  • Ocean currents: Toxicants dumped in coastal areas may spread across ocean basins.
  • Migratory species: Fish and birds can carry contaminants across regions.
  • Trade and waste export: Movement of e-waste and hazardous materials across borders contributes to global toxicant distribution.

4.3 Circulating Mechanisms and Exposure Pathway of Toxic Substances

4.3.1 Direct Exposure Pathways

  • Inhalation: Breathing in contaminated air or particulates (e.g., smoke, vapors).
  • Ingestion: Eating or drinking contaminated food and water.
  • Dermal contact: Absorption through skin (e.g., pesticides, industrial spills).

4.3.2 Indirect Exposure Pathways

  • Bioaccumulated contaminants in food: Fish containing mercury or PCBs.
  • Contaminated surfaces and dust: Especially relevant for children.
  • Secondary air pollution: Gases transformed into secondary toxic compounds (e.g., ozone from VOCs and NOx).

4.4 Degradable and Non-Degradable Toxic Substances in Food Chains

  • Degradable Toxicants: Break down through biological (biodegradation), chemical (oxidation, hydrolysis), or physical processes (sunlight).
    • Example: Certain pesticides degrade over days or weeks.
  • Non-Degradable Toxicants: Persist in the environment and accumulate in organisms.
    • Examples: Heavy metals (lead, mercury), POPs like DDT.

These toxicants can enter food chains, leading to bioaccumulation and biomagnification, particularly in higher trophic levels like predatory birds and humans.

4.5 Ecosystem Influence on Fate and Transport of Toxicants

4.5.1 Transport and Dispersion of Toxic Substances in Air

  • Dry and wet deposition: Toxicants fall with dust or rainfall.
  • Photochemical reactions: Sunlight can transform gases into harmful compounds.
  • Meteorological factors: Wind speed, humidity, and temperature influence dispersion.

4.5.2 Transport and Dispersion of Toxic Substances in Water

  • Runoff and leaching: Chemicals from agriculture or landfills reach rivers, lakes, and groundwater.
  • Turbulence and mixing: Spreads toxicants in water bodies.
  • Adsorption to sediments: Toxicants may bind to particles and settle at the bottom.

4.5.3 Dispersion on Land, Including Soil and Vegetation

  • Soil composition: Clay and organic matter may retain or slow down toxicants.
  • Plant uptake: Some plants absorb metals and pesticides, entering the food web.
  • Wind-blown dust: Can spread toxic particles over long distances.

4.6 Transformation Process of Toxic Substances

Toxic substances may undergo chemical, biological, or photochemical changes:

  • Biodegradation: Microorganisms break down organic pollutants.
  • Photolysis: Sunlight alters chemical structure (e.g., breaking down VOCs).
  • Hydrolysis and oxidation: Water and oxygen react with toxicants, sometimes making them more or less harmful.
  • Methylation: Converts inorganic mercury into methylmercury—more toxic and bioavailable.

4.7 Exposure Routes

Main exposure routes for humans and animals include:

  • Oral (ingestion): Contaminated food and water.
  • Inhalation: Airborne gases, dust, and vapors.
  • Dermal: Through skin contact with soil, water, or chemicals.
  • Transplacental: From mother to fetus through blood.

Understanding exposure routes is critical for designing effective public health and environmental policies.

4.8 Biosorption of Heavy Metals

Biosorption refers to the passive uptake of heavy metals by biological materials, especially microbial biomass and plant matter.

4.8.1 Biosorbent Materials

Examples include:

  • Algae (e.g., Spirulina): Bind metals through cell wall components.
  • Fungi (e.g., Aspergillus, Rhizopus): Use functional groups (hydroxyl, carboxyl).
  • Bacteria: Especially gram-negative strains.
  • Agricultural waste: Sawdust, rice husks, shells used as eco-friendly biosorbents.

These materials help remove heavy metals from wastewater in an economical and sustainable way.

4.9 Bioaccumulation

Bioaccumulation is the process by which organisms absorb toxic substances at a rate faster than they can eliminate them.

  • Occurs mainly in fat tissues.
  • Influenced by lifespan, metabolism, and diet.
  • Common in fish, amphibians, and top predators.

Example: Mercury bioaccumulates in tuna and swordfish, making them hazardous for frequent human consumption.

4.10 Biomagnification

Biomagnification refers to the increasing concentration of toxic substances as they move up the food chain.

  • Top predators (e.g., eagles, humans) often suffer the most.
  • Especially serious with substances like DDT, PCBs, and methylmercury.
  • Leads to reproductive failures, deformities, and neurological damage.

4.11 Let Us Sum Up

This unit examined the various pathways and mechanisms by which toxic substances disperse in the environment and impact living systems. Toxicants may travel long distances via air and water, enter food chains, and transform chemically or biologically in different ecosystems. Understanding their movement, transformation, and accumulation is vital to manage environmental health risks. Concepts like bioaccumulation, biomagnification, and biosorption are central to both the challenges and solutions of toxicant pollution.

4.12 Keywords

  • Dispersion: Spread of toxic substances through air, water, and soil.
  • Direct Exposure: Inhalation, ingestion, or skin contact.
  • Bioaccumulation: Accumulation of toxicants in an organism’s body over time.
  • Biomagnification: Increase in toxicant concentration at each trophic level.
  • Biosorption: Use of biological materials to absorb heavy metals from the environment.
  • Transformation: Chemical or biological change of toxicants into other compounds.

 

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