MEV 013: Unit 11 - Chemistry of Hazardous Substances and Wastes

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UNIT 11: CHEMISTRY OF HAZARDOUS SUBSTANCES AND WASTES

11.0 Introduction

Hazardous substances and wastes pose serious risks to human health and the environment due to their chemical reactivity, toxicity, flammability, and corrosiveness. Understanding their classification, properties, and impacts is crucial for safe handling, transport, storage, and disposal. This unit focuses on the chemical nature of hazardous materials, including combustible, reactive, corrosive, and toxic substances.

11.1 Objectives

After completing this unit, you will be able to:

  • Classify hazardous substances and wastes.
  • Describe the physical and chemical properties of combustible, reactive, corrosive, and toxic substances.
  • Explain the risks associated with these hazardous materials.
  • Understand the chemical mechanisms behind explosions, toxicity, and corrosion.

11.2 Classification of Hazardous Substances and Wastes

Hazardous substances are classified based on their:

  • Ignitability (e.g., flammable liquids)
  • Reactivity (e.g., unstable or explosive chemicals)
  • Corrosivity (e.g., strong acids or bases)
  • Toxicity (e.g., carcinogens, mutagens, poisons)

Hazardous wastes may be:

  • Industrial byproducts
  • Medical or chemical laboratory waste
  • Electronic waste (e-waste)
  • Household hazardous waste (e.g., cleaners, paints)

International regulations such as the Basel Convention, and national guidelines define these classifications for proper management.

11.3 Combustible Waste: Physical and Chemical Properties

Combustible wastes are materials that can catch fire and burn easily. They include:

  • Solvents (e.g., acetone, alcohol)
  • Fuels (e.g., gasoline, kerosene)
  • Packaging materials
  • Organic chemicals

Key characteristics:

  • Low flash point
  • High vapor pressure
  • Presence of volatile organic compounds (VOCs)

11.3.1 Dust Explosions and Combustion of Finely Divided Particles

Dust explosions occur when fine combustible particles (e.g., flour, coal dust, aluminum powder) are suspended in air and ignited.

Conditions required:

  1. Combustible dust
  2. Oxygen
  3. Ignition source
  4. Dispersion of dust
  5. Confinement of the explosion

These explosions can be devastating and are a common risk in grain silos, coal mines, and chemical plants.

11.3.2 Toxic Products of Combustion

Combustion of hazardous waste often leads to the formation of toxic byproducts such as:

  • Carbon monoxide (CO)
  • Dioxins and furans
  • Nitrogen oxides (NOₓ)
  • Polycyclic aromatic hydrocarbons (PAHs)

Incomplete combustion is especially dangerous as it increases toxicity.

11.4 Reactive Substances: Physical and Chemical Properties

Reactive substances can undergo violent chemical reactions, including:

  • Explosion
  • Rapid oxidation
  • Polymerization
  • Gas evolution

Examples include:

  • Peroxides
  • Nitrates
  • Metal hydrides
  • Cyanides

These substances may react dangerously with air, water, acids, or other chemicals.

Storage and handling require extreme care to prevent unwanted reactions.

11.5 Corrosive Substances: Physical and Chemical Properties

Corrosive substances degrade or destroy materials by chemical action, particularly:

  • Metals
  • Human tissue
  • Plastics

Two main categories:

  1. Acids (e.g., sulfuric acid, hydrochloric acid)
  2. Bases (e.g., sodium hydroxide, potassium hydroxide)

Physical properties:

  • Often liquid
  • Highly exothermic upon dilution
  • Reactive with metals (releasing hydrogen gas)

Protective gear and proper storage containers are essential for safety.

11.6 Toxic Substances: Physical and Chemical Properties

Toxic substances cause adverse health effects on exposure. They can be:

  • Acute toxins (e.g., cyanide)
  • Chronic toxins (e.g., lead, mercury)
  • Carcinogens (e.g., benzene, asbestos)
  • Neurotoxins (e.g., organophosphates)

Routes of exposure:

  • Inhalation
  • Ingestion
  • Dermal absorption

11.6.1 Types of Toxic Substances

  1. Organic toxins:
    • Pesticides
    • Dioxins
    • Volatile organic compounds (VOCs)
  2. Inorganic toxins:
    • Heavy metals (e.g., arsenic, lead, cadmium)
    • Cyanides
  3. Biotoxins:
    • Bacterial or fungal toxins

Toxicity depends on dose, duration, frequency of exposure, and biological sensitivity.

11.6.2 Risks of Using Toxic Substances

  • Environmental risks: contamination of water, soil, and air
  • Health risks: acute poisoning, long-term diseases, birth defects
  • Occupational hazards: workers in industries dealing with toxic substances are at high risk
  • Improper disposal leads to groundwater contamination and bioaccumulation in food chains

Risk mitigation includes:

  • Substitution with less toxic materials
  • Proper labeling, storage, and disposal
  • Use of PPE and engineering controls

11.7 Let’s Sum Up

In this unit, we explored the chemical properties and hazards associated with:

  • Combustible wastes (e.g., VOCs, dust explosions)
  • Reactive substances (e.g., oxidizers, unstable compounds)
  • Corrosive substances (e.g., acids, bases)
  • Toxic substances (e.g., heavy metals, pesticides)

Understanding these properties helps ensure safe handling, storage, transportation, and disposal, protecting both humans and the environment.

11.8 Glossary

  • Combustible Waste-Waste that can ignite and burn easily
  • Dust Explosion-Rapid combustion of fine particles suspended in air
  • Reactive Substance-A substance prone to violent or unexpected reactions
  • Corrosive Substance-A substance that destroys or damages other substances upon contact
  • Toxic Substance-A chemical that causes adverse health effects
  • Flash Point-The lowest temperature at which a liquid gives off enough vapor to ignite
  • VOCs-Volatile Organic Compounds; contribute to smog and health issues
  • Bioaccumulation-Accumulation of substances in living organisms
  • Chelation-The bonding of ions and molecules to metal ions
  • PPE-Personal Protective Equipment used to ensure safety

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