Module 96. Environmental Chemistry

Environmental chemistry is the scientific study of chemical processes occurring in the environment, including the composition, reactions, transport, effects, and fate of chemical substances in air, water, soil, and living organisms. It combines principles of chemistry, biology, physics, and earth sciences to understand the natural chemical cycles and the impact of human activities on them. The discipline plays a crucial role in addressing environmental challenges such as pollution, climate change, resource depletion, and ecological imbalance.

Historical Development

The roots of environmental chemistry can be traced back to the nineteenth century, when scientists first began to study air and water quality in relation to industrialisation. Early concerns about smoke emissions and contaminated waterways led to foundational research in atmospheric and aquatic chemistry. The development of analytical chemistry techniques in the twentieth century allowed for precise measurement of pollutants and their pathways in the environment.
The publication of Rachel Carson’s Silent Spring in 1962 marked a turning point, drawing attention to the harmful effects of pesticides and chemical pollutants on ecosystems. The subsequent rise of environmental awareness in the 1970s spurred legislation such as the Clean Air Act and Clean Water Act in the United States, alongside global efforts to regulate toxic chemicals and emissions. Since then, environmental chemistry has evolved into a multidisciplinary field encompassing global environmental monitoring, waste management, and green chemistry innovation.

Composition of the Environment

The environment consists of four primary components—atmosphere, hydrosphere, lithosphere, and biosphere—each with distinct chemical characteristics and interactions:

• Atmosphere: Composed mainly of nitrogen (78%), oxygen (21%), argon (0.9%), and trace gases such as carbon dioxide, methane, and ozone. Chemical reactions within the atmosphere influence weather, climate, and air quality.
• Hydrosphere: Includes all forms of water on Earth, from oceans and rivers to groundwater. It acts as a solvent and transport medium for numerous chemical species.
• Lithosphere: The solid outer layer of the Earth, consisting of minerals and rocks that contain chemical elements such as silicon, aluminium, and iron.
• Biosphere: Encompasses all living organisms, which interact with the other spheres through biochemical cycles.

Understanding the chemistry of these interconnected systems helps in evaluating environmental balance and the consequences of human interference.

Chemical Cycles in Nature

Natural chemical cycles maintain the Earth’s ecological stability by circulating essential elements through the environment. Key cycles include:

• Carbon Cycle: Regulates the exchange of carbon dioxide between the atmosphere, oceans, and living organisms. Excess carbon dioxide from fossil fuel combustion contributes to global warming.
• Nitrogen Cycle: Converts atmospheric nitrogen into biologically usable forms through nitrogen fixation and denitrification. Industrial fertilisers disrupt this balance, causing eutrophication.
• Sulphur Cycle: Involves the movement of sulphur through rocks, water, and air. Burning of fossil fuels releases sulphur dioxide, leading to acid rain.
• Phosphorus Cycle: Critical for plant growth, yet overuse of phosphate fertilisers leads to nutrient pollution and algal blooms in aquatic systems.

Disruption of these cycles due to anthropogenic activities results in significant environmental degradation.

Sources and Types of Pollution

Pollution arises from the introduction of harmful substances or energy forms into the environment. It can be categorised by the medium it affects or the nature of pollutants:

• Air Pollution: Caused by emissions of carbon monoxide, nitrogen oxides, sulphur dioxide, volatile organic compounds (VOCs), and particulate matter. Major sources include vehicles, industries, and power plants. Photochemical smog and acid rain are notable outcomes.
• Water Pollution: Results from discharge of domestic sewage, industrial effluents, agricultural runoff, and oil spills. Contaminants include heavy metals (lead, mercury), pesticides, and nitrates.
• Soil Pollution: Occurs through the deposition of chemicals such as pesticides, herbicides, and industrial waste, leading to loss of fertility and bioaccumulation of toxins.
• Noise and Thermal Pollution: Though non-chemical, these forms of pollution affect ecosystems and chemical equilibria in natural waters.
• Radioactive Pollution: Caused by nuclear activities and improper disposal of radioactive materials, posing long-term environmental and health risks.

Environmental Monitoring and Analysis

Monitoring environmental quality requires precise analytical methods to detect and quantify pollutants. Techniques such as spectrophotometry, gas chromatography, mass spectrometry, and atomic absorption spectroscopy are commonly employed. Sampling of air, water, and soil helps assess compliance with environmental standards and identify sources of contamination.
Bioindicators, such as lichens (for air quality) and aquatic invertebrates (for water quality), also provide valuable insight into ecosystem health. Environmental chemists interpret such data to design mitigation strategies and inform policymaking.

Green Chemistry and Pollution Control

Green chemistry, also known as sustainable chemistry, seeks to reduce or eliminate the use and generation of hazardous substances. It promotes environmentally friendly design, synthesis, and application of chemical products and processes. Key principles include:

• Use of renewable raw materials.
• Energy-efficient synthesis pathways.
• Development of biodegradable materials.
• Replacement of toxic solvents with safer alternatives.
• Waste minimisation through catalytic processes.

Pollution control strategies complement these efforts. Technologies such as catalytic converters, scrubbers, and filtration systems help remove pollutants before they enter the environment. Wastewater treatment plants utilise chemical and biological processes to purify effluents.

Global Environmental Issues

Environmental chemistry provides essential understanding of global environmental challenges that threaten ecological balance and human well-being:

• Climate Change: Driven by the enhanced greenhouse effect due to increased concentrations of carbon dioxide, methane, and nitrous oxide.
• Ozone Layer Depletion: Caused by chlorofluorocarbons (CFCs) and halons, leading to increased ultraviolet radiation reaching the Earth’s surface.
• Acid Rain: Formed by atmospheric reactions of sulphur dioxide and nitrogen oxides, leading to ecosystem damage and material corrosion.
• Eutrophication: Excessive nutrient enrichment of water bodies causing algal blooms and oxygen depletion.
• Plastic Pollution: Persistent plastics and microplastics contaminating oceans and food chains.
• Heavy Metal Contamination: Toxic metals accumulating in soils and water, adversely affecting health and agriculture.

International agreements such as the Paris Agreement, Montreal Protocol, and Kyoto Protocol illustrate the global commitment to mitigating these issues through collective action and scientific innovation.

Role in Sustainable Development

Environmental chemistry is integral to achieving sustainable development goals (SDGs), particularly those related to clean water, affordable energy, responsible consumption, and climate action. It aids in developing clean technologies, renewable energy sources, and efficient waste management systems.
Research in environmental chemistry contributes to the design of eco-friendly materials, such as biodegradable plastics, green fuels, and solar catalysts. Life-cycle assessment (LCA) and environmental impact analysis (EIA) help evaluate the sustainability of industrial processes and products.