Environmental chemistry is the branch of chemistry focused on the study of chemical processes and substances in the environment. It examines how chemicals interact with the natural world, how they affect ecosystems, and how they can be managed or mitigated to prevent or reduce environmental harm.

5.1 Introduction

Environmental chemistry is a branch of chemical science that deals with the production,transport, reactions, effects, and fates of chemical species in the water, air, terrestrial,and biological environment and the effects of human activities thereon.
A common pollutant species is used to illustrate the definition of environmental chemistry. Sulfur in coal is oxidized to sulfur dioxide gas, which is then released into the atmosphere. Sulfur dioxide gas can be oxidized to sulfur trioxide and eventually converted to sulfuric acid in the atmospheric, then fall back to earth as acid rain, affect a receptor like plants, and end up in a “sink” like a body of water or soil. shows a simplified schematic diagram that shown fate of pollutant species in the environment

5.1.1 Components of the environment

The environment consists of various compartments, including: Atmosphere, Hydrosphere, Lithosphere and Biosphere.

The atmosphere

One of the main components of Earth’s interdependent physical systems is the atmosphere. An atmosphere is the layers of gases surrounding a planet or other celestial body. Earth’s atmosphere is composed of about 78% nitrogen, 21% oxygen,and one percent other gases.including human being: such as protective blanket of gas surrounding the earth (0–50km), absorbs infrared (IR) radiation emitted by the sun and re-emitted from the earth, controls temperature of the earth, allows transmission of significant amounts of radiation from near UV (300 nm) to near IR (2500 nm) and blocks transmission of damaging UV radiation.
For example, the following important reactions occurred in the atmosphere:
I. Nitric oxide is oxidized by oxygen to nitrogen dioxide in the presence of ultraviolet light.

2NO g + O₂ (g) → 2NO₂ (g)

SO₂, SO₃ and NO₂ react with rainwater and form sulphurous acid(H₂SO₃), sulphuric acid (H₂SO₄) and nitric acid (HNO₃), respectively and cause acid rain.
II. Chlorofluorocarbons are used as refrigerants, solvents and plastic foam-blowing agents. When entering the atmosphere, they penetrate into the upper layers and interact with ultraviolet radiation as follows.
CF₂Cl₂ + UV → CF₂Cl + Cl
The free chlorine Cl- reacts with ozone to form chlorine monoxide and oxygen: Cl + O₃ = ClO + O₂

The Hydrosphere

The hydrosphere is the combined mass of water found on, under, and above the surface of the earth”. The hydrosphere includes water that is on the surface of the planet, underground, and in the air. And it is collective term for all different forms of water, including oceans, seas, rivers, lakes, streams, reservoirs, glaciers, and ground waters.

Only ~1% of global water supply is fresh water. Whereas, the greatest source of water on the planet is the ocean, which constitutes all salt water and at the same time, is the greatest source of water vapor. In the ocean, there are at least 77 important elements such as sodium and chlorine, magnesium and bromine,which are commercially exploited from seawater.

Some examples of chemical reaction in hydrosphere: Ammonia (NH₃)/Ammonium (NH₄⁺) that discharged from agriculture,aquaculture, industry and urban area into large water bodies result in toxicity to fish or aquatic ecosystem.
For example, the biological oxidation of NH₄⁺ to Nitrite(NO₂^–) and nitrate (NO₃^–) is a key part of the complex nitrogen cycle and a fundamental process in aquatic environments, having a profound influence on ecosystem stability and functionality.

Nitrate (NO3-)/Nitrite(NO2-) that discharged from agriculture, industry,aquaculture and sewage into water bodies result in accelerating aquatic plant growth leading to eutrophication.
For example, the organic form of nitrogen, ammonia, has been converted into an inorganic form of nitrogen, nitrate that plants can
use. The chemical equation: 2NH₃ + 3O₂ → 2NO₂^– 2H⁺ + 2H₂O
The equation summarizes the entire nitrification process.

The Lithosphere

The lithosphere consists of earth’s crust and upper mantle in which the crust part is the Earth’s outer skin that is accessible to humans being. This part of the earth i.e. the crust consists of rocks and soil (most important part to humans and the environment).
Some examples of important reaction in lithosphere are Different bacteria (mainly auto trophic i.e. not dependent on organic material for their carbon supply) can effect oxidation or reductions of mineral .

The Biosphere

It refers to the realm of living organisms and their interactions with the environment. (i.e., other compartments).

This compartment divided into smaller units called ecosystems. Each ecosystem contains dynamic interrelationships between living forms and their physical environment. These interrelations manifest as natural cycles, such as hydrologic, oxygen, nitrogen, phosphorous and sulfur. The natural cycles operate in a balanced manner providing a continuous circulation of essential constituents.

5.1.2 The natural cycle in the environment

The natural cycles, also known as bio-geochemical cycles, describe the movement and transformation of key elements through the Earth’s atmosphere, hydrosphere (water), lithosphere (land), and biosphere (living organisms). These cycles are essential for sustaining life and maintaining ecosystem balance. The major cycles include the hydrological cycle, oxygen cycle, nitrogen cycle, phosphorus cycle, sulfur cycle, and carbon cycle.

The Hydro Logical Cycle

The hydro logical cycle describes the continuous movement of water on, above, and below the Earth’s surface.

Evaporation: Water from oceans, lakes, rivers, and soil evaporates due to solar heat, turning into water vapor.

Condensation: The water vapor rises, cools, and condenses into clouds.

Precipitation: Water falls back to the Earth as rain, snow, sleet, or hail.Runoff and Infiltration: Water flows over the land as runoff into rivers, lakes, and oceans or infiltrates into the soil to replenish groundwater.

Transpiration: Plants also release water vapor into the atmosphere through transpiration.

Importance: The water cycle regulates freshwater availability, supports plant growth, drives weather patterns, and sustains ecosystems.

The Oxygen cycle

The oxygen cycle; involves the movement of oxygen within the atmosphere, biosphere, and lithosphere. Oxygen is essential for respiration and photosynthesis.

Photosynthesis: Plants, algae, and some bacteria use carbon dioxide and sunlight to produce oxygen during photosynthesis.
6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

Respiration: Organisms, including animals and plants, take in oxygen and use it to release energy from food, producing carbon dioxide as a byproduct.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Decomposition: De composers break down dead organic matter, consuming oxygen and releasing CO₂ and other gases.

Importance: The oxygen cycle supports life by providing the oxygen needed for respiration and sustaining ecosystems through photosynthesis.

The nitrogen cycle

The nitrogen cycle : describes the transformation and movement of nitrogen through the atmosphere, soil, water, and living organisms. Nitrogen is essential for the production of proteins and DNA.

Nitrogen Fixation: Atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or nitrate (NO₃⁻) by nitrogen-fixing bacteria in the soil or root nodules of legumes.

Nitrification: Soil bacteria convert ammonia into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), which plants can absorb.

Assimilation: Plants take up nitrates from the soil and incorporate them into organic molecules like amino acids.

Ammonification: Decomposers break down dead organisms, converting organic nitrogen back into ammonia.

Denitrification: In oxygen-poor conditions, bacteria convert nitrates back into nitrogen gas (N₂), releasing it into the atmosphere.

Importance: The nitrogen cycle provides usable nitrogen for plants and animals, crucial for building proteins and nucleic acids, while maintaining the balance of nitrogen in ecosystems.

The phosphorus cycle

The phosphorus cycle: involves the movement of phosphorus through rocks, soil, water, and living organisms. Phosphorus is essential for DNA, RNA, ATP, and cell membranes.

Weathering: Phosphorus is released from rocks into the soil and water through weathering.

Absorption by Plants: Plants absorb phosphorus in the form of phosphate ions (PO₄³⁻) from the soil.

Consumption by Animals: Animals obtain phosphorus by consuming plants or other animals.Decomposition: When plants and animals die, phosphorus is returned to the soil or water through decomposition.

Sedimentation: Phosphates can accumulate in sediments in water bodies and eventually form new rock formations over geologic timescales.

Importance: The phosphorus cycle is vital for energy transfer in cells (ATP), genetic material (DNA, RNA), and overall plant and animal growth.

The sulphur cycle

The sulfur cycle: involves the movement of sulfur through the atmosphere, lithosphere, hydrosphere, and biosphere. Sulfur is a component of certain amino acids and proteins.

Volcanic Emissions and Weathering: Sulfur is released into the atmosphere through volcanic eruptions or through the weathering of rocks containing sulfur compounds.

Absorption by Plants: Plants absorb sulfur from the soil in the form of sulfate ions (SO₄²⁻).

Decomposition: De-composers break down dead organisms, releasing sulfur back into the soil.

Human Activities: The burning of fossil fuels releases sulfur dioxide (SO₂) into the atmosphere, contributing to acid rain formation.Sulfur Dioxide and Acid Rain: SO₂ reacts with water vapor in the atmosphere to form sulfuric acid (H₂SO₄), which falls to the Earth as acid rain.

Importance: The sulfur cycle is essential for the production of amino acids and proteins, and it also influences soil fertility and environmental health.

The carbon cycle

The carbon cycle: involves the movement of carbon between the atmosphere, biosphere, hydrosphere, and lithosphere. Carbon is a building block of life, present in carbohydrates, proteins, fats, and nucleic acids.

Photosynthesis: Plants take in CO₂ from the atmosphere to produce glucose (C₆H₁₂O₆), storing carbon in organic form.

Respiration: Organisms release CO₂ back into the atmosphere during respiration.

Decomposition: Decomposers break down dead organic matter, releasing carbon back into the soil or atmosphere.

Combustion: The burning of fossil fuels (coal, oil, natural gas) releases stored carbon as CO₂ into the atmosphere. Ocean Absorption: Oceans absorb CO₂ from the atmosphere, where it can be used by marine organisms or stored as carbonates in sediments.

Importance: The carbon cycle regulates atmospheric CO₂ levels, influences climate, and provides carbon for the synthesis of organic molecules necessary for life.

5.1.3 Concept related to environmental Chemistry

1.Pollutant

A pollutant is a substance or energy that, when introduced into the environment, causes harm or has a negative impact on living organisms, ecosystems, or the atmosphere.Examples: Carbon monoxide (CO), sulfur dioxide (SO₂), plastics, heavy metals, and pesticides.

Impact: Pollutants can lead to air and water pollution, soil degradation, and health issues such as respiratory problems, cancer, and ecosystem imbalance.

2. Contaminant

A contaminant is any physical, chemical, biological, or radiological substance present in the environment where it does not naturally belong or is in excessive amounts.

Difference from Pollutant: A contaminant becomes a pollutant when it negatively impacts the environment or human health.Examples: Chemicals in drinking water, bacteria in food, and oil in oceans.

Impact: Contaminants may not always be harmful at low levels, but at higher concentrations, they can affect the quality of air, water, and soil.

3. Sink

In environmental science, a sink is a reservoir or process that absorbs, stores, or removes pollutants, contaminants, or chemicals from the environment.

Examples:Carbon sink: Forests and oceans absorb carbon dioxide (CO₂), reducing the amount in the atmosphere.Water bodies as sinks: Lakes and rivers can act as sinks for nutrients, chemicals, or waste products.

Importance: Sinks help mitigate environmental pollution by absorbing harmful substances, but they can become overloaded and lose their ability to filter or store pollutants effectively.

4. Dissolved Oxygen (DO)Definition:

Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water. It is a critical indicator of water quality and the health of aquatic ecosystems.

Significance: DO is essential for the survival of fish, plants, and other aquatic organisms. High DO levels indicate good water quality, while low levels can result in oxygen depletion, harming aquatic life.

Factors Affecting DO: Water temperature, salinity, pollution, and photosynthesis. Warmer water holds less oxygen, and pollutants can decrease oxygen levels.Ideal Range: A DO level of 5-6 mg/L is considered sufficient for aquatic life. Levels below 2 mg/L can cause stress and death for organisms.

5. Biological Oxygen Demand (BOD)

Biological Oxygen Demand (BOD) is the amount of dissolved oxygen required by aerobic microorganisms to break down organic matter in a water sample over a specific period (typically 5 days).

Indicator: BOD is used as an indicator of water pollution, especially from organic waste (e.g., sewage, food waste).High BOD Levels: High BOD indicates a high amount of organic pollution, which can lead to oxygen depletion in water, suffocating aquatic organisms.Low BOD Levels: Low BOD suggests good water quality with minimal organic pollution.Units: BOD is typically measured in milligrams per liter (mg/L).

6. Threshold Limit Value (TLV) Definition:

Threshold Limit Value (TLV) is the maximum concentration of a chemical substance to which workers or individuals can be exposed without experiencing harmful effects over a defined period (usually 8 hours per day or 40 hours per week).

Significance: TLVs are used to ensure safe working conditions by regulating exposure to hazardous substances in industries, laboratories, and other environments.