Environmental Chemistry

Understanding Environmental Chemistry

Environmental chemistry examines the chemical composition and transformations of substances in air, water, soil, and living organisms, with a particular focus on how human activities disturb natural cycles and balances. In this chapter, the emphasis is on principles and recurring patterns; detailed treatment of specific pollutants or cycles will appear in the following subsections.

Natural vs. Anthropogenic Chemicals

From the perspective of chemistry, there is no fundamental difference between “natural” and “synthetic” substances: atoms and bonds obey the same rules. What matters environmentally is:

• Origin
• Natural: formed by geological processes, organisms, natural atmospheric reactions (e.g. nitrogen oxides from lightning, methane from wetlands).
• Anthropogenic: produced or mobilized by human activities (industry, agriculture, transport, household use).
• Concentration
• Many substances are harmless or even essential at low concentrations, but harmful at elevated levels (e.g. nitrate in drinking water, ozone in the lower atmosphere).
• Persistence
• Readily degradable: broken down quickly by light, oxygen, or microorganisms.
• Persistent: remain for long times; may be transported far from the emission source (e.g. some halogenated organics, heavy metals which cannot be degraded at all but only transformed).
• Mobility and bioavailability
• Ability to move between environmental compartments (air, water, soil, biota) and to enter organisms (e.g. water-soluble ions vs. hydrophobic organics that accumulate in fats).

These properties, not the origin alone, determine the environmental relevance and risk of a substance.

Environmental Compartments and Interfaces

Environmental chemistry often distinguishes compartments, which are not perfectly separate but are useful for analysis:

• Atmosphere: mixtures of gases, aerosols, and particles.
• Hydrosphere: oceans, rivers, lakes, groundwater, snow and ice.
• Pedosphere (soil): mineral particles, organic matter, water, air, and organisms.
• Biosphere: all living organisms.

Substances can move between compartments across interfaces:

• Air–water (e.g. dissolution of gases in rain and oceans).
• Air–soil (e.g. adsorption of gases or deposition of particles onto surfaces).
• Water–soil (e.g. sorption of dissolved substances to mineral or organic matter).
• Environment–organism (uptake by respiration, ingestion, or through skin and plant surfaces).

Understanding these interfaces is crucial for predicting transport pathways and fate of pollutants.

Sources, Sinks, and Fluxes

For any chemical in the environment, three concepts are central:

• Sources: processes that introduce or generate the substance in a compartment.
• Natural: volcanic eruptions, weathering of rocks, forest fires, biological production.
• Anthropogenic: combustion processes, industrial emissions, fertilizer use, waste disposal.
• Sinks: processes that remove or transform the substance into a less available or different form within a compartment.
• Chemical reactions (oxidation, reduction, hydrolysis, photolysis).
• Biological uptake and degradation.
• Physical removal (sedimentation of particles, dissolution, burial in sediments).
• Fluxes: amounts of substance transported or transformed per unit time (e.g. kg/year), between compartments or within cycles.

A steady state (dynamic equilibrium) arises when total sources and sinks balance; environmental problems often emerge when human activities significantly increase sources or block sinks.

Chemical Speciation and Environmental Behavior

Speciation means the distribution of an element among different chemical forms (oxidation states, complexes, solid phases, etc.). Speciation strongly controls toxicity, mobility, and persistence.

Examples of speciation-dependent behavior:

• Metals and metalloids: different oxidation states and complexes differ in solubility and toxicity (e.g. $\text{Cr(VI)}$ vs. $\text{Cr(III)}$, inorganic vs. methylmercury).
• Nitrogen: exists as $\text{N}_2$, $\text{NH}_4^+$, $\text{NH}_3$, $\text{NO}_2^-$, $\text{NO}_3^-$, and various organic forms; each participates differently in environmental reactions and biological uptake.
• Carbon: $\text{CO}_2$, $\text{H}_2\text{CO}_3$, $\text{HCO}_3^-$, $\text{CO}_3^{2-}$, and organic species influence pH, buffering, and greenhouse effects.

Key environmental factors that affect speciation:

• pH: controls protonation/deprotonation and solubility (e.g. many metal hydroxides are less soluble at higher pH).
• Redox conditions (Eh): determine oxidation state; reducing conditions (e.g. in waterlogged sediments) favor reduced forms, oxidizing conditions favor higher oxidation states.
• Complexing agents: ligands such as organic acids, chloride, or humic substances can keep metals dissolved or immobilize them.
• Ionic strength and competing ions: affect equilibria of adsorption and complexation.

Speciation is particularly important when assessing risk: not just “how much” of an element is present, but “in what form”.

Chemical Transformations in the Environment

Substances in the environment undergo many transformations. Some key types:

• Photochemical reactions
• Driven by solar radiation, especially UV.
• Important for:
• Decomposition of organic compounds, especially at the surface of water bodies and in the atmosphere.
• Formation of reactive species (e.g. radicals) that start chain reactions.
• Typical in the troposphere and stratosphere, where they affect smog and ozone chemistry.
• Redox reactions
• Governed by the availability of oxidants (like $\text{O}_2$, $\text{NO}_3^-$) and reductants (organic matter, $\text{Fe}^{2+}$, etc.).
• Occur in:
• Surface waters and soils (often moderately oxidizing).
• Sediments and groundwater (often reducing, especially where organic matter is abundant and oxygen is depleted).
• Influence solubility of metals and nutrients, degradation of pollutants, and gas production (e.g. methane).
• Acid–base reactions
• Buffer systems (e.g. carbonate system) stabilize pH’s of natural waters and soils.
• Acidification (from atmospheric deposition or local inputs) can alter solubility of metals and nutrients and stress ecosystems.
• Hydrolysis and other substitution reactions
• Many reactive organics, especially with good leaving groups (e.g. esters, some pesticides), are broken down in water.
• Rates depend on pH and temperature.
• Sorption and desorption
• Binding of dissolved substances to soil or sediment surfaces (minerals, organic matter).
• Reduces concentration in water but may create a long-term reservoir that can release pollutants later.
• Biodegradation and biotransformation
• Carried out by microorganisms and plants.
• Range from complete mineralization to $\text{CO}_2$, water, and inorganic ions, to partial transformation into metabolites that may be more or less toxic.

These processes operate simultaneously; the net effect determines the environmental “lifetime” of a substance.

Bioaccumulation and Biomagnification

Environmental chemistry must consider how substances move through food webs:

• Bioaccumulation
• Uptake of substances by an organism faster than they are eliminated.
• Favored for:
• Lipophilic (fat-soluble) compounds with low water solubility.
• Metals that bind strongly to biological molecules and are poorly excreted.
• Biomagnification
• Increase in concentration of a substance from one trophic level to the next.
• Occurs when:
• The substance is efficiently absorbed with food and only slowly eliminated.
• Predators live longer and feed on many contaminated prey.

Consequences:

• Even low environmental concentrations can lead to high body burdens in top predators (including humans).
• Effects often appear far from the original source of pollution.

Environmental Fate and Half-Life

The environmental fate of a substance describes its pathways and transformations after release:

• Distribution among compartments (e.g. tendency to volatilize, dissolve, sorb to particles, enter organisms).
• Degradation by chemical, photochemical, and biological processes.
• Transport by air and water currents, particle movement, and organism migration.

To quantify persistence, several characteristic times are used:

• Reaction half-life: time for concentration to decrease by half due to a specified process, often approximated by first-order kinetics
  $$ t_{1/2} = \frac{\ln 2}{k} $$
  where:
• $t_{1/2}$ is the half-life
• $k$ is the first-order rate constant.
• Overall (environmental) half-life: effective half-life considering several parallel loss processes (e.g. photolysis, hydrolysis, biodegradation). If each has a rate constant $k_i$, an overall rate constant can often be approximated as
  $$ k_{\text{total}} = \sum_i k_i $$

Short half-lives limit long-range transport and long-term accumulation; long half-lives favor global distribution and long-lasting impacts.

Environmental Risk and Thresholds

Assessing chemical impacts requires connecting chemical properties with biological and ecosystem responses:

• Exposure: concentration, duration, and pathway (inhalation, ingestion, dermal contact).
• Effect: change caused at molecular, cellular, organism, or ecosystem level.

Some useful concepts:

• Toxicity benchmarks (e.g. LC50, EC50) are obtained from laboratory tests and indicate concentrations causing defined effects in test organisms.
• Threshold values and guidelines: concentrations in air, water, soil, or food set by authorities to protect human health and ecosystems.
• Mixture effects: in reality, organisms are often exposed to mixtures of substances, where effects may be additive, synergistic (stronger together), or antagonistic (weaker together).

From a chemical perspective, risk assessment combines:

• Knowledge of sources and emissions (how much is released).
• Transport and fate modeling (where does it go, how long does it stay).
• Speciation and transformation (which forms are present, how harmful they are).

Monitoring and Indicators in Environmental Chemistry

To understand and manage environmental quality, monitoring is essential:

• Sampling of air, water, soil, and biota at regular intervals or at critical sites.
• Analytical methods to detect and quantify chemicals, often at very low concentrations (link to analytical methods chapters).

Common chemical indicators:

• Nutrients (e.g. nitrate, phosphate, ammonium) as indicators of eutrophication potential.
• pH and alkalinity in waters and soils as indicators of buffering capacity and acidification.
• Dissolved oxygen as a central parameter for aquatic ecosystem health.
• Total organic carbon (TOC) or related measures as rough indicators of organic pollution.
• Specific pollutants (e.g. certain metals, organic compounds) monitored against legal or guideline values.

In addition, bioindicators (organisms that reflect environmental quality through their presence, abundance, or health) add a biological dimension to chemical data.

Environmental Chemistry and Sustainable Use of Chemicals

Chemistry can contribute to environmental protection by:

• Designing “greener” chemicals and processes
• Lower toxicity and persistence.
• Higher degradability into innocuous products.
• Efficient use of raw materials and energy.
• Closing material cycles
• Recycling and recovery of valuable elements.
• Reducing waste generation and emissions.
• Substitution and reduction
• Replacing particularly problematic substances with safer alternatives.
• Minimizing the use of chemicals where possible (e.g. targeted application of pesticides, optimized dosages in industrial processes).

Environmental chemistry provides the necessary understanding of how substances behave after they leave the production site or the product. This knowledge is essential for:

• Developing meaningful regulations and limits.
• Designing monitoring programs.
• Evaluating remediation strategies for contaminated environments.
• Supporting a transition to more sustainable chemistry and materials use.

The following subsections will discuss in more detail how chemicals are released into the environment, how they interact with global biogeochemical cycles, and how specific forms of air, water, and soil pollution arise and can be addressed.