Water Pollution

Types and Sources of Water Pollution

Water pollution refers to the introduction of substances or forms of energy into water bodies that impair their natural functions, their suitability as habitats, and their use by humans. In the context of environmental chemistry, the focus is on the chemical nature, behavior, and fate of pollutants in aquatic systems.

Natural Water Systems and Their Sensitivity

Natural waters (groundwater, rivers, lakes, oceans) are not chemically pure. They naturally contain dissolved gases (e.g. $O_2$, $CO_2$), ions (e.g. $Ca^{2+}$, $HCO_3^-$, $Na^+$, $Cl^-$), and organic matter (e.g. humic substances). These components are in dynamic equilibrium with:

• the atmosphere (gas exchange),
• soils and rocks (weathering, ion exchange),
• living organisms (uptake, excretion, decomposition).

Water pollution becomes relevant when anthropogenic inputs exceed the natural buffering and self‑purification capacity of the system, leading to persistent or harmful changes in composition and properties.

Major Classes of Water Pollutants

1. Oxygen-Demanding Substances

Oxygen-demanding substances include:

• biodegradable organic matter (e.g. sewage, food-processing wastes),
• reduced inorganic substances (e.g. $NH_4^+$, $Fe^{2+}$, $S^{2-}$).

Microorganisms oxidize these substances using dissolved oxygen:

$$\text{organics} + O_2 \longrightarrow CO_2 + H_2O + \text{biomass}$$

Consequences:

• Consumption of dissolved oxygen (DO); if input is high, DO can drop below levels needed for fish and invertebrates.
• Formation of anoxic zones, promoting anaerobic processes (e.g. sulfate reduction to $H_2S$), which can generate toxic or malodorous products.

The total oxygen demand is often characterized by:

• Biological Oxygen Demand (BOD): oxygen needed by microorganisms to biodegrade organic matter over a defined time (e.g. 5 days).
• Chemical Oxygen Demand (COD): oxygen equivalent required to chemically oxidize organic matter with a strong oxidant (e.g. $K_2Cr_2O_7$).

Elevated BOD or COD indicates heavy organic loading.

2. Nutrients and Eutrophication

Key nutrients:

• Nitrogen (e.g. $NO_3^-$, $NO_2^-$, $NH_4^+$),
• Phosphorus (primarily $PO_4^{3-}$ and related species).

Anthropogenic sources:

• agricultural fertilizers (field runoff),
• domestic wastewater and detergents,
• certain industrial effluents.

Excess nutrient input often triggers eutrophication, a process characterized by:

1. Accelerated growth of algae and aquatic plants (algal blooms),
2. Increased production of organic biomass,
3. Subsequent decomposition consuming oxygen,
4. Oxygen depletion in bottom waters,
5. Possible fish kills and loss of biodiversity.

Chemically, eutrophication alters:

• redox conditions (shift to reducing conditions in sediments),
• speciation of metals (more soluble reduced forms),
• pH and carbonate system equilibria.

Nitrogen compounds can also undergo transformation between different oxidation states via microbial processes:

• Nitrification (oxidation of $NH_4^+$ to $NO_3^-$),
• Denitrification (reduction of $NO_3^-$ to $N_2$ under anoxic conditions).

These processes influence both water quality and greenhouse gas emissions (e.g. $N_2O$).

3. Toxic Inorganic Substances

This group includes:

• Heavy metals (e.g. $Hg$, $Cd$, $Pb$, $As$, $Cr$, $Ni$, $Cu$, $Zn$),
• Other inorganic toxins (e.g. cyanides, fluorides).

Sources:

• mining and ore processing,
• electroplating and metal finishing,
• pigment and battery production,
• coal combustion (atmospheric deposition),
• corrosion of pipes and fittings.

Important aspects in aquatic systems:

• Speciation: The chemical form (free ion, complex, precipitate) determines toxicity and mobility. For example:
• $Cr(VI)$ as chromate $CrO_4^{2-}$ is generally more toxic and mobile than $Cr(III)$,
• $Hg^{2+}$ can form highly toxic organomercury compounds (see below).
• pH-dependence: Solubility of many metal hydroxides and carbonates is strongly pH-dependent; acidification can mobilize metals bound in sediments.
• Complexation: Ligands like $Cl^-$, $OH^-$, $CN^-$, or organic chelators can form complexes, altering bioavailability.

Metals are non-degradable; they can be transformed or redistributed but not destroyed, leading to long-term accumulation in sediments and organisms.

4. Organic Micropollutants

Organic micropollutants are anthropogenic organic compounds typically present at low concentrations (µg/L or ng/L) but potentially with significant effects.

Major classes:

• Pesticides (herbicides, insecticides, fungicides),
• Industrial chemicals (e.g. PCBs, phenols, plasticizers like phthalates),
• Pharmaceuticals and personal care products (e.g. analgesics, antibiotics, hormones, UV filters),
• Persistent organic pollutants (POPs) with resistance to degradation (e.g. certain chlorinated hydrocarbons).

Characteristics:

• Hydrophobicity: Many are poorly soluble in water and prefer organic phases. This is often quantified with the octanol–water partition coefficient $K_{ow}$. High $K_{ow}$ tends to correlate with:
• strong adsorption to sediments and organic matter,
• potential for bioaccumulation in organisms.
• Persistence: Slow biodegradation and resistance to hydrolysis or photolysis lead to long environmental half-lives.
• Toxicity and specific modes of action: For example, some compounds act as endocrine disruptors, interfering with hormonal systems even at very low concentrations.

Transport pathways:

• Runoff from treated soils,
• Discharge from wastewater treatment plants (often incomplete removal),
• Atmospheric deposition (for volatile or semi-volatile organics).

5. Suspended Solids and Sediment-Associated Pollutants

Suspended solids encompass:

• inorganic particles (clay, silt),
• organic detritus (particulate organic matter).

Chemical significance:

• Turbidity reduces light penetration, affecting aquatic photosynthesis.
• Particles provide surfaces for adsorption of metals and hydrophobic organics.
• Pollutants may be transported attached to particles and later deposited as contaminated sediments.

In sediments, reduced conditions often prevail, influencing:

• metal speciation (e.g. formation of metal sulfides),
• release or immobilization of pollutants depending on redox and pH changes.

Resuspension events (storms, dredging) can reintroduce sediment-bound pollutants into the water column.

6. Salts and Salinization

Increased salt content (salinization) is particularly relevant in:

• arid and semi-arid regions with intensive irrigation,
• regions influenced by seawater intrusion into groundwater,
• discharge of industrial brines and mining waters.

Chemically, salinization involves elevated concentrations of ions such as:

• $Na^+$, $K^+$, $Ca^{2+}$, $Mg^{2+}$,
• $Cl^-$, $SO_4^{2-}$, $HCO_3^-$.

Consequences:

• Osmotic stress for freshwater organisms,
• Impairment of drinking water quality (taste, corrosion),
• Changes in water hardness and scaling tendencies (precipitation of carbonates and sulfates).

7. Acidification and pH Changes

Deviations from the natural pH range affect:

• the carbonate equilibrium,
• solubility and speciation of metals and nutrients,
• biological processes (enzyme activity, membrane stability).

Sources of acidification:

• Acid mine drainage (oxidation of sulfide minerals producing $H_2SO_4$),
• Atmospheric deposition of acids (from $SO_2$, $NO_x$),
• Industrial discharges.

Conversely, strongly alkaline discharges (e.g. from cement or certain cleaning processes) can shift pH to high values, with their own ecological impacts.

8. Thermal Pollution

Thermal pollution arises mainly from:

• cooling water discharges from power plants and industrial facilities.

Even without introducing new chemicals, elevated temperature affects:

• solubility of gases (lower $O_2$ solubility),
• reaction kinetics (faster biochemical reactions, including microbial oxygen consumption),
• the balance of species that are adapted to narrow temperature ranges.

Chemically, temperature modifies equilibria and rates of redox reactions and decomposition processes, indirectly influencing pollutant behavior.

Chemical Processes in Polluted Waters

Self-Purification and Natural Attenuation

Natural waters possess some capacity for “self-purification” through:

• Dilution and dispersion,
• Sedimentation of particles and attached pollutants,
• Adsorption on mineral and organic surfaces,
• Biodegradation of organic matter,
• Chemical and photochemical transformation (e.g. oxidation by $O_2$, $H_2O_2$, or radicals).

However, this capacity is finite and pollutant-specific; persistent, bioaccumulative substances or large loads can exceed it.

Redox Processes and Stratification

Many water bodies exhibit vertical stratification:

• Surface layers: often oxic, exposed to light and atmosphere.
• Deep layers or sediments: often anoxic, dominated by reducing conditions.

Key redox processes include:

• Aerobic respiration (oxidation of organics with $O_2$),
• Nitrification–denitrification (interconversion between $NH_4^+$, $NO_3^-$, $N_2$),
• Sulfate reduction ($SO_4^{2-} \rightarrow S^{2-}$),
• Methanogenesis ($CO_2 \rightarrow CH_4$ under strongly reducing conditions).

These processes determine the dominant forms of carbon, nitrogen, sulfur, and many metals, as well as the production of gases such as $CO_2$, $N_2O$, $H_2S$, and $CH_4$.

Complexation and Chelation

In polluted waters, numerous ligands are present:

• inorganic (e.g. $Cl^-$, $OH^-$, $CN^-$),
• organic (natural organic matter, synthetic chelating agents like EDTA).

Formation of complexes:

$$M^{n+} + L^{m-} \rightleftharpoons ML^{(n-m)+}$$

influences:

• solubility (complexation may increase solubility of metals),
• mobility (metal–ligand complexes can be transported further),
• toxicity (in some cases reduced, in others enhanced).

Chelating agents used in industrial processes or detergents can accidentally increase the mobility of heavy metals in the aquatic environment.

Bioaccumulation and Biomagnification

Some hydrophobic and persistent pollutants (e.g. certain POPs, methylmercury) tend to:

• accumulate in organism tissues (bioaccumulation),
• increase in concentration at higher trophic levels (biomagnification).

From a chemical perspective, important properties are:

• lipophilicity (high $K_{ow}$),
• low susceptibility to metabolic transformation,
• strong binding to biological macromolecules (e.g. proteins, lipids).

These processes can lead to high internal doses in top predators (including humans) even when environmental concentrations are low.

Human and Ecological Impacts

The chemical changes described above translate into a variety of impacts:

• Potable water issues: taste, odor, hardness, toxic contaminants (e.g. nitrates, metals, organics),
• Ecotoxicological effects: acute or chronic toxicity to aquatic organisms, reproductive and developmental effects,
• Loss of ecosystem services: reduced fishery yields, loss of recreational value, degradation of habitats (e.g. wetlands).

Many water-quality standards define maximum permissible concentrations for specific substances, often based on toxicological data and chemical behavior (e.g. speciation, bioavailability).

Chemical Approaches to Control and Remediation

While technical and policy aspects are treated in other contexts, several chemical principles underlie water pollution control and treatment.

Removal or Transformation of Pollutants

Examples of chemical and physicochemical treatment steps include:

• Precipitation of metals:

$$M^{n+} + n\,OH^- \rightarrow M(OH)_n \downarrow$$

or as sulfides using $S^{2-}$.

• Coagulation–flocculation: addition of salts (e.g. $Fe^{3+}$, $Al^{3+}$) to neutralize charges and aggregate colloids and dissolved organics into settleable flocs.
• Adsorption: binding of organics and some metals to activated carbon or other sorbents.
• Oxidation processes:
• chlorination,
• ozonation,
• advanced oxidation processes (AOPs) involving hydroxyl radicals $\cdot OH$ to degrade persistent organics.
• Ion exchange: targeted removal of specific ions (e.g. nitrates, hardness ions).

Biological treatment (e.g. in wastewater treatment plants) relies heavily on the microbiological transformation of organic matter and nutrients, but its efficiency is constrained by the chemical structure and biodegradability of pollutants.

Prevention and Substitution

From a chemical standpoint, prevention strategies include:

• Development of less persistent and less toxic chemicals (e.g. more biodegradable surfactants and pesticides),
• Reduction of nutrient contents in detergents (phosphate-free formulations),
• Process modifications to minimize the generation or release of hazardous substances.

Green chemistry principles guide the design of chemicals and processes with reduced potential for water pollution.

Monitoring and Indicators

Environmental chemists use a combination of:

• bulk parameters (BOD, COD, total organic carbon, total nitrogen, total phosphorus),
• specific determinations (e.g. $NO_3^-$, $PO_4^{3-}$, $Pb$, $Cd$, selected pesticides, pharmaceuticals),
• physicochemical parameters (pH, electrical conductivity, DO, temperature, turbidity),

to assess water quality. Analytical methods for these determinations are addressed in detail in the chapters on analytical chemistry, but their application is central to understanding and managing water pollution.

Overall, water pollution illustrates how chemical substances and reactions interact with physical and biological processes in complex environmental systems, and how chemical knowledge is essential both to diagnose problems and to design effective solutions.