Environmental Chemistry - Theory & Concepts

Environmental Chemistry

The chemical processes occurring in the environment and the impacts of human activities.

Environmental chemistry is crucial for civil engineers involved in water resources, sanitation, and sustainable design. It deals with chemical processes occurring in the environment and the impacts of human activities.

Water Chemistry

Water is the "universal solvent" and plays a vital role in biological and geological processes. Its solvent properties make it highly susceptible to contamination.

Water Hardness

Hardness is caused by dissolved minerals, primarily calcium (

Ca^{2+}

) and magnesium (

Mg^{2+}

) ions. It is an engineering concern because it causes scale buildup in pipes and boilers.

Temporary Hardness: Caused by bicarbonate ( $HCO_3^-$ ). Can be removed by boiling.
$Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3(s) + H_2O + CO_2$
Permanent Hardness: Caused by sulfate ( $SO_4^{2-}$ ), chloride ( $Cl^-$ ), or nitrate ( $NO_3^-$ ). Requires chemical treatment.
Treatment: Lime softening ( $Ca(OH)_2$ ) or Ion Exchange (Water Softeners).

Biochemical Oxygen Demand (BOD)

A calculation of the rate at which microorganisms consume oxygen in a water sample.

BOD_t = L_0 (1 - e^{-kt})

Variables

Symbol	Description	Unit
$BOD_t$	BOD exerted at time t (mg/L)	-
$L_0$	Ultimate BOD (maximum oxygen demand) (mg/L)	-
$k$	BOD rate constant (days^-1)	-
$t$	Time (days)	-

Water Quality Parameters

Dissolved Oxygen (DO): Essential for aquatic life. Low DO indicates pollution, often leading to fish kills.
Biochemical Oxygen Demand (BOD): Amount of oxygen required by bacteria to decompose organic matter over a specific period (usually 5 days, $BOD_5$ ). High BOD $\rightarrow$ High organic pollution (like raw sewage).
Chemical Oxygen Demand (COD): Oxygen required to chemically oxidize all organic and inorganic pollutants. Always higher than and faster to measure than BOD.
Turbidity: Cloudiness caused by suspended particles; high turbidity interferes with disinfection processes by shielding pathogens from UV or chlorine.

Water Treatment Processes

Converting raw water into potable water involves several chemical steps:

Procedure

STEP-BY-STEP

Coagulation & Flocculation: Adding coagulants like Alum ( $Al_2(SO_4)_3$ ) to neutralize negative charges on suspended particles so they clump together into larger "flocs".
Sedimentation: Allowing the heavy flocs to settle to the bottom of a basin by gravity.
Filtration: Passing water through sand, coal, or membrane layers to physically remove remaining micro-particles.
Disinfection: Killing pathogens using Chlorine ( $Cl_2$ ), Ozone ( $O_3$ ), or UV light.
- Chlorination: $Cl_2 + H_2O \rightleftharpoons HOCl + HCl$ . Hypochlorous acid ( $HOCl$ ) is the active pathogen killer.

Atmospheric Chemistry

The atmosphere consists of layers: Troposphere (where weather and pollution occur), Stratosphere (Ozone Layer), Mesosphere, Thermosphere.

Air Pollutants

Primary Pollutants: Emitted directly from a source ( $CO$ , $SO_2$ , $NO_x$ , Particulate Matter).
Secondary Pollutants: Formed in the atmosphere through chemical reactions ( $O_3$ , Sulfuric Acid).
Photochemical Smog: Reaction of Nitrogen Oxides ( $NO_x$ ) + Volatile Organic Compounds (VOCs) + Sunlight $\rightarrow$ Ground-level Ozone ( $O_3$ ).

Global Environmental Issues

Ozone Depletion: Chlorofluorocarbons (CFCs) release Chlorine atoms in the stratosphere, which catalyze ozone destruction, allowing harmful UV radiation to reach the surface.
$Cl + O_3 \rightarrow ClO + O_2$
Greenhouse Effect: Gases like $CO_2$ , $CH_4$ (methane), and $N_2O$ trap outgoing infrared radiation from the Earth, raising global temperatures.
Acid Rain: $SO_2$ and $NO_x$ from fossil fuel combustion react with atmospheric water vapor to form acids ( $H_2SO_4$ , $HNO_3$ ), lowering rain pH below 5.6. This corrodes concrete and steel structures.

Solid and Hazardous Waste

Waste Management Basics

Leachate: The toxic liquid that drains from landfills. Civil engineers must design clay and synthetic liners to prevent it from contaminating groundwater.
Bioremediation: Using microorganisms (bacteria/fungi) to degrade organic pollutants (e.g., oil spills, soil contamination) into harmless substances like $CO_2$ and water.

Key Takeaways

Hardness (Ca and Mg ions) causes scale deposition in municipal pipes; it is removed via chemical softening.
BOD/COD measure the organic pollution load in wastewater; BOD follows a first-order exponential curve.
Chlorination is the most common water disinfection method but relies on maintaining free $HOCl$ .
Acid Rain ( $SO_2$ / $NO_x$ ) corrodes limestone ( $CaCO_3$ ) structures and harms aquatic ecosystems by lowering pH.

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