MEV 013: Unit 01 - Environmental Chemistry-I

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 UNIT 1: ENVIRONMENTAL CHEMISTRY – I

1.1 Introduction

Environmental Chemistry is the study of the chemical and biochemical phenomena that occur in natural places. It integrates the principles of chemistry to understand the environment, the impact of human activities on natural processes, and how chemical changes affect air, water, and soil systems. This unit introduces key chemical principles that are foundational to understanding environmental processes.

1.2 Objectives

By the end of this unit, you will be able to:

  • Understand the scope of environmental chemistry.
  • Apply stoichiometric and equilibrium concepts to environmental processes.
  • Understand the basics of reaction kinetics and reaction mechanisms.
  • Identify key types of environmental chemical reactions including hydrolysis, oxidation, and reduction.
  • Understand the roles of catalysis and adsorption in environmental systems.

1.3 Concept and Scope of Environmental Chemistry

Environmental chemistry is concerned with the origin, transport, reactions, and fates of chemical species in water, air, terrestrial, and biological environments. Its scope includes:

  • Monitoring pollutants and understanding their chemical behavior.
  • Studying the fate of contaminants (e.g., pesticides, heavy metals).
  • Assessing risk and designing chemical solutions for remediation.
  • Understanding biogeochemical cycles (carbon, nitrogen, phosphorus).

1.4 Fundamentals of Elemental Stoichiometry

Stoichiometry is the quantitative study of reactants and products in a chemical reaction. It forms the basis for calculating chemical yields, pollutant concentrations, and biochemical cycles in environmental systems.

Applications in Environment:

  • Calculating oxygen demand in water treatment.
  • Estimating carbon dioxide emissions.
  • Balancing redox reactions in groundwater chemistry.

1.5 Chemical Equilibrium

Environmental systems often operate under dynamic equilibria—especially in water and atmospheric chemistry. Equilibrium concepts are essential in predicting the extent of reactions such as dissolution, precipitation, or gas exchange.

1.5.1 Open and Closed Systems

  • Open System: Exchange of matter and energy (e.g., a lake interacting with the atmosphere).
  • Closed System: Exchange of energy but not matter (e.g., sealed water bottle).

1.5.2 Reversible Reactions

Reversible reactions are fundamental in nature; they occur simultaneously in both directions until equilibrium is reached.

1.6 Chemical Potential

Chemical potential is a thermodynamic quantity representing the ability of a substance to undergo a change (reaction, phase change, or transport).

  • Drives diffusion and phase transitions.
  • Influences chemical partitioning in ecosystems.

1.7 Chemical Kinetics

Kinetics describes how fast a chemical reaction proceeds, which is vital in understanding pollutant degradation, transformation, and environmental remediation.

1.7.1 Kinetics of Reactions of Different Orders

  • Zero-order: Rate is independent of reactant concentration.
  • First-order: Rate is proportional to one reactant.
  • Second-order: Depends on two reactants or square of one.

1.8 Simple Reaction Mechanisms

Mechanisms describe the stepwise sequence of elementary reactions.

  • Helps identify rate-determining steps.
  • Explains intermediate species in pollutant breakdown.

1.9 Order and Molecularity of Chemical Reactions

1.9.1 Reaction Order

  • Experimental concept.
  • Sum of powers of concentration terms in the rate law.

1.9.2 Molecularity of the Reaction

  • Theoretical number of molecules participating in an elementary step.
  • Can be unimolecular, bimolecular, or termolecular.

1.10 Chemical Reactions in Environmental Systems

Chemical transformations in nature involve various reactions that degrade, detoxify, or produce environmentally significant species.

1.10.1 Hydrolysis

  • Water reacts with a compound, breaking bonds.
  • Example: Hydrolysis of esters or halogenated organics.

1.10.2 Reduction Reactions

Reduction involves gain of electrons or a decrease in oxidation state.

1.10.2.1 Reductive Dehalogenation

  • Removal of halogens from organic compounds (e.g., chlorinated solvents in groundwater).

1.10.2.2 Nitroaromatic Reduction

  • Important for degradation of TNT and dinitro compounds.

1.10.2.3 Aromatic Azo Reduction

  • Cleavage of azo bonds (-N=N-) in dyes and pollutants.

1.10.2.4 N-Nitrosamine Reduction

  • Transformation of carcinogenic nitrosamines.

1.10.2.5 Sulfoxide Reduction

  • Involves sulfur compounds in atmospheric and soil chemistry.

1.10.2.6 Quinone Reduction

  • Plays roles in redox cycling of organic matter.

1.10.2.7 Reductive Dealkylation

  • Important in degradation of pesticides and pharmaceuticals.

1.10.3 Oxidation Reactions

Oxidation is the loss of electrons or an increase in oxidation state.

  • Occurs in biodegradation, photochemical smog formation.
  • Includes advanced oxidation processes (AOPs) in water treatment.

1.11 Catalysis

Catalysis accelerates chemical reactions without being consumed.

  • Homogeneous Catalysis: Catalyst in same phase (e.g., acid-catalyzed hydrolysis).
  • Heterogeneous Catalysis: Catalyst in different phase (e.g., catalytic converters in vehicles).

Environmental applications:

  • Catalytic degradation of pollutants
  • Industrial emission control

1.12 Adsorption in Catalysis

Adsorption involves the adhesion of atoms or molecules to a surface, often a critical step in heterogeneous catalysis.

  • Important in removing contaminants in water/air filters.
  • Affects the bioavailability of pollutants in soils.

Types:

  • Physisorption: Weak van der Waals forces
  • Chemisorption: Strong chemical bonding

1.13 Let Us Sum Up

This unit introduced the foundational principles of environmental chemistry, including stoichiometry, chemical equilibrium, kinetics, and types of chemical reactions relevant to natural and polluted environments. The detailed understanding of reaction mechanisms, catalysis, and adsorption is essential for designing effective environmental management strategies, pollutant degradation pathways, and sustainability practices.

1.14 Keywords

·         Environmental Chemistry-Study of chemical processes in the environment

·         Stoichiometry-Quantitative relationships in chemical reactions

·         Chemical Equilibrium-State where forward and reverse reactions balance

·         Chemical Potential-Energy driving a substance’s change or reaction

·         Reaction Kinetics-Study of reaction rates and mechanisms

·         Molecularity-Number of molecules in a reaction step

·         Hydrolysis-Reaction involving water breaking chemical bonds

·         Reductive Dehalogenation-Removal of halogen atoms by reduction

·         Oxidation-Loss of electrons or gain of oxygen

·         Catalysis-Increase in reaction rate by a catalyst

·         Adsorption-Molecule attachment to a surface

 

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