2.4 Degradation and transformation processes of contaminants
3 min read•august 7, 2024
Contaminants don't stick around forever. They break down through various processes in the environment. Biological degradation, chemical reactions, and physical changes all play a role in transforming pollutants into different compounds.
Understanding these breakdown processes is crucial for assessing environmental risks and cleaning up contaminated sites. Some degradation products can be harmless, while others may be even more toxic than the original pollutant.
Degradation Processes
Biological and Chemical Breakdown
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Biodegradation occurs when microorganisms break down organic contaminants into simpler compounds
Bacteria and fungi are the primary agents of biodegradation (aerobic and anaerobic)
Biodegradation rates depend on factors such as temperature, oxygen availability, and nutrient levels
Examples of biodegradable contaminants include petroleum hydrocarbons and certain pesticides
Photodegradation is the breakdown of contaminants by exposure to light, particularly ultraviolet (UV) radiation
Photodegradation can occur directly when a contaminant absorbs light energy or indirectly through reactions with photochemically generated species (hydroxyl radicals)
Photodegradation is an important process for contaminants in surface waters and the atmosphere
Examples of photodegradable contaminants include polycyclic aromatic hydrocarbons (PAHs) and some pharmaceuticals
Abiotic Transformations
Hydrolysis is a chemical reaction in which a molecule is cleaved by water, resulting in the formation of new bonds
Hydrolysis can be catalyzed by acids, bases, or enzymes
The rate of hydrolysis depends on factors such as pH, temperature, and the chemical structure of the contaminant
Examples of contaminants susceptible to hydrolysis include organophosphate pesticides and some plastics
Oxidation is a chemical reaction in which a substance loses electrons, often resulting in the formation of new compounds
Oxidation can be mediated by strong oxidizing agents (hydrogen peroxide, ozone) or by microorganisms
Oxidation is an important process in the degradation of many organic contaminants, such as phenols and aromatic amines
Reduction is a chemical reaction in which a substance gains electrons, often resulting in the formation of new compounds
Reduction can occur under anaerobic conditions and is mediated by microorganisms or chemical reducing agents
Reductive dehalogenation is an important process in the degradation of chlorinated solvents (tetrachloroethene, trichloroethene)
Degradation Products
Intermediates and End Products
Metabolites are intermediate products formed during the degradation of a parent compound
Metabolites can be more or less toxic than the parent compound, depending on their chemical structure and properties
The formation of metabolites can be an important consideration in risk assessment and remediation strategies
Examples of metabolites include the breakdown products of pesticides (DDT to DDE) and the transformation products of pharmaceuticals
Mineralization is the complete breakdown of an organic compound into inorganic constituents, such as carbon dioxide, water, and mineral salts
Mineralization represents the ultimate goal of biodegradation and is essential for the complete removal of contaminants from the environment
The rate of mineralization depends on the chemical structure of the contaminant and the availability of suitable microorganisms and environmental conditions
Examples of contaminants that can be mineralized include simple hydrocarbons (methane) and some organic acids (acetic acid)
Microbial Interactions
Collaborative Degradation and Bioavailability
Cometabolism is the degradation of a contaminant by microorganisms that do not use the contaminant as a primary energy or carbon source
Cometabolism requires the presence of a primary substrate that supports the growth and activity of the microorganisms
Cometabolism can be an important process in the degradation of recalcitrant compounds (chlorinated solvents) that are not readily biodegradable
Examples of cometabolic substrates include methane, propane, and aromatic compounds (toluene)
Bioavailability refers to the extent to which a contaminant is accessible to microorganisms for degradation
Bioavailability can be limited by factors such as sorption to soil or sediment particles, incorporation into organic matter, or low aqueous solubility
Enhancing bioavailability through the use of surfactants, solvents, or mechanical agitation can improve the efficiency of bioremediation
Examples of contaminants with limited bioavailability include polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs) in aged soil or sediment