2.1 Physical and chemical properties of environmental contaminants

Environmental contaminants have unique physical and chemical properties that determine their behavior in ecosystems. These properties, like solubility, polarity, and partitioning, affect how pollutants spread and interact with living organisms.

Understanding these properties is crucial for predicting a contaminant's environmental fate. Factors such as persistence, degradation, bioaccumulation, and volatility influence how long pollutants stick around and their potential impact on wildlife and human health.

Chemical Properties

Solubility and Polarity

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• Solubility measures the ability of a substance to dissolve in a solvent, typically water for environmental contaminants
• Highly soluble contaminants more readily disperse and transport in aquatic environments
• Polarity influences solubility as polar substances dissolve better in polar solvents like water (methanol), while nonpolar substances prefer nonpolar solvents (hexane)
• Contaminants with high solubility and polarity tend to have lower bioaccumulation potential as they do not partition well into organic tissues

Partitioning and Molecular Characteristics

• Octanol-water partition coefficient (Kow) quantifies the partitioning of a contaminant between octanol (organic phase) and water
• Higher Kow values indicate greater lipophilicity and potential for bioaccumulation in fatty tissues of organisms
• Molecular weight affects various properties and fate of contaminants
• Lower molecular weight substances tend to have higher volatility and solubility, facilitating their transport and dispersion
• Acid dissociation constant (pKa) describes the pH at which an ionizable contaminant is 50% dissociated
• pKa determines the degree of ionization and influences solubility, partitioning, and reactivity based on environmental pH conditions

Environmental Fate

Persistence and Degradation

• Persistence refers to a contaminant's ability to remain in the environment without undergoing significant transformation or degradation
• Persistent contaminants resist biotic and abiotic degradation processes, leading to long-term contamination and potential for long-range transport (PCBs, dioxins)
• Half-life quantifies the time required for 50% of a contaminant to degrade or transform
• Longer half-lives indicate greater persistence and potential for accumulation in environmental media and biota
• Degradation processes include photolysis (sunlight), hydrolysis (water), and biodegradation (microbial activity)

Bioaccumulation and Volatility

• Bioconcentration factor (BCF) represents the ratio of a contaminant's concentration in an organism to its concentration in the surrounding environment or water
• Contaminants with high BCF values tend to accumulate in organisms and biomagnify up the food chain (DDT, mercury)
• Lipophilic contaminants with high Kow values are more likely to bioaccumulate in fatty tissues
• Volatility describes a contaminant's tendency to evaporate or sublimate from the solid or liquid phase into the atmosphere
• Henry's law constant (H) quantifies the partitioning of a contaminant between air and water at equilibrium
• Volatile contaminants can disperse through the atmosphere and undergo long-range transport (PCBs)
• Volatilization from water or soil surfaces is a key process influencing the fate and distribution of contaminants in the environment