Fate and Bioavailability of Contaminants

Fundamental Concepts of Contaminant Fate and Bioavailability

• Objective and Scope: The study of contaminants in the environment focuses on their fate (where they go and how they change) and their bioavailability (the extent to which they can be taken up by living organisms).

• Sources of Contaminants:

  • Point Sources: Specific, localized sources of pollution, such as a factory pipe or a wastewater treatment plant outlet.

  • Diffuse Sources: Broad, non-specific sources where contaminants are spread over a large area, such as agricultural runoff or atmospheric deposition.

• Factors Determining Environmental Fate:

  • Chemical Properties: Includes persistence (resistance to degradation), solubility (how well it dissolves in water), and vapor pressure (tendency to evaporate).

  • Transport and Transformation: The movement of chemicals across local to global scales and their chemical or biological alteration.

  • Biotic Uptake: The process by which chemicals enter living organisms.

Persistent, Bioaccumulative, and Toxic (PBT) Contaminants

• Definition of PBTs: A special class of organic contaminants characterized by three traits:

  • Persistent: Molecules are stable and do not break down easily in the environment.

  • Bioaccumulative: Resistant to biochemical degradation within organisms, leading to accumulation over time.

  • Toxic: Capable of causing harm to organisms.

• Key Examples:

  • PBDE (Tetrabrominated diphenylether): An organobromine compound used as a flame retardant.

  • PCB (Polychlorinated biphenyl): A group of man-made organic chemicals consisting of carbon, hydrogen, and chlorine atoms.

• Chemical Composition: Often consist of aromatic hydrocarbons combined with halogens (Chlorine $\text{Cl}$, Fluorine $\text{F}$, Bromine $\text{Br}$, or Iodine $\text{I}$). The addition of halogens stabilizes the molecule, increases its persistence, and enhances its hydrophobicity (water-repelling nature).

Chemical Partitioning and the Partition Coefficient ($K_{ow}$)

• Partitioning Definition: The distribution of a chemical between two phases (e.g., water and octanol) at thermodynamic equilibrium.

• Log $K_{ow}$ (Octanol-Water Partition Coefficient):

  • Formula: $K_{ow} = \frac{C_{octanol}}{C_{water}}$

  • Example: If $C_{octanol} = 2291$ and $C_{water} = 1$, then $K_{ow} = 2291$, and $\log K_{ow} = 3.36$.

• Log $K_{ow}$ Values by Chemical Group:

  • Polycyclic aromatic hydrocarbons (PAH): $3$ – $7$

  • Halogenated aliphatic hydrocarbons: $1$ – $3$

  • Organic pesticides: $0$ – $7$

  • Polychlorinated biphenyls (PCB): $4$ – $10$

  • Methyl mercury: $0.3$

• Hydrophobicity Thresholds:

  • Very Hydrophobic: \log K_{ow} > 5

  • Super Hydrophobic: \log K_{ow} > 7

  • Bioaccumulation Potential: Generally observed in compounds with $\log K_{ow} \ge 3$.

• Solubility Correlations: As the number of chlorine atoms in a PCB molecule increases, its water solubility decreases, and its solubility in non-polar substances (lipids/octanol) increases.

• Other Partition Coefficients:

  • $K_{oc}$: Concentration on organic matter vs. concentration in water.

  • $K_{aw}$: Concentration in air vs. concentration in water ($10^{-1}$ to $10^{-3}$ for POPs).

  • $K_{oa}$: Concentration in octanol vs. concentration in air ($10^{6}$ to $10^{12}$ for POPs).

  • $K_{ow}$ for POPs: Typically ranges from $10^{4}$ to $10^{7}$.

Environmental Transport and Transformation

• Transformation Processes:

  • Abiotic (Chemical):

    • Photolysis: Degradation via light, specifically UV radiation.

    • Hydrolysis: Reaction with water or hydroxyl groups ($\text{OH}$).

    • Dissociation: Dependent on pH levels.

    • Oxidation/Reduction: Electronic transitions affecting molecular structure.

  • Biotic (Biological):

    • Microbial (Aerobic): Often involves ring cleavage of aromatic structures.

    • Microbial (Anaerobic): Often involves dehalogenation.

    • Macrobial: Metabolism via the $P_{450}$ monooxygenase system.

• The Two Phases of Enzymatic Metabolism:

  • Phase I: Functionalization (e.g., adding an $\text{OH}$ group) involving enzymes like $P_{450}$.

  • Phase II: Conjugation (e.g., adding Glucuronic acid, Cysteine, or Glycine) to make the compound more water-soluble for excretion.

• Global Transport - The Grasshopper Effect:

  • In warmer regions, Organochlorines (OCs) and Persistent Organic Pollutants (POPs) evaporate (volatilization).

  • Winds carry these OCs in the atmosphere toward colder regions.

  • In cold temperatures, the OCs condense and fall back to Earth (deposition).

  • This process repeats, leading to the accumulation of POPs in the Arctic and Antarctic, even if they were never used there.

• Phase Transitions:

  • Contaminants move between Atmosphere, Soil, Water, and Sediment through desorption, resuspension, sorption, sedimentation, volatilization, and deposition.

Sorption and Sediment Dynamics

• Sorption Definition: The association between contaminants and particles (Dissolved Organic Matter - DOM, or Particulate Organic Matter - POM).

• Influencing Factors:

  • Chemical properties of the contaminant.

  • Quality and chemical properties of the organic matter.

  • Total quantity of organic matter.

• Particle Size Relationship: Small particles have a high surface-to-volume ratio and generally contain higher organic content, leading to higher relative contaminant concentrations.

• Sediments as Sinks: Sediments are the primary long-term storage (sinks) for persistent contaminants due to sedimentation of POM.

Bioturbation and Contaminant Fate

• Bioturbation Processes:

  • Particle Mixing: Organisms (e.g., Arenicola marina) move sediment-associated contaminants.

  • Irrigation: Organisms flush sediment with water, affecting dissolved contaminants.

  • Redox Conditions: Changes in oxygen levels stimulate aerobic microbial degradation.

• Case Study - Marenzelleria spp.:

  • An invasive species that arrived in the southern Baltic Sea in the mid-1980s.

  • Features: Tolerates low oxygen/high sulfide; burrows up to $35\,\text{cm}$ (standard fauna only reaches $5\,\text{cm}$); densities up to $40,000\,\text{ind./m}^2$.

  • Impact: Their deep burrowing increases the release of PCBs from sediments back into the water column (‘reintroducing old sins’).

Biouptake Mechanisms

• Bioconcentration: Accumulation specifically from water exposure (C_{biota} > C_{water}).

  • BCF (Bioconcentration Factor): The net accumulation at steady state.

  • Dynamics: Defined by the rate of accumulation ($k_u C_w$) vs. the rate of elimination/depuration ($k_e C_T$).

• Bioaccumulation: Accumulation from all exposure routes, including water, food, and sediment (C_{biota} > C_{water/particles}).

  • BAF (Bioaccumulation Factor): The net accumulation from all sources at steady state.

• Biomagnification: The increase in contaminant concentration through trophic levels (C_{biota1} < C_{biota2} < C_{biota3}).

  • Top Predators: Often air-breathers who cannot eliminate contaminants efficiently, leading to high concentrations.

• Routes of Uptake: Respiration (air/water), diffusion over skin, and ingestion of food.

• Routes of Depuration: Respiration, skin diffusion, feces, metabolic conversion, reproduction (gametes, lactation, offspring), and growth (dilution).

Metals in the Environment

• Definitions:

  • Light Metals (Low Density): Essential (e.g., $Mg$); Non-essential (e.g., $Al$, $Be$, $Ti$).

  • Heavy Metals (High Density): Essential (e.g., $Co$, $Cr$, $Cu$, $Fe$, $Mn$, $Ni$, $Zn$); Non-essential (e.g., $Ag$, $Cd$, $Hg$, $Pb$).

• Uptake of Metals:

  • Free metal ions enter through ion channels, pumps, or carriers.

  • Organometals (like Methylmercury) and neutral complexes can pass straight through cell membranes.

• Assimilation Efficiency Examples: $Zn$ ($41.2\,\%$), $Co$ ($46.2\,\%$), $Cd$ ($17.6\,\%$), $Ag$ ($7.0\,\%$).

• Factors Affecting Metal Bioavailability:

  • pH: Influences speciation. In sea water, predominant forms include aquo-ions ($Ni$, $Zn$), chlorides ($Cd$, $Hg(II)$), carbonates ($Cu(II)$, $Pb(II)$), and hydroxides ($Fe(III)$, $Al(III)$).

  • Salinity: Specifically for species like Anadara granosa, $Cd$ uptake is a function of salinity and ambient concentration.

  • Dissolved Organic Carbon (DOC): Binds free metal ions, reducing bioavailability. This capacity is decreased by UV radiation which breaks down DOC.

  • Carbon Quality: Autochthonous (produced within the system) vs. Allochthonous (imported from outside) carbon affects toxicity. Juvenile trout (Oncorhynchus mykiss) show higher survival with carbon present.

• Metal Concentrations and Safety Levels: Natural oceanic levels are often very low (e.g., $Hg$ at $0.002\,\mu g/l$), while safety levels range significantly (e.g., $1-10\,\mu g/l$ for $Cr$).

Environmental Monitoring and Quality Standards

• Objectives: Assessment of environmental status, time trends, and the impact of legislation (e.g., monitoring eggshell thickness to track PCB-like compound toxicity).

• Quality Standard Parameters:

  • NOEC: No Observed Effect Concentration.

  • LOEC: Lowest Observed Effect Concentration.

  • $EC_{50}/LC_{50}$: Concentration causing effect/lethality in $50\,\%$ of the population.

• Key Definitions:

  • EQS: Environmental Quality Standard.

  • AA-QS: Annual Average Concentration.

  • MAC-QS: Maximum Acceptable Concentration.

• Assessment Factors (AF): Used to derive Quality Standards from laboratory data:

  • AF = 1000: When at least one short-term $L(E)C_{50}$ is available from three trophic levels (fish, daphnia, algae).

  • AF = 100: One long-term $EC_{10}$ or NOEC.

  • AF = 50: Two long-term results from two trophic levels.

  • AF = 10: Long-term results from at least three species representing three trophic levels.

• Monitoring Challenges:

  • Heterogeneity in Space: Contaminant levels vary significantly across sediment surfaces, burrows, and feces.

  • Heterogeneity in Time: Concentration levels (e.g., $Cd$, $Hg$) change yearly, requiring long-term data for trend analysis.

  • Analytical Quality: Must account for accuracy (closeness to true value), precision (reproducibility), and Limit of Detection (LOD).