Lecture 3: Fate and exposure

Partition coefficient

The ratio of a chemical that is dissolved in two different phases at equilibrium, used to predict the environmental distribution of chemicals at this equilibrium and thus evaluate potential risks

What is partitioning?

How the distribution of chemicals in the environment (air, water, soil biota) is governed by physio-chemical properties, and can be described by their partition coefficients (K)

List different partition coefficients

• K_ow is distribution between octanol (fatty alcohol lipid) and water

• K’_H is Henry’s Law Constant, the distribution between air and water

• K_oc is distribution between soil organic carbon and water

Why do we use octanol instead of lipid?

• Structure of octanol is very similar to the lipid octanoic acid and they share many physico-chemical properties

• The octanol – water partition coefficient is used as a
  surrogate for lipid – water

Logging partition coefficients

Partition coefficient values are converted to log 10 values

• e.g.

  • Acetone is polar due to CO

  • molec wgt = 59.08 amu

  • aq. sol. 100, 000 mg/L

Octanol-water partition coefficient (K_ow)

• The ratio between the concentration of a substance in octanol (nonpolar) and water (polar) at equilibrium

• K_ow is a measure of lipophilicity (solubility of chemical in lipid)

• Higher K_ow suggests greater tendency to bioaccumulate in organisms and sorb to solids (e.g. soils)

  • range from <0.001 to >10,000,000 (log K_ow: -3 to 7)

• The mass of a chemical in octanol (M_octanol) and water (M_water) depends on the volume of each phase (V_octanol and V_water)

Rule of thumb (log K_ow)

• < 2.7 = low bioaccumulation

• 2.7 - 3 = moderate

• > 3.0 = high

Henry’s Law Constant (K’_H)

• Ratio between the concentration of s substance in air and its concentration in water at equilibrium (neutral dilute solutes)

• Higher K’_H suggests the chemical readily volatises from water

• Increased temperature generally increases K’_H (C_water decreases)

• Increased pressure decreases K’_H (C_water increases)

Phys-chem properties affecting the fate of chemical in the environment

• Air (photolysis, biodegradation, oxidation)

  • volatility

  • adsorption

  • solubility

  • half-life

• Water (biodegradation, hydrolysis)

  • solubility

  • adsorption

  • volatility

  • half-life

• Soil (hydrolysis, oxidation)

  • adsorption

  • solubility

  • volatility

  • half-life

Environmental factors affecting the fate of chemicals in environment

• Air

  • temperature

  • pressure

  • humidity

  • light (UV)

  • wind

  • rainfall,

  • particles (dust)

• Water

  • temperature

  • particles (turbidity)

  • pH

  • light (UV)

• Soil

  • temperature

  • light (UV)

  • O₂ (aerobic/anaerobic)

  • pH

  • H₂O

  • Eh

  • composition

Spatial and temporal impacts

• Temporal (secs to millenia)

  • Source volume and continuity of use/release

  • Persistency

• Spatial (local to global)

  • Type of source and propensity to disperse in air, water, soil

  • Persistency

Sorption

• The sorption partition coefficient (K_d) is the distribution between solid and water (affinity of chemicals to be sorbed)

• Sorption in soil/sediment predominantly due to organic carbon (K_oc)

• Sorption governs the potential for chemical to leach through soil into ground water

• Sorption influences the potential for partitioning of chemical between water and suspended solids and sediment

  • Transport in water and volatilisation from water (C_w and C_A)

• Sorption affects bioavailability

What factors affect chemical sorption?

• Physico-chemical properties

  • hydrophobicity (nonpolar & uncharged polar
    species)

  • pK_a (ion species)

• Environmental conditions

  • soil organic carbon content

  • particle size

  • surface area,

  • soil composition – functional groups, cation
    exchange capacity, temperature, pH

What is bioavailability?

Measure by which substances in the environment enter into living organisms; it is commonly the limiting factor in the production of crops (due to solubility limitation or absorption of plant nutrients to soil colloids) and in the removal of toxic substances from the food chain by microorganisms (due to sorption to or partitioning of otherwise degradable substances into inaccessible phases in the environment).

Arsenic bioavailability as a case study

• Pathogen rich surface water, partially as a result of high population density, results in high infant mortality due to water borne diseases

• Solution is sourcing water from groundwater (via 40 mio wells), which reduced infant mortality

• However, ~50 mio rely on groundwater > 50 ug/L arsenic (WHO guideline = 10 ug/L); ~20 mio people poisoned (many mild-moderate)

  • ~3000 deaths/year and the long-term effects and extent of exposure are poorly understood

Eh

Redox potential; tendency of a chemical species to acquire electrons (to be reduced). This depends on the species’ affinity for electrons (increased Eh means increased affinity for e-).

Oxidation of chemicals

Oxidation is the loss of electrons. It can involve the incorporation of oxygen or increase the oxidation state of the chemical (e.g. 4Fe + 3O₂ → 2Fe₂O₃ rusting).

Oxidants

• Chemicals with elements of:

  • High oxidation state (e.g. H₂O₂, MnO₄−, CrO₃,)

  • Highly electronegative elements (O₂, F₂, Cl₂, Br₂) that gain extra electrons by oxidizing other substances

  • Mineral surfaces also catalyze many oxidative reactions (clays and oxides of silicon, aluminum, iron, and manganese provide surface active sites that increase oxidation)

The Grasshopper Effect

Several volatilisation and deposition cycles across the globe causing global distillation (travel distance according to phys-chem properties and temp gradient).

Photolysis of chemicals

Initiated by the absorption of a photon; energy required for
excitation and reaction are characteristic for each molecule. Dependent on light intensity (at specific λ)° and time (day / year), location, weather.

Direct and indirect photolysis

• Direct photolysis

  • The reactive molecule directly absorbs light

• Indirect photolysis

  • A light absorbing molecule transfers its excess energy to an acceptor molecule

  • Acceptor molecule reacts

Hydrolysis of chemicals

The chemical reaction of a compound with water (e.g. CO₂ + H₂O → H₂CO₃ (carbonic acid) → H⁺ + HCO₃^- (bicarbonate ions) → H⁺ + CO₃^2- (carbonate ions)

What mechanisms allow hydrolysis to occur?

• H₂O (neutral hydrolysis)

• Depend on pH, require less energy, can accelerate reaction and give different products

  • H⁺ acid catalysis

  • OH^- base catalysis

• Hydrolysis can occur in surface waters, groundwater, atmosphere, soil and biota

Exposure

The amount of a chemical at the outer boundary of the body available for exchange. Exposure routes can be oral, inhaled or dermal.

Factors influencing exposure

Concentrations in air, water, or soil, combined with estimates of the frequency and duration of human contact with the contaminated media:

• Infants

  • consume breastmilk or formula (lactational and gestational exposure)

• Toddlers

  • eat dirt, are near ground, hand-to-mouth behaviour, high food consumption per body weight

• Fishermen

  • high consumption of local seafood compared to general population

• Industry workers and their families

  • can be exposed to contaminated fumes, dust, clothing

• Vegans

  • less exposed to chemicals that accumulate in meats, dairy, seafood

• Home farmers

  • can be exposed to localised contamination (e.g. free-range eggs)

Exposure assessment

• Biomonitoring measures contaminants within the human body (e.g. blood, urine)

  • concentrations dependant on bioavailability and t_1/2

• Biomarkers of exposure measures the product of an interaction between a chemical and target molecules or cells in the human body (e.g. metabolites in urine, receptor activity in liver)

  • needs good information on mechanisms