Lecture 2: Environmental chemicals

What is a chemical substance?

A form of matter having constant chemical composition and characteristic properties.

What is matter?

A physical substance in general, which occupies space and possesses rest mass, especially as distinct from energy.

What is mass?

The quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field.

What is chemical composition?

The identities and relative numbers of the elements that make up any particular compound.

What is an element?

A substance whose atoms all have the same number of protons — all of a particular element’s atoms have the same atomic number. Elements are chemically the simplest substances and hence cannot be broken down using chemical reactions.

What is atomic number?

The number of protons in the nucleus of an atom, which is characteristic of a chemical element and determines its place in the periodic table.

What is a nucleus?

The centre of an atom. It is made up of nucleons called (protons and neutrons) and is surrounded by the electron cloud.

What is a proton?

A stable subatomic particle occurring in all atomic nuclei, which a positive electric charge equal in magnitude to that of an electron.

What is an electron?

A stable subatomic particle with a negative electric charge, found in all atoms and acting as the primary carrier of electricity in solids.

What is a neutron?

A subatomic particle of about the same mass as a proton but without and electric charge, present in all atomic nuclei except those of ordinary hydrogen.

What is the electron shell or atomic/molecular orbitals?

The outside part of an atom around the atomic nucleus where the electrons are, and is a group of atomic orbitals with the same value of the principal quantum number n.

• Electron shells have one or more electron subshells, or sublevels

• These sublevels have two or more orbitals with the same angular momentum quantum number

• Electron shells make up the electron configuration of an atom

What is the valence shell?

The valence shell is the outermost shell of an atom in its uncombined state, which contains the electrons most likely to account for the nature of any reactions involving the atom and of the bonding interactions it has with other atoms

Why is the atomic mass not an integer?

The mass of an atom of a chemical element is approximately
equivalent to the number of protons and neutrons in the atom (the
mass number) or to the average number allowing for the relative
abundances of different isotopes.

What is an isotope?

Variants of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.

Why would an environmental scientist care about atomic mass?

Atomic mass is important because we are using scales to measure chemicals in the environment.

Xenobiotics

Foreign chemicals in an organisms (not normally produced or expected to be present)

Pollutants

Substances that are introduced to the environment and have undesired effects or adversely affect environmental resources.

Contaminants

Unwanted constituents in the environment that render air, water, soil, biota impure and — in sufficient concentration — are harmful.

Poisons

Substances that cause disturbance to organisms when absorbed at a sufficient quantity.

Toxicants

Poisons made or introduced by humans.

Toxins

Poisons made naturally by organisms.

Venoms

Toxins injected via e.g.a bite or sting

Chemical regulation in Australia

• Therapeutic Goods Administration

• Food Standards Australia New Zealand

• Australian Pesticides and Veterinary Medicines Authority

• Australian Industrial Chemicals Introduction Scheme

Why does environmental toxicology even exist?

• It exists because there is a myriad of synthetic chemicals on the market (and formed as byproducts)

• Only approx. 10% of chemicals are thoroughly tested

• Only approx. 2% screened for carcinogenic and mutagenic properties

Terms for chemicals

• Industrial chemicals

  • solvents

  • consumer products

  • fertilisers

  • surfactants

  • plastics

  • additives

  • polymers

  • pigments

  • intermediates

• Natural toxicants

• Pharmaceuticals

• Pesticides

• Biocides

• Unintentional by-products

• Metals and organometallics

Classification groups

• By origin or use:

  • source

    • anthropogenic (e.g. industrial, agricultural), natural (e.g. dinoflagellates, bacteria)

  • occurrence

    • marine, atmospheric, soil, indoor

  • purpose

    • surfactant, pesticide, plasticiser, solvent

• By chemistry:

  • chemical groups

    • organic, inorganic, heavy metals

  • base structure

    • organochlorine, carbamate

  • physicochemical properties

    • polar, water soluble, lipophilic, volatile

  • fate

    • bio-accumulative, persistent, long-range transport

• By toxicology:

  • toxic end-point:

    • carcinogen, neurotoxin, EDC

  • mode of action

    • acetylcholinesterase inhibitors, cytotoxic, photosystem II inhibitors

What is a functional group?

• A group of atoms that have a predictable chemical behaviour

• Many different functional groups in organic chemistry

  • e.g. alkene has a carbon-carbon double bond

  • e.g. alyne has a carbon-carbon triple bond

  • e.g. arene has an aromatic ring

  • e.g. alkyl halide has a halogen

  • e.g. alcohol has an OH group (hydroxyl group)

  • e.g. thiol has an SH group (sulfhydryl)

  • e.g. ether has an oxygen with an R group on either side

  • e.g. sulphide has a sulphur atom with an R group on either side

  • e.g. amine has a nitrogen with three R groups or hydrogens

What does risk assessment (monitoring) and environmental forensics seek to address?

Risk assessment seeks to address the impact/risk of particular chemicals whereas environment forensics concerns which chemical/source has caused the observed effects.

Why classify chemicals?

• To inform sampling/monitoring and enable target analysis

• Similar chemicals have similar requirements for sampling and can be analysed using similar analytical approaches – related to their physicochemical properties

  • e.g. no point analysing a liquid if a chemical is volatile (gaseous), it is better to conduct a headspace analysis

‘Organic chemical’

A chemical with a carbon hydrogen backbone.

Physicochemical properties

• Govern the behaviour of chemicals in the environment (and can be used to predict their toxicity)

  • How will a chemical distribute and in what environmental compartment may it become a problem?

  • How could it enter organisms and does it have
    the potential to bioaccumulate?

    • e.g. polarity, solubility, octanol-water partition coefficient (K_OW)

  • How long will a chemical persist, i.e. can regular but low releases cause problems?

    • Resistance to chemical and biochemical transformation (half-life in soil, air, water, biota)

  • How readily will a chemical evaporate, i.e. can it disperse by air?

    • Vapor pressure, Henry’s Law Constant (K_OH)

Chemical polarity

• Separation of electric charge in a molecule (→ dipole moment, μ)

• ‘Like dissolves like” as polar chemicals tend to dissolve well in water and nonpolar chemicals tend to dissolve well in oils (lipids)

• It is the difference in electronegativity (how much an element attracts e):

  • Polar = ΔEN 0.5-1.6

  • Nonpolar = ΔEN <0.5

• If ΔEN is very high, electrons can be completely lost to the other atom:

  • Ionic = ΔEN >2.0 (e.g. Na+Cl- = 2.1)

Solubility (𝑪^𝒔𝒂𝒕)

• Maximum concentration of a chemical that can be dissolved in a solvent (e.g. water); solubility of chemicals in water or lipids governs their fate in the environment

• Can range from “insoluble” (<μg/L) to fully miscible (soluble without limits)

• Strongly influenced by temperature

• Water soluble chemicals can transport through soil, disperse in rivers, oceans, are often less stable, and less likely to volatilise from water

• Lipid soluble chemicals tend to partition to soil/sediments, may accumulate in organisms, are often persistent and have relatively low solubility in water (hydrophobic)

• Rule of thumb S_W (mg/L; 25 C):

  • <= 50 (low)

  • 50 - 500 (moderate)

  • > 500 (high)

Vapor pressure (P⁰)

• Tendency of liquid (or solid) to volatilise; pressure at which a liquid (or solid) is in equilibrium with its vapor (closed system, given temperature)

• The higher vapor pressure, the more volatile

• Characteristic for each substance and increases with temperature

• When vapour pressure = atmospheric pressure → substance boils

• Rule of thumb P₀ (mPa; 25 ⁰C):

  • < 1 x 10-6 (0.000001) (non-volatile)

  • 1 x 10-4 to 1 x 10-6 (intermediate state (semi-volatile))

  • > 1 x 10-4 (0.0001) (volatile)

Volatile chemicals (VOCs)

Volatilise readily at room temperature; generally have low to moderate water solubility (e.g. paints, solvents (BTEX), fuels) – important indoor air pollutants.

Semivolatile chemicals (SVOCs)

Are often relatively stable and have lower water solubility: have the potential for long-range atmospheric transport (e.g. POPs, pesticides).

Stability

• Resistance to chemical and biochemical degradation: the more stable, the longer its residence time, the greater the distance it can travel, and the more it can accumulate over time.

• Common transformation processes:

  • Oxidation (loss of electrons)

  • Hydrolysis (break of chemical bonds in presence of
    water)

  • Microbial degradation and other biotransformation

  • Photodegradation (photochemical transformation into smaller fragments)

• Stability of chemicals also depends on environmental factors (temperature, pH, radiation, oxygen, etc)

• Some transformations lead to increased toxicity!

• Rule of thumb T_1/2 (days; soil):

  • < 30 = (non-persistent)

  • 30 - 100 (moderately persistent)

  • 100 - 365 (persistent)

  • > 365 = (very persistent)