Aliphatic and Aromatic Hydrocarbons Flashcards
What is the difference between petrogenic and pyrogenic sources?
Petrogenic = associated with fossil fuels. E.g. fuels, oil spills. Pyrogenic = stems from incomplete combustion. Can be forest fires, or car engines.
What are the chemical strutures of apliphatic hydrocarbons and PAHs?
Aliphatic hydrocarbons: linear, branched or cyclic.
PAHs: fuses benzene-rings. whit/whitout substitution and heteroatoms.
Talk a bit about aliphatic hydrocarbons and PAHs.
Aliphatic:
Chemistry: alkanes - linear, branched or cyclic.
Source: petrogenic i.e. that they are associated with fossil fuels. Example: if oil spill, then
they will be found here.
PAHs:
-Chemical structure: Composed of fused aromatic rings in various configurations.
-Pyrogenic origin (incomplete combustion)
∗ Formed from fused radicals, resulting from high temperature pyrolysis (decomposi-
tion) of organic materials.
∗ Associated with the soot of a wood-burning stove.
∗ Un-substituted ring-structure.
∗ Main source of PAHs in the environment.
– Petrogenic origin (oil)
∗ Formed during low to moderate temperature formation of fossil fuels from organic
materials (catagenesis).
∗ Substituted ring structure, low-molecular-weight
– (by)products of no/limited commercial value.
- Hydrophobic compounds: log Kow 3-7.
• Carcinogenic and mutagenic
Talk a bit about the transportation of aliphatic hydrocarbons and PAHs.
Aliphatics
The lower chain hydrocarbons will dominate in the air, whereas the longer chain more heavy
hydrocarbons will stay in the water.
• Considering an oil spill, here are the processes that will take place:
• Evaporation: Some will evaporate. Here, the individual molecules will evaporate based on
they physico-chemical properties.
• Dispersion: this will be done so as a mixture, as we don’t expect any particular sorting of the
chemical-mixture.
• Spreading: will depend on the viscosity on the material, and less the individual constituents.
• Some will sorb to the sediments.
Polycyclic aromatic hydrocarbons (PAHs)
• Most PAHs are released to air from pyrogenic sources.
• Transport depends on molecular size.
– Low MW PAHs (2-3 rings) are ”freely dissolved” in air.
– High MW Pahs (≥ 4 rings) are bound to soot particles.
• As they can travel trough atmospheric transport, they will deposit in both terrestrial and
aquatic environments by either wet or dry deposition. This will can go from being local to
global transport.
– Dry deposition: The particles just come down.
– Wet deposition: The particles go down with the rain.
• Microscale: Diffusion and partitioning of freely dissolved molecules into/onto organic matter
or black carbon. These processes occur in both (pore) air and (pore) water.
• Local to global scale: Low MW PAHs are transformed along the way. High MW PAHs are
co-transported in particulate form (sorbed in/on dissolved organic matter or black carbon) in
air and water.
Talk a bit about the distribution of aliphatic and PAHs.
Aliphatics
• Aliphatic and monoaromatic hydrocarbons will volatilize to the air: High KH is a main char-
acteristic for aliphatic hydrocarbons
PAHs:
- The PAHs have a tendency to stay in the water due to low KH
- Distributed in 3 fractions:
– 1) Freely dissolved fraction.
– 2) Readily desorbable fraction – can become freely dissolvable over time.
– 3) Desorption resistant fraction – irreversible bound under given conditions - This will
mainly be higher MW PAHs.
-When they are sorbed to the soil or sediment, then they can be very persistent – not this
when they are in the freely dissolved. They can only be degraded when they are in the freely
dissolved – not when sorbed. As they are regarded to be carcinogenic, this explains why people
will make strict policies regarding soil pollution.
A great span in physicochemical properties
– Decreasing KH with increasing MW (linear): relatively good fit.
– Increasing Kow with increasing MW (linear): relatively good fit.
– Decreasing solubility with increasing MW: fair amount of outliers
- Most important physicochemical properties controlling environmental distribution of organic
compounds: aqueous solubility, hydrophobicity, and Henry’s Law constant.
-Hydrophobic sorption is prevailing for high-molecular-weight PAHs. The sorption does not
involve breaking or formation of chemical bonds and includes:
-Adsorption to high-affinity sites on black carbon (dominating at low concentrations).
– Absorption (partitioning) into organic matter (dominating at high concentrations).
Oil spills are usually at the surface, making up an oil film. Compounds with increasing vapor
pressure and decreasing solubility will volatilize so the composition in the oil film will change,
resulting in increased concentrations.
Talk a bit about aliphatics and PAHs in regards to Transformation.
Aliphatics
- A big part of the hydrocarbons will be biodegraded around 50%.
- Some will also undergo photolysis.
PAHs
- Freely dissolved PAHs are degradable – often limited by sorption.
- Transformation usually detoxifies the compound – this can be the opposite for PAHs.
- There are mainly 2 types of transformation:
- Photodegradation/activation: =¿ photoactivated toxicity.
- Biological degradation/activation.
• PAHs are known for undergoing photolysis. Since the oil lies in the water surface, it will be
exposed to sunlight