Sub-module 4A, page 1
Some chemistry terms and environmental loss
mechanisms.
Equilibrium Partitioning parameters
You are already aware of the concept
of vapor pressure and volatility. "Volatile" substances evaporate quickly.
A puddle of gasoline on my garage floor is soon gone. On the other
hand, a puddle of diesel fuel may stay on the floor for a long
time, and the old motor oil in a pan, from when I changed the
oil in September, is still there. Gasoline is more volatile than
diesel; motor oil might be described as non-volatile. Volatility
is a qualitative term. The vapor pressure of a liquid is closely
related to the volatility. Go to this site and read about volatility
and vapor pressure.
http://www.ilpi.com/msds/ref/vaporpressure.html
Several units are commonly used for pressure (and consequently
vapor pressure): the mm of mercury (mm Hg), which is the same
as a torr. The atmosphere (atm), which is 760 mm Hg. The preferred
unit is the pascal (Pa) which is a very small unit, so kilopascal
kPa is commonly used. In the British system, which many of us
still think in, the units of pressure are pound per square inch
(psi). Go back to site linked above and find the "pressure
converter calculator" and use it to find the vapor pressure
of n-octane, which is 1880 Pa, expressed as: atm, kPa, torr, mm
Hg, and psi. (You will need the answer for torr in HW problem
1. )
While the concept of volatility and vapor pressure are important,
more often in environmental contamination problems, we are interested
in a contaminant that is dissolved in water. The rate at which
a volatile contaminant leaves the water phase and enters the air
is a function of both the vapor pressure and the solubility of
the substance in water. The Henry's Law constant describes this
air-water partitioning. You can approximate the Henry's Law Constant
of hydrophobic chemicals with the vapor pressure of the pure chemical
divided by it's solubility in water. For chemicals that are more
than a few percent soluble, the approximation might not be very
accurate.
Solubility
Let's start by dividing all the liquid chemicals into two groups,
polar
and non-polar. If we had a molecular microscope, we could see
that some molecules, while electrically neutral as a whole, have
distribution of electric charge within the molecule. A molecule
might have a location that is positive and a location that is
negative, or at least positive relative to the negative part.
A pure substance composed of such molecules is called a polar
substance. Water is a polar
substance. A substance composed
of molecules that have no positive or negative portions is called
non-polar. Hydrocarbons
such as the chemicals in gasoline are non-polar.
In general, "like dissolves like." That is, polar substances
tend to dissolve in polar substances, like water, and non-polar
substances tend to dissolve in non-polar substances like gasoline.
We sometimes use the terms "hydrophilic" to describes substances that readily dissolve
in water, and hydrophobic for substances that do not dissolve in water. (Although
polar is not a synonym of hydrophilic, the words are often used
that way.) If you are not a hair-splitter, you can use the lipophilic
("fat loving") a synonym for non-polar or hydrophobic
substances.
I
Chemists like to describe polar versus non-polar with the dipole
moment. For environmental issues, the Octanol-Water Partition Coefficient is often more useful.
Octanol is a oily non-polar chemical that is hydrophobic. If we
took a sealed jar, half filled with water and half filled with
octanol, and shook it, then let it sit for a short while, we would
observe two phases, the Octanol on top and the water on the bottom.
Now if we put a chemical of interest in the jar, cyclohexane for
example, and again shook it, then let it sit, we would again observe
the two layers. Where would the cyclohexane be? Some of it would
be in the Octanol and some would be in the water. In round numbers,
there would be 1000 times more cyclohexane in the octanol than
in the water. Octanol is about as hydrophobic as animal tissue.
Therefore, if we spill the solvent cyclohexane to the surface
of a lake and let the system come to equilibrium, concentration
of cyclohexane that is dissolved in the water is about 55 mg /
L and the amount that is dissolved in the fat tissues of the fish
will be about 5,500 mg / L. (Note we are not saying how long
it takes to get to equilibrium.)
Because there is such a great range of octanol-water partition
coefficients, it is common to use the logarithm of the coefficient,
log Kow . (The OW is
properly written as a subscript, but I will have to use lower
case.) For example, the log Kow of DDT is 6.19. That means in
the jar experiment, if we had 1 mg of DDT in the water, we would
have 10^6.19 mg, or 1,550,000 mg in the octanol. (Must have used
a big jar.) The log Kow for many substances has been tabulated.
(For your homework, if the log Kow of Toluene is 2.69, and we
repeat the experiment and find 1 mg/L of toluene in the water,
how much is in the octanol)
Here a slick site: http://esc.syrres.com/interkow/kowdemo.htm If
you know the CAS number of a compound, you can get both a theoretical
Kow and an experimental value, if it is known. You will need this
site, probably, for your homework. To summarize, substances with
high Kow (above 2) tend
to enter animal tissue from the
environment.
Two related items. Charcoal is mostly carbon. Charcoal is very non-polar. Non-polar substances are absorbed in charcoal.
Similarly soils and sediments absorbed non-polar chemicals in
proportion to the amount of the organic
carbon in them. Ions, such as Na + or Cl - are
extremely hydrophilic.
Environmental Loss
Contaminants are lost from a local environment by advection and/or
diffusion. Diffusion refers to the process by which molecules of a fluid,
gas or liquid, move from regions where they are more concentrated
to regions where they are less concentrated. Advection refers to
bulk transport of the medium in which the contaminants are present.
If I drop a bucket of formaldehyde with holes in it into a river,
the bucket and the formaldehyde are advected down the river by
the current. The bulk of formaldehyde that leaks from the bucket
at first forms a region of high concentration in the river near
the bucket. The molecules of formaldehyde then diffuse away from
high concentration region. The formaldehyde is both diffused and
advected. Of course the same amount of contaminant is still in
the environment, someplace, just further from where we originally
spilled it and in lower concentrations.
Many contaminants become bound to environmental media or taken into plants or
animals. Again, the amount of the contaminant is the same, but
it may be sequestered and therefore not be available to other
organisms in the environment. The binding might hinder extraction
or chemical analysis of the contaminant and thus its concentration
may appear to be less.
Transformations will remove many contaminants. These transformations
are chemical reactions that change the contaminant into a different
chemical. Many chemical contaminants are organic chemicals that
are rich in carbon and hydrogen. Ideally these are completely
changed via oxidation into carbon dioxide and water. This process
is called mineralizing of the organic compound. Nature is not always benign.
The mercury poisoning at Minamata were
caused by methyl mercury, a compound formed by bacteria in the
bay from the elemental mercury that was first discharged. The
methyl mercury was more toxic than the elemental mercury. Radioactive
atoms remain so regardless of chemical treatment. Many of the
transforming chemical reactions follow incredibly complex pathways.
We will just mention the general categories of reactions: biodegradation,
hydrolysis, photolysis, and oxidation. Biodegradation requires
enzymes, typically produced by microorganisms or in cells
of higher animals. This process is exactly analogous to the metabolism
of chemicals discussed in Tox Tutor II, which you are referred
to. Biodegradation is sensitive to factors that affect the microorganisms,
such as the presence of nutrients, temperature, pH and so on.
Hydrolysis, photolysis, and oxidation are chemical reactions that
can occur independently of enzymes. Hydrolysis is the splitting
of a larger molecule by addition of water, it requires hydrogen
ion and hydroxyl ion and sensitive to pH. Photolysis refers to
the effect of sunlight which causes certain chemicals to degrade.
Oxidation, the addition of oxygen to organic molecules, can happen
external to cells via a free radical mechanism. An apple core
turning brown is such a process.
Bioconcentration versus bioaccumulation.
The term bioconcentration is commonly used to describe
chemicals that have higher concentrations in plants or animals
than in the surrounding medium, generally water. In the example
above the cyclohexane would be bioconcentrated in the fish, relative
to the water. Bioaccumulation is related to the persistence of
a contaminant in the environment. Bioaccumulation has two
meanings, as I use the word. First, within an organism, it means
the chemical is not excreted or metabolized. (You will learn about
this in Sub-module 4B.) DDT, for example, is not quickly altered
in the body. So if an individual is exposed to DDT each day, the
amount in the body increases. Within an ecosystem, the term bioaccumulation
refers to this same persistence, but now this bioaccumulation
continues up the food chain. That is, contaminated organisms lower
in the food chain are ingested by animals further up the chain,
who cannot metabolize or excrete it either, and so it bioaccumulates
in them. Eventually the concentration in tissue may be much higher
towards the top of the food chain. The use of term bioaccumulation
is sometimes referred to a biomagnification.