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 [Vapor pressures of Volatile Chemicals, an online calculator at the USDA.] 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 a 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 term lipophilic ("fat loving")
as 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 an 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 and there were
no losses, the 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 55,000 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'll be lazy and just 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 106.19 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/fatepointer/search.asp by entering the CAS a screen pops with some choices, try PHYSPROP, which comes up with physical-chemcial values including log Kow, although it is expressed as “log P (octanol-water)." Note that values of Kow vary quite a bit in the literature. 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 absorb 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
is 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. That use of the term bioaccumulation
is sometimes referred to a biomagnification.