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.
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.
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.
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.
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.
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