Sub-module 4D, Closure

*** In our discussion of literature, I forgot to mention some compendiums or standard reference books. These are not textbooks, in the sense they are used in a college course, but rather reference books in common use by similarly situated professionals. For example, (I'm not recommending you buy these, but looking around my book and paper strewn office): The Merck Index, has 3 or 4 paragraph descriptions of about 8000 chemicals in common use. Casarett and Doull's Toxicology has much basic and advanced information. NIOSH has its Pocket Guide to Chemical Hazards. The ACGIH produces the TLV booklet. There are others that I know folks use: Sax's Dangerous Properties of Industrial Materials is another commonly used book.

***Q. About the term paper; am I correct in reading that you want at least two references that are not from the internet? It seems that you are saying that you expect most of our references from the internet. I have never heavily relied on the internet for reference because it is usually considered an unreliable source. At most, I have used two, maybe three internet sources which I thought to be highly reputable and would not disappear in a few months as most web sites do.

A. You do not need to use any references from the Internet. I do expect you to do some web searching. The computerized library searching is accessed via the web, but the articles you are looking for are typically in paper.


*** Often different sources in the secondary literature will have different values for the same constant. Log Kow for example. The herbicide 2,4-D (see for more on this chemical) has a log Kow listed from 1.57 to 4.88; that is over a thousand-fold difference. Those of you lucky enough to get to the Kow site, often found two numbers listed, an "experimental" number (or numbers) and a "Log Kow estimated number," based on an estimating technique. For example for cyclohexanol the experimental number is 1.23 and the estimated is 1.64. Often you find only one number in the secondary literature, but that is NOT a sure sign that the matter is definite, only that all the secondary literature writers used the same primary source. I have been horrified to follow up a secondary source back to the primary and find out that everyone has been using the same number from original research that was done in the 1920's and 1930's. That doesn't mean the old number was wrong, but it makes you wonder. On the other hand it is completely impractical to check these numbers yourself. One advantage with computer modeling is that you can easily do a sensitivity analysis by sticking in the highest and lowest values from the literature for the various constants into the model, and see if they are critical or not. We will do a little of this in a few weeks.

*** Q. Since Octanol is as hydrophobic as animal tissue, I understand how it can be used as an indicator of which substances tend to enter animal tissue. The text states that substances with high Kow (above 2) tend to enter animal tissue. Was the number 2 derived from observation, or experimentation, or what? In my model, tetrachloroethylene has a Kow above 2, but a relatively small amount entered the fish. Can we assume that a higher Kow means that a comparatively higher amount of substance would enter the fish?

A. Yes, all other things being equal, the higher the Kow, the faster chemical will enter tissue from the environment. Some of the other things that might not be equal are volatility and solubility. Some chemicals will quickly enter the air, and then be swept away by wind and/or diffusion. Some chemicals are solid and remain in a lump, unless they are leached by rain. If they are insoluble, they do not leach, or do not go very far. At the cellular level, very large molecules do not enter the cells as fast as small molecules. Having said that, Kow is still a useful indicator and in common use.


*** Q. Speaking of water solubility…The model requires input of both water solubility (g/m3) and log Kow….I am having a difficult time with the difference between water solubility and log Kow. Don't they both basically measure the same thing? Will you ever have a chemical with both a high water solubility and a high Log Kow? If they are directly related, why does the model require both as input?
(and related question) This is more of a statement… I have never heard lipophilic, non-polar, and hydrophobic used as synonyms. They aren't really are they?

A. Put a liter of liquid chemical X and a liter of water in a two-liter jar, shake it, and let it stand for a while. When you come back, if there is only one layer, you say chemical X is "miscible." If you have two layers of about one liter each, you say chemical X is "hydrophobic." If you have two layers, but the layer with X is less than one liter, you say X is "soluble in water." You can describe this exactly, as 5.0 grams of X per 100 mL of water, implying that if you try to mix more than 5.0 grams with 100 mL of water the second layer will form, if you mix less than 5.0 there will not be a second layer. Often qualitative terms such as "poorly soluble" are used. Most hydrophobic substances are non-polar and most non-polar substances are hydrophobic, but the word "hydrophobic" is qualitative and hydrophobicity is relative. Both methanol (CH3OH) and octanol (C8H17OH) have the same polar character (how about "amount of polarity," hmmm.) in the OH or alcohol group. Octanol has a lot more non-polar character in the rest of the molecule than methanol, by an amount of 1:8 (roughly). So octanol is hydrophobic and non-polar (as a whole), methanol is both miscible with water and non-polar substances, like hydrocarbons. Likewise the word "lipophilic" is qualitative. The analysis of Kow is a method of measuring lipophilicity. But in general most non-polar substances tend to be hydrophobic, and have a high Kow and thus are lipophilic. No they are not synonyms.

*** Q. Although there I had completed my chemical simulation prior without too many problems with the log Kow website, I still had some issues. For one, in an attempt to find another website, I checked the MSDS sheets for the physical properties where the log Pow was listed. How is this related to logKow?

A. Some authors use "Pow" or just "P" where we use Kow. I am more familiar with the "K" usage.


Q. Doing Module4 I found a smart web source for chemicals. It even shows 3d structure of molecules! (Although it takes to download a small plug-in for either MS Explorer are Netscape Navigator) I got really excited about it.

A. Yes, a neat site indeed.


Question: What is the maximum amount of toxin (percentage) that can be exerted from the body via sweat.

A. Sweat and tears are very minor excretion pathways and neglected in most analyses. Mother's milk is a pathway for excretion of lipophilic and persistent compounds from the mother, but unfortunately an important exposure route for nursing infants.


Q. After having the Kow link not work, I actually found an alternative Kow in other literature that was different than the one from the second reference you gave us. How does one evaluate the various values found in the literature? This is all secondary literature as far as I can tell and it all seems to be appropriately referenced. There is a point at which running two or three simulations is probably faster and cheaper than looking through piles of reference data, although I am not sure what that point is. The differences in the log Kow were less than an order of magnitude, but not a lot less.
(See top paragraph)

Q. This afternoon, you directed us to another website for "chemical specific factors" to find log Kow. This website stated the Log Kow for 1, 1 dichloroethylene is zero. I had several other sources that gave me a value of 2.13 (which is what I used in my model).
(See top paragraph.)


Q. In Facilitated Diffusion of Xenobiotics, the protein acts as a CARRIER MEDIUM which facilitates Active Transport process, which is forceful, faster and can be in direction opposite to that of the gradient. Can this situation be used for stopping Xenobiotics from entering cells. So does this mean having such carrier proteins will eliminate Xenobiotics from the cells.

A. The boundary between the interior of a human cell and the environment around it is called the cell membrane; it is a "lipid bilayer" and essentially lipid. This lipid bilayer is connected to a vast network of lipid bilayer tubes in the cell called the endoplasmic reticulum. (Endoplasm is the juice inside the cell and reticulum means net.) Lipophilic substances freely diffuse through the membrane into the cell via the endoplasmic reticulum. This is true for both lipophilic nutrients and lipophilic xenobiotics; the transport is controlled by diffusion (in most cases). Polar substances on the other hand cannot get through the membrane on their own. Sodium or calcium for example need special transporters to enter and leave the cell. These transporters are designed for particular ions, but they can transport other substances, just not as efficiently. For example, lead is transported by some of the transporters that move calcium. The mechanisms of many xenobiotics, both therapeutic drugs and toxic chemicals require uptake by these natural transporters. Because they serve some other useful purpose in cell physiology it is not practical to block them as a means of protecting people from toxic chemicals in the environment. It is often done in therapy, for example the emergency treatment of poisonings.


Q. What would be a practical application of the Carbon/charcoal?

A. I put something about that in module 5, basically organic chemicals tent to "stick" to soil in proportion to the amount of carbon in the soil; same with sediment.

Q. With all this talk of diffusion, advection, and concentrations, would it be appropriate to ask if the solution is dilution????? [grin]

A. Shame on you, dilution is a no - no. However, "natural attenuation" which might be mostly dilution, is OK, and is often the only practical solution. But that is risk management, not risk assessment.


Q. Could you expand on the Henry's Law Constant? Why would the approximation not be very accurate if the solubility is really low (or do you mean higher than that)?

A. Henry's Law is always true for dilute solutions because it is just the definition of the vapor pressure of the solute (assume non-miscible liquid with a vapor pressure, like most of our contaminants of interest) over the solution. You could always measure it. (It might not be a straight line or linear relationship, especially at higher concentrations.) The common estimate of Henry's Law is the vapor pressure of the pure substance divided by the solubility of the pure substance in water. That is convenient because both solubility and vapor pressure are easily measure and are often tabulated. That estimate is not always a good, because it assumes the presence of water does not affect the vapor pressure.


Q. About biomagnification; I have been told that radioisotopes biomagnify from one organism to another, ex. Vegetations is polluted with radioisotopes and a caribou eats some of the radioactive vegitation and the isotopes biomagnify from the vegitation to the caribou and eventually from the caribou to a grizzly or human, but I have not heard of any other chemicals that do, what are some general chemicals that biomagnify within the scope of the example given?

A. Radioisotopes and metals do not biodegrade, so they are very persistent in the environment. Some organic chemicals are likewise persistent, such as many chlorinated solvents and pesticides. In order to bio-magnify, the chemicals must be taken up (ingested, typically) by an organism, then absorbed, then NOT be excreted or bio-transformed by the organism. Finally the organism is in the food chain and an organism at a higher trophic level ingests the first organism and so on. We tend to look at this on the big animal scale, but bio-degradation starts for many substances with microorganisms. If they absorb the contamination and do not biotransform it, it will likely work its way up the food chain. This is very chemical specific. PCBs, pesticides such as DDT, TCDD, and many others tend to biomagnify. Higher organisms have more complex and varied methods of biotransforming xenobiotics that microorganisms.


Q. Regarding one of the questions on the quiz. I am paraphrasing here, because I don't remember the exact wording. Why are constituents in soil more difficult to (analyze? Study? Determine the properties of?) than in water?

A. The question was "Many contaminants become bound to organic material in the soil and sediments. This makes chemical analysis of these contaminants much easier. " And it is false. The chemicals must first be separated ("extracted") from the soil. Organic contaminants are typically those that are bound to organic material in the soil. When you go to extract the organic contaminant you will also extract the organic material. Its all very doable, but is several steps more complicated then the equivalent extraction from a contaminant in water, and much more prone to error than an extraction from a "clean" soil such as sand or silt.

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