Expert: George Maxwell - 11/15/2007

Question
QUESTION: Hello,

I am a fellow "expert". I answer questions in the physics category. I need help with a personal issue. There are several aspects to what I don't know. Sorry for the multiple parts.

1. I believed that the behavior of compounds would not have any relation to the behavior of the component elements. For example hydrogen burns, oxygen supports combustion, water puts out fires. But it seems that this doesn't always apply. For example it seems that compounds involving arsenic are poisonous. Please confirm/deny and explain briefly.

2. Are all compounds involving mercury toxic like the elemental form?

3. I would like to know how mobile mercury is as environmental contamination. I hear and read that, among other sources, mercury comes through the air across the ocean from coal fires in China. I guess that it must come as a compound that is much less dense than the elemental form. True?

4. My particular interest is once it gets into a lake. Would the mercury migrate to downstream lakes (via shallow streams between deep lakes) easily? At all? One possibility (for a biologist to speculate on I suppose): could larva of insects be carriers as they migrate downstream?

Any insight greatly appreciated. I'm now retired and live on a Minnesota lakeshore.
Steve

ANSWER: Hi, and thanks for your question. I hope the following is useful, even though it's not complete.

1) In most cases, you're right that the behavior of a compound does not mirror the behavior of the constituent elements, but there is some link. For example, the reason that water doesn't burn is that it's made up of molecules that are basically oxidised hydrogen - you can't oxidise it any further. Compare this to Hydrogen Sulfide (H2S) which is a flammable toxic gas at room temperature.
So although the behavior is different, the properties of a compound are determined by the behavior of the constituent elements when they react.

2) All common mercury compounds are recognised as toxic (to a greater or lesser degree) by the EPA, though elemental mercuy is one of the most toxic forms.

3) The release and travel of mercuric compounds in the atmosphere doesn't depend on bulk density, because it occurs via a fine dispersion of particulate matter (just like smoke from a fire - the 'visible' smoke isn't gaseous at all, it's a collection of sooty particles.)

Normally, this happens because mercury in common products (e.g. barometers, some types of battery) has been incinerated when the products are disposed of. The fine ash of mercuric oxides and similar compounds is released in to the air and can travel great distances.

The mobility of mercury compounds in an aqueous environment largely depends on their solubility. Indeed, so does the toxicity, because more toxic compounds tend to be more soluble and so more easily absorbed in to the human body. The "mad hatter" in "Alice in Wonderland" is a reference to the use of mercuric chloride by hat-makers (to smooth down felt). Chlorides are generally soluble, and so the hatters absorbed the mercury and suffered from the neurologic symptoms of mercury poisioning which were misdiagnosed as "madness".

4) Depends on the lake and on the form of mercury. Elemental mercury in any lake would be very dangerous: small particles of the mercury would begin to disperse from the main contamination, and would flow around the body of the water. In slack water, the dspersion would be slow, but the contaminated area would retain very high amounts of mercury for a long time. In fast-flowing water, the mercury would be spread quickly, but would therfore be diluted quicker as well.

The mercury particles would enter the bodies of fish via their gills, albeit in small quantiies. However, the contamination would multiply up the food chain as follows:

Suppose that small fish each acumulate about 1mg of mercury each. A large fish comes along and eats 1 small fish, and so now has a dose of 10mg mercury, and so on. Eventually, by the time the contamination has reached the end of the food chain (Humans) the amounts of mercury are in the lethal dose area.
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Mercury compounds would be simiarly dispersed, but here, the solbility would play a major part in determining how far the contamination spreads.

The physical spread of contamnaton is unlikely to occur though insect transportation  because there is no way that insects (even collectively) could carry enough mercury to
be a hazard. However, transfer between animals, as described above, is an important factor after physical dispersion has taken place.

Hope this helps!




---------- FOLLOW-UP ----------

QUESTION: Thank you for your reply. I'd like to make a comment (with a question tacked on) and ask a followup.

From what I have read, coal-fired power plants is the main source of our mercury contamination. Yes there is mercury in products (switches, barometers, thermometers) that are disposed of too, but my understanding is that these sources are overwhelmed by what comes from coal. Does this conflict with what you know?

I have heard the "end of the food chain increase in toxicity" discussion before. But it seems to assume that regardless of how big the fish is, I'll eat one and only one. Actually I might eat 4 small fish or 1/4 of a big one. To me that begs the question: Does the concentration (ppm or equivalent) increase as you go up the food chain?

Answer
Thanks for the follow-up

Re: the build-up of concentration: the build - up occurs through the whole food chain. A single fish will eat 50 -100 small fish or plankton every few days, and if each one of those has 1mg of mercury, you're very quickly up in to the lethal levels, so however many fish you eat, you're in danger. You're right that if the contamination was only entering the food chain near the top, you probably wouldn't have a significant build-up.

Researchers have tracked the flow of mercury through aqueous food chains, following the Minemata disster (in which low concntrations of mercury in a secluded bay were concenrated through the food chain and caused human fatalities).

Re: the coal-spread contamination - natural coal doesn't contain an appreciable levels of mercury. However, if the coal is contaminated then burning it will relase it to the atmosphere. The contamination may come from minerals in the mine environment, but can also be absorbed from the surroundings. Coal (finely-divided carbon)is great at absorbing materials from the atmosphere, hence the use of activated charcoal in filter systems.