Where Did That Chemical Go?

Jan 3, 2019

Environmental Enlightenment #189

Acknowledgement is given to a book of this name by Ronald E. Ney, Jr., Ph.D., Van Nostrand Reinhold, New York, 1990, on which this material is based.

To dissipate in the environment means to reduce to the point of disappearing.

Dissipate derives from Latin dissipare “to disperse,” from dis- “apart” + supare “to throw.”

For example, in the images at the right, the dust devil in this picture will eventually collapse and dissipate.

The plume of smoke from the brushfire is dissipating into the surrounding air.

A chemical can dissipate in a variety of ways:

It can break down to other chemicals, more toxic or less.

Take tetrachloroethylene (also known as PCE, perc and other names), a carcinogenic solvent used in dry cleaning and for degreasing metals.

Left for consumption by naturally occurring bacteria, it will become trichloroethylene (TCE), then dichloroethane, then vinyl chloride (highly toxic), and down to ethene (or ethane); all the way releasing CO2 at every turn.

A chemical can scatter.

See how the ink diffuses (scatters) in water.

It might adsorb (attach) to soil particles

It might evaporate.

It could be sucked out by plants or animals.

We measure the rate of dissipation by the half-life of the chemical in the environment.

Half-life is the time it takes for the chemical to be reduced by one-half of its original amount. The lost 50% could be in the form of breakdown products.

For example, the time required for the body to eliminate one-half of the total amount of caffeine consumed varies widely. In healthy adults, caffeine’s half-life is approximately 4.9 hours (Ref. Wikipedia).

This is why breakdown products must be identified and studied in the same way the parent chemical is studied.

The dissipation of a chemical may be real, as in the breakdown of the parent chemical to its daughters.

The dissipation may not be real, as in the case when the chemical migrated from the source area elsewhere.

One must always remember this: the fact that a chemical has not been detected does not necessarily mean it is not there.


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