The EPA’s Citizen’s Guide to Cleanup Technologies Series 9: In Situ Chemical Reduction

Jun 28, 2017

Environmental Enlightenment #210

See our Newsletter, Environmental Enlightenment #200, for introduction to one highly valuable, plainly written series of Citizen’s Guides published by the EPA at

With this issue we introduce the EPA’s Citizen’s Guide to Phytoremediation. You can reach it through the hyperlink under the blue header below.

The building blocks of matter are atoms; and atoms have nuclei (cores) surrounded by shells of rotating electrons.

Licensed under GNU Free Documentation License, Version 1.2

The word oxidation originally implied reaction with oxygen. Ultimately, the meaning was generalized to include all processes involving loss of electrons.

When oxygen combines with iron to make rust, the iron loses electrons to the oxygen; thus, the iron is oxidized.

Photograph by Tony Fischer, licensed under Creative Commons Attribution-Share Alike 2.0 Generic License

When sodium combines with chlorine to make table salt, the sodium loses electrons to the chlorine; thus, we say that the sodium has been “oxidized” even though no oxygen is present.

The word reduction originally referred to the loss in weight upon heating a metallic ore to extract the metal. In other words, ore was “reduced” to metal.

Photograph by Hans Splinter and licensed under Creative Commons Attribution-Share Alike 2.0 Generic License

Antoine Lavoisier (1743-1794) showed that this loss of weight was due to the loss of oxygen as a gas.

Later, scientists realized that the metal atom gains electrons in this process. The meaning of reduction then became generalized to include all processes involving gain of electrons.

Oxidation and reduction go together. He who donates is oxidized. He who gains is reduced.

Even though “reduction” seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, which was its historical meaning.

reducing agent is a chemical that causes reduction. In the case of rust, the iron acts as a reducing agent on the oxygen.

Density is the amount of mass that an object contains per unit volume.

Examples for units of volume are ounce, quart, teaspoon, cubic centimeter, liter, bushel, barrel, cord, golf bag, Sydney Harbor, and more.

Photograph by La Citta Vita ( and licensed under Creative Commons Attribution-Share Alike 2.0 Generic License

The universal force of gravity is pull between masses.

The more material we have per unit of volume, the stronger is the gravity pull; thus, denser is heavier.

And Archimedes tells us that objects denser than water sink in water.

“Titanic Sinking” by Willy Stöwer (1912)

DNAPLs stands for “dense non-aqueous phase liquids.” These are other-than- water (non-aqueous) liquids denser (therefore heavier) than water.

When DNAPLs get in groundwater they sink to the bottom of the aquifer.

Perchloroethylene (PCE) (a dry cleaning solvent) is a DNAPL.

(Source: Washoe County NV Water Resources Department)

Examples of other DNAPLs include coal tar, PCBs, mercury and extra-heavy crude oil.

In chemistry, valence indicates the number of electrons that a chemical reaction has added or taken away from an element. The higher the valence, the stronger is the combining power of that element.


Illustration from Hints to Golfers by Niblick, published 1902

The origin of the word valence is from a Latin valere meaning strength. Words like valiant, value and valor derive from valere.

The image below represents a molecule of water: two hydrogens in connection with one oxygen.

The oxygen has pulled one electron from each hydrogen into the outer electronic shell of the oxygen.

We say that the oxygen is in a valence of -2 and each of the hydrogens is in a valence of +1.

Chromium can appear in a valence of 3 in one chemical and in a valence of 6 in another chemical. In its 3-valence chromium is benign, while in the 6-valence it is a harsh carcinogen.

Source: NASA

Metals in pure form are not stable in the environment (gold is an exception). We have to extract them from naturally occurring ores, and we invest great energy in the process.

Paraburdoo iron ore mine in the Pilbara Region of Western Australia; Photo by Calistemon and licensed under the Creative Commons Attribtion-Share Alike 3.0 Unported License

Zero Valent Metals are metals in their pure form. They can’t wait to impart electrons to others. Thus we use them as reducing agents.

A common reducing agent is Zero Valent Iron (ZVI).

Go to the EPA’s Citizen’s Guide to In Situ Chemical Reduction at for the rest. It is simple and fun to read.

A 700g individual Chinga iron meteorite (Ataxite, class IVB).  Source: Wikimedia Commons (H. Raab) and licensed under GNU Free Documentation License, Version 1.2


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