Redox Reactions

The membrane biofilm reactor and the microbial fuel cell are based on simple scientific principles. Both devices work using a class of chemical reactions that involve the transfer of electrons from one molecule to another.

Everything is made up of atoms. Atoms, in turn, are composed of protons, neutrons, and electrons. The protons and neutrons are bunched together in the center of the atom to form a nucleus. Electrons orbit around the nucleus in a cloud. Some of the electrons in the cloud can be used to make chemical bonds with other atoms. Molecules are the result.

One class of chemical reactions is known as oxidation-reduction. Scientists and chemistry students refer to them as redox reactions. Redox reactions involve a transfer of electrons from one atom to another.

Atoms or molecules that lose electrons during a redox reaction are said to be oxidized. Those that that gain electrons are reduced.

“The world is full of oxidized materials that are big problems. Reduce those oxidized materials and they are big problems no longer,” says Bruce Rittmann, an ASU professor of civil and environmental engineering.

Reducing problem materials is the simple idea behind the membrane biofilm reactor developed by researchers in Rittmann’s laboratory.

Hydrogen gas is an electron donor; it loses electrons. These electrons react with oxidized contaminants in the water to form reduced compounds that are not dangerous to human health. The microorganisms in the biofilm make this reaction happen.

The situation is a bit different inside a microbial fuel cell. The electron donors are organic materials in waste that is pumped into the fuel cell. The microorganisms perform half of a redox reaction. They free electrons from the waste compounds. But those electrons are sent through a circuit instead of being immediately transferred to other molecules. This circuit provides power for the device containing the fuel cell.

At the other end of the circuit, the electrons react with oxygen in the second half of the redox reaction. Clean water is the final product.—Linley Erin Hall