Reduction-Oxidation (Redox) Potential (Eh)

5.2.3 Reduction-Oxidation (Redox) Potential (Eh)

Reduction and oxidation can be broadly defined as a gain of electrons and loss of elec- trons, respectively. For a particular chemical reaction, an oxidizing agent is any material that gains electrons, and a reducing agent is any material that loses electrons. The reduction process is illustrated with the following expression (Hem, 1989):

(5.5) where ferric iron (Fe 3+ − ) is reduced to the ferrous state by gaining one electron. The symbol

Fe 3+ + − e = Fe 2+

“e ” represents the electron, or unit negative charge. This expression is a “half-reaction” for the iron reduction-oxidation couple; for the reduction to take place there has to be

a source of electrons, i.e., another element has to be simultaneously oxidized (lose elec- trons). Together with the hydrogen ion activity (pH), reduction and oxidation reactions play key role in solubility of various ionic substances. Microorganisms are involved in many of the reduction-oxidation reactions, and this relationship is especially important when studying the fate and transport of contaminants subject to biodegradation.

The electric potential of a natural electrolytic solution with respect to the standard hydrogen half-cell measuring instrument is expressed (usually) in millivolts or mV. This

GroundwaterQuality

measured potential is known as reduction-oxidation potential or redox and is denoted with Eh (h stands for hydrogen). Observed Eh range for groundwater is between +700 and –400 mV. This positive sign indicates that the system is oxidizing, and the negative that the system is reducing. The magnitude of the value is a measure of the oxidizing or reducing tendency of the system. Eh, just like pH, should be measured directly in the field. Concentration of certain elements is a good indicator of the range of possible Eh

values. For example, a notable presence of H 2 S (>0.1 mg/L) always causes negative Eh. If oxygen is present in concentrations greater than 1 mg/L, Eh is commonly between 300 and 450 mV. Generally, an increase in content of salts decreases the Eh of the solution.

The redox state is determined by the presence or absence of free oxygen in ground- water. Newly percolated (recharge) water often supplies oxygen to groundwater in the range from 6 to 12 mg/L. As groundwater moves away from the recharge zone, oxygen can be consumed in a number of different geochemical reactions, the most direct being oxidation of iron and manganese compounds. Microbial activity also consumes oxygen and may rapidly create a reducing environment in a saturated zone with an excess of dissolved organic carbon (DOC) (which is a nutrient for microbes) such as in cases of groundwater contamination with organic liquids.

Determining redox potential in an aquifer is particularly important in contaminant fate and transport and remediation studies. For example, oxidizing (aerobic) conditions fa- vor biodegradation of petroleum hydrocarbons such as gasoline, while reducing (anaero- bic) conditions favor biodegradation of chlorinated compounds such as tetrachloroethene (PCE). Based on oxygen demand of the various bacterial species, an oxygen content be- ◦

C water temperature has been commonly defined as threshold oxygen concentration for the boundary between oxidizing and reducing con- ditions. However, field observations suggest that reducing conditions may appear at considerably higher oxygen contents (Matthess, 1982).

tween 0.7 and 0.01 mg O 2 /L at 8

The redox potential generally decreases with rising temperature and pH, and this decrease results in an increasing reducing power of the aqueous system. Reducing sys- tems, in addition to the absence or very much reduced oxygen content, have a noticeable content of iron and manganese; occurrence of hydrogen sulfide, nitrite, and methane; an absence of nitrate; and often a reduction or absence of sulfate (Matthess, 1982).