Session II-2: Oxidation - Reduction Principles

MODULE II: CONTAMINANT BIOREMEDIATION

Session II-2

Oxidation - Reduction Principles

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Continue reading chapter 4 Wiedemeyer, Section 4.2, pages through 178. Cross-reference this material with the following text and figures comprising session II-2.

Ecological Niches.

A niche is a microenvironment occupied by a particular population that is specially adapted to that microenvironment. The concept of ecological niches in the subsurface simply means that different microbial species and/or populations occupy different environments that are controlled by factors such as temperature, pH, groundwater and/or soil chemistry, etc.

The take-home message here is:

Different environments will possess different metabolic cababilities!

Example: oxic vs. anoxic environs:

OXIC (O₂ present): Low molecular weight hydrocarbons readily degraded in oxygen-rich environments, particularly when those hydrocarbons are unsubstituted (i.e., contain only carbon and hydrogen, like alkanes, alkenes, and polycyclic aromatic hydrocarbons). The primary reason for this is that these hydrocarbons tend to be in a less oxidized form, and oxygen provides the necessary energy to carry out an oxidation reaction that transforms these compounds into their more oxidized byproducts.

ANOXIC (little or no O₂): Degradation rates of unsubstituted hydrocarbons tend to be very slow in anoxic environs. However, low-oxygen regions are a good environment for the degradation of halogenated (and other substituted) hydrocarbons such as trichloroethylene, pentachlorophenol, etc.

Metabolic variability arises from organism dependent capabilites. Some organisms are strict aerobes--i.e., they require oxygen in order for survival. Other organisms are strict anaerobies--if oxygen is present, their metabolic machinery will shut down (oxygen is even toxic to some organisms). Still other organisms are facultative, meaning that they can adapt their metabolic machinery to suit the level of oxygen present.

KEY POINT: Different environments select for different types of organisms!

Oxidation - Reduction (cross reference with section 4.1 page 165 Wiedemeyer)

Oxidation-reduction reactions are commonly referred to as "redox" reactions. They are named as such because in a redox reaction, one compound is oxidized (loses electrons) while another compound is reduced (gains electrons).

Oxidation of an organic chemical releases electrons (e^-)

Another substance must accept the electrons (i.e., be "reduced") to allow the oxidation reaction to be completed

For example:

e^- + O₂ --> CO₂ (Oxygen Reduction)

e^- + Fe³⁺ --> Fe²⁺ (Iron Reduction)

e^- + NO₃^- --> N₂ (Nitrate Reduction)

e^- + SO₄^2- --> H₂S (Sulfate Reduction)

e^- + CO₂ --> CH₄ (Methanogenesis)

Redox Reaction Energetics (cross reference sec. 4.2 pg 170 Wiedemeyer)

Aerobic respiration utilizes oxygen as the terminal electron acceptor and provides the most energy from the degradation of a carbon source.

Anaerobic respiration uses (in order of decreasing available energy) nitrate, iron, manganese, sulfate, or finally CO₂ as the terminal electron acceptor. As shown in the following table, methanogenesis provides only 1% of the energy of aerobic respiration or anaerobic respiration utilizing nitrate.

**The "Tower of Power"**
Electron Acceptor	DG^o (kj/mol toluene mineralized)
O₂	-3913
NO₃^-	-3778
Fe³⁺	-2175
SO₄^2-	-358
CO₂	-37

Redox Potential (cross reference sec 4.3 pg 175 Wiedemeyer)

Figure II-9 shows the relative energy involved for a variety of compounds to act as electron acceptors.

NOTE: In the presence of a suite of electron acceptors, the one that will be used first will be the one that has the highest energy associated with it. This energy is termed the "redox potential" of the electron acceptor, and is denoted by E_h (mv).

Figure II-9. Redox Potentials for Various Classes of Biotransformation Reactions

Thus, it is safe to assume that if all of these electron acceptors were present in a given microenvironment, oxygen would be used first. When the oxygen is depleted, nitrate would be used next, followed by Fe(III), SO₄, etc.

In general, a redox reaction employing a high-energy electron acceptor will proceed at a faster rate than a redox reaction employing a low-energy electron acceptor. This is why the injection of oxygenated air ("air-sparging") is a common and very effective technology for improving bioremediation at sites contaminated by petroleum hydrocarbons (which tend to be composed primarily of unsubstituted compounds like alkanes, alkenes, and polycyclic aromatic hydrocarbons).

Measurement of the oxidation/reduction potential (redox potential) provides a method for evaluating what respiration reaction may be predominant in the zone surrounding a monitoring well. Measuring the redox potential can be thought of as measuring the electron "density" in the reaction zone.

Electrons are a transient intermediary in each of the respiration reactions. The higher the electron density measured, the slower the respiration reaction occurring; thus, the fewer electrons measured, the faster the reaction rate.

A HIGH redox potential indicates LOW electron density concentrations, and LOW redox potential indicates HIGH electron density concentrations.

The redox potential provides a measurable parameter to determine which electron acceptor reaction is predominant. Unfortunately, this parameter is not diagnostic. Considerable overlap exists in the redox potential for the various electron acceptor reactions (see Figure II-9). However, general trends can be observed.

For example, if the redox potential was observed to be less than 0 within the plume, and up-gradient measurements are greater than 0, then it is safe to assume that anaerobic processes are dominant in the contaminated zone.

Water Chemistry in Biotransformation Reactions: Aerobic Processes

Aerobic Processes Aerobic reactions provide the most energy during microbial respiration. The following chemical reaction:

Toluene + 9O₂ --> 7CO₂ + 4H₂O

is balanced for a 7-carbon contaminant (e.g., toluene).

Complete oxidation of the contaminant will produce CO₂ and H₂O. The ratio of oxygen to contaminant required for complete oxidation varies for each contaminant based on the carbon concentration.

On average, approximately 9 moles of oxygen are required to degrade a single mole of BTEX (the common name for benzene-toluene-ethylbenzene-xylene, a major gasoline signature).

During the aerobic degradation of BTEX, the loss of dissolved oxygen in the groundwater system is the only measurable indictor of this reaction.

Water Chemistry in Biotransformation Reactions: Anaerobic Processes

Anaerobic respiration can occur using a number of different electron acceptors. Reaction preference is based on the amount of energy generated during biodegradation. Complete oxidation of the carbon source for each electron acceptor results in the production of carbon dioxide, a reduced form of the electron acceptor, and in most cases, water.

Table II-1 indicates the key reactions involved in various metabolic redox strategies:

Denitrification:

5 (BTEX) + 36 NO₃^- --> 18 N₂ + 35 CO₂ + 38 H₂O

Manganese Reduction:

(BTEX) + 18 Mn⁴⁺ + 29 H⁺ --> 18 Mn²⁺ +7 CO₂ + 22 H₂O

Iron Reduction:

(BTEX) + 36 Fe³⁺ + 21 H₂O --> 36 Fe²⁺ + 7 CO₂ + 7 H₂O

Sulfate Reduction:

8 (BTEX) + 35 SO₄^2- --> 35 S^2- + 56 CO₂ + 28 H₂O

Methanogenesis:

8 (BTEX) + 42 H₂O --> 35 CH₄ + 21 CO₂

Acetogenesis:

(BTEX) + H₂O --> CH₃COOH + CO₂

Table II-1. Reaction in various anaerobic redox reactions.

QUESTION: What compounds would you monitor in a well as an indicator of each reaction type?

Water Chemistry Monitoring Strategies for Anaerobic Processes

NITRATE REDUCTION. Monitoring of the reaction products may not be possible. Reduction of nitrate produces nitrogen gas, for example, which will tend to "off-gas", and is difficult to measure. However the reactant, nitrate, is water soluble and can be measured in a water sample.

IRON/MANGANESE REDUCTION. During reduction of iron or manganese, the reactants are insoluble and therefore not measurable. However, the reaction products (Fe²⁺, Mn²⁺) are soluble, and can be measured in a water sample.

SULFATE REDUCTION. Dissolved sulfate concentrations can be measured in groundwater; however produced sulfide ions typically precipitate with iron to form pyrite. This solid phase then becomes unavailable for measurement via down-well techniques.

METHANOGENESIS. Methane formation can be evaluated from a ground water sample; however, its high vapor pressure and limited solubility may inhibit accurate measurement. The Henry's Law Constant must be evaluated under the physical conditions present in the subsurface to determine the solubility of methane in the water sample. The solubility of methane in water is generally very low, on the order of ten times less than benzene*, increasing the difficulty of obtaining a valid methane concentration and production rate from in situ biodegradation processes.

*NOTE: The solubility of benzene is actually quite high on the order of 1700 mg/L, making methane solubility on the order of 170 mg/L. These values are still quite high when compared to organic contaminants, where solubilities of ug/L are common.

Measurement Techniques

Measurement techniques for determination of the various electron acceptor or reaction product concentrations have been described in the literature. A short list of analysis options is presented in Table II-2 along with a reference citation.

Parameter	Measurement Location	Technique	Reference
O₂	field	dissolved oxygen probe	common
O₂	lab	Winkler titration	common
NO₃^-	lab	anion exchange with chromatographic conductivity detection	Chapelle et al., 1995
SO₄^2-	lab	anion exchange with chromatographic conductivity detection	Chapelle et al., 1995
Fe²⁺	lab	filtration (0.1 mm) colorometric or FerroZine™ method	Stookey, 1970
CH₄	lab	gas chromatography (FID detector)	Vroblesky, 1994
H₂	field	bubble strip method	Chapelle & McMahon, 1991

Table II-2. Measurement techniques for electron acceptors or reaction products.

Redox Zones in the Field

Provided that the various electron acceptors were present in adequate concentrations, a cross-sectional area map of a contaminant plume undergoing biodegradation processes would typically show various reaction zones (Figure II-10). Monitoring either electron acceptor or reaction product concentrations can provide the necessary information to indicate which terminal electron acceptor processes are occurring in the region surrounding the monitoring well, and allow one to construct a redox "map" like the one shown in Figure II-10:

Figure II-10. Typical redox zones in a field plume indicated by redox couple.

Electron Acceptor Observations

Water chemistry monitoring can provide information to evaluate both the dominant terminal electron acceptor process and the rate of biodegradation. Comparison of electron acceptor concentrations and/or biodegradation reaction product concentrations over time or with uncontaminated monitoring well data can indicate the prevalent biodegradation reactions.

Several terminal electron acceptor zones can occur within the contaminant plume. At pseudo-steady state, step-wise consumption of the most energetically favorable electron acceptors will produce an "onion-skin" effect of electron acceptor reduction around the contaminant plume (see Figure II-11).

Consumption of dissolved oxygen from the ground-water will occur first around the contaminant plume and continue in regions of oxygen ingress (near surface, and at outer reaches of contaminant zone.) Contained within this outer skin of oxygen respiration, will be an anaerobic zone containing regions of anaerobic respiration in which electron acceptors of decreasing potential energy will be consumed as the distance from the contaminant source decreases.

Figure II-11. Plan view of typical redox zones in a field plume indicated by electron acceptor.

Use of Spatial Profiles to Identify Redox Zones

A cross-sectional diagram of the concentrations of contaminant, electron acceptors, and products drawn along the length of the plume typically indicates decreasing contaminant concentrations down-gradient, with the disappearance of the electron acceptors in the order of energy preference (see Figure II-12).

Appearance of reaction products down-gradient is indicative of biodegradation processes. Electron acceptors will again appear at some point down-gradient from the contaminant release, where the consumption reaction rate does not exceed the replenishment rate due to transport from groundwater recharging or infiltration.

Measurable reaction products may accumulate down-gradient (such as Fe²⁺, sulfate/iron products, or methane). Methane accumulation in the vadose zone may reach significant levels even as shallow as tens of feet below grade.

Figure II-12. 2-D spatial concentration profiles of key constituents that can be used to monitor the efficacy of in-situ bioremediation: the substrate (BTEX), electron acceptors (SO₄, NO₃, O₂), and redox reaction products (e.g., Fe²⁺, S^2-, Mn²⁺).

The Electron Acceptor:Carbon Source Ratio

The molar and mass ratios of electron acceptor to carbon source for complete degradation of BTEX contaminants allows for the assessment of the potential biodegradation capacity of a particular groundwater. The mass ratio provides information on the concentration of electron acceptor necessary for complete degradation of a contaminant (in this case BTEX). For example, if a concentration of 1 ml/Liter of BTEX is found in the ground water, a corresponding electron acceptor concentration of 3.1 mg/Liter of oxygen, or 4.8mg/Liter nitrate, etc. is required for complete oxidation (see Table II-3):

Electron Acceptor	Molar Ratio	Mass Ratio
O₂	9	3.1
NO₃^-	7.2	4.8
Mn⁴⁺	18	11
Fe³⁺	36	22
SO₄^2-	4.3	4.5

Table II-3. Mass of electron acceptor required to biodegrade a unit mass if BTEX

This information provides a method for evaluating the quantity of contaminant degraded by monitoring the disappearance of various electron acceptors.

It is important to remember, however, that changes in electron acceptor and contaminant concentrations may not correlate due to sorption and desorption of contaminants to soil solids.

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