Oxidation-Reduction Reactions

ENVIRONMENTAL GEOCHEMISTRY AT TEXAS A&M UNIVERSITY

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Oxidation-Reduction Reactions

Bruce HerbertGeology & Geophysics



Redox Reactions in the Subsurface

■ Reduction-oxidation(redox) reactions are chemical reactions where electrons are transferred from one species to another

■ Every oxidation is accompanied by a reduction reaction

■ Redox reactions are important for the following elements:

■ Macroelements: C, N, O, S, Mn, and Fe

■ Contaminants: As, Se, Cr, Hg, and Pb




Equilibrium calculations of redox reactions give only a semi-quantitative picture of redox patterns that are actually observed in the subsurface




■ Redox reactions are combinations of reduction and oxidation half reactions

■ O2 + 4H+ + 4e- = 4H2O reduction: gain e-

■ 4Fe2+ = 4Fe3+ + 4e- oxidation: lose e-

■ O2 + 4H+ + 4Fe2+ = 4H2O + 4Fe3+

■ Where

■ While these equilibrium constants are not written for full redox reactions, they have thermodynamic significance and can be calculated from Standard State chemical potentials



Redox Reactions in the Subsurface■ In the problem above, the oxidation states of Fe and O were changed.

Oxidation state is the hypothetical charge of an atom or the number of electrons per atom. The oxidation state of an atom is determined using the following rules:

■ The oxidation state of an atom in its elemental form is zero

■ The oxidation state of monatomic species is equal to its valence

■ The oxidation state of atoms in molecules is determined by assigning electrons in each bond to the more electronegative element. The sum of the oxidation states equals the net charge of the molecule.



■ In addition to these rules, there are some specific cases:

■ Hydrogen has an oxidation state of +1, except in metal binary compounds

■ Fluorine always has an oxidation state of -1

■ Elements of Group IA (except H) always have an oxidation state of +1

■ Elements of Group IIA always have an oxidation state of +2

■ In binary compounds, elements of Group VIIA have an oxidation state of -1

■ Oxygen has an oxidation state of -2, except in O(I)F2, H2O(-I)2, and O(-1/2)2





■ The oxidation of organic carbon with the reduction of sulfate. This redox reaction may be important in anaerobic systems such as deep ground waters or flooded soils.

■ Combine several reactions of organic carbon to give the oxidation of organic carbon (acetate) to CO2(g).

■ Since the log K for this reaction is positive, the ∆G˚<0. This indicates that the products are more stable than the reactants when all species are in their Standard States.




■ In general we can write any redox reaction in the following form:



Nonequilibrium for Redox Reactions

■ The thermodynamics of the two examples above indicate that these redox reactions can occur in natural systems, not that they will occur. These reactions may not be favored kinetically.

■ Redox reactions may not occur in a reasonable amount of time in natural systems because:

■ Most redox reactions are kinetically slow compared to other geochemical reactions.

■ Redox reactions are often poorly coupled. This means that reduction and oxidation reactions are not well linked. This could be because of slow diffusion of species of one microenvironment to another. This can create localized redox environments.



Nonequilibrium for Redox Reactions■ As an example, the following redox reaction describes the oxidation of dissolved

glucose (an analog of dissolved organic matter).

Log K=21.0 (7.18)

■ Thermodynamics predicts complete oxidation of the carbon at any pH value. But of the course, the long term stability of Coca Cola, as well as organics in the subsurface, shows that this reaction is kinetically slow.

■ The existence of redox equilibrium in the subsurface is dependent on catalysis. Most often catalysis is achieved through:

■ Microbial processes

■ The presence of reactive mineral surfaces such as manganese and titanium oxides.

■ Microbial organisms and reactive surfaces affect the rate of a redox reaction, not its equilibrium constant.



Electron Activity and pE■ The free electron activity is described in much the same way as the H+

activity.

pH = -log (H+) pE = -log (e-)

pE favor the existence of oxidized species while small values of pE favor reduced species. Redox environments are classified based on pE.

Oxic: >7 Suboxic: 7 > pE > 2 Anoxic: pE < 2

■ The range of possible pE valuse in the subsurface is defined by two redox reactions involving water.■ The upper range of pE is defined through the decomposition of water.

■ The lower range of pE is defined by the H+: H2 couple.



Measuring pE and pH

■ For a given reaction, the activity ratio of the redox couple can be used to calculate pE:

(7.27)

(7.28)

■ Uncertainty with the calculated value of pE derives from the following:

■ Uncertainty in the thermodynamic parameters such as Kr or activity

■ The assumption of equilibrium may not be correct

■ Different redox couples may be present in the system. Each redox couple can produce a different value of pE



Measuring pE and pH

■ One example of an electrochemical cell has the following electrodes

■ clean platinum (Pt) electrode: this responds to oxidation

■ saturated calomel electrode: this responds to reduction

log Kr = 4.53 (7.30)

■ The electrochemical cell needs to be calibrated like a pH electrode.



Measuring pE and pH

■ The measurement of pE through EH has great uncertainties due to:

■ the Pt electrode can respond to more than one redox half-reaction

■ concentrations of redox species can be too low to provide e- transfers detectable by the Pt electrode

■ the redox species are not electroactive (N, S, and C species)

■ the Pt electrode can become contaminated by oxide coatings

■ the value of the liquid junction potential in the calibration solution can be very different than the potential in a natural system. The liquid junction potential is the potential between the solution and the salt bridge, which is a part of the electrochemical cell. Differences in the liquid junction potential influence overall EH measurement.



Sequence of Reduction Reactions

■ Given a system effectively a closed system with abundant carbon to support microbial activity

■ There is a specific range of pE for the initiation of the reduction of the element.

These half reactions are coupled with the oxidation of carbon. In this way, the redox elements act as electron acceptors in C oxidation.



Sequence of Reduction Reactions

■ Changes in pE induce both chemical reduction sequences as well as sequences in microbial ecology.

■ Aerobic organisms that utilize O2 do not function below pE of 5

■ Denitrifying bacteria function in the pE range of +10 to 0

■ Sulfate-reducing bacteria live at pE's below 2



Case Study: As in Sediments

Masscheleyn et al., 1991. Environ. Sci. Technol. 25:1414-19



Case History: Brookhaven Landfill Plume

■ A landfill near Brookhaven, New York has contaminated a glacial outwash aquifer. The plume is suboxic. We will examine the spatial relationship of contaminant speciation in the aquifer plume as a function of redox.

Wexler, EJ, 1988, Ground-Water Flow and Solute Transport at a Municipal Landfill Site on Long Island, New York: Part I; Hydrogeology and Water Quality, US Geological Survey Water Resources Investigations Report 86-4070, Syosset, NY, 52 pp.

http://pbisotopes.ess.sunysb.edu/lig/Conferences/abstracts-03/tonjes.htm



Toxic Plume From Brookhaven Landfill, NEWS12 Exclusive (April 17, 2009)

http://www.youtube.com/watch?v=Aopcz-HTc5s








Case History: Brookhaven Landfill Plume■ Given the water analyses, we can tell that the groundwater is experiencing

reduced conditions through the following factors:

■ NH4+ / NO3

- ratio.

■ At a pE < 8, electrons are available to reduce NO3-. At this pE, bacteria

produce NO2-, N2, N2O, and NH4

+.

■ Presence of Fe and Mn in the dissolved phase.

■ As the pE drops between 7 and 5, Fe and Mn are reduced. Fe reduction does not occur until all O2 and NO3

- are depleted. Fe(II) is much more soluble than Fe(III), therefore a large increase in dissolved Fe indicates Fe reduction.

■ Presence of reduced S species in the dissolved phase.

■ As the pE drops below 0, electrons are available for sulfate reduction.

■ Products of SO42- reduction include H2S (hydrogen sulfide), HS- (bisulfide), and S2O3

- (thiosulfate).




■ We can also use the water analyses, in particular the NH4+ / NO3

- ratio, to estimate redox potential at particular points along the groundwater plume.

■ Given the following reaction linking nitrate and ammonium:




■ We can also use the water analyses, in particular the NH4+ / NO3

- ratio, to estimate redox potential at particular points along the groundwater plume.



Subsurface Biodegradation as a Redox Process

■ The most important microorganisms are bacteria, though fungi are important in the biodegradation of certain compounds

■ Microorganisms possess enzymes which degrade organics. Enzymes change the kinetics of organic compound oxidation, not the thermodynamics.

■ Bacteria are small organisms, roughly 1 µm in size. They are in intimate contact with contaminants. Bacteria can exist in both oxic and suboxic conditions.

■ Fungi are restricted to the soil surface. Fungi exist in oxic environments.




■ Microorganisms can degrade almost all organic compounds and many inorganic contaminants

■ The transformation of organics compounds occur because organisms often can use the metabolism of the organic for growth and reproduction. Metabolism of organic compounds provide a source of C and electrons which are used in energy cycles.

■ Organics represent reduced C, N, and S compounds that are ultimately created by photosynthesis and/or anthropogenic energy sources.

■ Oxidation of these reduced species liberate energy.




■ Microbes gain energy by catalyzing energy-releasing chemical reactions that involve bond breakage and electron transfers from the contaminant (contaminant is oxidized).

■ The contaminant is the electron donor; oxygen, nitrate, Fe, or S is typically the electron acceptor. The electron donor/acceptors are called primary substrates.

■ Those organisms which dominate in a given microenvironment are those that are capable of utilizing the best electron acceptor available. The best electron acceptor is the one that exhibits the most positive reduction potential. (see table)




■ If O2 is used, then the microbial reaction is called aerobic respiration. Organic C is transformed to CO2, O2 is transformed to H2O.

■ Anaerobic respiration involves the use of nitrate, sulfate, Fe3+, Mn4+ or CO2 are electron acceptors. Byproducts of anaerobic respiration include N2, H2S, reduced metals, and CH4 depending on the electron acceptor used. (ex. N2 is a product of the reduction of nitrate).

■ Inorganic compounds can also be electron donors and oxidized during microbial enzymatic reactions, including NH4+, NO2

-, Fe2+, Mn2+, and H2S. In these reactions, O2 is usually the electron acceptor and C originates from CO2.




■ Fermentation is a microbial process where the organic contaminant is both the electron donor and acceptor. Products of fermentation include acetate, propionate, ethanol, H2, and CO2. These products can then be degraded by other microbes to CO2, CH4 and H2O.

■ Secondary Utilization and Cometabolism is the process where microbes transform contaminants without benefit to the microbe.

■ The general case is secondary utilization.

■ Cometabolism is the transformation of a contaminant in an incidental reaction catalyzed by enzymes involved in normal cell metabolism or detoxification.

■ The contaminants are secondary substrates. (ex. cometabolism of chlorinated substrates occurs when CH4 (also toluene and phenol) are primary electron donors.




■ Reductive dehalogenation is the detoxification of chlorinated solvents based on the replacement of a halogen atom by H on the contaminant through a reduction process.

■ Electron donors involved in reductive dehalogenation include H2, lactate, acetate, methanol, and glucose. There is usually no energy gain but may detoxify environment.

■ Regardless of mechanism, microbes require essential nutrients (50% C, 14% N, 3% P, 2% K, 1% S, 0.2% Fe, and 0.5% of Ca, Mg, and Cl)




■ There are several types of bioremediation

■ Intrinsic bioremediation is a set of techniques used to manage the innate capabilities of naturally occurring microbial communities to degrade pollutants without trying to enhance the biodegradation rate.

■ Engineered bioremediation is a set of techniques used to acceleration the microbial degradation of a contaminant using engineering techniques

■ Bioaugmentation seeks to introduce organisms to a site.



Geologic Control of Bioremediation

■ The geologic environment has a strong influence on the effectiveness of in situ bioremediation.

■ It is important to recognize the important geologic variables in order to predict in situ bioremediation effectiveness.

■ geochemical site characteristics

■ contaminant bioavailability

■ toxicity to organisms

■ hydrologic site characteristics

Oxidation-Reduction Reactions

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