Studies on Redox Potential of Marine Sediments

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INTRODUCTION

The oxidation-reduction or redox potential is believed to have a pronounced effect upon the composition, chemical reactivity, diagenesis, color, biological population, and other properties of recent sediments. Being a quantitative measure of the tendency of a given system to oxidize or reduce susceptible substances, the redox potential of sediments provides a criterion as to whether certain constituents occur in an oxidized or a reduced state. For example, iron could be expected to occur in the metallic or ferrous state in a highly reducing environment, whereas it ordinarily would occur in the ferric state in an oxidizing environment. This applies to a large number of other reversibly oxidizable or reducible constituents of marine sediments, both organic and inorganic.

Besides indicating the state of such constituents, the redox potential of sediments indicates whether new materials being deposited are more likely to be oxidized or reduced. The solubility of such substances as iron, manganese, copper, and certain other reversibly oxidizable minerals is influenced by the redox potential. The state of both manganese and iron in marine sediments was found by Brujevicz¹ to be a function of the redox potential. Pearsall and Mortimer² report that the state of iron, sulphur, and certain nitrogen compounds in water-logged soils is influenced by the redox potential. Allgeier et al.³ find that the principal effect of the redox potential of lake deposits is on the content of ferrous iron, hydrogen sulphide, and organic matter. The redox potential of sediments is a most important factor in determining the stability and biochemical transformation of organic matter.

A good many chemical reactions which influence the diagenesis and morphology of sediments are influenced by the redox potential. According to Keaton and Kardos,⁴ the redox potential of a soil may be used in the study and interpretation of the general nature of the chemical processes in the soil and the changes in these processes as affected by some external factor. Any chemical reaction which involves the exchange of electrons (and this is generally true of all oxidation and reduction reactions) will be influenced by the redox potential. . . .

The redox potential has much to do with determining the kinds, distribution and physiological activities of bacteria and allied microorganisms in sediments and there are many ways in which the activities of microorganisms influence the diagenesis of sediments.⁵ Bacteria themselves appear to be the principal dynamic agents which affect the redox potential of soil and sediments. . . .

In addition to its multiple effects on the diagenesis of sediments, the redox potential is believed to influence the formation and preservation of petroleum. Highly reducing conditions favor the biochemical hydrogenation or reduction of organic matter, a process which tends to convert certain kinds of organic matter into petroleum hydrocarbons or substances which are more hydrocarbon-like than the parent substance. In oxidizing environments, organic matter is more likely to be carbonized or oxidized by microorganisms to carbon dioxide and water.

Porfiriev⁶ expressed the view that the same organic matter could have been converted into either coal or petroleum, depending on the mode of fossilization and whether conditions were oxidizing or reducing. According to Stutzer and Noé⁷ the redox potential of peat, which gives rise to coal, is quite different from that of the sapropel from which petroleum is believed to be derived. Since preliminary observations indicate that petroleum occurs only in highly reducing environments, it is believed that the redox potential may prove to be a significant characteristic of source sediments. Detailed data on the redox potentials of sediments may make it possible to determine the more exact nature of the diagenetic processes in general.

The potential differences across the contacts of sandstone and shale, which Dickey⁸ calls "natural potentials" as differentiated from the potentials caused by electro-endosmosis and concentration differences, may be due in part to the redox potential of sedimentary rocks. While it may be far easier to measure the redox potentials of sediments (and such measurements are attended by many pitfalls) than to interpret the results, detailed studies of the redox potentials of source sediments seem to be indicated. This paper is devoted primarily to a discussion of the concepts of redox potentials and the methods of determining such potentials of sedimentary materials.

DEFINITIONS

Oxidation-reduction potential may be defined as a quantitative measure of the energy of oxidation or the electron-escaping tendency or fugacity of a reversible oxidation-reduction system. For short, it is often referred to as the redox potential and is commonly abbreviated O/R potential. It is sometimes called the reduction potential, the oxidation potential, or the electrode potential, although these terms are not necessarily synonymous. Throughout this paper, redox potential is used as synonymous with oxidation-reduction or O/R potential.

The redox potential is the degree of oxidation or reduction of a reversible O/R system, or it is a measure of how reducing or how oxidizing the system is with reference to some standard. When referred to hydrogen, the redox potential is commonly expressed as E_h in terms of volts, E_h being the potential difference between the standard hydrogen electrode and the system of which the redox potential is being measured. In some respects the E_h of a system is analogous to the pH, and the two are closely related. Whereas the pH is an expression of the hydrogen-ion

concentration, or the relative acidity or alkalinity of a system, the E_h is an expression of the tendency of a reversible redox system to be oxidized or reduced.

Unlike the pH scale, on which neutrality is defined as pH 7.0, there is no true neutrality on the E_h or redox potential scale. Likewise there are no readily definable upper or lower limits on the E_h scale as there are on the pH scale. While it is customary to regard the E_h values of the theoretical hydrogen and oxygen electrodes as the lower and upper limits respectively on the redox potential scale, there are numerous oxidizing agents such as acidic bichromate and persulfate, for example, which are more oxidizing than

and there are numerous systems which are more reducing than ⁹

The redox potential of a standard normal hydrogen electrode ( a solution of

at one atmosphere pressure and pH 0) is at 25 ^° C. The theoretical oxygen electrode at pH 0 has been shown to be volts. As discussed later, the E_h is partly a function of the pH. At pH 7.0 the redox potential of the hydrogen electrode is volt. At pH 7.0 the redox potential of the oxygen electrode is volt.

According to early concepts, oxidation was regarded as a chemical reaction involving the addition of oxygen to an oxidizable substance such as ferrous oxide; for example:

The reverse process, or the removal of oxygen, is reduction. Later it was learned that certain substances could be oxidized or reduced without oxygen being involved. For example, on heating in the absence of oxygen, ethane is oxidized to ethylene and hydrogen:

In this case oxidation involves the loss of hydrogen. The reverse process, or the addition of hydrogen, is reduction. Thus reduction may be defined as the addition of hydrogen or the removal of oxygen, and oxidation may be defined as the addition of oxygen or the removal of hydrogen. However, certain substances may be oxidized or reduced without either oxygen or hydrogen being involved. For example, when treated with chlorine, ferrous chloride is oxidized to ferric chloride:

When written in the ionized form, it is observed that the oxidation of iron has involved the exchange of an electron. Art inspection of other oxidation or reduction reactions reveals that such reactions always involve the exchange of electrons regardless of whether oxygen or hydrogen is involved. If the iron system is considered alone, the reaction may be written:

where e represents an electron. It should be borne m mind that an electron is a negative charge. Substances or systems undergoing oxidation lose electrons while those undergoing reduction gain electrons. For every oxidation there must be a corresponding reduction.

PHYSICOCHEMICAL CONSIDERATIONS

Since oxidation and reduction reactions are electronic migrations involving the exchange of electric charges, the intensity of redox reactions can be measured in terms of e.m.f., or electric potential differences. When an unattackable electrode (such as platinum or gold metal) is immersed in a reversible redox system, a potential difference is set up at the electrode which can be measured potentiometrically. The more highly oxidized a system is the higher will be the electrode potential, and the more reduced a system is the more negative will the potential be. Since we are dealing with reversible systems, the electrons may flow in either direction, depending on prevailing conditions. Consider the system of ferrous-ferric ions, for example:

Applying the mass action chemical equilibrium equation, we get

where e_s is the concentrations of free electrons in the system and k is a constant. The parentheses indicate activity concentrations. If e_s is increased, reaction (1) proceeds from fight to left, or ferric iron will be reduced to ferrous iron until equilibrium is established. If e_s in reaction (1) is decreased, there is a tendency for ferrous iron to be oxidized to the ferric state.

An unattackable electrode immersed in the reversible redox system does not participate in the reaction but acts merely as an inert conductor of electrons to or from the system. Such an electrode can be considered to be a store of electrons of fixed concentration, e_m. Since the concentration or escaping tendency of electrons in the unattackable electrode, e_m. is different from that in the reversible redox system, e_s, a potential difference is set up at the electrode. It can be shown from physicochemical considerations that the work done in transferring an equivalent of electrons from the redox system to the electrode is:

where R is the gas constant equal to 1.99 calories per degree, T is the absolute temperature and In is natural logarithms. The work is equal to the quantity of electricity transferred multiplied by the potential at which the transfer is made:

where n is the number of equivalents transferred, E is the potential at which the transfer is made, and F is the conversion factor, a faraday of electricity. Now combining equations (3) and (4), we get:

and solving for the redox potential, E:

Since the concentration of electrons (e_m) in the unattackable electrode is a constant, k₁, equation (6) may be written:

Now returning to equation (1) and making it applicable to reversible redox systems in general, rather than merely to the ferrous-ferric iron system, we may designate the reduced form of a system by Red. and the oxidized form by Ox. so that:

where n is the number of electrons. Making similar substitutions in equation (2) we get:

Equation (9) may be rewritten:

and substituting this value for e_s in equation (7), we obtain:

where k₂ is a constant.

It is not possible to measure a single potential difference, E, at an electrode because this constitutes only a half-cell, but if the circuit is completed by including a standard half-cell, the e.m.f. of the completed cell may be measured. If the standard half-cell is fixed as a solution containing one atmosphere of hydrogen

and one normal hydrogenion concentration, we have a normal hydrogen electrode which is the standard of reference. Electrode potentials referred to this standard are measured in volts and designated E_h.

where k₃ is the potential of the normal hydrogen electrode. Then by substituting this value for E in equation (11):

Now let

a constant for the system:

This is the general electrode equation of Peters. According to Clark,

volt coulombs, T, the absolute temperature at 30°C. = 303°, and coulombs. Substituting these values for a system in which two electrons are concerned (n = 2), at constant pH the equation becomes:

in which log represents Briggsian logarithms

E_h is measured in volts and E₀ is a constant for the system. (Ox.) and (Red.) are the concentrations of the oxidized and the reduced forms respectively of the redox substance. From equation (14) it is evident that the E_h increases as (Ox.) increases and as (Red.) decreases. When the redox substance or system is 50 per cent oxidized (Ox.) = (Red.) and In other words, E₀ is the redox potential of a system which is 50 per cent in the oxidized form and 50 per cent in the reduced form.

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DISCUSSION

These studies, which are more exploratory than intensive or extensive in nature, indicate that with proper precautions it is possible to estimate the redox potential of recent marine sediments with sufficient precision to be of descriptive significance in characterizing and studying sediments. The range of redox potentials found in sediments is far greater than the range of experimental error in estimating the potentials. Redox potentials ranging from

volt have been observed in bottom deposits, homologous samples of which give E_h values that are reproducible to within ±0.01 to 0.05 volt.

One of the most disconcerting features of redox potential measurements of recent marine sediments is that the redox potential of such material is a dynamic property which is in a state of constant flux. The E_h of poorly poised sedimentary materials changes rapidly and appreciably with oxygen tension, bacterial activity, dilution with water, temperature, and other factors. Bacterial or enzymatic activity appears to be the most important dynamic factor which affects the redox potential of bottom deposits.

The negative drift in the redox potential of soil samples has been attributed by Heintze¹⁰ to the organic content. It has been shown by Burrows and Cordon,¹¹ however, that the drift is primarily a function of bacterial activity and that the potential may be influenced by both the numbers and kinds of bacteria present. Bacterial activity is influenced by the con\-centration and decomposability of organic matter.

Bacterial activity in marine muds and the attendant changes in E_h are appreciably accelerated by certain changes which occur during the collection of samples. A ten-fold increase in bacterial population was observed by ZoBell (1938) in mud samples stored for 7 days at 4°C., and much greater and more rapid changes occurred in mud stored at higher temperatures. This is a commentary on the necessity of making E_h measurements on mud samples as soon as possible after their collection in order to obtain values which are representative of O/R conditions in situ.

To date our efforts to find a means of stabilizing the redox potential of sediment samples have been unsuccessful because all of the substances tried for inhibiting bacterial and enzymatic activity have had a direct effect themselves on the redox potential. Volk¹² experimented with mercury compounds, copper compounds, toluene, alcohol, heat, and refrigeration as preservatives to prevent a drift in the redox potential of soil samples by bacterial activity from the time the samples were collected until E_h measurements could be made. Cooling the samples to just above the freezing point and excluding atmospheric oxygen by means of nitrogen proved to be the only procedure which was at all satisfactory.

In view of the multiplicity and complexity of the factors which influence the redox potential of sediment samples, it is noteworthy that values characteristic for each type of sediment are obtainable. The results probably would be viewed with considerable skepticism by the physical chemist, though, because of the large experimental errors involved and particularly because the interpretation of redox potentials of sediments is affected by so many unknown factors. Wartenburg, for example, claims that any correlations between redox potentials and soil properties that have been reported are due to special circumstances and to particular methods employed, and are not explainable by the physical chemist on a basis of true theoretical considerations. The purpose of this paper, however, is to point out that while the E_h values obtained for sediment samples are more descriptive than physicochemically exact, such values may prove to be a useful means of characterizing sediments, since so many chemical and biological processes which affect the diagenesis of sediments are influenced by the redox potential. The capacity factor (poise) as well as the intensity factor (E_h ) must be taken into consideration in the characterization of sediments.

Since exploratory observations have shown oil-bearing sands and other petroliferous sediments to have a relatively high reducing intensity and a rather low reducing capacity as compared with other sediments, it is believed that these properties may be proved a significant characteristic of source beds of petroleum or producing horizons. . . .

Bacteria have a pronounced effect on the E_h of the medium in which they are growing, and in turn the E_h of the medium influences the growth and metabolism of bacteria. It is claimed by many workers that the growth of anaerobic bacteria is determined by the E_h and not by the oxygen tension. It is a commonly reported observation that growth of anaerobes is possible in the presence of air when the E_h is sufficiently low. If the E_h is above the critical point, free oxygen interferes with the oxidation-reduction processes of anaerobes.

Whether the predominating type of bacterial activity is aerobic or anaerobic has a marked effect on the transformation of organic matter and certain inorganic constituents in sediments. Carbon dioxide is the principal product resulting from the aerobic attack of organic matter; anaerobic processes produce hydrogen, hydrogen sulphide, and methane along with lesser quantities of carbon dioxide. As far as is known, hydrogen, hydrogen sulphide, and methane are produced only under anaerobic conditions or at a low E_h. The formation of these gases is believed to be associated with petroleum genesis. Moreover, there is accumulating evidence that petroleum hydrocarbons will be produced by bacteria or accumulate in recent sediments only when the E_h is low. Nearly all kinds of hydrocarbons are susceptible to bacterial oxidation under aerobic conditions, according to ZoBell et al.,¹³ but under anaerobic conditions hydrocarbons are attacked very slowly by bacteria or not at all. For purposes of this discussion conditions may be regarded as anaerobic when the redox potential is negative to E_h − 0.1 volt.

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CONCLUSIONS

The E_h of recent marine sediments ranges from +0.350 to −0.500 volt and pH ranges from 6.4 to 9.5. Each type of sediment appears to have its own characteristic E_h and pH. Bottom deposits rich in organic matter and bacteria are generally reducing. Negative E_h values or reducing conditions are also a property of fine sediments; coarser sediments are generally less reducing. Positive E_h values are found in well-oxygenated bottoms. As a very general rule the E_h and redox capacity decrease with core depth; the pH increases. The reducing conditions found in bottom deposits are attributed to the activity of bacteria which oxidize organic matter. Once created, the reducing conditions are maintained by certain organic compounds, ferrous iron, reduced manganese, hydrogen sulphide, and other inorganic constitents in sediments.

It is believed that detailed data on the redox potential of sediments will contribute to an understanding of the morphology, general nature, and diagenesis of sediments. Such data may find their most important application in the study and characterization of source sediments of petroleum.

^* From American Association of Petroleum Geologists Bulletin 30 (1946), 477–513.

¹ S. W. Brujevicz. "Oxidation-reduction potentials and pH of sea bottom deposits." Verhandl. Intern. Vereinigung Theor. Angew. Limnologie 8 (1937), 35–49.

² W. H. Pearsall and C. H. Mortimer, "Oxidation-reduction potentials in water-logged soils, natural waters and muds," J. Ecol. 27 (1939), 483–501.

³ R. J. Allgeier, B. C. Hafford, and C. Juday, "Oxidation-reduction potentials and pH of lake waters and of lake sediments," Trans. Wisconsin Acad. Sci. 33 (1941) 115–133.

⁴ C. M. Keaton and L. T. Kardos, "Oxidation-reduction potentials of arsenate-arsenite systems in sand and soil mediums," Soil Sci. 50 (1940), 189–207.

⁵ C. E. ZoBell, "Changes produced by microorganisms in sediments after deposition," J. Sed. Petrol. 12 (1942), 127–136; "Influence of bacterial activity on source sediments," Oil Weekly 109 (1943), 15–26.

⁶ V. B. Porfiriev, "The mode of formation of oil fields in the Central Asiatic part of the Thetis," Abstract of Papers, Intern. Cong., 17th Sess. (Moscow, 1937), p. 7.

⁷ O. Stutzer and A, C. Noé Geology of Coal, University of Chicago Press, 1940.

⁸ P. A. Dickey, "Natural potentials in sedimentary rocks," Am. Inst. Min. Met. Eng. Tech. Publ. 1625 (1943), pp. 1–10.

⁹ C. D. Hodgman, Handbook of Chemistry and Physics, 27th ed. (Cleveland: Chemical Rubber Publ. Co., 1943), p. 1345.

¹⁰ S. G. Heintze, "The use of the glass electrode in soil reaction and the oxidation-reduction potential measurements," J. Agric. Sci. 24 (1934), 28–41.

¹¹ W. Burrows and T. C. Cordon, "The influence of the decomposition of organic matter on the oxidation-reduction potentials of soils," Soil Sci. 42 (1936), 1–10.

¹² N. J. Volk, "The determinaton of redox potentials in soils," J. Am. Soc. Agron. 31 (1939), 344–351.

¹³ C. E. ZoBell, C. W. Grant, and H. F. Haas, "Marine microorganisms which oxidize petroleum hydrocarbons," Bull. Am. Assn. Petrol. Geol. 27 (1943), 1175–1193.