Equilibrium Concepts in Natural Water Systems

15

Equilibria

a n d Nonequilibria in Organic

Geochemistry MAX BLUMER

Downloaded by CORNELL UNIV on May 27, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0067.ch015

Woods Hole Oceanographic Institution, Woods Hole, Mass. Carbon compounds may have existed at or near thermodynamic equilibrium on the early, prebiological earth. The activity of organisms has converted most carbon in the earth's crust into thermodynamically unstable but long-lived compounds. During diagenesis a gradual, irreversible approach towards equilibrium occurs, but complete equilibrium between all components is not reached within time spans approaching the age of the earth. This does not preclude the participation of many metastable fossil organic compounds in reversible equilibria. Further study of these may lead to a tool for investigating chemical environments in the past. Tnorganic geochemistry is dominated by equilibrium processes. Most reactions are rapid, and thermodynamic equilibria are established within geologically short time spans. Many reactions which appear slow in the laboratory still proceed sufficiently fast to influence the composi tion of geological systems. Noteworthy exceptions involve certain metastable ions like CO3 ", S0 ~, and P0 ~, which may persist even in unfavorable environments for millions if not billions of years. The equilibrium nature of most natural inorganic systems allows one to predict the stable ionic or mineral components of an environment if its pH, redox potential, pressure, and temperature are defined. Con versely, the reconstruction of a fossil physicochemical environment is often possible by studying fossil mineral assemblages (J, 7). The organic geochemist encounters a quite different situation. Most organic products of organisms are thermodynamically unstable. When incorporated into sediments, they persist for long time spans because of the high degree of metastability inherent in carbon compounds under terrestrial environment conditions. The lack of equilibrium in the sediA

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312 Stumm; Equilibrium Concepts in Natural Water Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


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mentary organic matter is manifest within molecules, between different organic molecules, and between them and their inorganic matrix. Compounds with different oxidation states of carbon in one molecule, like propionic acid, are unstable; their disproportionation is thermodynamically favored but prevented by slow kinetics. A similar degree of metastability exists for mixtures of compounds of different oxidation state, both in purely organic systems and i n mixed organic-inorganic systems. Thus, a 1:1 mixture of 1-pentane and formic acid is thermodynamically unstable at room temperature; likewise, the polynuclear aromatic hydrocarbons i n manganese nodules (8) are preserved i n association with M n ( l V ) only because of their great degree of metastability.

Figure 1. Thermodynamic equilibrium in atmospheres of varying elemental proportions. The ternary diagram shows all compositions of systems containing carbon, hydrogen, and oxygen (each point represents 100% of the three components). Lower curves indicate the potential formation of solid carbon if equilibrium could be attained. Dashed curve holds at 500°K., the continuous one at 700°K. The upper lines indicate the "asphalt threshold," the dashed one at 500° K., and the continuous one at 700°K. Above this threshold, thermodynamic equilibrium favors the formation of large proportions of polycyclic aromatic compounds ("asphalt") and a lesser increase of most of the other families of compounds. The dots through points A to C indicate the points used in the computations for Figure 2 (6).

Stumm; Equilibrium Concepts in Natural Water Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


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The thermodynamic equilibrium concentrations of numerous geochemically interesting compounds of C , H , O, and N have been computed by Dayhoff, Lippincott, and Eck for temperatures and pressures ranging from 300° to 1000°K. and 10~° to 300 atm. (6). These authors conclude that starting from a hydrogen rich composition an increase i n carbon content in this system (see Figure 1) leads to a threshold where the potential formation of elemental carbon becomes possible. The activation energy required to form solid carbon is quite high. A further increase in the carbon content of the system may therefore produce a metastable accumulation of tars and asphalt without the appearance of free carbon. Beyond this "asphalt boundary" large polycyclic aromatic systems become stable, and the concentration of many lower molecular weight organic compounds increases drastically (Figure 2). Relative to the aromatic hydrocarbons, aliphatics—especially the higher homologs of methane and ethane—are present in negligible concentrations. No combination of pressure, temperature, and elemental compositions was found which favors aliphatic over aromatic hydrocarbons. This study lays the foundation for any consideration of the equilibrium composition of organic mixtures both on a prebiological earth and in biogenic sediments remote in time and space from the influence of living organisms. It is interesting to note the great similarity between the composition of the computed equilibrium mixture and the composition of organic compounds found in carbonaceous meteorites. This has been interpreted as suggesting the formation of these compounds by equilibrium reactions at an early stage of the formation of the solar system ( I I , 12). DayhofFs data quantitatively confirm earlier qualitative descriptions of petroleum (13) and of kerogen as nonequilibrium mixtures. Generally, sedimentary organic matter, especially at the time of deposition, is quite remote from equilibrium. During diagenesis the more reactive of the unstable compounds or substituents are gradually eliminated. The increasing aromaticity of coal and kerogen with increasing age and burial depth results in a gradual approach to equilibrium of the sedimentary organic matter. This process is extremely slow; thermodynamically unstable structures like higher normal alkanes, isoprenoids, steranes, and porphyrins have survived in sediments since the Precambrian (5). The gross change in elemental composition and aromaticity of the sedimentary organic matter is much better understood than the detailed molecular changes leading to it. M u c h of our knowledge of sedimentary organic reactions comes from the study of the gradual conversion of sensitive biochemical products, especially pigments in the subsurface environment with increasing age and depth of burial (2, 3, 4, 5, 9, 10). Typical reactions ( Figure 3 ) are eliminations of oxygen functions—often


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in sterically unfavorable positions—decarboxylations, hydrogénations, isomerizations, and some cleavage reactions. Many of these lead to a destruction of the ordered building pattern of biochemical structures.

Figure 2. Equilibrium concentrations in mole fractions of selected compounds at 500°K. and 1 atm. with composition of 40% oxygen, the indicated percentage of carbon, and the rest hydrogen. To this basic composition is added an amount of nitrogen equal to the amount of carbon. The nitrogen remains primarily as N but produces significant quantities of some interesting compounds. The free energy of carbon in the system equals that of graphite at the composition indicated by the arrow. At this point solid carbon would be precipitated if it could be formed; there is no inflection of the curves at this point. The "asphalt threshold" is shown as a sharp inflection, sharpest of all for the aromatic and related heterocyclic compounds. If an atmosphere such as this were to condense, there would be about 1 molecule of glycine per droplet of condensate (6). 2


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Inherently such reactions are irreversible: it is quite unlikely that under abiotic conditions a reverse reaction would possess the specificity to recreate the ordered building pattern initially created through biochemical pathways. In favorable environments geochemically irreversible reactions proceed to completion, and reaction intermediates are either unstable or metastable; their occasional preservation arises from kinetic, not thermodynamic factors. Type


Reaction R-H R - H + shorter-chain acids and hydrocarbons

R-COOH R-COOH

Decarboxylation Cooper-Bray (6) degradation

Irreversible

Cleavage Cleavage Reduction Reduction Reduction Reduction Reduction

Irreversible Irreversible Irreversible Irreversible Irreversible Irreversible Irreversible

Reduction

Reversible

Reduction

Reversible

Reduction

Reversible

R-C-R R-C R..HC-CHR.J RCH RCH 3

f l

R.rCH.. R-H

-

Reversibility

o

Complexation Hydrolysis

R-H ?± R - M e R-COOR 5± R - C O O H

Reversible? Reversible

Figure 3. Geochemically reversible and irreversible reactions. Only the reactions are listed for which we have evidence from the fringelites, the fossil porphyrins, and the phytol-derived hydrocarbons (5). The mixture of sedimentary organic compounds, as a whole, is not in thermodynamic equilibrium. This does not preclude the participation of many metastable but long-lived compounds i n reversible equilibria. A typical case is acetic acid; as a thermodynamically unstable compound it should disproportionate: CH3COOH - » C H + C 0 4

2

However, the d i s p r o p o r t i o n a t e is so slow that it does not preclude the participation of the acid i n the rapid equilibrium: CH3COOH ^± CH3COO- + H



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In the same sense, many acid-base reactions i n the sediments— possibly including esterifications and hydrolyses—are reversible. This also is true of some complexation reactions involving organic ligands, though stable complexes like those of porphyrins appear not to be i n equilibrium with their components. M a n y purely organic redox systems may react sufficiently fast for equilibration with the environment to take place within geological time spans. A t low redox potentials aromatic hydrocarbons and heteroaromatic compounds like porphyrins appear to equilibrate with partially hydrogenated derivatives (2, 5, 10). In geochemically reversible reactions, environmental conditions determine the final ratio of starting material to end product; if conditions in the environment change, this ratio adjusts to the change. In cases of slow equilibration a molecular ratio of starting material to end product may be preserved, which is indicative of past environmental conditions. More data are needed to test this idea, but it is conceivable that analyses of aromatic and hydroaromatic hydrocarbons and pigments might be interpreted i n terms of a fossil environment. These considerations then lead us to the following conclusions: organic geochemistry, i n contrast to inorganic geochemistry, is dominated by nonequilibrium processes. Equilibrium processes are believed to account for the composition of meteoritic organic matter. They also may have played a role on the prebiological earth. Abiotic equilibrium-type mixtures of organic compounds have not yet been found on earth. Their existence i n igneous or metamorphic rocks appears possible and worthy of a systematic search. Most carbon i n the earth's crust has been cycled through organisms; as a result, it has been incorporated into unstable but long-lived biochemical structures. During diagenesis the less stable compounds are eliminated by reactions which generally lead to a scrambling of the ordered structures created by organisms. Diagenesis results in gradual equilibration of the sedimentary organic matter, but at moderate temperatures equilibrium is not reached within time spans comparable to the age of the earth. Certain metastable but long-lived organic compounds may participate i n acid-base, complexation and redox equilibria. The rates with which these equilibria adjust to changing environmental conditions varies widely i n nature. Certain slow equilibrations may be of diagnostic value in determining fossil environmental conditions. Acknowledgments This work is supported by a research contract with the U.S. Office of Naval Research (Contract Nonr-2196(00) ) and a grant from the National Science Foundation (GP-3250). W e thank M . O . Dayhoff for permission to reproduce Figures 1 and 2, the former in a corrected version of that originally published ( 6 ) .


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Literature Cited


(1) (2) (3) (4) (5) (6) (7)

Blumer, M., Helv. Chim. Acta 33, 98 (1950). Blumer, M., Omenn, G. S., Geochim. Cosmochim. Acta 25, 81 (1961). Blumer, M., Geochim. Cosmochim. Acta 26, 225 (1962). Ibid., p. 228. Blumer, M., Science 149, 722 (1965). Dayhoff, M. O., Lippincott, E. R., Eck, R. V., Science 146, 1461 (1964). Garrels, R. M., Christ, Ch., "Solutions, Minerals and Equilibria," Harper & Row, New York, 1965. (8) Thomas, D. W., Blumer, M., Science 143, 39 (1964). (9) Thomas, D. W., Blumer, M., Geochim. Cosmochim. Acta 28, 1147 (1964). (10) Ibid., p. 1467. (11) Studier, M. H., Hayatsu, R., Anders, E., Science 149, 1455 (1965). (12) Studier, M. H., Hayatsu, R., Anders, E., Science, in press. (13) Van Nes, K., Van Westen, H. A., "Aspects of the Constitution of Mineral Oils," Elsevier, Amsterdam, 1951. RECEIVED April 4, 1966. Contribution No. 1785 of the Woods Hole Oceano graphic Institution.


Equilibrium Concepts in Natural Water Systems

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