Application of Thermodynamic Calculations to Geochemical Processes Involving Organic Acids

Abstract

A review with many refs. summarizes some of the insights gained about the behavior of org. acids in geochem. processes through thermodn. calcns. Such calcns. are possible because of the combination of numerous exptl. studies on aq. org. acids and theor. equations of state, which allow accurate extrapolations of the measurements as well as predictions of thermodn. properties of aq. org. acids for which data have not been measured. Ests. of thermodn. data for aq. org. acids allow quant. tests of several hypotheses concerning the role of org. acids in geochem. processes. For example, thermodn. calcns. described in this chapter indicate that decarboxylation of org. acids is unlikely to proceed to any significant extent under sedimentary basin conditions. At the partial pressures of CO2 and CH4 assocd. with oil-field brines, equil. consts. for the decarboxylation of acetic acid require the acid concns. to be many orders of magnitude lower than reported values. Although the concns. of acetic acid in oil-field brines cannot be in equil. with both CO2 and CH4, they may be in redox equil. with CO2 as demonstrated by addnl. calcns. described in this chapter. This means that there is an enormous kinetic barrier blocking reactions between acetic acid and CH4 under sedimentary basin conditions. Therefore, acetic acid is preserved in a metastable state in oil-field brines, and appears to be in metastable equil. with CO2. In addn., thermodn. evaluation of acetic acid and propanoic concns. in many brines indicates that these acids are in homogeneous metastable equil. As a consequence, the ratio of the concns. of acetic and propanoic acids in basinal brines can be used as a tracer of the oxidn. state of sedimentary basins. Addnl. thermodn. calcns. described in this chapter allow tests of the plausibility of the hypothesis that the complex mixt. of liq. hydrocarbons found in petroleum may buffer the oxidn. state recorded by the acid ratios. It is found that this is a plausible argument not only for sedimentary basins but for hydrous pyrolysis expts. as well. Thermodn. calcns. can only demonstrate whether compds. are in equil. with one another (stable or metastable) and reveal nothing about the reaction mechanisms through which such equil. states are reached and maintained. In the case of metastable equil. among petroleum hydrocarbons, org. acids, and CO2 in sedimentary basins, thermophilic microorganisms may catalyze otherwise sluggish reactions so that the geol. observable metastable state is reached. If so, then many of the reactions shown to be in metastable equil. may represent overall metabolic processes, and the application of thermodn. calcns. enters the area of geochem. bioenergetics. Some preliminary examples for dehydrogenation, hydrogenation, sulfate redn., and methanogenesis reactions involving org. acids are discussed in this context at the end of this chapter. It appears that chem. reactions, which supply energy to microorganisms at low temps., provide considerably more energy at the elevated temps. encountered in sedimentary basins. [on SciFinder (R)]