Monocyclic aromatic compounds (MAC) represent a significant fraction of the
total organic carbon in many geologic environments such as hydrothermal sy
stems and petroleum reservoirs, yet the factors that control their abundanc
e in these environments remain highly uncertain. In order to evaluate wheth
er aqueous reactions involving MAC have a significant impact on their occur
rence, laboratory experiments were conducted to observe the reactivity of s
everal simple MAC under hydrothermal conditions that included the presence
of mineral redox buffers. Aqueous solutions of individual MAC were heated a
t 300 to 330 degreesC and 350 bar in flexible gold reaction vessels with ti
tanium fittings. Toluene in aqueous solution was found to gradually decompo
se during heating to form primarily benzene plus CO2, indicating the decomp
osition proceeded by an oxidative decarboxylation pathway. The rate of this
reaction was enhanced by the presence of dissolved sulfur compounds and re
latively oxidizing conditions, suggesting that intermediate oxidation state
sulfur compounds (such as thiosulfate or polythionates) could play a role
in promoting the reaction by facilitating the transfer of electrons among r
eactants. Benzoic acid decomposed rapidly to benzene plus CO2, suggesting t
hat formation of benzoic acid is the rate-limiting step in the overall conv
ersion of toluene to benzene. Both benzene and phenol were found to be esse
ntially unreactive. An assessment of the reaction products was performed to
evaluate whether reactions among MAC attained metastable thermodynamic equ
ilibrium. The results of this assessment, however, were equivocal, with som
e observations suggesting approach to thermodynamic equilibrium while other
data indicate that criteria to demonstrate equilibrium were not met. The l
aboratory results demonstrate that aqueous reactions can play a role in con
trolling the abundance of aromatic compounds in geologic environments. Copy
right (C) 2001 Elsevier Science Ltd.