A method is presented to evaluate the influence of redox conditions and pH on mineralization of polycyclic aromatic hydrocarbons (PAH) in subsurface environments. Microbial yields based on the free energy liberated from heterotrophic PAH metabolism are estimated under various environmental conditions using a simple bioenergetic growth model. The types and chemical forms of electron acceptors addressed in this paper include oxygen, nitrate, sulfate, carbon dioxide, iron (Fe+3and FeOOH), and manganese (Mn+4 and MnO2). PAHs addressed include naphthalene (2-fused aromatic rings), anthracene (3-fused aromatic rings), phenanthrene (3-fused aromatic rings), and pyrene (4-fused aromatic rings).
Calculated free energy changes demonstrated that sequential utilization of electron acceptors will follow the order Mn+4, O2, NO3−1, Fe+3, MnO2, FeOOH, SO4−2, and CO2. The behavior in microbial growth yield predictions were found to mimic the change in free energy liberated with the use of different electron acceptors. Despite release of free energy under all conditions evaluated, the small energy liberated during PAH mineralization under sulfate-reducing and methanogenic conditions suggests that heterotrophic degradation of PAH compounds under these conditions is unlikely to occur.
The large microbial growth yields associated with reduction of free metal species (Mn+4, Fe+3) during PAH oxidation suggest a selective advantage for microbes that can tolerate acid conditions and/or participate in metal chelation processes.