Rapid Microbial Mineralization of Toluene and 1,3-Dimethylbenzene in the Absence of Molecular Oxygen

Abstract

Up to 0.4 mM 1,3-dimethylbenzene (m-xylene) was rapidly mineralized in a laboratory aquifer column operated in the absence of molecular oxygen with nitrate as an electron acceptor. Under continuous flow conditions, the degradation rate constant (pseudo-first order) was >0.45 h⁻¹. Based on a carbon mass balance with [ring-¹⁴C]m-xylene and a calculation of the electron balance, m-xylene was shown to be quantitatively (80%) oxidized to CO₂ with a concomitant reduction of nitrate. The mineralization of m-xylene in the column also took place after reducing the redox potential, E′, of the inflowing medium with sulfide to <−0.11 V. Microorganisms adapted to growth on m-xylene were also able to degrade toluene under denitrifying conditions. These results suggest that aromatic hydrocarbons present in anoxic environments such as lake sediments, sludge digestors, and groundwater infiltration zones from landfills and polluted rivers are not necessarily persistent but may be mineralized in the absence of molecular oxygen.