DEGRADATION OF BTEX AND THEIR AEROBIC METABOLITES BY INDIGENOUS MICROORGANISMS UNDER NITRATE-REDUCING CONDITIONS

Batch incubations, seeded with four different aquifer materials, were used to survey the catabolic capacity of indigenous microorganisms under nitrate reducing conditions. Benzene, toluene, ethylbenzene, xylenes (BTEX), and selected potential metabolites of their incomplete aerobic degradation, were tested as substrates for nitrate-based respiration. Toluene and its potential aerobic metabolites, benzoate, protocatechuate, 3-methylcatechol, 4-methylcatechol, succinate, and adipate were degraded in strictly anoxic (O-2 < 0.1 mg/l) nitrate reducing incubations. Toluene degradation was directly coupled to nitrate reduction. Ortho-xylene removal was toluene dependent. Meta- and para-xylenes were degraded in nitrate reducing enrichments from only one of the four aquifer samples. Benzene, ethylbenzene, catechol and gentisate were not degraded within up to four months in any of the incubations, even though nitrate reduction occurred. Anaerobic benzene degradation was not observed. Incubations receiving nitrate as an adjunct electron acceptor to oxygen degraded significantly more benzene than incubations amended with only oxygen, although benzene was only degraded until the dissolved oxygen was depleted. Possibly, more oxygen was available to degrade benzene when nitrate was added because denitrifiers utilizing nitrate as terminal electron acceptor oxidized benzoate, which had been added to increase the biochemical oxygen demand of the system. Benzoate oxidation with nitrate apparently spared oxygen for benzene degradation.