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 und er nitrate reducing conditions. Benzene, toluene, ethylbenzene, xylene s (BTEX), and selected potential metabolites of their incomplete aerob ic degradation, were tested as substrates for nitrate-based respiratio n. Toluene and its potential aerobic metabolites, benzoate, protocatec huate, 3-methylcatechol, 4-methylcatechol, succinate, and adipate were degraded in strictly anoxic (O-2 < 0.1 mg/l) nitrate reducing incubat ions. Toluene degradation was directly coupled to nitrate reduction. O rtho-xylene removal was toluene dependent. Meta- and para-xylenes were degraded in nitrate reducing enrichments from only one of the four aq uifer samples. Benzene, ethylbenzene, catechol and gentisate were not degraded within up to four months in any of the incubations, even thou gh nitrate reduction occurred. Anaerobic benzene degradation was not o bserved. Incubations receiving nitrate as an adjunct electron acceptor to oxygen degraded significantly more benzene than incubations amende d with only oxygen, although benzene was only degraded until the disso lved oxygen was depleted. Possibly, more oxygen was available to degra de benzene when nitrate was added because denitrifiers utilizing nitra te as terminal electron acceptor oxidized benzoate, which had been add ed to increase the biochemical oxygen demand of the system. Benzoate o xidation with nitrate apparently spared oxygen for benzene degradation .