Soil Slurry Microcosms Research Articles

Sequential anaerobic-aerobic biodegradation of PCBs in soil slurry microcosms.

Many industrial locations have identified the need for treatment of polychlorinated biphenyl (PCB) wastes and remediation of PCB-contaminated sites. Biodegradation of PCBs is a potentially effective technology for treatment of PCB-contaminated soils and sludges; however, a practicable remediation technology has not yet been demonstrated. In laboratory experiments, soil slurry microcosms inoculated with microorganisms extracted from PCB-contaminated Hudson River sediments have been used for anaerobic dechlorination of weathered Aroclor 1248 in contaminated soil with a low organic carbon content. Anaerobic incubation was then followed by exposure to air, addition of biphenyl, and inoculation with Pseudomonas sp. LB400, an aerobic PCB degrader. The sequential anaerobic-aerobic treatment constituted an improvement compared to anaerobic or aerobic treatment alone by reducing the total amount of PCBs remaining and decreasing the tendency for end products to accumulate in humans. A 70% reduction of PCBs was observed during sequential treatment with products containing fewer chlorines and having a shorter half-life in humans than the original PCBs. The aerobic treatment alone was also quite effective as a stand-alone treatment reducing the PCBs by 67%. The results represent a case in which anaerobic river sediment organisms have been successfully transferred to a matrix free of river or lake sediments.

Reductive dehalogenation of chlorinated benzenes and toluenes under methanogenic conditions

The anaerobic metabolism of chlorinated benzenes and toluenes was evaluated in soil slurry microcosms under methanogenic conditions. A mixture of hexachlorobenzene, pentachlorobenzene, and 1,2,4-trichlorobenzene (TCB) in soil slurries was biotransformed through sequential reductive dechlorination to chlorobenzene (CB). The metabolic pathway for hexachlorobenzene and pentachlorobenzene decay proceeded via 1,2,3,4-tetrachlorobenzene (TTCB)-->1,2,3-TCB + 1,2,4-TCB-->1,2-dichlorobenzene (DCB) + 1,4-DCB-->CB. In a mineral salts medium, the CB-adapted soil microorganisms dehalogenated individual 1,2,4,5-TTCB, 1,2,3,4-TTCB, 1,2,3-TCB, and 1,2,4-TCB but not 1,2,3,5-TTCB or 1,3,5-TCB. Similarly, a mixture of 2,3,6-trichlorotoluene (TCT), 2,5-dichlorotoluene (DCT), and 3,4-DCT was reductively dechlorinated in soil slurries to predominantly toluene and small amounts of 2-, 3-, and 4-chlorotoluene (CT). Toluene was further degraded. When tested individually in a mineral salts medium, the CT-adapted soil microorganisms dechlorinated several TCT and DCT isomers. Key metabolic routes for TCTs followed: 2,3,6-TCT-->2,5-DCT-->2-CT-->toluene; 2,4,5-TCT-->2,5-DCT + 3,4-DCT-->3-CT + 4-CT-->toluene. Among DCTs tested, 2,4-DCT and 3,4-DCT were dechlorinated via the removal of o- and m-chlorine, respectively, to 4-CT and subsequently to toluene via p-chlorine removal. Likewise, 2,5-DCT was dechlorinated via 2-CT to toluene. Evidently, microorganisms capable of removing o-, m-, and p-chlorines are present in the soil system, as reflected by the dechlorination of different isomers of CBs and CTs to CB and toluene, respectively. These findings help clarify the metabolic fate of chlorinated benzenes and toluenes in anaerobic environments.