Bioaugmentation with Immobilized Microorganisms to Enhance Phytoremediation of PCB-Contaminated Soil

The aim of this study was evaluate the effect of bioaugmentation by free and immobilized strains of microbial consortium on the phytoremediation of polychlorinated biphenyl (PCB)-contaminated soil using the Avena sativa, Brachiaria decumbens, Brassica juncea, and Medicago sativa plants. Alginate and biochar were used as carrier materials and free cells were used as the control. PCBs 44, 66, 118, 138, 153, 170, and 180 were chosen as indicator PCB congeners. After 60 days of plant growth, the concentration of each congener and the survival of the microbial inoculum were evaluated. The removal of the PCB congener was greater in B. juncea planted treatments and using biochar as a carrier material. PCB 66 was the congener with the highest removal percentage in all using biochar and alginate-immobilized microorganisms and free microorganisms, while PCB 170 had the lowest removal percentage in all treatments. The largest removal percentage for all congeners was obtained using biochar as a carrier material (7.2–30.3%) and the lowest with planted treatments using free microorganisms (2.3–6.8%). Real-time polymerase chain reaction (PCR) showed that the microbial inoculum survived when it was immobilized using both alginate and biochar without any significant differences between treatments; however, PCB removal percentages were obtained with biochar, which demonstrated that this carrier material has a positive effect on microbial activity.     

Dechlorination of Fenclor 54 and of a synthetic mixture of polychlorinated biphenyls by anaerobic microorganisms

Microorganisms obtained from a contaminated experimental soil were found to reductively dechlorinate the polychlorobiphenyls (PCBs) of ex-commercial Fenclor 54 and of a synthetic mixture of single congeners, under laboratory anaerobic conditions. The dechlorination rate and extent tended to increase as the chlorination degree of F 54 congeners increased. Several penta-chlorinated congeners temporarily accumulated during the final period of incubation. Dechlorination occurred primarily from the meta and para positions while ortho-sustituted congeners accumulated in the medium during incubation. The dechlorination pattern observed with these unacclimated microorganisms in both PCB mixtures could be only partially compared to patterns reported in the literature. The low product yield deriving from reductive dechlorination of PCBs, i.e. di-and tri-chlorinated biphenyls, and the slow rate of PCB biotransformation can be attributed to a lower dehalogenation capability of artificially contaminated soil microorganisms and, perhaps, also to the inadequacy of the adopted anaerobic medium. Correspondence to: F. Fava     

Rapid assay for screening and characterizing microorganisms for the ability to degrade polychlorinated biphenyls

We designed a rapid assay that assesses the polychlorinated biphenyl (PCB)-degradative competence and congener specificity of aerobic microorganisms, identifies strains capable of degrading highly chlorinated biphenyls, and distinguishes among those that degrade PCBs by alternative pathways. Prior attempts to assay PCB-degradative competence by measuring disappearance of Aroclors (commercial PCB mixtures) have frequently produced false-positive findings because of volatilization, adsorption, or absorption losses. Furthermore, these assays have generally left the chemical nature of the competence obscure because of incomplete gas chromatographic resolution and uncertain identification of Aroclor peaks. We avoided these problems by using defined mixtures of PCB congeners and by adopting incubation and extraction methods that prevent physical loss of PCBs. Our assay mixtures include PCB congeners ranging from dichloro- to hexachlorobiphenyls and representing various structural classes, e.g., congeners chlorinated on a single ring (2,3-dichlorobiphenyl), blocked at 2,3 sites (2,5,2'5'-tetrachlorobiphenyl), blocked at 3,4 sites (4,4'-dichlorobiphenyl), and lacking adjacent unchlorinated sites (2,4,5,2',4',5'-hexachlorobiphenyl). The PCB-degrative ability of microorganisms is assessed by packed-column gas chromatographic analysis of these defined congener mixtures following 24-h incubation with resting cells. When tested with 25 environmental isolates, this assay revealed a broad range of PCB-degradative competence, highlighted differences in congener specificity and in the extent of degradation of individual congeners, predicted degradative competence on commercial PCBs, and (iv) identified strains with superior PCB-degradative ability.     

Repeated application of carvone-induced bacteria to enhance biodegradation of polychlorinated biphenyls in soil

Carvone, the principal component of spearmint oil, induces biodegradation of polychlorinated biphenyls (PCB) by Arthrobacter sp. strain B1B. This study investigated the effectiveness of the repeated application of carvone-induced bacteria for bioremediation of Aroclor-1242-contaminated soil. Control treatments compared a single inoculation of carvone-induced cells, repeated applications of noninduced cells, and repeated applications of cell-free carvone/fructose medium. The results showed that repeated application of carvone-induced bacteria was the most effective treatment for mineralizing PCB, resulting in 27 ± 6% degradation of Aroclor 1242 after 9 weeks; whereas a single application of cells resulted in no significant degradation. Addition of cell-free, carvone/fructose medium resulted in 10% degradation of PCB, which suggests that this treatment stimulated biodegradation of PCB by the indigenous microflora. The di- and trichlorobiphenyls were the most readily degraded congeners. More highly chlorinated congeners, which had been previously shown to be degraded in liquid culture, were not substantially degraded in soil, indicating that low bioavailability may have limited their degradation. With the development of new technology, which permits automated in situ fermentation and delivery of degrader microorganisms, the repeated application of carvone-induced bacteria may facilitate bioremediation of PCB-contaminated soils. Received: 7 January 1998 / Received revision: 18 June 1998 / Accepted: 27 June 1998     

Growth of the phototrophic purple sulfur bacterium Thiocapsaroseopersicina under oxic/anoxic regimens in the light

Abstract The phototrophic purple sulfur bacterium Thiocapsa roseopersicina was grown in sulfide-limited continuous cultures exposed to oxic/anoxic regimens in continuous light. Synthesis of bacteriochlorophyll a (BChl a ) did not occur during the oxic periods, but started immediately upon the creation of anoxic conditions. In contrast, protein synthesis continued during both oxic and anoxic periods. Consequently, the specific content of BChla fluctuated. Despite the presence of oxygen and the fluctuating BChl a content, growth occurred predominantly in a phototrophic mode and respiration was virtually zero.
BChl a synthesis continued at high rates during anoxic periods, thus compensating for the lack of synthesis during oxic periods. When cultivated under regimens with oxic periods shorter than 12 h the highest specific BCh a content was 27 μg·mg⁻ protein. In contrast, when cultivated under regimens with oxic periods longer than 12 h the specific BChl a content was always lower than 27μg·mg⁻ length of the oxic periods. During the anoxic periods, BChl a synthesis occurred at the maximal velocity of 1.2 μg·mg⁻¹ protein·h⁻, but the length of the anoxic periods was not sufficient to allow the BChl a content to reach the maximum level.
Cultivation under continuously oxic conditions eventually resulted in pigmentless cells growing chemolithotrophically. The BChl a synthesizing ability was not lost during prolonged exposure to oxygen.
It was concluded that T. roseopersicina is very well adapted to oxic/anoxic cycles.     

Bacterial degradation of a mixture obtained through the chemical modification of polychlorinated biphenyls by polyethylene glycols

Polychlorinated biphenyls (PCBs), also known by the trade name Sovol, are toxic industrial wastes. They have been subjected to chemical treatment by polyethylene glycols (PEGs) and potassium hydroxide. As a result of the interaction of the Sovol with various molecular mass PEGs (MM_PEG-4 ～ 200, MM_PEG-22 ～ 1000), water-soluble mixtures M1 and M2 containing mono(polyethylene glycol)oxy-derivatives (PCB-PEG-4 and PCB-PEG-22), polychlorobiphenylols, and unreacted PCB congeners (PCB 44, PCB 47, PCB 49, PCB 52, and PCB 66) were obtained. It was shown for the first time that mixtures M1 and M2 are susceptible to bacterial degradation without their fractionation. According to the gas-liquid chromatography with flame-ionization and mass-spectrometric detection, the Rhodococcus wratislaviensis KT112-7 strain degraded all of the chemical compounds occurring in the mixtures. In a 5-day experiment, it was found that the KT112-7 strain decomposes mono(polyethylene glycol)oxy-derivatives completely (by 100%) and polychlorobiphenylols and PCB congeners by 90–95% in the M1 and M2 mixtures. The culture medium did not contain transformation products, whereas free chlorine ions were accumulated (72–94% of the maximum possible amount). Thus, the use of the chemical modification and consecutive bacterial degradation provided an effective destruction of technical PCB mixtures with a high content of highly chlorinated congeners.     

Effects of a transient oxic period on mineralization of organic matter to CH4 and CO2 in anoxic peat incubations

Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH₄ and CO₂, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH₄ and CO₂ to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH₄ production, oxic conditions during period 2 did not result in a lower potential CH₄ production rate during period 3, although production was delayed ～1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH₄, oxic conditions during period 2 resulted in zero production of CH₄ during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH₄ production, the oxic period had a negligible effect on anoxic CO₂ production during period 3, in surface as well as deep peat. In both surface and deep peat, CO₂ production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO₂ production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.     

Aerobic biodegradation of biphenyl and polychlorinated biphenyls by Arctic soil microorganisms.

We examined the degradation of biphenyl and the commercial polychlorinated biphenyl (PCB) mixture Aroclor 1221 by indigenous Arctic soil microorganisms to assess both the response of the soil microflora to PCB pollution and the potential of the microflora for bioremediation. In soil slurries, Arctic soil microflora and temperate-soil microflora had similar potentials to mineralize [14C]biphenyl. Mineralization began sooner and was more extensive in slurries of PCB-contaminated Arctic soils than in slurries of uncontaminated Arctic soils. The maximum mineralization rates at 30 and 7 degrees C were typically 1.2 to 1.4 and 0.52 to 1.0 mg of biphenyl g of dry soil-1 day-1, respectively. Slurries of PCB-contaminated Arctic soils degraded Aroclor 1221 more extensively at 30 degrees C (71 to 76% removal) than at 7 degrees C (14 to 40% removal). We isolated from Arctic soils organisms that were capable of psychrotolerant (growing at 7 to 30 degrees C) or psychrophilic (growing at 7 to 15 degrees C) growth on biphenyl. Two psychrotolerant isolates extensively degraded Aroclor 1221 at 7 degrees C (54 to 60% removal). The soil microflora and psychrotolerant isolates degraded all mono-, most di-, and some trichlorobiphenyl congeners. The results suggest that PCB pollution selected for biphenyl-mineralizing microorganisms in Arctic soils. While low temperatures severely limited Aroclor 1221 removal in slurries of Arctic soils, results with pure cultures suggest that more effective PCB biodegradation is possible under appropriate conditions.     

Degradation of anaerobic reductive dechlorinationproducts of Aroclor 1242 by four aerobic bacteria

We studied the aerobic degradation of eight PCB congeners which comprise from 70 to 85% of the anaerobic dechlorination products from Aroclor 1242, including2-, 4-, 2,4-, 2,6-, 2,2'-, 2,4'-, 2,2',4-, and2,4,4'-chlorobiphenyl (CB), and the biodegradation of their mixtures designed to simulate anaerobic dechlorination profiles M and C. StrainsComamonas testosteroni VP44 and Rhodococcus erythreus NY05 preferentially oxidizeda para-substituted ring, while Rhodococcus sp. RHA1, similar to well known strain Burkholderia sp. LB400, preferably attackedan ortho-chlorinated ring. Strains with ortho-directed attack extensively degraded2,4'- and 2,4,4'-CB into 4-chlorobenzoate, while bacteria with para-directed attack transformed these congeners mostly into potentially problematicmeta-cleavage products. The strains that preferentiallyoxidized an ortho-substituted ring readily degradedseven of the eight congeners supplied individually; only 2,6-CB was poorly degraded. Degradationof 2,2'- and 2,4,4'-CB was reduced when present in mixtures M and C. Higher efficiencies of degradation of the individual congeners and defined PCB mixtures M and C and greater production of chlorobenzoates were observed with bacteria that preferentially attackan ortho-substituted ring. PCB congeners 2,4'-, 2,2',4-, and 2,4,4'-CB canbe used to easily identify bacteria with ortho-directed attack whichare advantageous for use in the aerobic stage of the two-phase (anaerobic/aerobic)PCB bioremediation scheme.     

Effect of dissolved oxygen regime on growth dynamics of Pseudomonas spp during benzene degradation

We investigated the effect of different oxygen regimes on growth patterns of Pseudomonas spp. during benzene degradation in microcosm batch studies. Benzene degradation was induced by limiting oxygen available for microbial activity, which consists of three initial-dissolved oxygen (DO) levels of oxic, hypoxic, and anoxic conditions. Batch experiments were performed for cell growth and benzene degradation by inoculating three strains of Pseudomonas spp. (Pseudomonas aeruginosa, Pseudomonas fluorescens, Pseudomonas putida) in mineral salt medium containing aqueous benzene. Results showed that all strains were capable to grow and degrade benzene under all oxygen regimes but in a different manner. The highest cell growth of P. aeruginosa and P. fluorescens was achieved under oxic and anoxic condition, respectively, but there was no substantial difference on benzene degradation between the oxygen treatments with about 25% reduction for both strains. P. putida showed a facultative process for both cell growth and benzene degradation. This reveals that care should be taken in selection of microorganisms with regard to environmental studies since they exhibit different responses for given environmental conditions such as DO levels.     

Comparison of anaerobic and aerobic biodegradation of mineralized skeletal structures in marine and estuarine conditions

The knowledge of the biodegradation rates is essential to studies of the biogeochemistry and ecology of aquatic systems. It helps us to quantify the production and uptake rates of chemical components and their recycling, and to understand the mechanisms and rates of organic matter accumulation in sediments. Experimental studies of biodegradation processes in six types of mineralized skeletons were performed in shallow-marine waters of Calvi Bay, Corsica and in estuarine waters of Roscoff, Brittany. Three types of mollusk shells, sea urchin skeletal plates, crab cuticle and fish vertebrae were exposed to oxic and anoxic conditions over periods of 15 days to 30 months. After recovery of the substrates, protein assays, bacterial counts and organic carbon analyses were performed.Quantitative protein assays and bacterial counts indicate that biodegradation of mineralized skeletal structures occurs at a slower rate in anoxic conditions than in oxic conditions. Bacterial analysis showed that in anoxic environment, less than 0.5% of the consumed organic matter is converted into bacterial biomass. The aerobic biodegradation rate was positively correlated with the organic content of the skeletons.Anoxic biodegradation of skeletons occurred at much slower rates in estuarine sediments than in shallow marine sediments. Preservation of skeletal structures in estuarine conditions appears to be correlated with the abundance of dissolved organic matter rather than with high sedimentation rates.     

Degradation of polychlorinated biphenyl (PCB) by a consortium obtained from a contaminated soil composed of Brevibacterium,Pandoraea and Ochrobactrum

An indigenous polychlorinated biphenyl (PCB)-degrading bacterial consortium was obtained from soils contaminated by transformer oil with a high content of PCBs. The PCB degrader strains were isolated and identified as Brevibacterium antarcticum, Pandoraea pnomenusa, and Ochrobactrum intermedium by 16S rRNA gene sequence phylogenetic analysis. The PCB-degrading ability of the consortium and of individual strains was determined by using GC/MS. The PCB-degrading capacities of the consortium were evaluated for three concentrations of transfomer oil ranging from 55 to 152 μM supplemented with 0.001% biphenyl and 0.1% of Tween 80 surfactant. PCB biodegradation by the consortium was favored in the presence of both additives and the greatest extent of biodegradation (67.5%) was obtained at a PCB concentration of 55 μM. Each bacterial species exhibited a particular pattern of degradation relating to specific PCB congeners. Isolated strains showed a moderate degradation capability towards tetra-, hepta-, and octa-chlorobiphenyls; although no effect on penta-, hexa-, and nona-chlorobiphenyls was observed. Recently, PCB degradation capacity was recognized in a Pandorea member; however, this is the first study that describes the ability of Brevibacterium and Ochrobactrum species to degrade PCBs.     

Survival and Recovery of Methanotrophic Bacteria Starved under Oxic and Anoxic Conditions

The effects of carbon deprivation on survival of methanotrophic bacteria were compared in cultures incubated in the presence and absence of oxygen in the starvation medium. Survival and recovery of the examined methanotrophs were generally highest for cultures starved under anoxic conditions as indicated by poststarvation measurements of methane oxidation, tetrazolium salt reduction, plate counts, and protein synthesis. Methylosinus trichosporium OB3b survived up to 6 weeks of carbon deprivation under anoxic conditions while maintaining a physiological state that allowed relatively rapid (hours) methane oxidation after substrate addition. A small fraction of cells starved under oxic and anoxic conditions (4 and 10%, respectively) survived more than 10 weeks but required several days for recovery on plates and in liquid medium. A non-spore-forming methanotroph, strain WP 12, displayed 36 to 118% of its initial methane oxidation capacity after 5 days of carbon deprivation. Oxidation rates varied with growth history prior to the experiments as well as with starvation conditions. Strain WP 12 starved under anoxic conditions showed up to 90% higher methane oxidation activity and 46% higher protein production after starvation than did cultures starved under oxic conditions. Only minor changes in biomass and morphology were seen for methanotrophic bacteria starved under anoxic conditions. In contrast, starvation under oxic conditions resulted in morphology changes and an initial 28 to 35% loss of cell protein. These data suggest that methanotrophic bacteria can survive carbon deprivation under anoxic conditions by using maintenance energy derived solely from an anaerobic endogenous metabolism. This capability could partly explain a significant potential for methane oxidation in environments not continuously supporting aerobic methanotrophic growth.     

Hydrolyzed polyacrylamide biotransformation in an up-flow anaerobic sludge blanket reactor system: key enzymes,functional microorganisms,and biodegradation mechanisms

Hydrolyzed polyacrylamide (HPAM) biotransformation in an up-flow anaerobic sludge blanket reactor including biodegradation performances, biodegradation mechanisms, key enzymes, and functional microorganisms was explored. Response surface methodology was applied to further improve HPAM degradation. The predicted degradation ratios of HPAM and COD_Cr were 46.2% and 83.4% under the optimal conditions. HPAM biodegradation ratio and total organic carbon removal ratio reached 40.5% and 38.9%. Total nitrogen concentration was dramatically decreased with the increasing fermentation time during the fermentation, while low ammonia nitrogen (NH₄⁺–N) and nitrite nitrogen (NO₂⁻–N) were generated. NH₄⁺–N and NO₂⁻–N increased slightly on the whole. Enzyme activity change was correlated with HPAM biodegradation. Dehydrogenase activity had a decline of 21.3–41.0%, and the minimum value occurred at 300 mg/L of HPAM. Urease activity was varied from 28.7 to 78.7% and the maximal inhibition ratio occurred at 200 mg/L of HPAM. Mechanisms for the biodegradation of HPAM were also explored by FT-IR, HPLC, and SEM. The results indicated that long-chain HPAM was broken into micromolecule compounds and the amide groups of HPAM were transformed into carboxyl groups. Based on the sequencing results on an Illumina MiSeq platform, Proteobacterias, Bacteroidetes, and Chloroflexi were turned out to be the critical microorganisms involved in HPAM degradation. This work lays a basis for HPAM-containing wastewater treatment and offers a support for water saving and emission reduction. It is of great significance to the sustainable development of oilfield.

     

Root exudates and plant secondary metabolites of different plants enhance polychlorinated biphenyl degradation by rhizobacteria

Polychlorinated biphenyls (PCBs) are toxic and persistent compounds that are difficult to break down and biodegrade. Plant secondary metabolites (PSMs) on root exudates can act as inducers of the biphenyl catabolic pathway, enhancing PCB biodegradation. In this study, the authors evaluated the effect of root exudates and PSMs obtained from Avena sativa, Brachiaria decumbens, Medicago sativa, and Brassica juncea on the biodegradation of PCB 44, PCB 66, PCB 118, PCB 138, PCB 153, PCB 170, and PCB 180 by a microbial consortium isolated from the rhizosphere of plants grown on soil contaminated with Aroclor 1260. Microorganisms were identified as Pseudomonas sp. and Stenotrophomonas sp. based on their 16S rRNA sequence. The plant root exudates increased the degradation percentage of PCB 44, PCB 66, and PCB 118, which were used as carbon source by the microorganisms. Flavanone, flavone, isoflavone, 7-hydroxyflavanone, 7-hydroxyflavone, and 6-hydroxyflavone were the PSMs identified in the root exudates, which increased the degradation percentage of all seven PCB congeners; they were also used as growth substrates by microbial consortium. These results showed the importance of the interaction between plants and microorganisms for achieving the removal of persistent pollutants such as PCBs from soil.     

Microbial reductive dechlorination of aroclor 1260 in Baltimore harbor sediment microcosms is catalyzed by three phylotypes within the phylum Chloroflexi

The specific dechlorination pathways for Aroclor 1260 were determined in Baltimore Harbor sediment microcosms developed with the 11 most predominant congeners from this commercial mixture and their resulting dechlorination intermediates. Most of the polychlorinated biphenyl (PCB) congeners were dechlorinated in the meta position, and the major products were tetrachlorobiphenyls with unflanked chlorines. Using PCR primers specific for the 16S rRNA genes of known PCB-dehalogenating bacteria, we detected three phylotypes within the microbial community that had the capability to dechlorinate PCB congeners present in Aroclor 1260 and identified their selective activities. Phylotype DEH10, which has a high level of sequence identity to Dehalococcoides spp., removed the double-flanked chlorine in 234-substituted congeners and exhibited a preference for para-flanked meta-chlorines when no double-flanked chlorines were available. Phylotype SF1 had similarity to the o-17/DF-1 group of PCB-dechlorinating bacteria. Phylotype SF1 dechlorinated all of the 2345-substituted congeners, mostly in the double-flanked meta position and 2356-, 236-, and 235-substituted congeners in the ortho-flanked meta position, with a few exceptions. A phylotype with 100% sequence identity to PCB-dechlorinating bacterium o-17 was responsible for an ortho and a double-flanked meta dechlorination reaction. Most of the dechlorination pathways supported the growth of all three phylotypes based on competitive PCR enumeration assays, which indicates that PCB-impacted environments have the potential to sustain populations of these PCB-dechlorinating microorganisms. The results demonstrate that the variation in dechlorination patterns of congener mixtures typically observed at different PCB impacted sites can potentially be mediated by the synergistic activities of relatively few dechlorinating species.     

Microbial reductive dechlorination of PCBs

Reductive dechlorination is an advantageous process to microorganisms under anaerobic conditions because it is an electron sink, thereby allowing reoxidation of metabolic intermediates. In some organisms this has been demonstrated to support growth. Many chlorinated compounds have now been shown to be reductively dechlorinated under anaerobic conditions, including many of the congeners in commercial PCB mixtures. Anaerobic microbial communities in sediments dechlorinate Aroclor at rates of 3 µg Cl/g sediment × week. PCB dechlorination occurs at 12° C, a temperature relevant for remediation at temperate sites, and at concentrations of 100 to 1000 ppm. The positions dechlorinated are usually meta > para > ortho. The biphenyl rings, and the mono-ortho- and diorthochlorobiphenyls were not degraded after a one year incubation. Hence subsequent aerobic treatment may be necessary to meet regulatory standards. Reductive dechlorination of Arochlors does reduce their dioxin-like toxicity as measured by bioassay and by analysis of the co-planar congeners. The most important limitation to using PCB dechlorination as a remediation technology is the slower than desired dechlorination rates and no means yet discovered to substantially enhance these rates. Long term enrichments using PCBs as the only electron acceptor resulted in an initial enhancement in dechlorination rate. This rate was sustained but did not increase in serial transfers. Bioremediation of soil contaminated with Aroclor 1254 from a transformer spill was dechlorinated by greater than 50% following mixing of the soil with dechlorinating organisms and river sediment. It is now reasonable to field test reductive dechlorination of PCBs in cases where the PCB concentration is in the range where regulatory standards may be directly achieved by dechlorination, where a subsequent aerobic treatment is feasible, where any co-contaminants do not pose an inhibitory problem, and where anaerobic conditions can be established.This paper was presented at the Pacific Basin Conference on Hazardous Waste, April, 1992, Bangkok, Thailand. Published by permission of the Pacific Basin Consortium for Hazardous Waste Research, East-West Center, Honolulu, HI     

RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) Biodegradation in Aquifer Sediments under Manganese-Reducing Conditions

A shallow, RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine)-contaminated aquifer at Naval Submarine Base Bangor has been characterized as predominantly manganese-reducing, anoxic with local pockets of oxic conditions. The potential contribution of microbial RDX degradation to localized decreases observed in aquifer RDX concentrations was assessed in sediment microcosms amended with [U-¹⁴C] RDX. Greater than 85% mineralization of ¹⁴C-RDX to ¹⁴CO₂ was observed in aquifer sediment microcosms under native, manganese-reducing, anoxic conditions. Significant increases in the mineralization of ¹⁴C-RDX to ¹⁴CO₂ were observed in anoxic microcosms under NO₃-amended or Mn(IV)-amended conditions. No evidence of ¹⁴C-RDX biodegradation was observed under oxic conditions. These results indicate that microbial degradation of RDX may contribute to natural attenuation of RDX in manganese-reducing aquifer systems.