Anaerobic BTEX degradation in soil bioaugmented with mixed consortia under nitrate reducing conditions

A Column Study: Impact of Redox, Substrate Composition and Exposure Order on Toluene and Benzene Biodegradation and Microbial Communities

Aromatic compounds persist as hazardous contaminants in both aquatic and terrestrial environments, needing rapid and effective remediation strategies. This study evaluated toluene and benzene biodegradation under sulfate and nitrate-reducing conditions in column experiments, utilizing aquifer sediments from a contaminated site. Over a period of 36 weeks, four glass columns were operated simultaneously in an alternating flow-batch regime. Each column received either nitrate or sulfate as an electron acceptor while being exposed to different substrate compositions in varied exposure orders. A redox dependent contaminant removal efficiency was observed, with toluene removal efficiency at 81% under sulfate and 55% under nitrate-reducing conditions, and benzene removal efficiency approximately at 44% and 59%, respectively, within 4 to 6 weeks. The rapid removal under anaerobic conditions was attributed to the alternating flow-batch regime, allowing biomass growth in batch mode, and applying selection pressure to non-specific biodegraders during flow regime. Toluene removal remained unaffected by benzene's presence but exhibited slight inhibition in the presence of an aromatic mixture composed of BTEX, indene, indane, and naphthalene. Benzene removal efficiency dropped to 8% in the presence of toluene but remained unaffected by the mixture. Pre-exposure to a single compound enhanced breakdown efficiency when further faced with a more complex mixture. Additionally, beta-diversity analysis conducted on the four columns revealed distinct microbial community clustering between sulfate and nitrate-reducing conditions, emphasizing the determining role of redox conditions. Findings of this study can be used to develop more effective pollution cleanup strategies, specifically targeting parameters like redox conditions, substrate interactions, and pollution history, thus improving our ability to mitigate contamination across diverse environments.

Spatial distribution of volatile organic compounds in contaminated soil and distinct microbial effect driven by aerobic and anaerobic conditions

The vertical distribution of 35 volatile organic compounds (VOCs) was investigated in soil columns from two obsolete industrial sites in Eastern China. The total concentrations of ΣVOCs in surface soils (0-20 cm) were 134-1664 ng g-1. Contamination of VOCs in surface soil exhibited remarkable variability, closely related to previous production activities at the sampling sites. Additionally, the concentrations of ΣVOCs varied with increasing soil depth from 0 to 10 m. Soils at depth of 2 m showed ΣVOCs concentrations of 127-47,389 ng g-1. Among the studied VOCs, xylene was the predominant contaminant in subsoils (2 m), with concentrations ranging from n.d. to 45,400 ng g-1. Chlorinated alkanes and olefins demonstrated a greater downward migration ability compared to monoaromatic hydrocarbons, likely due to their lower hydrophobicity. As a result, this vertical distribution of VOCs led to a high ecological risk in both the surface and deep soil. Notably, the risk quotient (RQ) of xylene in subsoil (2 m, RQ up to 319) was much higher than that in surface soil. Furthermore, distinct effects of VOCs on soil microbes were observed under aerobic and anaerobic conditions. Specifically, after the 30-d incubation of xylene-contaminated soil, Ilumatobacter was enriched under aerobic condition, whereas Anaerolineaceae was enriched under anaerobic condition. Moreover, xylene contamination significantly affected methylotrophy and methanol oxidation functions for aerobic soil (t-test, p < 0.05). However, aromatic compound degradation and ammonification were significantly enhanced by xylene in anaerobic soil (t-test, p < 0.05). These findings suggest that specific VOC compound has distinct microbial ecological effects under different oxygen content conditions in soil. Therefore, when conducting soil risk assessments of VOCs, it is crucial to consider their ecological effects at different soil depths.

The negative effects of the excessive nitrite accumulation raised by anaerobic bioaugmentation on bioremediation of PAH-contaminated soil

Nitrite accumulation in anaerobic bioaugmentation and its side effects on remediation efficiency of polycyclic aromatic hydrocarbon (PAH)-contaminated soil were investigated in this study. Four gradient doses of PAH-degrading inoculum (10^4, 10^5, 10^6 and 10^7 cells/g soil) were separately supplied to the actual PAH-contaminated soil combining with nitrate as the biostimulant. Although bioaugmented with higher dose of inoculum could effectively improve the biodegradation efficiencies in the initial stage than sole nitrate addition but also accelerated the accumulation of nitrite in soil. The inhibition effects of nitrite swiftly occurred following the rapid accumulation of nitrite in each experiment group, restraining the PAH-degrading functionality by inhibiting the growth of total biomass and denitrifying functions in soil. This study revealed the side effects of nitrite accumulation raised by bioaugmentation on soil microorganisms, contributing to further improving the biodegrading efficiencies in the actual site restoration.

Microbial compositional and functional traits of BTEX and salinity co-contaminated shallow groundwater by produced water

Intrusion of salinity and petroleum hydrocarbons (e.g., benzene, toluene, ethylbenzene, and xylenes, BTEX) into shallow groundwater by so-called 'produced water' (the water associated with oil and gas production) has recently drawn much attention. However, how this co-contamination affects the groundwater microbial community remains unknown. Herein, geochemical methods (e.g., ion ratios) and high-throughput sequencing (amplicon and shotgun metagenomic) were used to study the contaminant source, hydrogeochemical conditions, microbial community and function in salinity and BTEX co-contaminated shallow groundwater in an oil field, northwest China. The desulfurization coefficient (100rSO₄^2-/rCl^-), coefficient of sodium and chloride (rNa⁺/rCl^-), and coefficient of magnesium and chloride (rMg²⁺/rCl^-) revealed an intrusion of produced water into groundwater, resulting in elevated levels of salinity and BTEX. The consumption of terminal electron acceptors (e.g., NO₃^-, Fe³⁺, and SO₄^2-) was likely coupled with BTEX degradation. Relative to the bacteria, decreased archaeal diversity and enriched community in produced water-contaminated groundwater suggested that archaea were more susceptible to elevated BTEX and salinity. Relative to the nitrate and sulfate reduction genes, the abundance of marker genes encoding fermentation (acetate and hydrogen production) and methanogenesis (aceticlastic and methylotrophic) was more proportional to BTEX concentration. The produced water intrusion significantly enriched the salt-tolerant anaerobic fermentative heterotroph Woesearchaeia in shallow groundwater, and its co-occurrence with BTEX-degrading bacteria and methanogen Methanomicrobia suggested mutualistic interactions among the archaeal and bacterial communities to couple BTEX degradation with fermentation and methanogenesis. This study offers a first insight into the microbial community and function in groundwater contaminated by produced water.

Coupling of biostimulation and bioaugmentation for enhanced bioremoval of chloroethylenes and BTEX from clayey soil

The bioremoval potential of Pseudomonas plecoglossicida toward mixed contaminants was explored through the coupled biostimulation and bioaugmentation in soil microcosm. Response surface methodology was employed to optimize nutrients and innoculum size for the cometabolic removal of two representative chloroethylenes, trichloroethylene (TCE) and cis-1,2-dichloroethylene (cis-DCE), mixed with benzene, toluene, ethylbenzene, and xylenes (BTEX). The interactive effects of nutrients [nitrogen (N) and phosphorus (P)] and inoculum size toward the bioremoval of mixture of BTEX (600 mg kg^-1), cis-DCE (10 mg kg^-1), and TCE (10 mg kg^-1) were estimated using principal component analysis and two-dimensional hierarchical cluster analysis. The optimal condition was confirmed with C:N:P ratio of 100:26.7:1.8-4.8 and higher inoculum size (≥25%), where 97.7% of benzene, 98.3% of toluene, 91.2% of ethylbenzene, 45.6% of m,p-xylene, 31.2% of o-xylene, 26.9% of cis-DCE, and 33.5% of TCE were bioremoved.

Bioaugmentation potential evaluation of a bacterial consortium composed of isolated Pseudomonas and Rhodococcus for degrading benzene, toluene and styrene in sludge and sewage

Bioaugmentation was conducted using a bacterial consortium of Pseudomonas putida SW-3 and Rhodococcus ruber SS-4, to test their ability to degrade benzene, toluene, and styrene (BTS). SW-3 and SS-4 were isolated from domestic sludge and sewage samples to establish a synthetic consortium with an optimized ratio of 2:1 to reach a degradation efficiency of 82.5-89.8% of BTS. The bacterial consortium was inoculated with sludge and sewage samples at a ratio of 2:1, resulting in a degradation efficiency of 97.9% and 92.7%, respectively, at a BTS concentration of 1800 mg·L^-1. Analysis of bacterial community structure following bioaugmentation indicated an increase in abundance of BTS-degrading bacteria, particularly Acinetobacter and Pseudoxanthomonas in sludge and Pseudomonas in sewage, enhancing the collective BTS degradation ability of the bacterial community. Principal component analysis demonstrated that a more balanced bacterial community structure was established following intervention. This indicated that the selected bacteria are excellent candidates for bioaugmentation.

BTEX biodegradation by Bacillus amyloliquefaciens subsp. plantarum W1 and its proposed BTEX biodegradation pathways

Benzene, toluene, ethylbenzene and (p-, m- and o-) xylene (BTEX) are classified as main pollutants by several environmental protection agencies. In this study, a non-pathogenic, Gram-positive rod-shape bacterium with an ability to degrade all six BTEX compounds, employed as an individual substrate or as a mixture, was isolated. The bacterial isolate was identified as Bacillus amyloliquefaciens subsp. plantarum strain W1. An overall BTEX biodegradation (as individual substrates) by strain W1 could be ranked as: toluene > benzene, ethylbenzene, p-xylene > m-xylene > o-xylene. When presented in a BTEX mixture, m-xylene and o-xylene biodegradation was slightly improved suggesting an induction effect by other BTEX components. BTEX biodegradation pathways of strain W1 were proposed based on analyses of its metabolic intermediates identified by LC–MS/MS. Detected activity of several putative monooxygenases and dioxygenases suggested the versatility of strain W1. Thus far, this is the first report of biodegradation pathways for all of the six BTEX compounds by a unique bacterium of the genus Bacillus. Moreover, B. amyloliquefaciens subsp. plantarum W1 could be a good candidate for an in situ bioremediation considering its Generally Recognized as Safe (GRAS) status and a possibility to serve as a plant growth-promoting rhizobacterium (PGPR).

Remediation of BTEX and Cr(VI) contamination in soil using bioelectrochemical system-an eco-friendly approach

Soil contamination with benzene, toluene, ethylbenzene and xylene isomers (BTEX) has raised increasing concern because of its high solubility in water and toxicity to biotic communities. This study aims at investigating the process and prospects of deploying bioelectrochemical system (BES) for the removal of BTEX from artificially contaminated soil using Pseudomonas putida YNS1, alongside the reduction of hexavalent chromium (Cr(VI)). The BES was setup with desired operating conditions: initial concentration of BTEX (50-400 mg/L in 100 mL of sterilized water), pH (4-10) and applied potential voltage (0.6-1.2 V) with 10 μL log-phase culture along with the addition of Cr(VI) (10 mg/L). Samples were collected at regular intervals and analysed for BTEX degradation using gas chromatography and Cr(VI) reduction using UV-Vis spectrophotometer. Under optimized conditions (initial BTEX concentration, 200 mg/L; pH 7; and applied voltage 0.8 V with Cr(VI) of 10 mg/L), 92% of BTEX was degraded and 90% Cr(VI) was reduced from the contaminated soil. The intermediates produced during degradation were analysed through gas chromatography-flame ionization detector (GC-FID), and the possible degradation pathway was elucidated. The results indicated that BES could be effective for simultaneous degradation of BTEX along with Cr(VI) reduction.

Benzene degradation in a denitrifying biofilm reactor: activity and microbial community composition

Benzene is an aromatic compound and harmful for the environment. Biodegradation of benzene can reduce the toxicological risk after accidental or controlled release of this chemical in the environment. In this study, we further characterized an anaerobic continuous biofilm culture grown for more than 14 years on benzene with nitrate as electron acceptor. We determined steady state degradation rates, microbial community composition dynamics in the biofilm, and the initial anaerobic benzene degradation reactions. Benzene was degraded at a rate of 0.15 μmol/mg protein/day and a first-order rate constant of 3.04/day which was fourfold higher than rates reported previously. Bacteria belonging to the Peptococcaceae were found to play an important role in this anaerobic benzene-degrading biofilm culture, but also members of the Anaerolineaceae were predicted to be involved in benzene degradation or benzene metabolite degradation based on Illumina MiSeq analysis of 16S ribosomal RNA genes. Biomass retention in the reactor using a filtration finger resulted in reduction of benzene degradation capacity. Detection of the benzene carboxylase encoding gene, abcA, and benzoic acid in the culture vessel indicated that benzene degradation proceeds through an initial carboxylation step.

Multivariate optimization for the simultaneous bioremoval of BTEX and chlorinated aliphatic hydrocarbons by Pseudomonas plecoglossicida

This study aimed to evaluate the effects of some major parameters on the cometabolic removal of cis-1,2-dichloroethylene (cis-DCE) and trichloroethylene (TCE), mixed with benzene, toluene, ethylbenzene, and xylenes, by an indigenous bacterial isolate Pseudomonas plecoglossicida. Such statistical methodologies as hierarchical cluster analysis heat map and principal component analysis were applied to better evaluate the effects of major parameters (soil pH, temperature, moisture, and cis-DCE/TCE concentrations) on the biological process. The bioremoval experiments were carried out in microcosms containing soil slurry, and the headspace concentrations of contaminants were analyzed by gas chromatography. The optimal bioremoval conditions for the mixture were soil water content >110%, pH 8-9, and temperature 15-20°C, while the cis-DCE/TCE concentration did not significantly affect the mixture bioremoval within the tested range (∼10mg per kg soil). Under the optimal conditions, benzene (97.7%), toluene (96.3%), and ethylbenzene (89.8%) were almost completely removed, while cis-DCE (24.5%), TCE (29.0%), m,p-xylene (36.3%), and o-xylene (29.6%) showed lower removal efficiencies. The obtained results would help to better design a remediation technology to be applied to the sites contaminated with mixed wastes, and the statistical methodologies used in this study appear to be very efficient and could serve as a template for optimization.

Aerobic and anaerobic microbial degradation of crude (4-methylcyclohexyl)methanol in river sediments

Cyclohexane and some of its derivatives have been a major concern because of their significant adverse human health effects and widespread occurrence in the environment. The 2014 West Virginia chemical spill has raised public attention to (4-methylcyclohexyl)methanol (4-MCHM), one cyclohexane derivative, which is widely used in coal processing but largely ignored. In particular, the environmental fate of its primary components, cis- and trans-4-MCHM, remains largely unexplored. This study aimed to investigate the degradation kinetics and mineralization of cis- and trans-4-MCHM by sediment microorganisms under aerobic and anaerobic conditions. We found the removal of cis- and trans-4-MCHM was mainly attributed to biodegradation with little contribution from sorption. A nearly complete aerobic degradation of 4-MCHM occurred within 14 days, whereas the anaerobic degradation was reluctant with residual percentages of 62.6% of cis-4-MCHM and 85.0% of trans-4-MCHM after 16-day incubation. The cis-4-MCHM was degraded faster than the trans under both aerobic and anaerobic conditions, indicating an isomer-specific degradation could occur during the 4-MCHM degradation. Nitrate addition enhanced 4-MCHM mineralization by about 50% under both aerobic and anaerobic conditions. Both cis- and trans-4-MCHM fit well with the first-order kinetic model with respective degradation rates of 0.46-0.52 and 0.19-0.31 day(-)(1) under aerobic condition. Respective degradation rates of 0.041-0.095 and 0.013-0.052 day(-)(1) occurred under anaerobic condition. One bacterial strain capable of effectively degrading 4-MCHM isomers was isolated from river sediments and identified as Bacillus pumilus at the species level based on 16S rRNA gene sequence and 97% identity. Our findings will provide critical information for improving the prediction of the environmental fate of 4-MCHM and other cyclohexane derivatives with similar structure as well as enhancing the development of feasible treatment technologies to mitigate these compounds.

Bioremediation of petroleum hydrocarbons from crude oil-contaminated soil with the earthworm: Hyperiodrilus africanus

A study on the bioremediation potentials of the earthworm Hyperiodrilusafricanus (Beddard) in soil contaminated with crude oil was investigated. Dried and sieved soils were contaminated with 5 ml each of crude oil with replicates and inoculated with earthworms and monitored daily for 12 weeks. Physicochemical parameters such as pH, total organic carbon, sulfate, nitrate, phosphate, sodium, potassium, calcium and magnesium were determined using standard procedures. Total petroleum hydrocarbon (TPH) was determined using atomic absorption spectrophotometer (AAS), while BTEX constituents and earthworms tissues were analyzed using Gas Chromatography with Flame Ionization Detector (GC-FID). The results showed that the earthworm significantly enhanced the physicochemical parameters of the contaminated soil resulting in a decrease of the total organic carbon (56.64 %), sulfate (57.66 %), nitrate (57.69 %), phosphate (57.73 %), sodium (57.69 %), potassium (57.68 %), calcium (57.69 %) and magnesium (57.68 %) except pH (3.90 %) that slightly increased. There was a significant decrease in the TPH (84.99 %), benzene (91.65 %), toluene (100.00 %), ethylbenzene (100.00 %) and xylene (100.00 %). Analyses of the tissues of the earthworm at the end of the experiment showed that the earthworms bioaccumulated/biodegraded 57.35/27.64 % TPH, 38.91/52.73 % benzene, 27.76/72.24 % toluene, 42.16/57.85 % ethylbenzene and 09.62/90.38 % xylene. The results showed that the earthworms H. africanus could be used to bioremediate moderately polluted soil with crude oil contamination in the Niger Delta region of Nigeria.

Anaerobic BTEX degradation in oil sands tailings ponds: Impact of labile organic carbon and sulfate-reducing bacteria

The extraction of bitumen from oil sands in Alberta (Canada) produces volumes of tailings that are pumped into large anaerobic settling-basins. Beside bitumen, tailings comprise fractions of benzene, toluene, ethylbenzene and xylenes (BTEX) that derive from the application of industrial solvents. Due to their toxicity and volatility, BTEX pose a strong concern for gas- and water-phase environments in the vicinity of the ponds. The examination of two pond profiles showed that concentrations of indigenous BTEX decreased with depth, pointing at BTEX transformation in situ. With depth, the relative contribution of ethylbenzene and xylenes to total BTEX significantly decreased, while benzene increased relatively from 44% to 69%, indicating preferential hydrocarbon degradation. To predict BTEX turnover and residence time, we determined BTEX degradation rates in tailings of different depths in a 180-days microcosm study. In addition, we evaluated the impact of labile organic substrates (e.g. acetate) generally considered to stimulate hydrocarbon degradation and the contribution of sulfate-reducing bacteria (SRB) to BTEX turnover. In all depths, BTEX concentrations significantly decreased due to microbial activity, with degradation rates ranging between 4 and 9 μg kg(-1) d(-1). BTEX biodegradation decreased linearly in correlation with initial concentrations, suggesting a concentration-dependent BTEX transformation. SRB were not significantly involved in BTEX consumption, indicating the importance of methanogenic degradation. BTEX removal decreased to 70-90% in presence of organic substrates presumptively due to an accumulation of acetate that lowered BTEX turnover due to product inhibition. In those assays SRB slightly stimulated BTEX transformation by reducing inhibitory acetate levels.

Application of nitrate to enhance biodegradation of gasoline components in soil by indigenous microorganisms under anoxic condition

Anaerobic/anoxic biodegradation of hydrocarbons offers an attractive approach to the removal of these compounds from polluted environments such as aquifers, aquatic sediments, submerged soils and subsurface soils. The application of nitrate was investigated to accelerate the degradation of gasoline components such as mono-aromatic hydrocarbons and total petroleum hydrocarbons (TPH) in soil by indigenous microorganisms under anoxic condition. The addition of nitrate had little effect on the degradation of mono-aromatic hydrocarbons m- & p-xylene, o-xylene, sec-butylbenzene and 1,2,4-trimethylbenzene, but facilitated the degradation of TPH (C6-C12) and mono-aromatic hydrocarbons toluene and ethylbenzene markedly. Furthermore, the more nitrate added, the higher the percentage of toluene, ethylbenzene and TPH (C6-C12) degraded after 180 days of anoxic incubation. Microorganisms capable of degrading toluene, ethylbenzene and TPH (C6-C12) with nitrate as the electron acceptor under anaerobic/anoxic condition are composed predominantly of Alpha-, Beta-, Gamma- or Delta-proteobacteria. Beta- and Gamma-proteobacteria were the main components of indigenous microorganisms, and accounted for 83-100% of the total amount of indigenous microorganisms in soil used in this study. Furthermore, the total amount of indigenous microorganisms increased with nitrate added. The addition of nitrate stimulated the growth of indigenous microorganisms, and therefore facilitated the degradation of toluene, ethylbenzene and TPH (C6-C12).

Nitrate addition to groundwater impacted by ethanol-blended fuel accelerates ethanol removal and mitigates the associated metabolic flux dilution and inhibition of BTEX biodegradation

A comparison of two controlled ethanol-blended fuel releases under monitored natural attenuation (MNA) versus nitrate biostimulation (NB) illustrates the potential benefits of augmenting the electron acceptor pool with nitrate to accelerate ethanol removal and thus mitigate its inhibitory effects on BTEX biodegradation. Groundwater concentrations of ethanol and BTEX were measured 2 m downgradient of the source zones. In both field experiments, initial source-zone BTEX concentrations represented less than 5% of the dissolved total organic carbon (TOC) associated with the release, and measurable BTEX degradation occurred only after the ethanol fraction in the multicomponent substrate mixture decreased sharply. However, ethanol removal was faster in the nitrate amended plot (1.4 years) than under natural attenuation conditions (3.0 years), which led to faster BTEX degradation. This reflects, in part, that an abundant substrate (ethanol) can dilute the metabolic flux of target pollutants (BTEX) whose biodegradation rate eventually increases with its relative abundance after ethanol is preferentially consumed. The fate and transport of ethanol and benzene were accurately simulated in both releases using RT3D with our general substrate interaction module (GSIM) that considers metabolic flux dilution. Since source zone benzene concentrations are relatively low compared to those of ethanol (or its degradation byproduct, acetate), our simulations imply that the initial focus of cleanup efforts (after free-product recovery) should be to stimulate the degradation of ethanol (e.g., by nitrate addition) to decrease its fraction in the mixture and speed up BTEX biodegradation.

Anaerobic benzene biodegradation by a pure bacterial culture of Bacillus cereus under nitrate reducing conditions

A pure culture using benzene as sole carbon and energy sources was isolated by screening procedure from gasoline contaminated soil. The analysis of the 16S rDNA gene sequence, morpholpgical and physiological characteristics showed that the isolated strain was a member of genus Bacillus cereus. The biodegradation performance of benzene by B. cereus was evaluated, and the results showed that benzene could be efficiently biodegraded when the initial benzene concentration was below 150 mg/L. The metabolites of anaerobic nitrate-dependent benzene oxidation by strain B. cereus were identified as phenol and benzoate. The results of substrate interaction between binary combinations for benzene, phenol and benzoate showed that the simultaneous presence of benzene stimulated the degradation of benzoate, whereas the addition of benzene inhibited the degradation of phenol. Benzene degradation by B. cereus was enhanced by the addition of phenol and benzoate, the enhanced effects were more pronounced at higher concentration. To our knowledge, this is the first report that the isolated bacterial culture of B. cereus can efficiently degraded benzene under nitrate reducing conditions.