Anaerobic BTEX biodegradation linked to nitrate and sulfate reduction

New perspectives on the anaerobic degradation of BTEX: Mechanisms, pathways, and intermediates

Aromatic hydrocarbons like benzene, toluene, xylene, and ethylbenzene (BTEX) can escape into the environment from oil and gas operations and manufacturing industries posing significant health risks to humans and wildlife. Unlike conventional clean-up methods used, biological approaches such as bioremediation can provide a more energy and labour-efficient and environmentally friendly option for sensitive areas such as nature reserves and cities, protecting biodiversity and public health. BTEX contamination is often concentrated in the subsurface of these locations where oxygen is rapidly depleted, and biodegradation relies on anaerobic processes. Thus, it is critical to understand the anaerobic biodegradation characteristics as it has not been explored to a major extent. This review presents novel insights into the degradation mechanisms under anaerobic conditions and presents a detailed description and interconnection between them. BTEX degradation can follow four activation mechanisms: hydroxylation, carboxylation, methylation, and fumarate addition. Hydroxylation is one of the mechanisms that explains the transformation of benzene into phenol, toluene into benzyl alcohol or p-cresol, and ethylbenzene into 1-phenylethanol. Carboxylation to benzoate is thought to be the primary mechanism of degradation for benzene. Despite being poorly understood, benzene methylation has been also reported. Moreover, fumarate addition is the most widely reported mechanism, present in toluene, ethylbenzene, and xylene degradation. Further research efforts are required to better elucidate new and current alternative catabolic pathways. Likewise, a comprehensive analysis of the enzymes involved as well as the development of advance tools such as omic tools can reveal bottlenecks degradation steps and create more effective on-site strategies to address BTEX pollution.

Shift in Bacterial Community Structure in the Biodegradation of Benzene and Toluene under Sulfate-Reducing Condition

Groundwater contaminated by benzene and toluene is a common issue, posing a threat to the ecosystems and human health. The removal of benzene and toluene under sulfate-reducing condition is well known, but how the bacterial community shifts during this process remains unclear. This study aims to evaluate the shift in bacterial community structure during the biodegradation of benzene and toluene under sulfate-reducing condition. In this study, groundwater contaminated with benzene and toluene were collected from the field and used to construct three artificial samples: Control (benzene 50 mg/L, toluene 1.24 mg/L, sulfate 470 mg/L, and HgCl2 250 mg/L), S1 (benzene 50 mg/L, toluene 1.24 mg/L, sulfate 470 mg/L), and S2 (benzene 100 mg/L, toluene 2.5 mg/L, sulfate 940 mg/L). The contaminants (benzene and toluene), geochemical parameters (sulfate, ORP, and pH), and bacterial community structure in the artificial samples were monitored over time. By the end of this study (day 90), approximately 99% of benzene and 96% of toluene could be eliminated in both S1 and S2 artificial samples, while in the Control artificial sample the contaminant levels remained unchanged due to microbial inactivation. The richness of bacterial communities initially decreased but subsequently increased over time in both S1 and S2 artificial samples. Under sulfate-reducing condition, key players in benzene and toluene degradation were identified as Pseudomonas, Janthinobacterium, Novosphingobium, Staphylococcus, and Bradyrhizobium. The results could provide scientific basis for remediation and risk management strategies at the benzene and toluene contaminated sites.

Polycyclic aromatic hydrocarbon removal from subsurface soil mediated by bacteria and archaea under methanogenic conditions: Performance and mechanisms

In situ anoxic bioremediation is an easy-to-use technology to remediate polycyclic aromatic hydrocarbon (PAH)-contaminated soil. Degradation of PAHs mediated by soil bacteria and archaea using CO₂ as the electron acceptor is an important process for eliminating PAHs under methanogenic conditions; however, knowledge of the performance and mechanisms involved is poorly unveiled. In this study, the effectiveness and efficiency of NaHCO₃ (CO₂) as an electron acceptor to stimulate the degradation of PAHs by bacteria and archaea in highly contaminated soil were investigated. The results showed that CO₂ addition (EC2000) promoted PAH degradation compared to soil without added CO₂ (EC0), with 4.18%, 9.01%-8.05%, and 6.19%-12.45% increases for 2-, 3- and 4-ring PAHs after 250 days of incubation, respectively. Soil bacterial abundances increased with increasing incubation time, especially for EC2000 (2.90 × 10⁸ g^-1 soil higher than EC0, p < 0.05). Different succession patterns of the soil bacterial and archaeal communities during PAH degradation were observed. According to the PCoA and ANOSIM results, the soil bacterial communities were greatly (ANOSIM: R = 0.7232, P = 0.001) impacted by electron acceptors, whereas significant differences in the archaeal communities were not observed (ANOSIM: R = 0.553, P = 0.001). Soil bacterial and archaeal co-occurrence network analyses showed that positive correlations outnumbered the negative correlations throughout the incubation period for both treatments (e.g., EC0 and EC2000), suggesting the prevalence of coexistence/cooperation within and between these two domains rather than competition. The higher complexity, connectance, edge, and node numbers in EC2000 revealed stronger linkage and a more stable co-occurrence network compared to EC0. The results of this study could improve the knowledge on the removal of PAHs and the responses of soil bacteria and archaea to CO₂ application, as well as a scientific basis for the in situ anoxic bioremediation of PAH-contaminated industrial sites.

Computational Investigations of Metal-Organic Frameworks as Sorbents for BTEX Removal

Sequestration of aromatic volatile organic compounds (VOCs) via metal-organic frameworks (MOFs) as sorbents is a viable means of environmental preservation. In this investigation, we shed light on the key features associated with MOFs that govern the selective uptake of a subclass of VOCs containing benzene, toluene, ethylbenzene, and xylenes (BTEX). We investigate, through a multistep computational framework including ab initio electronic structure and classical molecular dynamics simulations, the energetic and dynamical properties associated with BTEX capture in three MOFs: HKUST-1, ZIF-8, and MIL-53. Our work demonstrates the importance of considering both static and dynamical properties upon introduction of guest molecules in such computational investigations. We elucidate the key geometric factors associated with efficient capture of BTEX compounds and highlight possible postsynthetic modifications that can be used to produce next generation sorbents for BTEX capture.

Microbial bioremediation of produced water under different redox conditions in marine sediments

The discharge of produced water from offshore oil platforms is an emerging concern due to its potential adverse effects on marine ecosystems. In this study, we investigated the feasibility and capability of using marine sediments for the bioremediation of produced water. We utilized a combination of porewater and solid phase analysis in a series of sediment batch incubations amended with produced water and synthetic produced water to determine the biodegradation of hydrocarbons under different redox conditions. Significant removal of benzene, toluene, ethylbenzene and xylene (BTEX) compounds was observed under different redox conditions, with biodegradation efficiencies of 93-97% in oxic incubations and 45-93% in anoxic incubations with nitrate, iron oxide or sulfate as the electron acceptor. Higher biodegradation rates of BTEX were obtained by incubations dominated by nitrate reduction (104-149 nmolC/cm³/d) and oxygen respiration (52-57 nmolC/cm³/d), followed by sulfate reduction (14-76 nmolC/cm³/d) and iron reduction (29-39 nmolC/cm³/d). Chemical fingerprint analysis showed that hydrocarbons were biodegraded to smaller alcohols/acids under oxic conditions compared to anoxic conditions with nitrate, indicating that the presence of oxygen facilitated a more complete biodegradation process. Toxicity of treated produced water to the marine copepod Acartia tonsa was reduced by half after sediment incubations with oxygen and nitrate. Our study emphasizes the possibility to use marine sediment as a biofilter for treating produced water at sea without extending the oil and gas platform or implementing a large-scale construction.

Adsorption of benzene on soils under different influential factors: an experimental investigation, importance order and prediction using artificial neural network

The adsorption of benzene on soils is specifically associated with its migration and transformation. Although previous studies have proved that the adsorption of benzene is affected by various factors, studies simultaneously considering the effects of multiple factors are rare. This study aimed to identify the qualitative and quantitative relationships between multiple influential factors and the adsorption capacity of benzene (BC). Batch adsorption experiments considering different influential factors, including initial concentration (IC), pH, temperature (T), ion strength (IS) and organic matter content (OMC), were conducted in three kinds of soils collected in a chemical industry park. The correlation analysis between different influential factors and BC was carried out based on the experimental data. The artificial neural network (ANN) was applied to predict BC. The results showed that BC increased with the increase of T. As the pH increased, BCs on silty loam and loam increased, while that on sandy loam decreased. Besides, BCs on silty loam and loam raised with increasing OMC, while that on sandy loam remained unchanged. BCs on all three kinds of soils attained their peaks when IS was small and then become stable with an increase in IS. The sequence of correlation between BC and influential factors is listed as IC > OMC > T > IS > pH for silty loam, OMC > IC > T > IS > pH for loam and IC > T > IS > pH > OMC for sandy loam. ANN analysis showed satisfactory accuracy in predicting BC under different influential factors. These results help us understand the important factors affecting benzene adsorption and provide a tool to get the adsorption information easily in complex site conditions.

Aerobic and Anaerobic Biodegradability of Organophosphates in Activated Sludge Derived From Kitchen Garbage Biomass and Agricultural Residues

Organophosphates (also known as organophosphate esters, OPEs) have in recent years been found to be significant pollutants in both aerobic and anaerobic activated sludge. Food waste, such as kitchen garbage and agricultural residues, can be used as co-substrates to treat the active sludge in sewage treatment plants (STPs). We investigated the biodegradability of nine OPEs derived from kitchen garbage biomass and agricultural residues under different conditions. Under anaerobic conditions, the rate of removal of triphenyl ester OPEs was significantly higher than that of chloride and alkyl OPEs. The addition of FeCl₃ and Fe powder increased the rate of degradation of triphenyl ester OPEs, with a DT₅₀ for triphenyl ester OPEs of 1.7–3.8 d for FeCl₃ and 1.3–4.7 d for Fe powder, compared to a DT₅₀ of 4.3–6.9 d for the blank control. Addition of an electron donor and a rhamnolipid increased the rate of removal of chlorinated OPEs, with DT₅₀ values for tris(2-carboxyethyl)phosphine) (TCEP) and tris(1,3-dichloroisopropyl)phosphate (TDCPP) of 18.4 and 10.0 d, respectively, following addition of the electron donor, and 13.7 and 3.0 d, respectively, following addition of the rhamnolipid. However, addition of an electron donor, electron acceptor, surfactant, and Fe powder did not always increase the degradation of different kinds of OPEs, which was closely related to the structure of the OPEs. No treatment increased the removal of alkyl OPEs due to their low anaerobic degradability. Tween 80, a non-ionic surfactant, inhibited anaerobic degradation to some degree for all OPEs. Under aerobic conditions, alkyl OPEs were more easily degraded, chlorinated OPEs needed a long adaptation period to degrade and finally attain a 90% removal rate, while the rates of degradation of triphenyl ester OPEs were significantly affected by the concentration of sludge. Higher sludge concentrations help microorganisms to adapt and remove OPEs. This study provides new insights into methods for eliminating emerging pollutants using activated sludge cultured with kitchen garbage biomass and agricultural residues.

Peculiar attenuation of soil toluene at contaminated coking sites

In the soil of contaminated coking sites, polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene and xylene (BTEX) are typical indicator compounds. Generally, PAHs are enriched in the topsoil layer. BTEX, with higher water solubilities and lower organic carbon-water partitioning coefficients (K_oc), are distributed deeper than PAHs. However, current models have employed predictions using single compounds to mimic the migration of BTEX at contaminated coking sites. Such models have not considered the influence of the upper soil layer, where PAHs are enriched. An attempt to fill this gap was made by setting up a control soil column experiment in this study. One column was filled with undisturbed soil (column #1) and the other with PAH-contaminated soil (column #2) to simulate the theoretical and actual surface soil layers, respectively. The results showed that in column #2, the toluene gas concentration of the headspace and time required to reach steady state were notably greater than those in column #1. High-throughput sequencing revealed that there were large microbial community structure differences between the two soil columns throughout the experiment, while some genera that degrade toluene with high efficiency emerged noteworthily in column #2. This implied that the upper soil layer enriched with PAHs was conducive to the degradation of toluene vapor. Applying this finding to human health exposure assessment of toluene suggests that the potential exposure level should be reduced from the current predicted level given the unanticipated attenuation at contaminated coking sites.

The Co-occurrence of DNRA and Anammox during the anaerobic degradation of benzene under denitrification

It was revealed that Anammox process promotes the anaerobic degradation of benzene under denitrification. This study investigates the effect of dissimilatory nitrate reduction to ammonium (DNRA) and exogenous ammonium on anaerobic ammonium oxidation bacteria (AnAOB) during the anaerobic degradation of benzene under denitrification. The results indicate that anammox occurs synergistically with organisms using the DNRA pathway, such as Draconibacterium and Ignavibacterium. Phylogenetic analysis showed 64% (16/25) and 36% (5/25) hzsB gene sequences, a specific biomarker of AnAOB, belonged to Candidatus 'Brocadia fuldiga' and Candidatus 'Kuenenia', respectively. Exogenous ammonium addition enhanced the anammox process and accelerated benzene degradation at a 1.89-fold higher average rate compared to that in the absence of exogenous ammonium and AnAOB belonged to Ca. 'Kuenenia' (84%) and Ca. 'Brocadia fuldiga' (16%). These results indicate that Ca. 'Brocadia fuldiga' could also play a role in DNRA. However, the diversity of abcA and bamA, the key anaerobic benzene metabolism biomarkers, remained unchanged. These findings suggest that anammox occurrence may be coupled with DNRA or exogenous ammonium and that anammox promotes anaerobic benzene degradation under denitrifying conditions. The results of this study contribute to understanding the co-occurrence of DNRA and Anammox and help explore their involvement in degradation of benzene, which will be crucial for directing remediation strategies of benzene-contaminated anoxic environment.

Benzene homologues contaminants in a former herbicide factory site: distribution, attenuation, risk, and remediation implication

Benzene homologues often used as organic raw materials or as detergents in chemical industry are prone to accidental release into the environment which can cause serious long-term soil pollutions. In a large former herbicide factory site, we investigated 43 locations for benzene homologues contaminations in soil, soil gas, and groundwater and studied the hydrogeological conditions. An inverse distance weighted interpolation method was employed to determine the pollutants three-dimensional spatial distribution in the soils. Results showed that benzene homologues residues were mainly originated from the herbicide production workshop and that the pollution had horizontally expanded at the deeper soil layer. Contaminants had already migrated 15 m downward from ground surface. Contaminant phase distribution study showed that NAPL was the primary phase (> 99%) for the pollutants accumulated in the unsaturated zone, while it had not migrated to groundwater. The primary mechanism for contaminant transport and attenuation included dissolution of "occluded" NAPL into pore water and pollutant volatilization into soil pore space. Risk assessment revealed that the pollutants brought unacceptable high carcinogenic and non-carcinogenic risks to public health. In order to convert this former chemical processing factory site into a residential area, a remediation to the polluted production workshop sites is urgently required.

Novel insights into freshwater hydrocarbon-rich sediments using metatranscriptomics: Opening the black box

Baseline biogeochemical surveys of natural environments is an often overlooked field of environmental studies. Too often research begins once contamination has occurred, with a knowledge gap as to how the affected area behaved prior to outside (often anthropogenic) influences. These baseline characterizations can provide insight into proposed bioremediation strategies crucial in cleaning up chemical spill sites or heavily mined regions. Hence, this study was conducted to survey the in-situ microbial activity within freshwater hydrocarbon-rich environments cutting through the McMurray formation - the geologic strata constituting the oil sands. We are the first to report in-situ functional variations among these freshwater microbial ecosystems using metatranscriptomics, providing insight into the in-situ gene expression within these naturally hydrocarbon-rich sites. Key genes involved in energy metabolism (nitrogen, sulfur and methane) and hydrocarbon degradation, including transcripts relating to the observed expression of methane oxidation are reported. This information provides better linkages between hydrocarbon impacted environments, closing knowledge gaps for optimizing not only oil sands mine reclamation but also enhancing microbial reclamation strategies in various freshwater environments. These finding can also be applied to existing contaminated environments, in need of efficient reclamation efforts.

Comparative proteomic analysis and characterization of benzo(a)pyrene removal by Microbacterium sp. strain M.CSW3 under denitrifying conditions

High-molecular-weight polycyclic aromatic hydrocarbons are persistent organic pollutants with great environmental and human health risks and the associated bioremediation activities have always been hampered by the lack of powerful bacterial species under redox conditions. A Microbacterium sp. strain capable of using benzo(a)pyrene as sole carbon and energy sources under denitrifying conditions was isolated. The difference in protein expression during BaP removal and removal characterization were investigated. A total of 146 proteins were differentially expressed, 44 proteins were significantly up-regulated and 102 proteins were markedly down-regulated. GO and COG analysis showed that BaP removal inhibited the expression of proteins related to glucose metabolism at different levels and activated other metabolic pathway. The proteins associated with catalytic activity and metabolic process were altered significantly. Furthermore, the BaP removal might be occurred in certain organelle of M.CSW3. The strain removed BaP with a speed of 0.0657-1.0072 mg/L/day over the concentrations range 2.5-100 mg/L. High removal rates (>70%) were obtained over the range of pH 7-11 in 14 days. Carbohydrates and organic acids which could be utilized by the strain, as well as heavy metal ions, reduced BaP removal efficiency. However, phenanthrene or pyrene addition enhanced the removal capability of M.CSW3. The strain was proved to have practical potential for bioremediation of PAHs-contaminated soil and this study provided a powerful platform for further application by improving production of associated proteins.

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.

Combined iron and sulfate reduction biostimulation as a novel approach to enhance BTEX and PAH source-zone biodegradation in biodiesel blend-contaminated groundwater

The use of biodiesel as a transportation fuel and its growing mandatory blending percentage in diesel increase the likelihood of contaminating groundwater with diesel/biodiesel blends. A 100L-field experiment with B20 (20% biodiesel and 80% diesel, v/v) was conducted to assess the potential for the combined biostimulation of iron and sulfate reducing bacteria to enhance BTEX and PAH biodegradation in a diesel/biodiesel blend-contaminated groundwater. A B20 field experiment under monitored natural attenuation (MNA) was used as a baseline control. Ammonium acetate and a low-cost and sustainable product recovered from acid mine drainage treatment were used to stimulate iron and sulfate-reducing conditions. As a result, benzene and naphthalene concentrations (maximum concentrations were 28.1μgL^-1 and 10.0μgL^-1, respectively) remained lower than the MNA experiment (maximum concentrations were 974.7μgL^-1 and 121.3μgL^-1, respectively) over the whole experiment. Geochemical changes were chronologically consistent with the temporal change of the predominance of Geobacter and GOUTA19 which might be the key players responsible for the rapid attenuation of benzene and naphthalene. To the best of our knowledge, this is the first field experiment to demonstrate the potential for the combined iron and sulfate biostimulation to enhance B20 source-zone biodegradation.

Degradation of cis- and trans-(4-methylcyclohexyl) methanol in activated sludge

Crude (4-methylcyclohexyl)methanol (MCHM) caused extensive contamination of drinking water, wastewater, and the environment during the 2014 West Virginia Chemical Spill. However, information related to the environmental degradation of cis- and trans-4-MCHM, the main components of the crude 4-MCHM mixture, remains largely unknown. This study is among the first to investigate the degradation kinetics and transformation of 4-MCHM isomers in activated sludge. The 4-MCHM loss was mainly due to biodegradation to form carbon dioxide (CO2), plus acetic, propionic, isobutyric, and isovaleric acids with little contribution from adsorption. The biodegradation of 4-MCHM isomers followed the first-order kinetic model with half-lives higher than 0.50 days. Nitrate augmented the degradation of 4-MCHM isomers, while glucose and acetate decreased their degradation. One 4-MCHM-degrading bacterium isolated from activated sludge was identified as Acinetobacter bouvetii strain EU40 based on 16S rRNA gene sequences. This study will enhance the prediction of the environmental fate of 4-MCHM in water treatment systems.

Anodes Stimulate Anaerobic Toluene Degradation via Sulfur Cycling in Marine Sediments

Hydrocarbons released during oil spills are persistent in marine sediments due to the absence of suitable electron acceptors below the oxic zone. Here, we investigated an alternative bioremediation strategy to remove toluene, a model monoaromatic hydrocarbon, using a bioanode. Bioelectrochemical reactors were inoculated with sediment collected from a hydrocarbon-contaminated marine site, and anodes were polarized at 0 mV and +300 mV (versus an Ag/AgCl [3 M KCl] reference electrode). The degradation of toluene was directly linked to current generation of up to 301 mA m(-2) and 431 mA m(-2) for the bioanodes polarized at 0 mV and +300 mV, respectively. Peak currents decreased over time even after periodic spiking with toluene. The monitoring of sulfate concentrations during bioelectrochemical experiments suggested that sulfur metabolism was involved in toluene degradation at bioanodes. 16S rRNA gene-based Illumina sequencing of the bulk anolyte and anode samples revealed enrichment with electrocatalytically active microorganisms, toluene degraders, and sulfate-reducing microorganisms. Quantitative PCR targeting the α-subunit of the dissimilatory sulfite reductase (encoded by dsrA) and the α-subunit of the benzylsuccinate synthase (encoded by bssA) confirmed these findings. In particular, members of the family Desulfobulbaceae were enriched concomitantly with current production and toluene degradation. Based on these observations, we propose two mechanisms for bioelectrochemical toluene degradation: (i) direct electron transfer to the anode and/or (ii) sulfide-mediated electron transfer.

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).

Benzene homologues in environmental matrixes from a pesticide chemical region in China: Occurrence, health risk and management

The contamination status and health risks of benzene, toluene, ethylbenzene and xylene (BTEX) in air, soil, dust and groundwater were evaluated in a pesticide chemical region located in Hebei province, China. The concentrations of BTEX in air ranged from 7.80 to 238ug/m(3) and those in soil and dust ranged from lower than limit of detection (LOD) to 32,360ng/g dw, and those in groundwater varied from 2.68 to 98.6ug/L. Generally, the levels of BTEX in multimedia matrixes were all below the standards established in China. Health risk assessment was performed based on the monitoring data via inhalation, dermal contact and ingestion pathways and hazard quotient (HQ) was calculated to be on the order of 10(-7), below 10(-6), and Hazard index (HI) levels of BTEX were lower than 1.0. However, both HQ and HI ascended with an increase in work experience/exposure. Integrated risk management was proposed to eliminate BTEX pollution and to protect occupational health of workers in those industries.

Toxicity effects of hazardous and noxious substances (HNS) to marine organisms: acute and chronic toxicity of p-xylene to the amphipod Gammarus locusta

Despite the recent focus on hazardous and noxious substances (HNS) spills preparedness and responses, much remains to be done regarding the threat posed by HNS spills on marine biota. Among the identified priority HNS, p-xylene was selected to conduct ecotoxicological assays. The aim of this study was to assess the performance of the amphipod Gammarus locusta under acute and chronic exposure to p-xylene simulating conditions of a spill incident. In the acute exposure (96 h) the p-xylene LC50 was estimated. In the chronic bioassay (36 d), an integration of organism-level endpoints (survival, growth rate, and sex ratio) with biochemical markers indicative of oxidative stress including catalase (CAT), glutathione S-transferase (GST), and superoxide dismutase (SOD) activities and lipid peroxidation (LPO) levels was determined. The aim was to increase the xylene ecotoxicological database and better predict its impact in aquatic environments. p-Xylene induced several chronic toxicity effects in G. locusta. Significant alterations in antioxidant enzymes and lipid peroxidation levels as well as growth rate and biased sex-ratio were observed. p-Xylene significantly affected the activities of CAT, SOD, and GST in G. locusta and produced oxidative damage by increasing levels of LPO in males. Further, impacts in key ecological endpoints, that is, growth and sex ratio, were noted that might be indicative of potential effects at the population level in a spill scenario. The present data may be useful to assist relevant bodies in preparedness and response to HNS spills.

Impact of nitrate dose on toluene degradation under denitrifying condition

In this study, we investigated the impact of nitrate dose on toluene degradation by Pseudomonas putida to elucidate the upper limit of nitrate concentration and whether an optimum ratio of nitrate to toluene concentration exists. Batch microcosm studies were conducted in order to monitor toluene degradation for various ratios (2-20) of nitrate to toluene with nitrate concentrations ranging from 0 to 700 mg L(-1) for a given toluene concentration of 50 and 25 mg L(-1) during 4-day (short term) and 14-day (long term) incubation time, respectively. The short-term study revealed that nitrate concentration of 500 mg L(-1) was toxic to bacteria and the optimum concentration was 300 mg L(-1) yielding the highest toluene degradation rate (0.083 mg L(-1) h(-1)). In the batch study of long term, toluene degradation was limited to 6 days after which the nitrate at 50 mg L(-1) was depleted, indicating that nitrate was a necessary electron acceptor. For both batch studies, an optimum ratio of 6 was found yielding the highest toluene degradation rate. This indicates that an appropriate nitrate dose is essential for efficient degradation of toluene when bioremediation of groundwater contaminated with toluene is under consideration.

Anaerobic benzene oxidation by Geobacter species

The abundance of Geobacter species in contaminated aquifers in which benzene is anaerobically degraded has led to the suggestion that some Geobacter species might be capable of anaerobic benzene degradation, but this has never been documented. A strain of Geobacter, designated strain Ben, was isolated from sediments from the Fe(III)-reducing zone of a petroleum-contaminated aquifer in which there was significant capacity for anaerobic benzene oxidation. Strain Ben grew in a medium with benzene as the sole electron donor and Fe(III) oxide as the sole electron acceptor. Furthermore, additional evaluation of Geobacter metallireducens demonstrated that it could also grow in benzene-Fe(III) medium. In both strain Ben and G. metallireducens the stoichiometry of benzene metabolism and Fe(III) reduction was consistent with the oxidation of benzene to carbon dioxide with Fe(III) serving as the sole electron acceptor. With benzene as the electron donor, and Fe(III) oxide (strain Ben) or Fe(III) citrate (G. metallireducens) as the electron acceptor, the cell yields of strain Ben and G. metallireducens were 3.2 × 10(9) and 8.4 × 10(9) cells/mmol of Fe(III) reduced, respectively. Strain Ben also oxidized benzene with anthraquinone-2,6-disulfonate (AQDS) as the sole electron acceptor with cell yields of 5.9 × 10(9) cells/mmol of AQDS reduced. Strain Ben serves as model organism for the study of anaerobic benzene metabolism in petroleum-contaminated aquifers, and G. metallireducens is the first anaerobic benzene-degrading organism that can be genetically manipulated.

Fast start-up and stable performance coupled to sulfate reduction in the nitrobenzene bio-reduction system and its microbial community

Three laboratory-scale sequence batch reactors were set up in this study. It was found that a successfully established sulfate-reducing system - operating at COD/SO42- ratio of 5.0 - could speed up the start-up process, leading to a high performance. It took about 100 and 60 days, respectively, for a non-sulfate-reducing system and a sulfate-reducing control system to achieve stable and high removal of NB. However, it maintained a complete NB removal in sulfate-reducing system throughout the study. Rapid and stable NB reduction was coupled to the on-going sulfate reducing process. Denaturing gradient gel electrophoresis profile demonstrated that the predominant bacterial groups in the non-sulfate-reducing system and the sulfate-reducing control system were affiliated to Deltaproteobacterium and Acinetobacter, while in the sulfate-reducing system were sulfate-reducing and sulfur-oxidizing bacteria.

Anaerobic biodegradation of ethylthionocarbamate by the mixed bacteria under various electron acceptor conditions

Biodegradation behavior and kinetics of ethylthionocarbamate under nitrate, sulfate and ferric reducing conditions by mixed cultures enriched from the anaerobic digester sludge was investigated. The results showed that ethylthionocarbamate could be degraded independently by the mixed cultures coupled to nitrate, sulfate, and ferric reduction, and meanwhile, nitrite, sulfide, and ferrous were accumulated as a result of nitrate, sulfate and ferric reduction, respectively. Ferric was a more favorable terminal electron acceptor compared to nitrate and sulfate. The order of the electron acceptors with decreasing biodegradation rates of the ethylthionocarbamate was: ferric>nitrate>sulfate, and the corresponding maximum biodegradation rate was 7.240, 6.267, and 4.602 mg/(L·d), respectively. The anaerobic biodegradation of ethylthionocarbamate under various electron acceptor conditions can be accurately described by first order exponential decay kinetics.

Modeling of vapor intrusion from hydrocarbon-contaminated sources accounting for aerobic and anaerobic biodegradation

A one-dimensional steady state vapor intrusion model including both anaerobic and oxygen-limited aerobic biodegradation was developed. The aerobic and anaerobic layer thickness are calculated by stoichiometrically coupling the reactive transport of vapors with oxygen transport and consumption. The model accounts for the different oxygen demand in the subsurface required to sustain the aerobic biodegradation of the compound(s) of concern and for the baseline soil oxygen respiration. In the case of anaerobic reaction under methanogenic conditions, the model accounts for the generation of methane which leads to a further oxygen demand, due to methane oxidation, in the aerobic zone. The model was solved analytically and applied, using representative parameter ranges and values, to identify under which site conditions the attenuation of hydrocarbons migrating into indoor environments is likely to be significant. Simulations were performed assuming a soil contaminated by toluene only, by a BTEX mixture, by Fresh Gasoline and by Weathered Gasoline. The obtained results have shown that for several site conditions oxygen concentration below the building is sufficient to sustain aerobic biodegradation. For these scenarios the aerobic biodegradation is the primary mechanism of attenuation, i.e. anaerobic contribution is negligible and a model accounting just for aerobic biodegradation can be used. On the contrary, in all cases where oxygen is not sufficient to sustain aerobic biodegradation alone (e.g. highly contaminated sources), anaerobic biodegradation can significantly contribute to the overall attenuation depending on the site specific conditions.

Biotransformation of 17α-methyltestosterone in sediment under different electron acceptor conditions

17α-Methyltestosterone (MT), an anabolic androgenic steroid, is used widely in inducing an all male population in aquaculture farming of fish, such as Nile tilapia (Oreochromis niloticus). Current understanding of the occurrence and fate of MT in the sediments and the surrounding areas of the aquaculture ponds are very limited. Bioassay tests showed that MT was biotransformed under aerobic and sulfate-reducing conditions with a half-life of 3.8d and 5.3d, respectively, with complete disappearance of androgenic activity. However, under methanogenic condition, MT was found to biotransform but the androgenic activity continued to persist even after 45 d of incubation. In contrast, MT was found to transform slowly under iron(III)-reducing condition and was hardly transformed under nitrate-reducing condition. A possible reason for the lack of transformation of MT under nitrate-reducing condition is the presence of the methyl group at the C-17 position. The results of this study suggest that MT and its degradation products with androgenic activity may potentially accumulate in the sediments of fish farming ponds under iron(III)-reducing, nitrate-reducing and methanogenic conditions.

Oxidative stress and DNA damage in the earthworm Eisenia fetida induced by toluene, ethylbenzene and xylene

Superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), and the comet assay (SCGE) were used as biomarkers to evaluate the oxidative stress and genotoxicity of toluene, ethylbenzene and xylene in earthworms (Eisenia fetida). The results indicated that the exposure of the three pollutants caused a stress response of the three enzymes, an approximate bell-shaped change (a tendency of inducement firstly and then inhibition with increasing concentrations of the pollutants) was mostly found. The three enzymes tested differed in their sensitivity to different pollutants. While the activity of POD was not significantly changed within the concentration range, the concentration thresholds for significant (P < 0.05) responses to toluene based on SOD and CAT were 5 mg kg(-1), respectively. Similarly, the concentration thresholds for significant (P < 0.05) responses to ethylbenzene based on CAT and POD were 10 and 5 mg kg(-1), respectively, while the activity of SOD was not significantly changed within the concentration range. Significant responses to xylene based on CAT and POD were 5 mg kg(-1), respectively, while the activity of SOD was significantly (P < 0.05) induced at 10 mg kg(-1). The SCGE assay results showed that these three pollutants could significantly (P < 0.01) induce DNA damage in earthworms and the clear dose-dependent relationships were displayed, indicating potential genotoxic effects of toluene, ethylbenzene, and xylene on E. fetida. The inducement of DNA damage may be attributed to the oxidative attack of toluene, ethylbenzene, and xylene. Toluene seemed to be more genotoxic as it could induce the higher extent of DNA damage than ethylbenzene and xylene. The results suggest that the SCGE assay of earthworms is simple and efficient for diagnosing the genotoxicity of pollutants in terrestrial environment.

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.

In situ bioremediation of monoaromatic pollutants in groundwater: a review

Monoaromatic pollutants such as benzene, toluene, ethylbenzene and mixture of xylenes are now considered as widespread contaminants of groundwater. In situ bioremediation under natural attenuation or enhanced remediation has been successfully used for removal of organic pollutants, including monoaromatic compounds, from groundwater. Results published indicate that in some sites, intrinsic bioremediation can reduce the monoaromatic compounds content of contaminated water to reach standard levels of potable water. However, engineering bioremediation is faster and more efficient. Also, studies have shown that enhanced anaerobic bioremediation can be applied for many BTEX contaminated groundwaters, as it is simple, applicable and economical. This paper reviews microbiology and metabolism of monoaromatic biodegradation and in situ bioremediation for BTEX removal from groundwater under aerobic and anaerobic conditions. It also discusses the factors affecting and limiting bioremediation processes and interactions between monoaromatic pollutants and other compounds during the remediation processes.

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

Different concentrations of BTEX, including benzene, toluene, ethylbenzene, and three xylene isomers, were added into soil samples to investigate the anaerobic degradation potential by the augmented BTEX-adapted consortia under nitrate reducing conditions. All the BTEX substrates could be anaerobically biodegraded to non-detectable levels within 70 d when the initial concentrations were below 100 mg/kg in soil. Toluene was degraded faster than any other BTEX compounds, and the high-to-low order of degradation rates were toluene > ethylbenzene > m-xylene > o-xylene > benzene > p-xylene. Nitrite was accumulated with nitrate reduction, but the accumulation of nitrite had no inhibitory effect on the degradation of BTEX throughout the whole incubation. Indigenous bacteria in the soil could enhance the BTEX biodegradation ability of the enriched mixed bacteria. When the six BTEX compounds were simultaneously present in soil, there was no apparent inhibitory effect on their degradation with lower initial concentrations. Alternatively, benzene, o-xylene, and p-xylene degradation were inhibited with higher initial concentrations of 300 mg/kg. Higher BTEX biodegradation rates were observed in soil samples with the addition of sodium acetate compared to the presence of a single BTEX substrate, and the hypothesis of primary-substrate stimulation or cometabolic enhancement of BTEX biodegradation seems likely.