Control of petroleum-hydrocarbon contaminated groundwater by intrinsic and enhanced bioremediation

Evaluating Natural Source Zone Depletion and Enhanced Source Zone Depletion in laboratory columns via soil redox continuous sensing and microbiome characterization

To optimally employ Natural Source Zone Depletion (NSZD) and Enhanced Source Zone Depletion (ESZD) at sites impacted by light non-aqueous phase liquids (LNAPL), monitoring strategies are required. Emerging use of subsurface oxidation-reduction potential (ORP) sensors shows promise for tracking redox evolution, which reflects ongoing biogeochemical processes. However, further understanding of how soil redox dynamics relate to subsurface microbial activity and LNAPL degradation pathways is needed. In this work, soil ORP sensors and DNA and RNA sequencing-based microbiome analysis were combined to elucidate NSZD and ESZD (biostimulation via periodic sulfate addition and biosparging) processes in columns containing LNAPL-impacted soils from a former petroleum refinery. Results show expected relationships between continuous soil redox and active microbial communities. Continuous data revealed spatial and temporal detail that informed interpretation of the hydrocarbon biodegradation data. Redox increases were transient for sulfate addition, and sequencing revealed how hydrocarbon concentration and composition impacted microbiome structure and naphthalene degradation. Periodic biosparging did not result in fully aerobic conditions suggesting observed biodegradation improvements could be explained by alternative anaerobic metabolisms (e.g., iron reduction due to air oxidizing reduced iron). Collectively, data suggest combining continuous redox sensing with microbiome analysis provides insights beyond those possible with either monitoring tool alone.

Risk assessment of Benzene, Toluene, Ethyl benzene, and Xylene (BTEX) in the atmospheric air around the world: A review

Volatile organic compounds, such as BTEX, have been the subject of numerous debates due to their detrimental effects on the environment and human health. Human beings have had a significant role in the emergence of this situation. Even though US EPA, WHO, and other health-related organizations have set standard limits as unhazardous levels, it has been observed that within or even below these limits, constant exposure to these toxic chemicals results in negative consequences as well. According to these facts, various studies have been carried out all over the world - 160 of which are collected within this review article, so that experts and governors may come up with effective solutions to manage and control these toxic chemicals. The outcome of this study will serve the society to evaluate and handle the risks of being exposed to BTEX. In this review article, the attempt was to collect the most accessible studies relevant to risk assessment of BTEX in the atmosphere, and for the article to contain least bias, it was reviewed and re-evaluated by all authors, who are from different institutions and backgrounds, so that the insights of the article remain unbiased. There may be some limitations to consistency or precision in some points due to the original sources, however the attempt was to minimize them as much as possible.

An overview of in situ remediation for groundwater co-contaminated with heavy metals and petroleum hydrocarbons

Groundwater is an important component of water resources. Mixed pollutants comprising heavy metals (HMs) and petroleum hydrocarbons (PHs) from industrial activities can contaminate groundwater through such processes as rainfall infiltration, runoff and discharge, which pose direct threats to human health through the food chain or drinking water. In situ remediation of contaminated groundwater is an important way to improve the quality of a water environment, develop water resources and ensure the safety of drinking water. Bioremediation and permeable reactive barriers (PRBs) were discussed in this paper as they were effective and affordable for in situ remediation of complex contaminated groundwater. In addition, media types, technology combinations and factors for the PRBs were highlighted. Finally, insights and outlooks were presented for in situ remediation technologies for complex groundwater contaminated with HMs and PHs. The selection of an in situ remediation technology should be site specific. The remediation of complex contaminated groundwater can be approached from various perspectives, including the development of economical materials, the production of slow-release and encapsulated materials, and a combination of multiple technologies. This review is expected to provide technical guidance and assistance for in situ remediation of complex contaminated groundwater.

Chlorinated solvents source identification by nonlinear optimization method

In this work, chloride ions were used as conservative tracers and supplemented with conservative amounts of chloroethenes (PCE, TCE, Cis-DCE, 1,1-DCE), chloroethanes (1,1,1-TCA, 1,1-DCA), and the carbon isotope ratios of certain compounds, the most representative on the sites studied, which is a novelty compared to the optimization methods developed in the scientific literature so far. A location of the potential missing sources is then proposed in view of the balances of the calculated mixing fractions. A test of the influence of measurement errors on the results shows that the uncertainties in the calculation of the mixture fractions are less than 11%, indicating that the source identification method developed is a robust tool for identifying sources of chlorinated solvents in groundwater.

Toxicity prediction: An application of alternative testing and computational toxicology in contaminated groundwater sites in Taiwan

Groundwater contamination remains a global threat due to its toxic effects to humans and the environment. The remediation of contaminated groundwater sites can be costly, thus, identifying the priority areas of concern is important to reduce money spent on resources. In this study, we aimed to identify and rank the priority groundwater sites in a contaminated petrochemical district by combining alternative, non-animal approaches - chemical analysis, cell-based high throughput screening (HTS), and Toxicological Priority Index (ToxPi) computational toxicology tool. Groundwater samples collected from ten different sites in a contaminated district showed pollutant levels below the detection limit, however, hepatotoxic bioactivity was demonstrated in human hepatoma HepaRG cells. Integrating the pollutants information (i.e., pollutant characteristics and concentration data) with the bioactivity data of the groundwater samples, an evidence-based ranking of the groundwater sites for future remediation was established using ToxPi analysis. The currently presented combinatorial approach of screening groundwater sites for remediation purposes can further be refined by including relevant parameters, which can boost the utility of this approach for groundwater screening and future remediation.

The occurrence and partition of total petroleum hydrocarbons in sediment, seawater, and biota of the eastern sea area of Shandong Peninsula, China

To investigate the occurrence and partition of total petroleum hydrocarbons (TPHs), sediment, seawater, and biota of the eastern sea area of Shandong Peninsula (China) were sampled in 2016. Due to shipping activities and discharge of industrial wastes, the TPH levels in the sediment decreased from the coastal areas to the offshore area. Waterborne TPH concentrations showed a decreasing order of autumn > summer > spring > winter, probably caused by low input of pollutants from the river into the ocean in dry seasons and frequent aquaculture operations in wet seasons. The distribution coefficient (logKd) between sediment and seawater ranged from 2.00 to 5.05, suggesting that TPHs are preferentially retained by the sediment. Significant positive correlations between logKd and total organic carbon (TOC), mud (fine particles), and temperature, as well as negative correlations between logKd and dissolved oxygen (DO), hydrogen ion concentration (pH), and oxidation-reduction potential (Eh), suggest interactions between environmental behaviors of TPHs and physicochemical factors. Mean bioaccumulation factors (BAFs) of TPHs in fish, shrimps, and mollusks were determined to be 240 ± 200, 900 ± 1800, and 2630 ± 1280, respectively, suggesting that considerable bioaccumulation occurs in shrimps when TPHs are transported from the seawater to the shrimps. A negative linear correlation between logTPHs and trophic level indicated biodilution of TPHs along the marine food web. This is the first study that has focused on the accumulation of TPHs in the entire sediment-seawater-biota system.

Biodegradation of p-xylene-a comparison of three psychrophilic Pseudomonas strains through the lens of gene expression

p-Xylene is considered a recalcitrant compound despite showing a similar aromatic structure to other BTEXs (benzene, toluene, ethylbenzene, xylene isomers). This study evaluated the p-xylene biodegradation potential of three psychrophilic Pseudomonas strains (Pseudomonas putida S2TR-01, Pseudomonas synxantha S2TR-20, and Pseudomonas azotoformans S2TR-09). The p-xylene metabolism-related catabolic genes (xylM, xylA, and xylE) and the corresponding regulatory genes (xylR and xylS) of the selected strains were investigated. The biodegradation results showed that the P. azotoformans S2TR-09 strain was the only strain that was able to degrade 200 mg/L p-xylene after 60 h at 15 °C. The gene expression study indicated that the xylE (encoding catechol 2,3-dioxygenase) gene represents the bottleneck in p-xylene biodegradation. A lack of xylE expression leads to the accumulation of intermediates and the inhibition of biomass production and complete carbon recovery. The activity of xylene monooxygenase and catechol 2,3-dioxygenase was significantly increased in P. azotoformans S2TR-09 (0.5 and 0.08 U/mg, respectively) in the presence of p-xylene. The expression of the ring cleavage enzyme and its encoding gene (xylE) and activator (xylS) explained the differences in the p-xylene metabolism of the isolated bacteria and can be used as a novel biomarker of efficient p-xylene biodegradation at contaminated sites.

Investigating the potential of bioremediation in aged oil-polluted hypersaline soils in the south oilfields of Iran

To date, studies for bioremediation of oil-polluted hypersaline soils have been neglected or limited to specific spots. Hence, in this study, ten samples of oil field soils in the Khuzestan province of Iran were collected to evaluate bioremediation's feasibility. These samples were analyzed for their physicochemical properties as well as the most probable number of total and hydrocarbon-degrading bacteria. Thirty-nine hydrocarbon-degrading bacteria were isolated from these soils over a 1-month incubation in an MSM medium enriched with diesel oil as the sole source of carbon. As revealed by 16S-rRNA analysis, the identified strains belonged to the genera Ochrobactrum, Microbacterium, and Bacillus with a high frequency of Ochrobactrum species. Additionally, by using degenerate primers, the third group of alkB gene was detected in Ochrobactrum and Microbacterium isolates through the touchdown nested PCR method for the first time. Ochrobactrum species possessing the alkB gene showed the highest population, and therefore, the highest adaptation to harsh environmental conditions. Most isolates showed outstanding results in the ability to grow with crude and diesel oil and tolerate high salt percentages, biosurfactant production, and emulsification activity, which are considered the most effective factors in bioremediation of such environments. Considering the soil analysis, limiting factors in bioremediation like available phosphorous, and the abundance of bacteria with remediation traits in these soils, these extremely polluted environments can be refined.

Identifying remedial solutions through optimal bioremediation design under real-world field conditions

Over more than a century of intense industrial production and associated accidental release, petroleum products (e.g., gasoline, diesel, fuel oil) have contaminated a significant portion of the world's groundwater resources. Groundwater remediation is generally a complex task, especially where aquifers and the associated contaminant distribution are highly heterogeneous. The ability to predict the efficiency of such remediation is of crucial importance, as the costs are strongly linked to the treatment design and duration. In this study, a coupled simulation-optimization (S/O) framework, consisting of a process-based reactive transport simulation model linked with particle swarm optimization (PSO) was developed. It was subsequently applied for the design of a real-world in situ bio-treatment of a BTEX contaminated aquifer in France. In the application, the optimization framework was used to simultaneously determine optimal well locations and their optimal injection rates, both constituting key elements of the enhanced biodegradation design problem. The optimization of the treatment efficiency was examined in terms of three different regulatory objectives, (1) minimization of the residual NAPL mass of the key contaminant, i.e., benzene, in the source zone, (2) reduction of the maximum concentration of benzene in groundwater, and (3) minimization of the time required to reduce the benzene concentration in groundwater to below a threshold value. Our analysis of potential, optimal remediation strategies showed that: (i) the complexity of the biodegradation behavior at real sites may favor very different remediation options as a result of varying remediation targets, (ii) the long term behavior of the contaminants after the end of the active treatment period, which is often neglected, showed to have a significant influence on remediation design that requires increased attention, (iii) PSO has shown to be a very efficient algorithm in the context of the present study. The insights that can be gained from such a framework will provide decision support to select the most suitable remediation strategy while facing different regulatory objectives.

Reductive transformation of hexavalent chromium by ferrous ions in a frozen environment: Mechanism, kinetics, and environmental implications

The transformation between hexavalent chromium (Cr⁶⁺) and trivalent chromium (Cr³⁺) has a significant impact on ecosystems, as Cr⁶⁺ has higher levels of toxicity than Cr³⁺. In this regard, a variety of Cr⁶⁺ reduction processes occurring in natural environments have been studied extensively. In this work, we investigate the reductive transformation of Cr⁶⁺ by ferrous ions (Fe²⁺) in ice at -20 °C, and compare the same process in water at 25 °C. The Fe²⁺-mediated reduction of Cr⁶⁺ occurred much faster in ice than it did in water. The accelerated reduction of Cr⁶⁺ in ice is primarily ascribed to the accumulation of Cr⁶⁺, Fe²⁺, and protons in the grain boundaries formed during freezing, which constitutes favorable conditions for redox reactions between Cr⁶⁺ and Fe²⁺. This freeze concentration phenomenon was verified using UV-visible spectroscopy with o-cresolsulfonephthalein (as a pH indicator) and confocal Raman spectroscopy. The reductive transformation of Cr⁶⁺ (20 µM) by Fe²⁺ in ice proceeded rapidly under various Fe²⁺ concentrations (20-140 µM), pH values (2.0-5.0), and freezing temperatures (-10 to -30 °C) with a constant molar ratio of oxidized Fe²⁺ to reduced Cr⁶⁺ (3:1). This result implies that the proposed mechanism (i.e., the redox reaction between Cr⁶⁺ and Fe²⁺ in ice) can significantly contribute to the natural conversion of Cr⁶⁺ in cold regions. The Fe²⁺-mediated Cr⁶⁺ reduction kinetics in frozen Cr⁶⁺-contaminated wastewater was similar to that in frozen Cr⁶⁺ solution. This indicates that the variety of substrates typically present in electroplating wastewater have a negligible effect on the redox reaction between Cr⁶⁺ and Fe²⁺ in ice; it also proposes that the Fe²⁺/freezing process can be used for the treatment of Cr⁶⁺-contaminated wastewater.

Metagenomic Insights Into the Mechanisms for Biodegradation of Polycyclic Aromatic Hydrocarbons in the Oil Supply Chain

Petroleum is a very complex and diverse organic mixture. Its composition depends on reservoir location and in situ conditions and changes once crude oil is spilled into the environment, making the characteristics associated with every spill unique. Polycyclic aromatic hydrocarbons (PAHs) are common components of the crude oil and constitute a group of persistent organic pollutants. Due to their highly hydrophobic, and their low solubility tend to accumulate in soil and sediment. The process by which oil is sourced and made available for use is referred to as the oil supply chain and involves three parts: (1) upstream, (2) midstream and (3) downstream activities. As consequence from oil supply chain activities, crude oils are subjected to biodeterioration, acidification and souring, and oil spills are frequently reported affecting not only the environment, but also the economy and human resources. Different bioremediation techniques based on microbial metabolism, such as natural attenuation, bioaugmentation, biostimulation are promising approaches to minimize the environmental impact of oil spills. The rate and efficiency of this process depend on multiple factors, like pH, oxygen content, temperature, availability and concentration of the pollutants and diversity and structure of the microbial community present in the affected (contaminated) area. Emerging approaches, such as (meta-)taxonomics and (meta-)genomics bring new insights into the molecular mechanisms of PAH microbial degradation at both single species and community levels in oil reservoirs and groundwater/seawater spills. We have scrutinized the microbiological aspects of biodegradation of PAHs naturally occurring in oil upstream activities (exploration and production), and crude oil and/or by-products spills in midstream (transport and storage) and downstream (refining and distribution) activities. This work addresses PAH biodegradation in different stages of oil supply chain affecting diverse environments (groundwater, seawater, oil reservoir) focusing on genes and pathways as well as key players involved in this process. In depth understanding of the biodegradation process will provide/improve knowledge for optimizing and monitoring bioremediation in oil spills cases and/or to impair the degradation in reservoirs avoiding deterioration of crude oil quality.

Application of encapsulated magnesium peroxide (MgO2) nanoparticles in permeable reactive barrier (PRB) for naphthalene and toluene bioremediation from groundwater

One of the challenges in the petroleum hydrocarbon contaminated groundwater remediation by oxygen releasing compounds (ORCs) is to identify the remediation mechanism and determine the impact of ORCs on the environment and the intrinsic groundwater microorganisms. In this research, the application of encapsulated magnesium peroxide (MgO₂) nanoparticles in the permeable reactive barrier (PRB) for bioremediation of the groundwater contaminated by toluene and naphthalene was studied in the continuous flow sand-packed plexiglass columns within 50 d experiments. For the biodiversity studies, next generation sequencing (NGS) of the 16S rRNA gene was applied. The results showed that naphthalene was metabolized (within 20 days) faster than toluene (after 30 days) by microorganisms of the aqueous phase. By comparing the contaminant removal in the biotic (which resulted in the complete contaminant removal) and abiotic (around 32% removal for naphthalene and 36% for toluene after 50 d) conditions, the significant role of microorganisms on the decontamination process was proved. Furthermore, the attached microbial communities on the porous media were visualized by scanning electron microscopy (SEM). Microbial community structure analysis by NGS technique revealed that the microbial species which were able to degrade toluene and naphthalene such as P. putida and P. mendocina respectively were stimulated by addition of MgO₂ nanoparticles. The presented study resulted in a momentous insight into the application of MgO₂ nanoparticles in the hydrocarbon compounds removal from groundwater.

Effectiveness and mechanism of natural attenuation at a petroleum-hydrocarbon contaminated site

This study applied an integrated method for evaluating the effectiveness and mechanism of natural attenuation (NA) of petroleum-hydrocarbon contaminated groundwater. Site groundwater and soil samples were analysed to characterize spatial and temporal variations in petroleum hydrocarbons, geochemical indicators, microbial diversity and isotopes. The results showed that the area of petroleum hydrocarbon contamination plume decreased almost 60% in four years, indicating the presence of natural attenuation. The ¹⁴C content and sequence analysis indicate that there are more relatively 'old' HCO₃^- that have been produced from petroleum hydrocarbons in the upgradient portion of the contaminated plume, confirming that intrinsic biodegradation was the major factor limiting spread of the contaminated plume. The main degradation mechanisms were identified as sulfate reduction and methanogenesis based on the following: (1) more SO₄^2- have been consumed in the contamination source than downgradient, and the δ³⁴S values in the resident SO₄^2- were also more enriched in the contamination source, (2) production of more CH₄ in the contamination source with the δ¹³C values for CH₄ was much lower than that of CO₂, and the fractionation factor was 1.030-1.046. The results of this study provide significant insight for applying natural attenuation and enhanced bioremediation as alternative options for remediation of petroleum-hydrocarbon contaminated sites.

Ecotoxicity tests with Allium cepa to determine the efficiency of rice husk ash in the treatment of groundwater contaminated with benzene, toluene, ethylbenzene, and xylene

The validation of adsorption treatment based on toxicity assays aims to assess the actual environmental impact caused by effluents after treatment. This study describes the use of rice husk ash as adsorbent and evaluates the efficiency of adsorption treatment to remediate groundwater contaminated with benzene, toluene, ethylbenzene, and xylene (BTEX). The synthetic effluent was prepared with standard benzene, toluene, ethylbenzene, and xylene solutions. Adsorption was assessed at treatment times 0, 60, 120, and 240 min. Compounds were quantified by gas chromatography with flame ionization detector. The treatment was validated based on ecotoxicity assays using Allium cepa as indicator organism. For the treatment times stipulated, samples containing 25, 50, and 100% of BTEX were used. The dilutions were carried out with drinking water according to Fiskesjö (1985). The relative growth index (RGI), root inhibition index (Ii), and germination index (GI) confirmed the efficiency of the treatment approach tested. The best adsorption time for an initial BTEX concentration of 3.378 mg/L was 60 min. Critical level (EC50) and critical concentration that induced phytotoxic effect on A. cepa germination was observed only for the undiluted effluent.

Characteristics of petroleum-contaminated groundwater during natural attenuation: a case study in northeast China

The objective of this study was to investigate a petroleum-contaminated groundwater site in northeast China. We determined the physicochemical properties of groundwater that contained total petroleum hydrocarbons (TPH) with a view to developing a scientifically robust strategy for controlling and remediating pollution of groundwater already contaminated with petroleum. Samples were collected at regular intervals and were analyzed for dissolved oxygen (DO), iron (Fe³⁺), sulfate (SO₄^2-), electrical conductivity (Eh), pH, hydrogen carbonate (HCO₃^-), and enzyme activities of catalase (CAT), peroxidase (HRP), catechol 1,2-dioxygenase (C12O), and catechol 2,3-dioxygenase (C23O). We used factor analysis in SPSS to determine the main environmental characteristics of the groundwater samples. The results confirmed that the study site was slightly contaminated and that TPH levels were decreasing slightly. Some of the physicochemical variables showed regular fluctuations; DO, Fe³⁺, and SO₄^2- contents decreased gradually, while the concentrations of one of the microbial degradation products, HCO₃^-, increased. Microorganism enzyme activities decreased gradually. The microbiological community deteriorated noticeably during the natural attenuation process, so microbiological degradation of pollutants receded gradually. The HCO₃^- content increased and the pH and Eh decreased gradually. The groundwater environment tended to be reducing.

Benzene-contaminated groundwater remediation using calcium peroxide nanoparticles: synthesis and process optimization

Nano-size calcium peroxide (nCaO₂) is an appropriate oxygen source which can meet the needs of in situ chemical oxidation (ISCO) for contaminant remediation from groundwater. In the present study, an easy to handle procedure for synthesis of CaO₂ nanoparticles has been investigated. Modeling and optimization of synthesis process was performed by application of response surface methodology (RSM) and central composite rotatable design (CCRD) method. Synthesized nanoparticles were characterized by XRD and FESEM techniques. The optimal synthesis conditions were found to be 5:1, 570 rpm and 10 °C for H₂O₂:CaSO₂ ratio, mixing rate and reaction temperature, respectively. Predicted values showed to be in good agreement with experimental results (R ² values were 0.915 and 0.965 for CaO₂ weight and nanoparticle size, respectively). To study the efficiency of synthesized nanoparticles for benzene removal from groundwater, batch experiments were applied in biotic and abiotic (chemical removal) conditions by 100, 200, 400, and 800 mg/L of nanoparticles within 70 days. Results indicated that application of 400 mg/L of CaO₂ in biotic condition was able to remediate benzene completely from groundwater after 60 days. Furthermore, comparison of biotic and abiotic experiments showed a great potential of microbial stimulation using CaO₂ nanoparticles in benzene remediation from groundwater.

A compact remediation system for the treatment of groundwater contaminated with BTEX and TPH

Gas stations constitute important point sources of soil and groundwater pollution. The leaking of hydrocarbons into the soil is a significant environment issue due to the wide-ranging occurrence of leaks and the high levels and toxicity of pollutants involved in the contamination of groundwater for human use. This study introduces a compact system developed to treat groundwater contaminated with benzene, toluene, ethyl benzene, and xylenes (BTEX) and total petroleum hydrocarbon (TPH) leaked from gas station tanks. The system comprises three units: (1) suction and volatilization of volatile organic compounds (VOCs), (2) aeration tank (to remove volatile organic substances), and (3) an adsorption packed-bed filter (activated carbon (AC) and rice husk ash, 50% each, to remove TPH). Contaminated groundwater was characterized in a pilot study and in the field. Levels of BTEX and of TPH decreased by 96% with an 8-h retention time. The results obtained show that the remediation system is highly efficient and yielded water that met the discharge standards defined in the Brazilian legislation, that is, maximum benzene, toluene, and xylene levels of 5, 170, and 300 μg/L, respectively.

Elucidating the fate of a mixed toluene, DHM, methanol, and i-propanol plume during in situ bioremediation

Organic pollutants such as solvents or petroleum products are widespread contaminants in soil and groundwater systems. In-situ bioremediation is a commonly used remediation technology to clean up the subsurface to eliminate the risks of toxic substances to reach potential receptors in surface waters or drinking water wells. This study discusses the development of a subsurface model to analyse the performance of an actively operating field-scale enhanced bioremediation scheme. The study site was affected by a mixed toluene, dihydromyrcenol (DHM), methanol, and i-propanol plume. A high-resolution, time-series of data was used to constrain the model development and calibration. The analysis shows that the observed failure of the treatment system is linked to an inefficient oxygen injection pattern. Moreover, the model simulations also suggest that additional contaminant spillages have occurred in 2012. Those additional spillages and their associated additional oxygen demand resulted in a significant increase in contaminant fluxes that remained untreated. The study emphasises the important role that reactive transport modelling can play in data analyses and for enhancing remediation efficiency.

A novel three-stage treatment train for the remediation of trichloroethylene-contaminated groundwater

The proposed treatment train removed TCE and its by-products effectively and there was no problem with the connection of chemical oxidation and anaerobic bioremediation in the novel treatment train technology.

Exploiting the intrinsic microbial degradative potential for field-based in situ dechlorination of trichloroethene contaminated groundwater

Bioremediation of trichloroethene (TCE) polluted groundwater is challenging, with limited next generation sequencing (NGS) derived information available on microbial community dynamics associated with dechlorination. Understanding these dynamics is important for designing and improving TCE bioremediation. In this study, biostimulation (BS), biostimulation-bioaugmentation (BS-BA) and monitored natural attenuation (MNA) approaches were applied to contaminated groundwater wells resulted in ≥ 95% dechlorination within 7 months. Vinyl chloride's final concentrations in stimulated wells were between 1.84 and 1.87 μg L(-1), below the US EPA limit of 2.0 μg L(-1), compared to MNA (4.3 μg L(-1)). Assessment of the groundwater microbial community with qPCR showed up to ∼ 50-fold increase in the classical dechlorinators' (Geobacter and Dehalococcoides sp.) population post-treatment. Metagenomic assays revealed shifts from Gammaproteobacteria (pre-treatment) to Epsilonproteobacteria and Deltaproteobacteria (post-treatment) only in stimulated wells. Although stimulated wells were functionally distinct from MNA wells post-treatment, substantial dechlorination in all the wells implied some measure of redundancy. This study, one of the few NGS-based field studies on TCE bioremediation, provides greater insights into dechlorinating microbial community dynamics which should be useful for future field-based studies.

Enhanced natural attenuation of BTEX in the nitrate-reducing environment by different electron acceptors

Enhancing natural attenuation of benzene, toluene, ethylbenzene, and xylene (BTEX) in groundwater is a potential remediation technology. This study focused on selecting appropriate electron acceptors to promote BTEX degradation in a nitrate-reducing environment. Nitrate-reducing soil was obtained from simulated BTEX-contaminated column. Enhancing experiments were conducted in the microcosm with nitrate-reducing material and simulated BTEX-polluted groundwater to investigate the promoting feasibility of adding dissolved oxygen (DO), nitrate, chelated Fe(III), and sulphate as electron acceptors. The concentrations of BTEX, electron acceptors, and their reducing products were measured. The order of promoting BTEX degradation with four electron acceptors was nitrate>sulphate>chelated Fe(III)>DO, and the first-order decay coefficients were 0.0432, 0.0333, 0.0240, and 0.0155, respectively. Nitrate, sulphate, and chelated Fe(III) enhanced attenuation. Nitrate was the most effective electron acceptor under nitrate-reducing conditions. Selecting proper electron acceptor is significant in promoting BTEX degradation according to the biogeochemical characteristics of local underground environment.

Effect of salinity on the bioremediation of petroleum hydrocarbons in a saline-alkaline soil

AIMS

 The aim of this paper is to check the effect of salinity on the bioremediation process of petroleum hydrocarbons in the saline-alkaline soil.

METHODS AND RESULTS

 In this study, soil salinity was adjusted to different levels by water leaching method and the bioremediation process was conducted for 28 days. Soil pH increased after leaching and decreased during bioremediation process. At initial time, moderate salinity enhanced the biodegradation and addition of microbial consortium was not effective in enhancing degradation rate of petroleum hydrocarbons. At day of 28 days, higher degradation rate was found in treatments with more leaching times with a maximum value of 42·36%. Dehydrogenase activity increased with the progress of bioremediation and positive correlation was found between dehydrogenase activity and degradation rate of petroleum hydrocarbons. Denaturing gradient gel electrophoresis analysis result showed decreased microbial community diversity with increased salt content.

CONCLUSIONS

 The result suggested that salinity had great impact on bioremediation, and leaching and addition of inoculated consortium were effective in enhancing biodegradation of petroleum hydrocarbons in the saline-alkaline soil.

SIGNIFICANCE AND IMPACT OF THE STUDY

 The result of this study is important for understanding the bioremediation process of petroleum in contaminated soil. New remediation method of petroleum contaminated soil can be developed based on this study.

Application of In Situ Chemical Oxidation for the Remediation of TPH-Contaminated Soils

Soils at many existing and former industrial areas and disposal sites are contaminated by petroleum hydrocarbons. In this study, laboratory bench-scale experiments were performed to evaluate the effectiveness of applying in situ chemical oxidation (ISCO) on the treatment of petroleum-hydrocarbon contaminated soils. Three different oxidation processes including Fenton’s oxidation, persulfate oxidation, and permanganate oxidation were evaluated with initial total petroleum hydrocarbon (TPH) concentration of approximately 3,920 mg/kg. The major control factors were oxidant species (hydrogen peroxide, persulfate, permanganate) and soil to liquid volume ratios (1 to 3). The oxidant concentration was 5 wt.%. Ferrous iron was used as the catalyst during the Fenton’s oxidation and persulfate oxidation processes, and the oxidant to ferrous iron molar ratio was 1 to 0.1. Among these three oxidation processes, contaminated soils treated by permanganate oxidation had the highest TPH removal efficiency (94% of TPH removal) during 360 min of operation. Approximately 75 and 61% of TPH removal was observed in batch experiments applying Fenton’s oxidation and persulfate oxidation, respectively. Due to the consumption of ferrous iron (used as the catalytic chemical) in the early stage during the operational period, both persulfate and Fenton’s oxidation processes had less TPH removal efficiencies. Frequent supplement of catalyst is required when persulfate and Fenton’s oxidation is applied for field application. Results from this study indicate that the ISCO scheme is a feasible technology for the treatment of petroleum-hydrocarbon contaminated soils within a short treatment period. The experimental results can be used for a scale-up system for practical application.

In situ oxidation of petroleum-hydrocarbon contaminated groundwater using passive ISCO system

Groundwater contamination by gasoline spill is a worldwide environmental problem. Gasoline contains methyl tertiary-butyl ether (MTBE) (a fuel oxygenates) and benzene, which are the chemicals of concerns among the gasoline components. In this study, an in situ chemical oxidation (ISCO) barrier system was developed to evaluate the feasibility of applying this passive system on the control of MTBE and benzene plume in aquifer. The developed ISCO barrier contained oxidant-releasing materials, which could release oxidants (e.g., persulfate) when contact with water for the contaminants' oxidation in groundwater. In this study, laboratory-scale fill-and-draw experiments were conducted to determine the component ratios of the oxidant-releasing materials and evaluate the persulfate release rates. Results indicate that the average persulfate-releasing rate of 7.26 mg S(2)O(8)(2-)/d/g was obtained when the mass ratio of sodium persulfate/cement/sand/water was 1/1.4/0.24/0.7. The column study was conducted to evaluate the efficiency of in situ application of the developed ISCO barrier system on MTBE and benzene oxidation. Results from the column study indicate that approximately 86-92% of MTBE and 95-99% of benzene could be removed during the early persulfate-releasing stage (before 48 pore volumes of groundwater pumping). The removal efficiencies for MTBE and benzene dropped to approximately 40-56% and 85-93%, respectively, during the latter part of the releasing period due to the decreased persulfate-releasing rate. Results reveal that acetone, byproduct of MTBE, was observed and then further oxidized completely. Results suggest that the addition of ferrous ion would activate the persulfate oxidation. However, excess ferrous ion would compete with organic contaminants for persulfate, and thus, cause the decrease in contaminant oxidation rates. The proposed treatment scheme would be expected to provide a more cost-effective alternative to remediate MTBE, benzene, and other petroleum-hydrocarbon contaminated aquifers. Results from this study will be useful in designing a scale-up system for field application.