In-Situ Remediation Approaches for the Management of Contaminated Sites: A Comprehensive Overview

Hybrid and enhanced electrokinetic system for soil remediation from heavy metals and organic matter

The electrokinetic (EK) process has been proposed for soil decontamination from heavy metals and organic matter. The advantages of the EK process include the low operating energy, suitability for fine-grained soil decontamination, and no need for excavation. During the last three decades, enhanced and hybrid EK systems were developed and tested for improving the efficiency of contaminants removal from soils. Chemically enhanced-EK processes exhibited excellent efficiency in removing contaminants by controlling the soil pH or the chemical reaction of contaminants. EK hybrid systems were tested to overcome environmental hurdles or technical drawbacks of decontamination technologies. Hybridization of the EK process with phytoremediation, bioremediation, or reactive filter media (RFM) improved the remediation process performance by capturing contaminants or facilitating biological agents' movement in the soil. Also, EK process coupling with solar energy was proposed to treat off-grid contaminated soils or reduce the EK energy requirements. This study reviews recent advancements in the enhancement and hybrid EK systems for soil remediation and the type of contaminants targeted by the process. The study also covered the impact of operating parameters, imperfect pollution separation, and differences in the physicochemical characteristics and microstructure of soil/sediment on the EK performance. Finally, a comparison between various remediation processes was presented to highlight the pros and cons of these technologies.

Large-scale comparative analysis reveals different bacterial community structures in full- and lab-scale wastewater treatment bioreactors

The activated sludge process is widely used for biological wastewater treatment due to its low cost and high efficiency. Although numerous lab-scale bioreactor experiments have been conducted to investigate the microorganism performance and mechanisms in activated sludge, understanding the bacterial community differences between full- and lab-scale bioreactors has remained elusive. In this study, we investigated the bacterial communities in 966 activated sludge samples obtained from various bioreactors, including both full- and lab-scale ones, from 95 previous studies. Our findings reveal significant differences in the bacterial communities between full- and lab-scale bioreactors, with thousands of bacterial genera exclusive to each scale. We also identified 12 genera that are frequently abundant in full-scale bioreactors but rarely observed in lab-scale reactors. By using a machine-learning method, organic matter and temperature were determined as the primary factors affecting microbial communities in full- and lab-scale bioreactors. Additionally, transient bacterial species from other environments may also contribute to the observed bacterial community differences. Furthermore, the bacterial community differences between full- and lab-scale bioreactors were verified by comparing the results of lab-scale bioreactor experiments to full-scale bioreactor sampling. Overall, this study sheds light on the bacteria overlooked in lab-scale studies and deepens our understanding of the differences in bacterial communities between full- and lab-scale bioreactors.

Engineering Pseudomonas putida To Produce Rhamnolipid Biosurfactants for Promoting Phenanthrene Biodegradation by a Two-Species Microbial Consortium

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic contaminants that pose a significant environmental hazard. Phenanthrene is one of the model compounds for the study of biodegradation of PAHs. However, the biodegradation of phenanthrene is often limited by its low water solubility and dissolution rate. To overcome this limitation, we engineered a strain of Pseudomonas putida to produce rhamnolipid biosurfactants and thereby promote phenanthrene biodegradation by an engineered strain of Escherichia coli constructed previously in our lab. The E. coli-P. putida two-species consortium exhibited a synergistic effect of these two distinct organisms in degrading phenanthrene, resulting in an increase from 61.15 to 73.86% of the degradation ratio of 100 mg/L phenanthrene within 7 days. After additional optimization of the degradation conditions, the phenanthrene degradation ratio was improved to 85.73%. IMPORTANCE Polycyclic aromatic hydrocarbons (PAHs), which are recalcitrant, carcinogenic, and tend to bioaccumulate, are widespread and persistent environmental pollutants. Based on these characteristics, the U.S. Environmental Protection Agency has listed PAHs as priority contaminants. Although there are many methods to treat PAH pollution, these methods are mostly limited by the poor water solubility of PAHs, which is especially true for the biodegradation process. Recent evidence of PAH-contaminated sites suffering from increasingly severe impact has emerged. As a result, the need to degrade PAHs is becoming urgent. The significance of our study lies in the development of nonpathogenic strains of biosurfactant-producing Pseudomonas aeruginosa for promoting the degradation of phenanthrene by engineered Escherichia coli.

Current status of pesticide effects on environment, human health and it's eco-friendly management as bioremediation: A comprehensive review

Pesticides are either natural or chemically synthesized compounds that are used to control a variety of pests. These chemical compounds are used in a variety of sectors like food, forestry, agriculture and aquaculture. Pesticides shows their toxicity into the living systems. The World Health Organization (WHO) categorizes them based on their detrimental effects, emphasizing the relevance of public health. The usage can be minimized to a least level by using them sparingly with a complete grasp of their categorization, which is beneficial to both human health and the environment. In this review, we have discussed pesticides with respect to their global scenarios, such as worldwide distribution and environmental impacts. Major literature focused on potential uses of pesticides, classification according to their properties and toxicity and their adverse effect on natural system (soil and aquatic), water, plants (growth, metabolism, genotypic and phenotypic changes and impact on plants defense system), human health (genetic alteration, cancer, allergies, and asthma), and preserve food products. We have also described eco-friendly management strategies for pesticides as a green solution, including bacterial degradation, myco-remediation, phytoremediation, and microalgae-based bioremediation. The microbes, using catabolic enzymes for degradation of pesticides and clean-up from the environment. This review shows the importance of finding potent microbes, novel genes, and biotechnological applications for pesticide waste management to create a sustainable environment.

An insight on microbial degradation of benzo[a]pyrene: current status and advances in research

Benzo[a]pyrene (BaP) is a high molecular weight polycyclic aromatic hydrocarbon produced as a result of incomplete combustion of organic substances. Over the years, the release of BaP in the atmosphere has increased rapidly, risking human lives. BaP can form bonds with DNA leading to the formation of DNA adducts thereby causing cancer. Therefore addressing the problem of its removal from the environment is quite pertinent though it calls for a very cumbersome and tedious process owing to its recalcitrant nature. To resolve such issues many efforts have been made to develop physical and chemical technologies of BaP degradation which have neither been cost-effective nor eco-friendly. Microbial degradation of BaP, on the other hand, has gained much attention due to added advantage of the high level of microbial diversity enabling great potential to degrade the substance without impairing environmental sustainability. Microorganisms produce enzymes like oxygenases, hydrolases and cytochrome P450 that enable BaP degradation. However, microbial degradation of BaP is restricted due to several factors related to its bio-availability and soil properties. Technologies like bio-augmentation and bio-stimulation have served to enhance the degradation rate of BaP. Besides, advanced technologies such as omics and nano-technology have opened new doors for a better future of microbial degradation of BaP and related compounds.

Artificial Consortium of Three E. coli BL21 Strains with Synergistic Functional Modules for Complete Phenanthrene Degradation

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic and persistent organic pollutions that can accumulate in the environment. In this study, an aromatic ring cleavage module, a salicylic acid synthesis module, and a catechol metabolism module were respectively constructed in three Escherichia coli BL21 strains. Subsequently, the engineered strains were cocultured as an artificial consortium for the biodegradation of phenanthrene, a typical PHA. Single factor experiments and response surface methodology were used to identify the optimal degradation conditions, including an inoculation interval of 6 h, inoculation ratio of 1:1:1, and IPTG concentration of 2 mM. Under these conditions, the 7-day degradation ratio of 100 mg/L phenanthrene reached 72.67%. Moreover, the engineered Escherichia coli BL21 strains showed good phenanthrene degradation ability at substrate concentrations 10 mg/L up to 500 mg/L. Enzyme activity assays combined with gas chromatography-mass spectrometry measurements confirmed that the three engineered strains behaved as a synergistic consortium in the phenanthrene degradation process. Based on the analysis of the key metabolites, the engineered bacteria were supplemented at 7-day intervals in batches so that each engineered strain maintained its optimal degradation ability. The 21-day degradation ratio finally reached 90.66%, which was much higher than what was observed with simultaneous inoculation. These findings suggest that the three engineered strains with separate modules constructed in this study offer an attractive solution for removing PAHs from the environment.

Risk factors and assessment strategies for the evaluation of human or environmental risk from metal(loid)s - A focus on Ireland

Elevated human exposure to metals and metalloids (metal(loid)s) may lead to acute sickness and pose a severe threat to human health. The human body is exposed to metal(loid)s principally through food, water, supplements, and (occasionally) air. There are inherent background levels of many metal(loid)s in regional soils as a consequence of geological sources. Baseline levels coupled with anthropogenic sources such as regional application of biosolids may lead to increased levels of certain metal(loid)s in soil, leading to potential transfer to water sources and potential uptake by plants. The latter could potentially transfer into the feed-to-food chain, viz. grazing animals, and bio-transfer to food products resulting in human exposure. This study addresses health concerns due to excessive intake of metal(loid)s by conducting a traditional review of peer-reviewed journals between 2015 and 2019, secondary references and relevant websites. The review identified the most researched metal(loid)s as Cu, Zn, Pb, Cd, Ni, Cr, As, Hg, Mn, Fe in the environment. The potential uptake of metal(loid)s by plants (phytoavailability) is a function of the mobility/retainability of metal(loid)s in the soil, influenced by soil geochemistry. The most critical parameters (including soil pH, soil organic matter, clay content, cation exchange capacity, the capability of decomposition of organic matter by microbes, redox potential, ionic strength) influencing metal(loid)s in soil are reviewed and used as a foundation to build a framework model for ranking metal(loid)s of concern. A robust quantitative risk assessment model is recommended for evaluating risk from individual metal(loid)s based on health-based indices (Daily Dietary Index (DDI), No Observed Adverse Effect Level (NOAEL), and Lowest Observed Adverse Effect Level (LOAEL)). This research proposes a risk assessment framework for potentially harmful metal(loid)s in the environment and highlights where regulation and intervention may be required.

Isolation, characterization, and evaluation of a high-siderophore-yielding bacterium from heavy metal-contaminated soil

Heavy metal-resistant siderophore-producing bacteria (SPB) with plant growth-promoting traits can assist in phytoremediation of heavy metal-contaminated soil. We isolated siderophore-producing bacteria from Pb and Zn mine soil in Shangyu, Zhejiang, China. The isolate with the highest siderophore production, strain SX9, was identified as Burkholderia sp. Burkholderia sp. SX9 produced catecholate-type siderophore, with the highest production at a pH range of 6.0 to 8.0, a temperature range of 20 to 30 °C and NaCl concentration below 2%. Siderophore production was highest without Fe³⁺ and became gradually lower with increasing Fe³⁺ concentration. Minimal inhibitory concentrations (MIC) of Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ were 4000, 22000, 5000, and 2000 μmol L^-1, respectively. The strain had a strong metal solubilization ability: the contents of Cu²⁺, Zn²⁺, and Cd²⁺ in the supernatant were 47.4%, 133.0%, and 35.4% higher, respectively, in strain SX9-inoculated cultures than in the not inoculated controls. The siderophore produced by strain SX9 could combine with Fe³⁺, Zn²⁺, and Cd²⁺ with good effectiveness. The plant growth-promoting traits of the strain included indole acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, and phosphate solubilization capability. Compared to the uninoculated growth medium and SX9 culture supernatant, the germination rate of Lolium perenne seeds was higher when inoculated with strain SX9 culture. In the experiment of seed germination, adding bacterial culture or supernatant could alleviate the toxicity of heavy metals to L. perenne seed germination. Under Cu²⁺ and Zn²⁺ stress, strain SX9 promoted the germination rate. Taken together, Burkholderia sp. SX9 had properties beneficial in the microbial enhancement of phytoremediation of soil contaminated with heavy metals.

A risk management framework for Gentle Remediation Options (GRO)

Gentle Remediation Options (GRO) are remediation measures involving plants, fungi, bacteria, and soil amendments that can be applied to manage risks at contaminated sites. Several studies and decision-support tools promote the wider range of benefits provided by GRO, but there is still skepticism regarding GRO implementation. Key issues that need to be better communicated are the various risk mitigation mechanisms, the required risk reduction for an envisioned land use, and the time perspective associated with the risk mitigation mechanisms. To increase the viability and acceptance of GRO, the phytomanagement approach implies the combination of GRO with beneficial green land use, gradually reducing risks and restoring ecosystem services. To strengthen the decision basis for GRO implementation in practice, this paper proposes a framework for risk management and communication of GRO applications to support phytomanagement strategies at contaminated sites. The mapping of the risk mitigation mechanisms is done by an extensive literature review and the Swedish national soil guideline value model is used to derive the most relevant human health exposure pathways and ecological risks for generic green land use scenarios. Results indicate that most of the expected risk mitigation mechanisms are supported by literature, but that knowledge gaps still exist. The framework is demonstrated to support the identification of GRO options for the case study site given two envisioned land uses: biofuel park and allotment garden. A more easily understandable risk management framework, as proposed here, is expected to act as a communication tool to educate decision-makers, regulatory bodies and other stakeholders for better understanding of risk mitigation mechanisms and preliminary timeframes of various GRO, particularly in the early stages of a brownfield redevelopment project.

Bibliometric Analysis of Hydrocarbon Bioremediation in Cold Regions and a Review on Enhanced Soil Bioremediation

Simple Summary

Anthropogenic activities in cold regions require petroleum oils to support various purposes. With the increased demand of petroleum, accidental oil spills are generated during transportation or refuelling processes. Soil is one of the major victims in petroleum pollution, hence studies have been devoted to find solutions to remove these petroleum hydrocarbons. However, the remote and low-temperature conditions in cold regions hindered the implementation of physical and chemical removal treatments. On the other hand, biological treatments in general have been proposed as an innovative approach to attenuate these hydrocarbon pollutants in soils. To understand the relevancy of biological treatments for cold regions specifically, bibliometric analysis has been applied to systematically analyse studies focused on hydrocarbon removal treatment in a biological way. To expedite the understanding of this analysis, we have summarised these biological treatments and suggested other biological applications in the context of cold conditions.

Abstract

The increased usage of petroleum oils in cold regions has led to widespread oil pollutants in soils. The harsh environmental conditions in cold environments allow the persistence of these oil pollutants in soils for more than 20 years, raising adverse threats to the ecosystem. Microbial bioremediation was proposed and employed as a cost-effective tool to remediate petroleum hydrocarbons present in soils without significantly posing harmful side effects. However, the conventional hydrocarbon bioremediation requires a longer time to achieve the clean-up standard due to various environmental factors in cold regions. Recent biotechnological improvements using biostimulation and/or bioaugmentation strategies are reported and implemented to enhance the hydrocarbon removal efficiency under cold conditions. Thus, this review focuses on the enhanced bioremediation for hydrocarbon-polluted soils in cold regions, highlighting in situ and ex situ approaches and few potential enhancements via the exploitation of molecular and microbial technology in response to the cold condition. The bibliometric analysis of the hydrocarbon bioremediation research in cold regions is also presented.

Biotechnology and nanotechnology for remediation of chlorinated volatile organic compounds: current perspectives

Chlorinated volatile organic compounds (CVOCs) are persistent organic pollutants which are harmful to public health and the environment. Many CVOCs occur in substantial quantities in groundwater and soil, even though their use has been more carefully managed and restricted in recent years. This review summarizes recent data on several innovative treatment solutions for CVOC-affected media including bioremediation, phytoremediation, nanoscale zero-valent iron (nZVI)-based reductive dehalogenation, and photooxidation. There is no optimally developed single technology; therefore, the possibility of using combined technologies for CVOC remediation, for example bioremediation integrated with reduction by nZVI, is presented. Some methods are still in the development stage. Advantages and disadvantages of each treatment strategy are provided. It is hoped that this paper can provide a basic framework for selection of successful CVOC remediation strategies.

Intensification of Ex Situ Bioremediation of Soils Polluted with Used Lubricant Oils: A Comparison of Biostimulation and Bioaugmentation with a Special Focus on the Type and Size of the Inoculum

Used lubricant oils (ULOs) strongly bind to soil particles and cause persistent pollution. In this study, soil microcosm experiments were conducted to model the ex situ bioremediation of a long term ULO-polluted area. Biostimulation and various inoculation levels of bioaugmentation were applied to determine the efficacy of total petrol hydrocarbon (TPH) removal. ULO-contaminated soil microcosms were monitored for microbial respiration, colony-forming units (CFUs) and TPH bioconversion. Biostimulation with inorganic nutrients was responsible for 22% of ULO removal after 40 days. Bioaugmentation using two hydrocarbon-degrader strains: Rhodococcus quingshengii KAG C and Rhodococcus erythropolis PR4 at a small inoculum size (107 CFUs g-1 soil), reduced initial TPH concentration by 24% and 29%, respectively; the application of a higher inoculum size (109 CFUs g-1 soil) led to 41% and 32% bioconversion, respectively. After 20 days, all augmented CFUs decreased to the same level as measured in the biostimulated cases, substantiating the challenge for the newly introduced hydrocarbon-degrading strains to cope with environmental stressors. Our results not only highlight that an increased number of degrader cells does not always correlate with enhanced TPH bioconversion, but they also indicate that biostimulation might be an economical solution to promote ULO biodegradation in long term contaminated soils.

Spent lubricant oil-contaminated soil toxicity to Eisenia andrei before and after bioremediation

Bioremediation is very efficient in biodegrading petroleum hydrocarbons. However, the decrease in these target contaminants in soils is not necessarily followed by a decrease in toxicity. The remaining contaminants can be enough to retain toxicity, while incomplete degradation of several compounds can generate sub-products, which can be even more toxic. In this context, the aim of this study was to assess acute and chronic toxicity in Eisenia andrei exposed to soil contaminated with 5% spent lubricant oil before and after 22 months of bioremediation in 150 L aerobic reactors. Applied bioremediation strategies were biostimulation (BIOS), bioaugmentation by adding mature compost from municipal solid waste (BIOA₁) and bioaugmentation by adding non-mature compost from municipal solid waste (BIOA₂). After 22 months, total petroleum hydrocarbons (TPH) were reduced 71% in BIOS and 73% in both BIOA₁ and BIOA₂. Polycyclic aromatic hydrocarbons (PAH) were reduced in about 98% in all treatments (BIOS, BIOA₁ and BIOA₂). At the 14^th day of exposure, mortality rates were 7 ± 2, 20 ± 0, 75 ± 25, 93 ± 12 and 100 ± 0% for Eisenia andrei exposed to CONT (soil with no oil addition), BIOS, OLU (soil newly contaminated with 5% spent oil), BIOA₁ and BIOA₂, respectively. After 14 days, surviving specimens in both BIOS and OLU soils exhibited anatomic deformations, less biomass than the controls, and decrease in juvenile forms and coelomocytes. After 28 days, the mortality rate for BIOS and OLU soils increased to 97 and 100%, respectively. Therefore, even with a reduction of 71-73% for TPH and 98% for PAH, toxic effects remained in all soils bioremediated, probably due to the remaining hydrocarbons and/or hydrocarbon biodegradation products. The results indicate that both chemical analyses and toxicological monitoring are required to follow-up soil remediation progress.

Enhanced degradation of polycyclic aromatic hydrocarbons by indigenous microbes combined with chemical oxidation

In this study, the removal efficiency PAHs by chemical oxidation combined with microbe remediation was evaluated in two contaminated soils. The number of indigenous soil microbes decreased after the addition of chemical oxidants and then increased by nutrients addition. The total removal efficiencies of PAHs by chemical oxidation and nutrient addition followed the order: activated persulfate > potassium permanganate > modified Fenton reagent > Fenton reagent. There are 24.29-27.97%, 22.00-23.67%, 10.24-13.74% and 1.9-2.5% contributions separately due to nutrient treatment in Fenton, modified Fenton, activated persulfate and potassium permanganate treatment, which show significantly difference. The different chemical oxidants exhibited 78-90% removal efficiency for 5-6 rings PAHs, while 52-85% removal efficiency for 2-4 rings PAHs. With the addition of nutrients, the growth of indigenous microbes was enhanced significantly, and the contents of 2-4 rings PAHs in the soil were further decreased. Furthermore, the removal efficiencies of NAP and ANY were increased by more than 45%, while the removal efficiencies of ANE, FLE and PHE were about 30% at Fenton system. There was a complementary enhancing effect of microbial remediation for PAHs degradation after chemical oxidation.

Implications of Bioremediation of Polycyclic Aromatic Hydrocarbon-Contaminated Soils for Human Health and Cancer Risk

Bioremediation uses soil microorganisms to degrade polycyclic aromatic hydrocarbons (PAHs) into less toxic compounds and can be performed in situ, without the need for expensive infrastructure or amendments. This review provides insights into the cancer risks associated with PAH-contaminated soils and places bioremediation outcomes in a context relevant to human health. We evaluated which bioremediation strategies were most effective for degrading PAHs and estimated the cancer risks associated with PAH-contaminated soils. Cancer risk was statistically reduced in 89% of treated soils following bioremediation, with a mean degradation of 44% across the B2 group PAHs. However, all 180 treated soils had postbioremediation cancer risk values that exceeded the U.S. Environmental Protection Agency (USEPA) health-based acceptable risk level (by at least a factor of 2), with 32% of treated soils exceeding recommended levels by greater than 2 orders of magnitude. Composting treatments were most effective at biodegrading PAHs in soils (70% average reduction compared with 28-53% for the other treatment types), which was likely due to the combined influence of the rich source of nutrients and microflora introduced with organic compost amendments. Ultimately, bioremediation strategies, in the studies reviewed, were unable to successfully remove carcinogenic PAHs from contaminated soils to concentrations below the target cancer risk levels recommended by the USEPA.

Soil and brownfield bioremediation

Soil contamination with petroleum hydrocarbons, persistent organic pollutants, halogenated organic chemicals and toxic metal(loid)s is a serious global problem affecting the human and ecological health. Over the past half‐century, the technological and industrial advancements have led to the creation of a large number of brownfields, most of these located in the centre of dense cities all over the world. Restoring these sites and regeneration of urban areas in a sustainable way for beneficial uses is a key priority for all industrialized nations. Bioremediation is considered a safe economical, efficient and sustainable technology for restoring the contaminated sites. This brief review presents an overview of bioremediation technologies in the context of sustainability, their applications and limitations in the reclamation of contaminated sites with an emphasis on brownfields. Also, the use of integrated approaches using the combination of chemical oxidation and bioremediation for persistent organic pollutants is discussed.