Ex-Situ Remediation Technologies for Environmental Pollutants: A Critical Perspective

Application of microbial agents in organic solid waste composting: a review

The rapid growth of organic solid waste has recently exacerbated environmental pollution problems, and its improper treatment has led to the loss of a large number of biomass resources. Here, we expound the advantages of microbial agents composting compared with conventional organic solid waste treatment technology, and review the important role of microbial agents composting in organic solid waste composting from the aspects of screening and identification, optimization of conditions, mechanism of action, combination with other technologies and ultra-high-temperature and ultra-low-temperature microbial composting. We discuss the value of microorganisms with different growth conditions in organic solid waste composting, and put forward a seasonal multi-temperature composite microbial composting technology. Provide new ideas for the all-round treatment of microbial agents in organic solid waste in the future. © 2024 Society of Chemical Industry.

Diverting organic waste from landfills via insect biomanufacturing using engineered black soldier flies (Hermetia illucens)

A major roadblock towards the realisation of a circular economy are the lack of high-value products that can be generated from waste. Black soldier flies (BSF; Hermetia illucens) are gaining traction for their ability to rapidly consume large quantities of organic wastes. However, these are primarily used to produce a small variety of products, such as animal feed ingredients and fertiliser. Using synthetic biology, BSF could be developed into a novel sustainable biomanufacturing platform to valorise a broader variety of organic waste feedstocks into enhanced animal feeds, a large variety of high-value biomolecules including industrial enzymes and lipids, and improved fertiliser.

The role and mechanisms of microbes in dichlorodiphenyltrichloroethane (DDT) and its residues bioremediation

Environmental contamination with dichlorodiphenyltrichloroethane (DDT) has sever effects on the ecosystem worldwide. DDT is a recalcitrant synthetic chemical with high toxicity and lipophilicity. It is also bioaccumulated in the food chain and causes genotoxic, estrogenic, carcinogenic, and mutagenic effects on aquatic organisms and humans. Microbial remediation mechanism and its enzymes are very important for removing DDT from environment. DDT and its main residues dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD) can biodegrade slowly in soil and water. To enhance this process, a number of strategies are proposed, such as bio-attenuation, biostimulation, bioaugmentation and the manipulation of environmental conditions to enhance the activity of microbial enzymes. The addition of organic matter and flooding of the soil enhance DDT degradation. Microbial candidates for DDT remediation include micro-algae, fungi and bacteria. This review provide brief information and recommendation on microbial DDT remediation and its mechanisms.

Cost reduction strategies in the remediation of petroleum hydrocarbon contaminated soil

Petroleum hydrocarbon spill on land pollutes soil and reduces its ecosystem. Hydrocarbon transport in the soil is aided by several biological, physical, and chemical processes. However, pore characteristics play a major role in the distribution within the soil matrix. Restoring land use after spills necessitates remediation using cost-effective technologies. Several remediation technologies have been demonstrated at different scales, and research is ongoing to improve their performances towards the reduction of treatment costs. The process of removing the contaminants in the soil is through one or a combination of containment, separation, and degradation methods under the influence of biological, physical, chemical, and electrically-dominated processes. Generally, performance improvement is achieved through the introduction of products/materials and/or energy. Nevertheless, the technologies can be categorized based on effectiveness period as short, medium, and long term. The treatment cost of short, medium, and long-term technologies are usually in the range of $39 - 331/t (/tonne), $22 - 131/t, and $8 - 131/t, respectively. However, the total cost depends on other factors such as site location, capital cost, and permitting. This review compiles cost-saving strategies reported for different techniques used in remediating petroleum hydrocarbon polluted soil. We discuss the principles of contaminant removal, performance enhancing methods, and the cost-effectiveness analysis of selected technologies.

Effects of Salinity on the Biodegradation of Polycyclic Aromatic Hydrocarbons in Oilfield Soils Emphasizing Degradation Genes and Soil Enzymes

The biodegradation of organic pollutants is the main pathway for the natural dissipation and anthropogenic remediation of polycyclic aromatic hydrocarbons (PAHs) in the environment. However, in the saline soils, the PAH biodegradation could be influenced by soil salts through altering the structures of microbial communities and physiological metabolism of degradation bacteria. In the worldwide, soils from oilfields are commonly threated by both soil salinity and PAH contamination, while the influence mechanism of soil salinity on PAH biodegradation were still unclear, especially the shifts of degradation genes and soil enzyme activities. In order to explain the responses of soils and bacterial communities, analysis was conducted including soil properties, structures of bacterial community, PAH degradation genes and soil enzyme activities during a biodegradation process of PAHs in oilfield soils. The results showed that, though low soil salinity (1% NaCl, w/w) could slightly increase PAH degradation rate, the biodegradation in high salt condition (3% NaCl, w/w) were restrained significantly. The higher the soil salinity, the lower the bacterial community diversity, copy number of degradation gene and soil enzyme activity, which could be the reason for reductions of degradation rates in saline soils. Analysis of bacterial community structure showed that, the additions of NaCl increase the abundance of salt-tolerant and halophilic genera, especially in high salt treatments where the halophilic genera dominant, such as Acinetobacter and Halomonas. Picrust2 and redundancy analysis (RDA) both revealed suppression of PAH degradation genes by soil salts, which meant the decrease of degradation microbes and should be the primary cause of reduction of PAH removal. The soil enzyme activities could be indicators for microorganisms when they are facing adverse environmental conditions.

Soil Characteristics Constrain the Response of Microbial Communities and Associated Hydrocarbon Degradation Genes during Phytoremediation

Rhizodegradation is a promising cleanup technology where microorganisms degrade soil contaminants in the rhizosphere. A symbiotic relationship is expected to occur between plant roots and soil microorganisms in contaminated soils that enhances natural microbial degradation. However, little is known about how different initial microbiotas influence the rhizodegradation outcome. Recent studies have hinted that soil initial diversity has a determining effect on the outcome of contaminant degradation. To test this, we either planted (P) or not (NP) balsam poplars (Populus balsamifera) in two soils of contrasting diversity (agricultural and forest) that were contaminated or not with 50 mg kg^-1 of phenanthrene (PHE). The DNA from the rhizosphere of the P and the bulk soil of the NP pots was extracted and the bacterial genes encoding the 16S rRNA, the PAH ring-hydroxylating dioxygenase alpha subunits (PAH-RHDα) of Gram-positive and Gram-negative bacteria, and the fungal ITS region were sequenced to characterize the microbial communities. The abundances of the PAH-RHDα genes were quantified by real-time quantitative PCR. Plant presence had a significant effect on PHE degradation only in the forest soil, whereas both NP and P agricultural soils degraded the same amount of PHE. Fungal communities were mainly affected by plant presence, whereas bacterial communities were principally affected by the soil type, and upon contamination the dominant PAH-degrading community was similarly constrained by soil type. Our results highlight the crucial importance of soil microbial and physicochemical characteristics in the outcome of rhizoremediation.IMPORTANCE Polycyclic aromatic hydrocarbons (PAH) are a group of organic contaminants that pose a risk to ecosystems' health. Phytoremediation is a promising biotechnology with the potential to restore PAH-contaminated soils. However, some limitations prevent it from becoming the remediation technology of reference, despite being environmentally friendlier than mainstream physicochemical alternatives. Recent reports suggest that the original soil microbial diversity is the key to harnessing the potential of phytoremediation. Therefore, this study focused on determining the effect of two different soil types in the fate of phenanthrene (a polycyclic aromatic hydrocarbon) under balsam poplar remediation. Poplar increased the degradation of phenanthrene in forest, but not in agricultural soil. The fungi were affected by poplars, whereas total bacteria and specific PAH-degrading bacteria were constrained by soil type, leading to different degradation patterns between soils. These results highlight the importance of performing preliminary microbiological studies of contaminated soils to determine whether plant presence could improve remediation rates or not.

Recent Developments in Polymer Nanocomposite-Based Electrochemical Sensors for Detecting Environmental Pollutants

The human population is generally subjected to diverse pollutants and contaminants in the environment like those in the air, soil, foodstuffs, and drinking water. Therefore, the development of novel purification techniques and efficient detection devices for pollutants is an important challenge. To date, experts in the field have designed distinctive analytical procedures for the detection of pollutants including gas chromatography/mass spectrometry and atomic absorption spectroscopy. While the mentioned procedures enjoy high sensitivity, they suffer from being laborious, expensive, require advanced skills for operation, and are inconvenient to deploy as a result of their massive size. Therefore, in response to the above-mentioned limitations, electrochemical sensors are being developed that enjoy robustness, selectivity, sensitivity, and real-time measurements. Considerable advancements in nanomaterials-based electrochemical sensor platforms have helped to generate new technologies to ensure environmental and human safety. Recently, investigators have expanded considerable effort to utilize polymer nanocomposites for building the electrochemical sensors in view of their promising features such as very good electrocatalytic activities, higher electrical conductivity, and effective surface area in comparison to the traditional polymers. Herein, the first section of this review briefly discusses the most important methods for polymer nanocomposites synthesis, such as in situ polymerization, direct mixing of polymer and nanofillers (melt-mixing and solution-mixing), sol-gel, and electrochemical methods. It then summarizes the current utilization of polymer nanocomposites for the preparation of electrochemical sensors as a novel approach for monitoring and detecting environmental pollutants which include heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, nitrite, and hydrazine in different mediums. Finally, the current challenges and future directions for the polymer nanocomposites-based electrochemical sensing of environmental pollutants are outlined.

An analysis of the versatility and effectiveness of composts for sequestering heavy metal ions, dyes and xenobiotics from soils and aqueous milieus

The persistence and bioaccumulation of environmental pollutants in water bodies, soils and living tissues remain alarmingly related to environmental protection and ecosystem restoration. Adsorption-based techniques appear highly competent in sequestering several environmental pollutants. In this review, the recent research findings reported on the assessments of composts and compost-amended soils as adsorbents of heavy metal ions, dye molecules and xenobiotics have been appraised. This review demonstrates clearly the high adsorption capacities of composts for umpteen environmental pollutants at the lab-scale. The main inferences from this review are that utilization of composts for the removal of heavy metal ions, dye molecules and xenobiotics from aqueous environments and soils is particularly worthwhile and efficient at the laboratory scale, and the adsorption behaviors and effectiveness of compost-type adsorbents for agrochemicals (e.g. herbicides and insecticides) vary considerably because of variabilities in structure, topology, bond connectivity, distribution of functional groups and interactions of xenobiotics with the active humic substances in composts. Compost-based field-scale remediation of environmental pollutants is still sparse and arguably much challenging to implement if, furthermore, real-world soil and water contamination issues are to be addressed effectively. Hence, significant research and process development efforts should be promptly geared and intensified in this direction by extrapolating the lab-scale findings in a cost-effective manner.

Comparing performances, costs and energy balance of ex situ remediation processes for PAH-contaminated marine sediments

This study proposes a comparison of different ex situ technologies aimed at the removal of polycyclic aromatic hydrocarbons from marine sediments in terms of performances, costs and energy balance. In accordance with the principles of water-energy nexus, anaerobic bioremediation, soil washing and thermal desorption were investigated under low liquid phase and temperature conditions using phenanthrene (PHE) as model compound. After 42 days of anaerobic bioremediation, the highest PHE biodegradation of 68 and 64% was observed under denitrifying and methanogenic conditions, respectively, accompanied by N₂ and CH₄ production and volatile fatty acid accumulation. During soil washing, more than 97% of PHE was removed after 60 min using a solid-to-liquid ratio of 1:3. Along the same treatment time, low-temperature thermal desorption (LTTD) allowed a PHE removal of 88% at 200 °C. The economic analysis indicated that LTTD resulted in a higher cost (i.e. 1782 € m^-3) than bioremediation and soil washing (228 and 371 € m^-3, respectively). The energy balance also suggested that bioremediation and soil washing are more sustainable technologies as a lower required energy (i.e. 16 and 14 kWh m^-3, respectively) than LTTD (i.e. 417 kWh m^-3) is needed.

Evaluation of soil treatment techniques on remediated brine spill sites in semi-arid rangelands

Unconventional oil and gas development (UOG) generates high volumes of flowback and produced water, byproducts of hydraulic fracturing operations, that are often released or spilled on the soil surface. Soil contamination with these wastewaters, commonly referred to as brine, has the potential to inhibit vegetation growth indefinitely. Natural attenuation of brine is not expedient in arid and semi-arid regions where most United States UOG developments are located, including the Bakken region of North Dakota. In situ (at-site) and ex situ (off-site) soil treatment techniques are commonly employed to remediate brine-contaminated soils in the Bakken. However, little is known regarding each technique's efficacy despite differences in application, cost, and efficiency. We selected 10 sites previously remediated with chemical amendments (in situ) and 11 sites with topsoil excavation (ex situ) in the United States Forest Service Little Missouri National Grasslands. We paired each remediated site with a reference to examine the ability of each strategy to return brine-contaminated sites to conditions reflective of the current state of the surrounding semi-arid rangeland ecosystem. At each site, we quantified soil electrical conductivity (EC_e) as an indicator of soil salinity and measured vegetation cover, biomass production, bare ground, and litter. The difference between paired reference and remediated sites was used for analysis. Brine contamination was still evident as soil EC_e was similarly increased on chemical amendment and topsoil excavation remediated sites over paired references at all soil depths tested. Due to the nature of the topsoil excavation treatment, elevated EC_e in the 0-15 cm depth suggested resalinization of the new topsoil. Remediation techniques also resulted in similar plant community composition marked by an increase in exotic forb biomass, largely due to the invasion of kochia (Bassia scoparia) which was absent from reference sites. However, remediation techniques differed substantially in vegetation establishment. We found 15% more bare ground on sites remediated with chemical amendment treatment than paired references and 55% more with topsoil excavation. Our results indicate that in situ strategies may be more suitable than ex situ strategies for brine-spill remediation in semi-arid rangelands like the Bakken in North Dakota as they cause less disturbance and likely require less post-remediation management to establish adequate vegetation cover to protect the soil from further erosion.

Animal fat and glycerol bioconversion to polyhydroxyalkanoate by produced water bacteria

Oil reservoirs contain large amounts of hydrocarbon rich produced water, trapped in underground channels. Focus of this study was isolation of PHA producers from produced water concomitant with optimization of production using animal fat and glycerol as carbon source. Bacterial strains were identified as Bacillus subtilis (PWA), Pseudomonas aeruginosa (PWC), Bacillus tequilensis (PWF), and Bacillus safensis (PWG) based on 16S rRNA gene sequencing. Similar amounts of PHA were obtained using animal fat and glycerol in comparison to glucose. After 24 h, high PHA production on glycerol and animal fat was shown by strain PWC (5.2 g/ L, 6.9 g/ L) and strain PWF (12.4 g/ L, 14.2 g/ L) among all test strains. FTIR analysis of PHA showed 3-hydroxybutyrate units. The capability to produce PHA in the strains was corroborated by PhaC synthase gene sequencing. Focus of future studies can be the use of lipids and glycerol on industrial scale.

Remediation of Crude Oil-Polluted Soil by the Bacterial Rhizosphere Community of Suaeda salsa Revealed by 16S rRNA Genes

Crude oil pollution of soil is a serious environmental issue, and bioremediation using plants and microorganisms is a natural and sustainable method for its restoration. Pot incubation of a two-factor randomized block (plants with two levels, and crude oil with three levels) was designed to investigate the rhizosphere bacterial community of Suaeda salsa (L.) Pall. Crude oil contamination of soil was studied at different levels: 2 g/kg (low), 4 g/kg (medium), and 6 g/kg (high) levels. In this study, the physicochemical properties of the collected rhizosphere soil were analyzed. Moreover, the soil bacteria were further identified using the 16S rRNA gene. The effects of S. salsa and crude oil and their interaction on the physiochemical properties of the soil and crude oil degradation were found to be significant. Crude oil significantly influenced the diversity and evenness of bacteria, while the effects of S. salsa and interaction with crude oil were not significant. Proteobacteria were found to be dominant at the phylum level. Meanwhile, at the genera level, Saccharibacteria and Alcanivorax increased significantly in the low and medium contamination treatment groups with S. salsa, whereas Saccharibacteria and Desulfuromonas were prevalent in the high contamination treatment group. High crude oil contamination led to a significant decrease in the bacterial diversity in soil, while the effects of S. salsa and its interaction were not significant. Despite the highest abundance of crude oil degradation bacteria, S. salsa reduced crude oil degradation bacteria and increased bacteria related to sulfur, phosphorus, and nitrogen cycling in the low and high contamination group, whereas the opposite effect was observed for the medium contamination treatment group. The abundance of most crude oil degradation bacteria is negatively correlated with crude oil content. Nitrogen cycling bacteria are sensitive to the total nitrogen, total phosphorus, ammonia nitrogen, and nitrate nitrogen, and pH of the soil. Sulfur cycling bacteria are sensitive to aromatic hydrocarbons, saturated hydrocarbons, and asphaltene in soil. This research is helpful for further studying the mechanism of synergistic degradation by S. salsa and bacteria.

Advanced Oxidation Based Treatment of Soil Wash Water Contaminated with Sulfolane

This study investigates advanced oxidation processes (AOPs) as post-treatment techniques to degrade sulfolane in soil washing water. Soil washing experiments were conducted with different soil/water ratios, shaking times, and number of extraction cycles. The soil wash water containing sulfolane was treated with four AOPs including H2O2/ultraviolet (UV), O3/UV, alkaline ozonation, and neutral Fenton reagent. Results show that sulfolane can be effectively removed from the soil using water as a washing solvent, where optimum conditions were found with 30 min of vigorous shaking, using 1:2 soil/water mass ratio, and a three-cycle extraction procedure. Moreover, the sulfolane in the soil wash water was also effectively degraded using appropriate AOPs. Among the four AOPs investigated, neutral Fenton was the least effective method to treat sulfolane in the wash water, while H2O2/UV, O3/UV, and alkaline ozonation can achieve more than 99% of sulfolane degradation within 1 h.

Enhancement of gasworks groundwater remediation by coupling a bio-electrochemical and activated carbon system

Here, we show the electrical response, bacterial community, and remediation of hydrocarbon-contaminated groundwater from a gasworks site using a graphite-chambered bio-electrochemical system (BES) that utilizes granular activated carbon (GAC) as both sorption agent and high surface area anode. Our innovative concept is the design of a graphite electrode chamber system rather than a classic non-conductive BES chamber coupled with GAC as part of the BES. The GAC BES is a good candidate as a sustainable remediation technology that provides improved degradation over GAC, and near real-time observation of associated electrical output. The BES chambers were effectively colonized by the bacterial communities from the contaminated groundwater. Principal coordinate analysis (PCoA) of UniFrac Observed Taxonomic Units shows distinct grouping of microbial types that are associated with the presence of GAC, and grouping of microbial types associated with electroactivity. Bacterial community analysis showed that β-proteobacteria (particularly the PAH-degrading Pseudomonadaceae) dominate all the samples. Rhodocyclaceae- and Comamonadaceae-related OTU were observed to increase in BES cells. The GAC BES (99% removal) outperformed the control graphite GAC chamber, as well as a graphite BES and a control chamber both filled with glass beads.

Population characteristics and influential factors of nitrogen cycling functional genes in heavy metal contaminated soil remediated by biochar and compost

Sixteen treatments of soil contaminated by Cu, Pb, and Zn by the addition of a different percentage of biochar and compost were incubated for 120 days. The abundance of denitrifying genes such as narG, nirK, nirS and nosZ and the ammonia-oxidizing amoA genes of ammonia-oxidizing archaea/bacteria (AOA/AOB), soil nitrite reductase activity (S-NiR) and their shaping factors were also determined. The relationships between functional genes, S-NiR, and physico-chemical parameters were analyzed using the Pearson correlation method. The study found that the changes in physico-chemical parameters, including water-soluble organic carbon (WSC), nitrate (NO₃^-) and ammonium (NH₄⁺), were predominant in different treatments. The abundance of nirK and narG genes is most sensitive to the changes in the properties of the soil sample. Bacterial 16S rDNA gene abundance was significantly affected by NO₃^- and S-NiR (P < 0.05). Nitrifying genes were mainly correlated to WSC and S-NiR, while denitrifying genes were associated with pH, electrical conductivity, NO₃^- and S-NiR. The systematic study for the relationship between the genes and the environmental parameters will help us to deep understand the biological mechanisms of nitrogen cycle in heavy metal contaminated soils remediated by biochar and compost.

Treatment technologies for PAH-contaminated sites: a critical review

To reduce environmental and human health risks of contaminated sites, having a comprehensive knowledge about the polycyclic aromatic hydrocarbon (PAH) removal processes is crucial. PAHs are contaminants which are highly recognized to pose threats to humans, animals, and plants. PAHs are hydrophobic and own two or more benzene rings, and hence are resistant to structural degradation. There are various techniques which have been developed to treat PAH-contaminated soil. Four distinct processes to remove PAHs in the contaminated soil, thought to be more effective techniques, are presented in this review: soil washing, chemical oxidation, electrokinetic, phytoremediation. In a surfactant-aided washing process, a removal rate of 90% was reported. Compost-amended phytoremediation treatment presented 58-99% removal of pyrene from the soil in 90 days. Chemical oxidation method was able to reach complete conversion for some PAHs. In electrokinetic treatment, researchers have achieved reliable results in removal of some specific PAHs. Researchers' innovations in novel studies and advantages/disadvantages of the techniques are also investigated throughout the paper. Finally, it should be noted that an exclusive method or a combination of methods by themselves are not the key to be employed for remediation of every contaminated site but the field characteristics are also essential in selection of the most appropriate decontamination technique(s). The remedy for selection criteria is based on PAH concentrations, site characteristics, costs, shortcomings, and advantages.

What's the point? The contribution of a sustainability view in contaminated site remediation

Decision support tools (DST) are often used in remediation projects to aid in the complex decision on how best to remediate a contaminated site. In recent years, the sustainable remediation concept has brought increased attention to the often-overlooked contradictory effects of site remediation, with a number of sustainability assessment tools now available. The aim of the present study is twofold: (1) to demonstrate how and when different assessment views affect the decision support outcome on remediation alternatives in a DST, and (2) to demonstrate the contribution of a full sustainability assessment. The SCORE tool was used in the analysis; it is based on a holistic multi-criteria decision analysis (MCDA) approach, assessing sustainability in three dimensions: environmental, social, and economic. Four assessment scenarios, compared to a full sustainability assessment, were considered to reflect different possible assessment views; considering public and private problem owner perspectives, as well as green and traditional assessment scopes. Four real case study sites in Sweden were analyzed. The results show that the decision support outcome from a full sustainability assessment most often differs to that of other assessment views, and results in remediation alternatives which balance trade-offs in most of the scenarios. In relation to the public perspective and traditional scope, which is seen to lead to the most extensive and expensive remediation alternatives, the trade-off is related to less contaminant removal in favour of reduced negative secondary effects such as emissions and waste disposal. Compared to the private perspective, associated with the lowest cost alternatives, the trade-off is higher costs, but more positive environmental and social effects. Generally, both the green and traditional assessment scopes miss out on relevant social and local environmental secondary effects which may ultimately be very important for the actual decision in a remediation project.

Remediation of soils contaminated with heavy metals with an emphasis on immobilization technology

The major frequent contaminants in soil are heavy metals which may be responsible for detrimental health effects. The remediation of heavy metals in contaminated soils is considered as one of the most complicated tasks. Among different technologies, in situ immobilization of metals has received a great deal of attention and turned out to be a promising solution for soil remediation. In this review, remediation methods for removal of heavy metals in soil are explored with an emphasis on the in situ immobilization technique of metal(loid)s. Besides, the immobilization technique in contaminated soils is evaluated through the manipulation of the bioavailability of heavy metals using a range of soil amendment conditions. This technique is expected to efficiently alleviate the risk of groundwater contamination, plant uptake, and exposure to other living organisms. The efficacy of several amendments (e.g., red mud, biochar, phosphate rock) has been examined to emphasize the need for the simultaneous measurement of leaching and the phytoavailability of heavy metals. In addition, some amendments that are used in this technique are inexpensive and readily available in large quantities because they have been derived from bio-products or industrial by-products (e.g., biochar, red mud, and steel slag). Among different amendments, iron-rich compounds and biochars show high efficiency to remediate multi-metal contaminated soils. Thereupon, immobilization technique can be considered a preferable option as it is inexpensive and easily applicable to large quantities of contaminants derived from various sources.

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.

Remediation of metalliferous mines, revegetation challenges and emerging prospects in semi-arid and arid conditions

Understanding plant behaviour in polluted soils is critical for the sustainable remediation of metal-polluted sites including abandoned mines. Post-operational and abandoned metal mines particularly in semi-arid and arid zones are one of the major sources of pollution by soil erosion or plant hyperaccumulation bringing ecological impacts. We have selected from the literature 157 species belonging to 50 families to present a global overview of 'plants under action' against heavy metal pollution. Generally, all species of plants that are drought, salt and metal tolerant are candidates of interest to deal with harsh environmental conditions, particularly at semi-arid and arid mine sites. Pioneer metallophytes namely Atriplex nummularia, Atriplex semibaccata, Salsola kali, Phragmites australis and Medicago sativa, representing the taxonomic orders Caryophyllales, Poales and Fabales are evaluated in terms of phytoremediation in this review. Phytoremediation processes, microbial and algal bioremediation, the use and implication of tissue culture and biotechnology are critically examined. Overall, an integration of available remediation plant-based technologies, referred to here as 'integrated remediation technology,' is proposed to be one of the possible ways ahead to effectively address problems of toxic heavy metal pollution. Graphical abstract Integrated remediation technology (IRT) in metal-contaminated semi-arid and arid conditions. The hexagonal red line represents an IRT concept based on remediation decisions by combination of plants and microbial processes.