Remediation approaches for polycyclic aromatic hydrocarbons (PAHs) contaminated soils: Technological constraints, emerging trends and future directions

Contribution of dicarboxylic acids to pyrene biodegradation and transcriptomic responses of Enterobacter sp. PRd5

The colonization of degrading endophytic bacteria is an effective means to reduce the residues of polycyclic aromatic hydrocarbons (PAHs) in crops. Dicarboxylic acids, as the main active components in crops, can affect the physiological activities of endophytic bacteria and alter the biodegradation process of PAHs in crops. In this study, malonic acid and succinic acid were selected as the representatives to investigate the contribution of dicarboxylic acids to pyrene biodegradation by endophytic Enterobacter sp. PRd5 in vitro. The results showed that dicarboxylic acids improved the biodegradation of pyrene and altered the expression of the functional gene of strain PRd5. Malonic acid and succinic acid reduced the half-life of pyrene by 20.0% and 27.8%, respectively. The degrading enzyme activities were significantly stimulated by dicarboxylic acids. There were 386 genes up-regulated and 430 genes down-regulated in strain PRd5 with malonic acid, while 293 genes up-regulated and 340 genes down-regulated with succinic acid. Those up-regulated genes were distributed in the functional classification of signal transduction, membrane transport, energy metabolism, carbohydrate metabolism, and amino acid metabolism. Malonic acid mainly enhanced the central carbon metabolism, cell proliferation, and cell activity. Succinic acid mainly improved the expression of degrading gene. Overall, the findings of this study provide new insights into the regulation and control of PAH stress by crops. KEY POINTS: • Dicarboxylic acids improved the biodegradation of pyrene by Enterobacter sp. PRd5. • The degrading enzyme activities were stimulated by dicarboxylic acids. • There are different facilitation mechanisms between malonic acid and succinic acid.

Exploring bacterial communities through metagenomics during bioremediation of polycyclic aromatic hydrocarbons from contaminated sediments

The goal of this study was to evaluate the degradation effectiveness of PAHs degrading bacteria at the mesocosm level, including Stenotrophomonas maltophilia (SC), mixed culture (MC), and enriched native microflora (EC) at the mesocosm level. Maximum degradation was found in the mesocosm MC (26.67 %), followed by SC (25.08 %) and EC (18.25 %) after 60 days. Thus, mixed culture and Stenotrophomonas maltophilia could be a game changer in the PAHs bioremediation at the chronically contaminated sites. MiSeq sequencing has revealed dominancy of γ-Proteobacteria, α-Proteobacteria, β-Proteobacteria at class level and Sphingomonadales, oceanospirillales, Rhodothermales at Order level. Families Alcanivoracaceae, Alteromonadaceae, Nocardiaceae, Rhodospirillaceae and genus Stenotrophomonas, Alcanivorax, Methylophaga, Fluviicola and Rhodoplanes were considerably increased which play key role in the PAHs degradation. Dominant bacterial communities have revealed resilience community to enable potential PAHs degradation process in all the mesocosms. To the best our knowledge this is the first ever attempt in PAHs biodegradation study conducted at the mesocosm level mimicking natural environmental conditions. Consequently, this study could be a benchmark against which future progress studies for the policy makers and stakeholders to design appropriate bioremediation study for the historically PAHs polluted contaminate sites.

Reutilization of waste self-heating pad by loading cobalt: A magnetic and green peroxymonosulfate activator for naphthalene degradation

The disposal and recovery of solid wastes and the remediation of polycyclic aromatic hydrocarbons (PAHs) are the key issues of environmental pollution control. In this study, micro cobalt loaded on iron-carbon-vermiculite composite (Co-ICV) was prepared for the first time by the reutilization of waste self-heating pad as a carrier of cobalt catalyst, which exhibited better performance than bulk cobalt catalyst in peroxymonosulfate (PMS) activation for the degradation of naphthalene (NAP) in water. Above 98% of NAP (2.0 mg/L) was effectively eliminated within 15 min by the Co-ICV (0.2 g/L) activated PMS (0.5 mmol/L) in a pH range of 5.0-9.0. High magnetism and very limited cobalt leaching realized the convenient separation and stable reusability of Co-ICV. Mechanism investigation indicated that Co(II) species were the main active sites to activate PMS decomposition for the generation of SO₄^•- and ^•OH, contributing to the rapid degradation of NAP. Meanwhile, the NAP degradation pathways were deduced via combining the identification of intermediates and the calculation of frontier electron densities (FEDs). Furthermore, the ability of the Co-ICV/PMS system for the NAP degradation in actual lake water and the removal of other refractory pollutants demonstrated that the combination of Co-ICV and PMS was a prospective method for the removal of PAHs. Overall, Co-ICV is a green and promising activator of PMS, and the future development will provide more insights into the comprehensive utilization of solid wastes for the remediation of wastewater containing PAHs.

Coupling Effects of combined thermal desorption and stabilisation on stability of cadmium in the soil

Combined thermal desorption and stabilisation is a major choice for the remediation of soil polluted by heavy metals and organic toxicants. Coupled, these processes physically and chemically affect the stability of heavy metals. For this study, polluted soil containing cadmium (Cd) around a lead-zinc smelter was chosen as the subject. To determine the coupling influence of combining thermal desorption and stabilisation to stabilise the Cd in the soil, the stability of Cd in the soil after thermal desorption, stabilisation, and combined treatment was examined based on the leaching rate, chemical speciation, and soil microstructure. The results showed that the stability of Cd was directly related to the temperature of thermal desorption and the stabilisation agent dose. The influence of the two combined stabilisation‒thermal desorption and thermal desorption‒stabilisation processes on the stability of Cd was analysed. The proportion of residual Cd of the former was 1.14 times higher than those of the latter, and the soil particles in the former process crystalized more significantly than those in the latter. Multiple regression analysis was used to construct the Cd stability model. In order to make the stabilised fraction of Cd consistent, compared with the stabilisation‒thermal desorption process, the agent dose in the thermal desorption‒stabilisation process should increase by 1.39-5.55 times higher, or the desorption temperature should increase by 28.3 °C-69.5 °C. Therefore, the combined stabilisation‒thermal desorption process is more conducive to stabilising Cd in the soil, a phenomenon that saves energy and reduces carbon emissions.

Biochar application strategies for polycyclic aromatic hydrocarbons removal from soils

Polycyclic aromatic hydrocarbons (PAHs) are known as a hazardous group of pollutants in the soil which causes many challenges to the environment. In this study, the potential of biochar (BC), as a carbonaceous material, is evaluated for the immobilization of PAHs in soils. For this purpose, various bonding mechanisms of BC and PAHs, and the strength of bonds are firstly described. Also, the effect of impressive criteria including BC physicochemical properties (such as surface area, porosity, particle size, polarity, aromaticity, functional group, etc., which are mostly the function of pyrolysis temperature), number of rings in PAHs, incubation time, and soil properties, on the extent and rate of PAHs immobilization by BC are explained. Then, the utilization of BC in collaboration with biological tools which simplifies further dissipation of PAHs in the soil is described considering detailed interactions among BC, microbes, and plants in the soil matrix. The co-effect of BC and biological remediation has been authenticated by previous studies. Moreover, recent technologies and challenges related to the application of BC in soil remediation are explained. The implementation of a combined BC-biological remediation method would provide excellent prospects for PAHs-contaminated soils.

Spatial Distribution, Potential Sources, and Health Risk of Polycyclic Aromatic Hydrocarbons (PAHs) in the Surface Soils under Different Land-Use Covers of Shanxi Province, North China

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the environment and pose a serious threat to the soil ecosystem. In order to better understand the health risks for residents exposed to PAH-contaminated soil, 173 surface soil samples were collected in Shanxi Province, China, to detect the levels of 16 priority PAHs. The spatial distribution patterns of PAHs were explored using interpolation and spatial clustering analysis, and the probable sources of soil PAHs were identified for different land-use covers. The results indicate that the soil Σ16 PAH concentration ranged from 22.12 to 1337.82 ng g^-1, with a mean of 224.21 ng g^-1. The soils were weakly to moderately contaminated by high molecular weight PAHs (3-5 ring) and the Taiyuan-Linfen Basin was the most polluted areas. In addition, the concentration of soil PAHs on construction land was higher than that on other land-use covers. Key sources of soil PAHs were related to industrial activities dominated by coal burning, coking, and heavy traffic. Based on the exposure risk assessment of PAHs, more than 10% of the area was revealed to be likely to suffer from high carcinogenic risks for children. The study maps the high-risk distribution of soil PAHs in Shanxi Province and provides PAH pollution reduction strategies for policy makers to prevent adverse health risks to residents.

Highly Ca2+-Ion-Tolerant Biobased Zwitterionic Surfactant with High Interfacial Activity

The wide application of surfactants has a harmful effect on the environment, drawing more attention to the development and application of low-toxicity surfactants. A salt-tolerant and low-toxicity biobased zwitterionic surfactant, N,N-dimethyl-N-[2-hydroxy-3-sulfo-propyl]-N-benzyloxyoctadecanoyl-1,3-propanediamine (SPBOPA), was prepared from the oleic acid extracted from waste oils and anise ether extracted from the tarragon. The final surfactant structure was confirmed using gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and ¹H nuclear magnetic resonance (NMR) spectroscopy. The SPBOPA surfactant could reduce the interfacial tension between crude oil and formation brine to ultralow (5.2 × 10^-4 mN/m) at a low dosage without extra alkali. It still had good interfacial properties in NaCl up to 60 g/L, Ca²⁺ up to 2000 mg/L, and temperature up to 100 °C. Furthermore, SPBOPA had strong antidilution and antiadsorption properties with low toxicity as demonstrated by the high LD₅₀ value of >5000 mg/kg·BW. It could also enhance the wetting ability of crude oil surfaces. Meanwhile, it showed a high biodegradability in the environment. All of the results achieved in this work confirmed that the SPBOPA surfactant is a more robust and promising biobased surfactant candidate than traditional surfactants as an eco-friendly surfactant for enhanced oil recovery (EOR).

Rhamnolipid-Enhanced ZVI-Activated Sodium Persulfate Remediation of Pyrene-Contaminated Soil

In soil, polycyclic aromatic hydrocarbons (PAHs) are tightly bound to organic components, but surfactants can effectively transform them from a solid to a liquid phase. In this study, the biosurfactant rhamnolipid (RL) was selected as the eluent; shaking elution in a thermostatic oscillator improved the elution rate of pyrene, and the effects of RL concentration, temperature, and elution time on the elution effect were compared. After four repeated washings, the maximum elution rate was 75.6% at a rhamnolipid concentration of 20 g/L and a temperature of 45 °C. We found that 38 μm Zero-Valent Iron (ZVI) had a higher primary reaction rate (0.042 h^-1), with a degradation rate of 94.5% when 3 g/L ZVI was added to 21 mM Na₂S₂O₈ at 60 °C. Finally, electron paramagnetic resonance (EPR) detected DMPO-OH and DMPO-SO₄ signals, which played a major role in the degradation of pyrene. Overall, these results show that the combination of rhamnolipid elution and persulfate oxidation system effectively remediated pyrene-contaminated soil and provides some implications for the combined remediation with biosurfactants and chemical oxidation.

Degradation of phenol in the bio-cathode of a microbial desalination cell with power generation and salt removal

In this study, the performance of a three-chamber microbial desalination cell (MDC) was assessed to simultaneously remove salt (35 g.L^-1) from water and degrade phenol as a hazardous compound. Two parallel MDCs with the same configurations were run using glucose as the chemical oxygen demand (COD) at an initial concentration of 1.5 g.L^-1 as the anolyte. MDC#1 operated with 10 mM phosphate buffer solution (PBS), while MDC#2 operated with bio-cathode as the catholyte for the degradation of 100 mg.L^-1 of phenol. The use of MDC#1 resulted in a power density, desalination efficiency, and COD removal of 366.2 mW.m^-2, 50.3 ± 4.0 %, and 79.3 ± 2.2 %, respectively. All performance parameters were improved in MDC#2 with bio-cathode so that power density, desalination efficiency, and COD removal reached 660.1 mW.m^-2, 72.1 ± 3.0 %, and 92.6 ± 2.4 %, respectively. Also, more than 96 % of phenol was degraded using bio-cathode within 7 h of operation. Bio-cathode could enhance the performance of the MDC reactor through catalyzing the final reactions of electron acceptors compared to MDC#1 with a chemical cathode. In general, the results indicated that heterotrophic microorganisms, able to grow alongside autotrophic bacteria, could effectively extend the applications of MDC reactors to degrade hazardous compounds in cathode chambers.

Ecological risk assessment of priority PAHs pollutants in crude oil contaminated soil and its impacts on soil biological properties

Polycyclic aromatic hydrocarbons (PAHs) are one of the major toxic constituents of crude oil and therefore, an understanding on PAHs associated risks and their relationship with soil biological parameters are necessary for adopting effective risk-based and site specific remediation strategies in the contaminated soil. Here, risks evaluation of eight detected PAHs in terms of toxic equivalent concentration (TEQ_C), benzo(a)pyrene equivalent (BaPeq), contamination factor (CF), pollution load index (PLI), hazard quotient (HQ), hazard index (HI), toxic unit for individual PAHs (TU) and PAHs mixture (TU_m) have been evaluated. Besides, the effect of PAHs contamination on soil biological properties has also been investigated and correlated with PAHs concentrations. The TEQc of eight PAHs was recorded in the range of 0.06-5.0 mg kg^-1 soil_, whereas the BaPeq value was 25.3 mg kg^-1 soil which exceed the permissible limit. Similarly, CF (85.5-1668.2), PLI (322.8), HQ (311.7-8340.9), HI (26,443.8), TU (227.9-3821.6) and TU_m(7916.2) also exceed the permissible values for non-toxic conditions indicating carcinogenic risk for humans. Besides, activities of soil dehydrogenase, urease, alkaline-phosphatase, catalase, amylase and cellulase were decreased by 1.5-2.3 folds in the contaminated soil than control. The results of Pearson's correlation matrix also established negative impact of PAHs on the soil's biological properties.

Occurrence of polycyclic aromatic compounds and interdomain microbial communities in oilfield soils

Soil polycyclic aromatic compound (PAC) pollution as a result of petroleum exploitation has caused serious environmental problems. The unclear assembly and functional patterns of microorganisms in oilfield soils limits the understanding of microbial mechanisms for PAC elimination and health risk reduction. This study investigated the polycyclic aromatic hydrocarbons (PAHs) and substituted PAHs (SPAHs) occurrence, and their impact on the bacteria-archaea-fungi community diversity, co-occurrence network and functionality in the soil of an abandoned oilfield. The results showed that the PAC content in the oilfield ranged from 3429.03 μg kg^-1 to 6070.89 μg kg^-1, and risk assessment results suggested a potential cancer risk to children and adults. High molecular weight PAHs (98.9%) and SPAHs (1.0%) contributed to 99.9% of the toxic equivalent concentration. For microbial analysis, the abundantly detected degraders and unigenes indicated the microbial potential to mitigate pollutants and reduce health risks. Microbial abundance and diversity were found to be negatively correlated with health risk. The co-occurrence network analysis revealed nonrandom assembly patterns of the interdomain microbial communities, and species in the network exhibited strong positive connections (59%). The network demonstrated strong ecological linkages and was divided into five smaller coherent modules, in which the functional microbes were mainly involved in organic substance and mineral component degradation, biological electron transfer and nutrient cycle processes. The keystone species for maintaining microbial ecological functions included Marinobacter of bacteria and Neocosmospora of fungi. Additionally, benzo [g,h,i]pyrene, dibenz [a,h]anthracene, indeno [1,2,3-cd]perylene and total phosphorus were the key environmental factors driving the assembly and functional patterns of microbial communities under pollution stress. This work improves the knowledge of the functional pattern and environmental adaptation mechanisms of interdomain microbes, and provides valuable guidance for the further bioremediation of PAC-contaminated soils in oilfields.

Bioremediation of tetramethyl thiuram disulfide and resource utilization of natural rubber wastewater by WR-2 Bacillus-dominated microbial community

Tetramethyl thiuram disulfide (TMTD), an emerging pollutant with ecotoxicity and accumulation in rubber wastewater, is directly discharged by factories into the surrounding soil to save costs, and this disrupts the nearby ecosystem. In this study, an efficient bioremediation microbial community (WR-2) dominated by Bacillus was acclimatized and isolated from soil contaminated by rubber wastewater. After passing through the metabolic process of WR-2, the ecotoxic TMTD decomposes within 14 days. In the pot experiment, WR-2 not only completed the bioremediation of contaminated soil but also significantly improved the crop growth conditions and the product quality. These results show that WR-2 has broad application prospects in the bioremediation of soil contaminated by rubber wastewater. It also provides a theoretical framework for the resource utilization of the effluent at the end of the initial rubber processing.

Dissipation of a mix of priority PAHs in soils by using availability enhancers. Effect of aging and pollutant interactions

A remediation strategy using three non-toxic availability enhancers (two cyclodextrins and a rhamnolipid biosurfactant) was applied to various soils artificially contaminated with a mix of Polycyclic Aromatic Hydrocarbons (PAHs) considered priority pollutants at two levels of contamination: only with 7 low molecular weight PAHs (LMW PAHs, 5 with 3-ring and 2 with 4-ring - fluoranthene and pyrene) or with 14 PAHs (from 3 to 6 rings). Natural attenuation of PAHs in all soils showed degradation capacity for the LMW PAHs, with a final content of LMW PAHs <5% of their initial concentration. Conversely, the rest of PAHs (high molecular weight PAHs, HMW) remained in the soils (61% - 83.5%), indicating abiotic dissipation of HMW PAHs due to formation of non-extractable residues in soils. The influence of the presence of HMW PAHs on the degradation of the 7 LMW PAHs was also tested, showing a general decrease in the time to obtain 50% dissipation (DT₅₀), statistically significant for acenaphthene, acenaphthylene and fluorene. Availability enhancers showed different effects on PAHs dissipation. 2-hydroxypropyl-β-cyclodextrin (HP) decreased DT₅₀ of some of the lighter PAHs, whereas the rhamnolipid (RL) caused a slight DT₅₀ increase due to its initial toxicity on native soil microorganisms, but showing later high degradation rate for LMW PAHs. On the contrary, randomly methylated-β-cyclodextrin (RAMEB) slowed down PAHs degradation due to its high adsorption onto soil surface, blocking the desorption of PAHs from the soils. The high number of experimental factors not studied simultaneously before (soil type, co-contamination, availability enhancers and incubation time) allowed to conduct a statistical analysis which supported the conclusions reached. Principal Component Analysis separated the studied PAHs in 3 groups, in relation with their molecular weight and Kow. The first principal component was related with LMW PAHs, and separate the inefficient RAMEB from the other availability enhancers.

A comprehensive review of heavy metal pollution in the coastal areas of Bangladesh: abundance, bioaccumulation, health implications, and challenges

The coastal zone of Bangladesh, with a population density of 1278 people per square kilometer, is under serious threat due to heavy metal pollution. To date, many studies have been conducted on the heavy metal contamination in soils, water, aquatic animals, and plants in the coastal zone of Bangladesh; however, the available information is dispersed. In this study, previous findings on the contamination levels, distributions, risks, and sources of heavy metals in sediments and organisms were summarized for the first time to present the overall status of heavy metal pollution along coastal regions. Earlier research found that the concentrations of various heavy metals (HMs), particularly Co, Cd, Pb, Cu, Cr, Mn, Fe, and Ni in water, sediment, and fish in most coastal locations, were above their permissible limits. High concentrations of HMs were observed in sediments and water, like Cr of 55 mg/kg and 86.93 mg/l in the ship-breaking areas and Karnaphuli River, respectively, in coastal regions of Bangladesh. Heavy metals severely contaminated the Karnaphuli River estuary and ship-breaking area on the Sitakundu coast, where sediments were the ultimate sink of high concentrations of metals. Sedentary or bottom-dwelling organisms like gastropods and shrimp had higher levels of heavy metals than other organisms. As a result, the modified PRISMA review method was used to look at the critical research gap about heavy metal pollution in Bangladesh's coastal areas by analyzing the current research trends and bottlenecks.

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.

Fate of pyrene on mineral surfaces during thermal remediation as a function of temperature

There is evidence that contaminants can transform at the elevated temperatures of thermal remediation; however, the contribution of redox active minerals to transformation has not been investigated. Three redox active minerals (i.e., birnessite (MnO₂), magnetite (Fe₃O₄), and hematite (Fe₂O₃)) and one redox inactive mineral (Ottawa sand (SiO₂)) were spiked with pyrene and thermally treated. Under dry, anoxic conditions, 100%, 75% ± 3%, 70% ± 15%, and 14% ± 28% of the initial pyrene mass was removed with birnessite, magnetite, hematite, and Ottawa sand, respectively, after treatment at 250 °C for 30 min. Under wet, oxic conditions, 92% ± 8%, 86% ± 12%, 79% ± 4%, and 42% ± 7% was removed for the same minerals, respectively, after treatment at only 150 °C for 30 min. Baseline studies with Ottawa sand resulted in volatilization alone of pyrene with no transformation observed. Increased pyrene loading was used to evaluate potential transformation pathways based on identified by-products, demonstrating that both oxidative and reductive pathways were operative depending on the conditions. Reaction products in the presence of redox active minerals indicate transformation was dominated by reduction via hydrogenation in dry experiments, and by oxidation via hydroxyl radicals in wet experiments. The latter was unexpected, because only low hydroxyl radical concentrations have been detected in mineral-water systems at ambient temperature. These results indicate that understanding dominant reaction pathways and products is advantageous for the design of efficient and safe thermally enhanced treatment systems.

Efficient bioremediation of PAHs-contaminated soils by a methylotrophic enrichment culture

Bioaugmentation effectively enhances microbial bioremediation of hazardous polycyclic aromatic hydrocarbons (PAHs) from contaminated environments. While screening for pyrene-degrading bacteria from a former manufactured gas plant soil (MGPS), the mixed enrichment culture was found to be more efficient in PAHs biodegradation than the culturable pure strains. Interestingly, analysis of 16S rRNA sequences revealed that the culture was dominated by a previously uncultured member of the family Rhizobiaceae. The culture utilized C1 and other methylotrophic substrates, including dimethylformamide (DMF), which was used as a solvent for supplementing the culture medium with PAHs. In the liquid medium, the culture rapidly degraded phenanthrene, pyrene, and the carcinogenic benzo(a)pyrene (BaP), when provided as the sole carbon source or with DMF as a co-substrate. The efficiency of the culture in the bioremediation of PAHs from the MGPS and a laboratory waste soil (LWS) was evaluated in bench-scale slurry systems. After 28 days, 80% of Σ16 PAHs were efficiently removed from the inoculated MGPS. Notably, the bioaugmentation achieved 90% removal of four-ringed and 60% of highly recalcitrant five- and six-ringed PAHs from the MGPS. Likewise, almost all phenanthrene, pyrene, and 65% BaP were removed from the bioaugmented LWS. This study highlights the application of the methylotrophic enrichment culture dominated by an uncultured bacterium for the efficient bioremediation of PAHs.

Enhanced oil removal from oily sand by injecting micro-macrobubbles in swirl elution

Environmental contamination by petroleum hydrocarbons was exacerbated by oil pipeline breaks, marine oil spills and discharges from industrial production. To further improve the removal performance of petroleum hydrocarbons in solid particles, the deoiling experiments of swirl elution with micro-macrobubbles on oily sands were carried out in this paper. Experiment results indicated that when particles fell from the center of the bubble, the collision efficiency was 99.3%. The instantaneous contact angle (ICA) between the macrobubbles and the oil layer was improved in the presence of microbubbles. Furthermore, the maximum ICA of bubbles attaching to the oil layer was found to occur at pH 9 in the system of oily sand mixtures ranging from pH 5 to pH 14. This finding indicated that the slightly alkaline solution was more advantageous for bubbles to attach to the oil layer than the highly alkaline solution. The optimum condition for the elution of oily sand in the mixture of pH 7-14 was pH 12, and the oil removal efficiency was 85.4% for 10 min. The oil removal efficiency of swirl elution (SE) with bubbles on oily sand at pH 12 for 10 min was superior to either SE without bubbles or air flotation (AF). The results show that the swirl elution with bubbles can effectively enhance the oil removal efficiency of oily sands and provide guidance for controlling the environmental petroleum hydrocarbon contamination and reducing the usage of surfactants.

Assessment for combined phytoremediation and biomass production on a moderately contaminated soil

Once previous industrial activity has ceased, brownfields are found in urban and suburban environments and managed in different ways ranging from being left untouched to total reconversion. These situations apply to large surface areas often impacted by residual diffuse pollution. Though significant and preventing any sensitive use, residual contamination does not necessarily require treatment. Moreover, conventional treatments show their technical and economic limits in these situations and gentle remediation options such as phytomanagement might appear more relevant to the management of those sites. Thus, these sites face up two major issues: managing moderate contamination levels and providing an alternative use of economic interest. This work proposes to assess a management strategy associating the phytoremediation of organic pollution along with the production of biomass for energy generation production. A 16-week controlled growth experiment was conducted on a soil substrate moderately impacted by multiple pollution (trace elements, mainly Zn and Pb, and hydrocarbons), by associating rhizodegradation with Medicago sativa or biomass production with Robinia pseudoacacia or Alnus incana in monocultures. The effect of a microbial inoculum amendment on the performances of these treatments was also evaluated. Results showed total hydrocarbons (TH), and to a lesser extent polycyclic aromatic hydrocarbons (PAH), concentrations decreased over time, whatever the plant cover. Good biomass production yields were achieved for both tree species in comparison with the control sample, even though R. pseudoacacia seemed to perform better. Furthermore, the quality of the biomass produced was in conformity with the thresholds set by the legislation concerning its use as a renewable energy source.

Decrypting the synergistic action of the Fenton process and biochar addition for sustainable remediation of real technogenic soil from PAHs and heavy metals

The objective of this study was to demonstrate the feasibility and the relevance of combining biochar with the Fenton process for the simultaneous improvement of polycyclic aromatic hydrocarbons (PAHs) degradation and immobilization of heavy metals (HMs) in real soil remediation processes at circumneutral pH. The evaluation of PAHs degradation results was performed through multivariate statistical tools, including principal component analysis (PCA) and partial least squares (PLS). PCA showed that the level of biochar amendment decisively affected the degree of degradation of total PAHs, highlighting the role of biochar in catalyzing the Fenton reaction. Moreover, the PLS model was used to interpret the important features of each PAH's physico-chemical properties and its correlation to degradation efficiency. The electron affinity of PAHs correlated positively with the degradation efficiency only if the level of biochar amendment sat at 5%, explained by the ability of biochar to transfer the electrons to PAHs, improving the Fenton-like degradation. Moreover, the addition of biochar reduced the mobilization of HMs by their fixation on their surface, reducing the Fenton-induced metal leaching from the destruction of metal-organic complexes. In overall, these results on the high immobilization rate of HMs accompanied with additional moderate PAHs degradation highlighted the advantages of using a biochar-assisted Fenton-like reaction for sustainable remediation of technogenic soil.

Efficient degradation of anthracene in soil by carbon-coated nZVI activated persulfate

The easy passivation defect of nano zero-valent iron (nZVI) greatly limits its application in site pollution remediation. Carbon coating can effectively inhibit the passivation of nZVI, but its effectiveness in the soil is still unknown. This study investigated the feasibility of carbon-coated nZVI (Fe⁰@C) as a persulfate (PS) activator to degrade anthracene (ANT) in soil. The results show that the Fe⁰@C/PS system can remove 51.6% of ANT in the soil after 0.5 h of reaction, and reach 76.4% after 12 h of reaction. Not only that, the Fe⁰@C/PS system shows a good removal effect on ANT within the initial pH range of 3-9. Free radical scavenging experiments show that superoxide radicals (O₂^•-) and singlet oxygen (¹O₂) are mainly responsible for the removal of ANT, while O₂^•- may be mainly used as a precursor for the generation of ¹O₂. The activation of PS by Fe⁰@C can generate a large number of free radicals, and soil components (such as β-MnO₂) can promote the conversion of O₂^•- to ¹O₂. Furthermore, the possible degradation pathway of ANT was also proposed. The findings are of great significance to fill up the knowledge gaps in the application of nZVI in soil remediation.

Petroleum-contaminated soil: environmental occurrence and remediation strategies

Soil is an environmental matrix that carries life for all living things. With the rise of human activities and the acceleration of population, the soil has been exposed in part to pollution by the discharge of various xenobiotics and persistent pollutants into it. The disposal of toxic substances such as polycyclic aromatic hydrocarbons (PAHs) alters soil properties, affects microbial biodiversity, and damages objects. Considering the mutagenicity, carcinogenicity, and toxicity of petroleum hydrocarbons, the restoration and clean-up of PAH-polluted sites represents an important technological and environmental challenge for sustainable growth and development. Though several treatment methods to remediate PAH-polluted soils exist, interesting bacteria, fungi, and their enzymes receive considerable attention. The aim of the present review is to discuss PAHs' impact on soil properties. Also, this review illustrates physicochemical and biological remediation strategies for treating PAH-contaminated soil. The degradation pathways and contributing factors of microbial PAH-degradation are elucidated. This review also assesses the use of conventional microbial remediation compared to the application of genetically engineered microorganisms (GEM) that can provide a cost-effective and eco-friendly PAH-bioremediation strategy.

Nematodes As Soil Stress Indicators for Polycyclic Aromatic Hydrocarbons: a Review

Polycyclic aromatic hydrocarbons (PAHs) are an important group of organic pollutants present in all parts of the environment, affecting ecosystems and human health. PAHs, which have a strong affinity for organic carbon, are found in large quantities in soil, which is one of the most important sinks for these contaminants. Their impact on the soil biotic compartments depends on a number of different factors in combination with PAH behaviour and can be assessed using soil monitoring. Soil fauna have already shown excellent properties for biomonitoring of contaminants with most promising indicator frameworks based on nematodes, which are involved in essential processes in this environment. Nematodes respond to PAHs at multiple levels, including molecular, individual and community levels. At the molecular level, this is associated with activation of metabolic pathways for xenobiotics and increased demand for energy and resources. At the individual level, this is reflected in the slowing down of various physiological processes, which has consequences at the individual and community level for sensitive taxa. In this review, the toxicity and the direct and indirect effects of PAHs on soil nematode communities are discussed. It also considers the perspectives and challenges in assessing the toxicity of PAHs and their indication using soil nematodes.

Sphingomonas Relies on Chemotaxis to Degrade Polycyclic Aromatic Hydrocarbons and Maintain Dominance in Coking Sites

Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants widely present in industrial sites. Microbial degradation is an effective method of removing PAHs. The identification of microorganisms that have important ecological functions at the site is of great significance for PAH removal. We collected soil samples at three depths in the range of 0-100 cm at 70-day intervals at the coking site and explored the degradation of PAHs. We combined molecular ecology networking, metagenomics, and genome assembly to search for microorganisms that persist, dominate, and affect the microbial community construction in the degradation process and analyzed their adaptation strategies. The results showed that 15.78 mg/kg of PAHs naturally decayed, and 13.33 mg/kg of PAHs migrated from 30-100 cm to 0-30 cm in the soil. Sphingomonas, which occupied a niche advantage, was both the core and keystone microorganism, and its spatial distribution pattern and temporal change dynamics were consistent with those of PAHs. We assembled the genome of Sphingomonas sp., revealing its multiple potential for degrading PAHs and other pollutants. Additionally, flagellar assembly and bacterial chemotaxis genes ranked high in the assembled genome of Sphingomonas sp., which might help it obtain a competitive advantage in the soil. The findings underscored the strategy of Sphingomonas to maintain dominance, enriched the understanding of PAH-degrading microorganisms in site soil, and provided references for the remediation of PAHs.

A simple empirical model for phenanthrene adsorption on soil clay minerals

Soil clay minerals are effective substrate adsorbents of polycyclic aromatic hydrocarbons (PAHs) in natural soil. The adsorbed PAHs result in long-term contamination of soils. In this paper, a typical PAH phenanthrene (Phe) and nine high purity clay minerals are selected as representative PAH pollutants and adsorbents, respectively. A series of experiments have been conducted to disclose the relationship between the Phe adsorption effect of these clay minerals and their physical properties, including specific surface area (SSA), cation exchange capacity (CEC) and contact angle (CA). Molecular simulation methods are performed to explore the mechanism of clay mineral structure on Phe adsorption at the molecular level. Density functional theory (DFT) calculation suggests that the adsorption of Phe on clay minerals is mainly due to the van der Waals effect. The strength of the O-H-π effect is greater than that of the hydrophobic effect of Phe adsorption. Molecular dynamic (MD) simulations imply that the hydration effect of cations hinders the Phe hydrophobic adsorption by occupying the adsorption sites. Based on the mechanism explored, a simple empirical model is proposed, and the adsorption distribution coefficient K_d of clay mineral and water phases can be precisely predicted by the three physical properties of clay minerals, without rigorous quantitative analysis of soil clay minerals.

Strategy to Promote the Biodegradation of Phenanthrene in Contaminated Soil by a Novel Bacterial Consortium in Slurry Bioreactors

Polycyclic aromatic hydrocarbons (PAHs) are typical high-risk, persistent organic pollutants. Biological slurry reactors are widely used for enhanced bioremediation. In this experiment, a highly efficient phenanthrene-degrading bacteria group was obtained through screening and domestication, and the community was named MZJ_21. After the addition of MZJ_21 to the aerobic slurry bioreactor, with the optimum conditions of the temperature, stirring speed, and aeration rate of 30 °C, 120 rpm, and 1 L/min, respectively, the phenanthrene degradation ratio reached 95.41% within 48 h. The exploration of the degradation of phenanthrene by MZJ_21 indicated that most MZJ_21 communities adsorbed on the soil particle, mainly because MZI_21 could secrete extracellular polymers, which could stably adhere MZJ_21 on the solid phase. At the same time, the distribution ratio of phenanthrene in the solid phase is increased, so that the efficient phenanthrene degradation reaction takes place in the solid phase.

Effect of soil contamination with polycyclic aromatic hydrocarbons from drilling waste on germination and growth of lawn grasses

In many studies, grasses were used to increase the biodegradation of polycyclic aromatic hydrocarbons (PAHs) in soil because they are the most common plant species on the ground level and are quite resistant to contamination with these compounds. One of the main failures in PAH remediation in soil using plant species was the negative impact on germination and seedling growth. The objective of this study was to evaluate grass seed germination and seedling growth affected by drill cuttings to determine the resistance of selected grass species to the impact of PAH and their suitability for an effective phytoremediation of soils contaminated with waste that contain compounds from this group. In the study four grass species: tall fescue (Festuca arundinacea), red fescue (Festuca rubra), perennial ryegrass (Lolium perenne) and common meadow-grass (Poa pratensis). The germination energy of all species decreased as the amount of drill cuttings increased. Among the species studied, the highest germination energy and capacity were found in Lolium perenne (54.1 and 73.2 respectively), and the lowest - in Poa pratensis (16.7 and 23.3 respectively). With an increasing amount of drill cuttings, the root and seedling height were decreased. Festuca arundinacea seedlings were distinctly the highest and had the longest roots (96.7 and 52.7, respectively), while Poa pratensis seedlings showed the significantly slowest seedling and root elongation rate (30.4 and 12.4, respectively). However, the strongest decrease in seedling height and root length compared to the control was observed in Festuca rubra. Based on IC50, the greatest tolerance to the addition of drilling waste to the substrate was found for Festuca arundinacea and Festuca rubra. The conducted investigation indicates that Festuca arundinacea and Lolium perenne are grass species that are least sensitive to drilling waste in the substrate because no significant differences were found in root length and seedling height between the control soil and the soil where a PAH dose of 5% and 10% was applied.

Biodegradation of Naphthalene and Anthracene by Aspergillus glaucus Strain Isolated from Antarctic Soil

Biotechnologies based on microbial species capable of destroying harmful pollutants are a successful way to solve some of the most important problems associated with a clean environment. The subject of investigation is the Antarctic fungal strain Aspergillus glaucus AL1. The culturing of the examined strain was performed with 70 mg of wet mycelium being inoculated in a Czapek Dox liquid medium containing naphthalene, anthracene, or phenanthrene (0.3 g/L) as the sole carbon source. Progressively decreasing naphthalene and anthracene concentrations were monitored in the culture medium until the 15th day of the cultivation of A. glaucus AL1. The degradation was determined through gas chromatography–mass spectrometry. Both decreased by 66% and 44%, respectively, for this period. The GC-MS analyses were applied to identify salicylic acid, catechol, and ketoadipic acid as intermediates in the naphthalene degradation. The intermediates identified in anthracene catabolism are 2-hydroxy-1-naphthoic acid, o-phthalic acid, and protocatechuic acid. The enzyme activities for phenol 2-monooxygenase (1.14.13.7) and catechol 1,2-dioxygenase (1.13.11.1) were established. A gene encoding an enzyme with catechol 1,2-dioxygenase activity was identified and sequenced (GeneBank Ac. No KM360483). The recent study provides original data on the potential of an ascomycete’s fungal strain A. glaucus strain AL 1 to degrade naphthalene and anthracene.

A Comprehensive DFT Investigation of the Adsorption of Polycyclic Aromatic Hydrocarbons onto Graphene

To better understand graphene and its interactions with polycyclic aromatic hydrocarbons (PAHs), density-functional-theory (DFT) computations were used. Adsorption energy is likely to rise with the number of aromatic rings in the adsorbates. The DFT results revealed that the distance between the PAH molecules adsorbed onto the G ranged between 2.47 and 3.98 Å depending on the structure of PAH molecule. The Non-Covalent Interactions (NCI) plot supports the concept that van der Waals interactions were involved in PAH adsorption onto the Graphene (G) structure. Based on the DFT-calculated adsorption energy data, a rapid and reliable method employing an empirical model of a quantitative structure–activity relationship (QSAR) was created and validated for estimating the adsorption energies of PAH molecules onto graphene.

A novel electrokinetic remediation with in-situ generation of H2O2 for soil PAHs removal

Electrokinetic-Fenton (EK-Fenton) technology requires a high dose of H₂O₂ to produce •OH radicals, which adds a high cost to the remediation process and raises safety concerns during transportation and storage of H₂O₂. Moreover, the remediation efficiency of the conventional EK-Fenton process is low due to the meaningless consumption of H₂O₂ on the electrodes and the alkaline environment near the cathode. In this work, a modified CMK3-gas diffusion electrode (CMK3-GDE) is fabricated. This cathode can continuously generate H₂O₂, and the cumulative H₂O₂ concentration can reach 0.23 M during 10 days of the test. The utilization of cation exchange membranes (CEMs) efficiently restricts the decomposition of H₂O₂ on the electrodes and prevents the alkalization of the soil near the cathode, resulting in a 13.7-43.2% increase of the removal efficiency of polycyclic aromatic hydrocarbons (PAHs). In this new treatment process, PAHs are mainly oxidized into quinones, ketones, alcohols, and small molecule acids, and all these products have lower toxicities than PAHs. The EK-Fenton/CMK3-GDE-CEM system exhibits excellent remediation efficiency for treating PAHs polluted soil, which could be a sustainable, eco-friendly, and low-cost strategy for soil remediation.

The relationships between soil physicochemical properties, bacterial communities and polycyclic aromatic hydrocarbon concentrations in soils proximal to coking plants

Microbial degradation of polycyclic aromatic hydrocarbons (PAHs) is the major channel for their decontamination from different environments. Aerobic and anaerobic biodegradations of PAHs in batch reactors with single or multiple bacterial strains have been intensively studied, but the cooperative mechanism of functional PAH-degrading populations at the community level under field conditions remains to be explored. We determined the composition of PAH-degrading populations in the bacterial community and PAHs in farmland and wasteland soils contaminated by coking plants using high-throughput sequencing and high-performance liquid chromatography (HPLC), respectively. The results indicated that the PAH content of farmland was significantly lower than that of wasteland, which was attributed to the lower content of low molecular weight (LMW) PAHs and benzo [k]fluoranthene. The soil physicochemical properties were significantly different between farmland and wasteland. The naphthalene content was related to the soil organic carbon (SOC) and pH, while phenanthrene was related to the nitrate nitrogen (NO₃^--N) and water content (WC). The pH, nitrite (NO₂^--N), SOC, NO₃^--N and WC were correlated with the content of high molecular weight (HMW) PAHs and total PAHs. The relative abundances of the phyla Actinobacteria, Chloroflexi, Acidobacteria, and Firmicutes and the genera Nocardioides, Bacillus, Lysobacter, Mycobacterium, Streptomyces, and Steroidobacter in farmland soil were higher than those in wasteland soil. The soil physicochemical characteristics of farmland increased the diversities of the PAH degrader and total bacterial communities, which were significantly negatively related to the total PAHs and LMW PAHs. Subsequently, the connectivity and complexity of the network in farmland were lower than those in wasteland, while the module containing a module hub capable of degrading PAHs was identified in the network of farmland soil. Structural equation modelling (SEM) analysis showed that the soil characteristics and optimized abundance and diversity of the bacterial community in farmland were beneficial for the dissipation efficiency of PAHs.

Insights into the effects of Fenton oxidation on PAH removal and indigenous bacteria in aged subsurface soil

Combined chemical oxidation and bioremediation is a promising method of treating polycyclic aromatic hydrocarbon (PAH) contaminated soil, wherein indigenous soil bacteria play a critical role in the subsequent biodegradation of PAHs after the depletion of the oxidant. In this study, different Fenton conditions were applied by varying either the oxidation mode (conventional Fenton (CF), Fenton-like (LF), modified Fenton (MF), and graded modified Fenton (GMF)) or the H₂O₂ dosage (0%, 3%, 6%, and 10% (v/v)) to treat PAH contaminated soil. The results revealed that when equal dosages of H₂O₂ are applied, PAHs are significantly removed following oxidation treatment, and the removal percentages obeyed the following sequence: CF > GMF > MF > LF. In addition, higher dosages of H₂O₂ improved the PAH removal from soil treated with the same oxidation mode. The ranges of total PAHs removal efficiencies in the soil added 3%, 6%, and 10% of H₂O₂ (v/v) were 18.04%∼59.48%, 31.88%∼71.83%, and 47.56%∼78.16%, respectively. The PAH removal efficiency decreased with increasing ring numbers for the same oxidation treatment. However, the negative influences on soil bacterial abundance, community composition, and function were observed after Fenton treatment. After Fenton oxidation, the bacterial abundance in the soil received 3%, 6%, and 10% of H₂O₂ (v/v) decreased 1.96-2.69, 2.44-3.22, and 3.09-3.42 orders of magnitude compared to the untreated soil. The soil bacterial abundance tended to be impacted by the oxidation mode and H₂O₂ dosage simultaneously. While the main factor influencing the soil bacterial community composition was the H₂O₂ dosages. The results of this study showed that different oxidation mode and H₂O₂ dosage exhibited different effects on PAHs removal and soil bacteria (including abundance, community composition, and function), and there was a trade-off between the removal of PAHs and the adverse impact on soil bacteria.

Remediation of soil polluted with petroleum hydrocarbons and its reuse for agriculture: Recent progress, challenges, and perspectives

Petroleum hydrocarbons (PHs) are used as raw materials in many industries and primary energy sources. However, excessive PHs act as soil pollutants, posing serious threats to living organisms. Various ex-situ or in-situ chemical and biological methods are applied to restore polluted soil. However, most of the chemical treatment methods are expensive, environmentally unfriendly, and sometimes inefficient. That attracts scientists and researchers to develop and select new strategists to remediate polluted soil through risk-based analysis and eco-friendly manner. This review discusses the sources of PHs, properties, distribution, transport, and fate in the environment, internal and external factors affecting the soil remediation and restoration process, and its effective re-utilization for agriculture. Bioremediation is an eco-friendly method for degrading PHs, specifically by using microorganisms. Next-generation sequencing (NGS) technologies are being used to monitor contaminated sites. Currently, these new technologies have caused a paradigm shift by giving new insights into the microbially mediated biodegradation processes by targeting rRNA are discussed concisely. The recent development of risk-based management for soil contamination and its challenges and future perspectives are also discussed. Furthermore, nanotechnology seems very promising for effective soil remediation, but its success depends on its cost-effectiveness. This review paper suggests using bio-electrochemical systems that utilize electro-chemically active microorganisms to remediate and restore polluted soil with PHs that would be eco-friendlier and help tailor-made effective and sustainable remediation technologies.

Concentration, source apportionment and potential carcinogenic risks of polycyclic aromatic hydrocarbons (PAHs) in roadside soils

PAHs are organic pollutants that have carcinogenic and mutagenic impacts on human health and are a subject of great concern. The soil-bound polycyclic aromatic hydrocarbons (PAHs) in the urban areas can be very lethal to human health. The concentrations, sources, and possible cancer risks of 15 PAHs were analysed by collecting roadside soil samples in Lucknow, India. The range of ∑₁₅PAHs was found to be 478.94 ng/g to 8164.07 ng/g with a mean concentration of 3748.23 ng/g. The highest contribution (32.5%) was found to be from four-ring PAHs, followed by six-ring (24.5%) and five-ring (16.7%) PAHs. The source apportionment through diagnostic ratios ANT/(ANT + PHE) against FL-2/(FL-2+PYR) highlighted the dominance of petroleum, wood, coal, and grass combustion as sources of PAHs in the study area. Source apportionment was also done through positive matrix factorization, confirming the dominance of 'vehicular emissions' (49%), followed by 'coal and biomass combustion' (∼39%), and 'leakages, volatilization and petroleum combustion' (∼12%) as potential sources. The results from lifetime cancer risks (ILCR) varied in the range of 7.5 × 10^-4 and 1.3 × 10 × ^-2 illustrating 'high cancer risk'. The total cancer risk susceptibility of children was found to be 31% more than that of adults. The highest risk associated with toxic equivalent concentration (TEQ) was found at site S8 highlighting the impact of the presence of an international airport, and huge traffic load. The present study will prove to be useful for information related to human exposure to PAHs content in soil in the study area and as baseline study for policy makers, stakeholders, and researchers.

Remediation of diesel-contaminated soil by alkoxyethanol aqueous two-phase system

The increasing diesel pollution accidents pose a serious threat to the ecological environment and human health. Remediation of diesel-contaminated soil (DCS) has attracted widespread attention during the past few decades. This work proposed an approach for the remediation of DCS by alkoxyethanol aqueous two-phase extraction (ATPE), which was an application of this small molecule aqueous two-phase system (ATPS). In addition, the influence of temperature, stirring speed, stirring time, and solid-liquid ratio on the removal of diesel was explored respectively. The removal efficiency of diesel could reach more than 97.18% in 18 min. Meanwhile, ATPS had high reusability, and the removal efficiency remained above 85.17% in the reuse process. Alkoxyethanol ATPE could effectively remove diesel hydrocarbons with different carbon chain lengths and the remediation process hardly caused residual organic solvents on the soil surface according to the analysis of gas chromatography-mass spectrometry (GC-MS) and Fourier transforms infrared (FT-IR), which could be regarded as the distinct advantage compared to the traditional surfactant washing method and organic solvent extraction method. The study of soil physicochemical properties and wheat germination proved that the soil structure and properties changed little after ATPE remediation. And finally, the mechanism of alkoxyethanol ATPE was intensively discussed according to the remediation characteristic. This work provided an efficient method for the remediation of DCS and widened the application fields of alkoxyethanol ATPS as well.

Review on the contamination and remediation of polycyclic aromatic hydrocarbons (PAHs) in coastal soil and sediments

The rapid economic and population growth in coastal areas is causing increasingly serious polycyclic aromatic hydrocarbons (PAHs) pollution in these regions. This review compared the PAHs pollution characteristics of different coastal areas, including industrial zones, commercial ports, touristic cities, aquacultural & agricultural areas, oil & gas exploitation areas and megacities. Currently there are various treatment methods to remediate soils and sediments contaminated with PAHs. However, it is necessary to provide a comprehensive overview of all the available remediation technologies up to date, so appropriate technologies can be selected to remediate PAHs pollution. In view of that, we analyzed the characteristics of the remediation mechanism, summarized the remediation methods for soil or sediments in coastal areas, which were physical repair, chemical oxidation, bioremediation and integrated approaches. Besides, this review also reported the development of new multi-functional green and sustainable systems, namely, micro-nano bubble (MNB), biochar, reversible surfactants and peracetic acid. While physical repair, expensive but efficient, was regarded as a suitable method for the PAHs remediation in coastal areas because of land shortage, integrated approaches would produce better results. The ultimate aim of the review was to ensure the successful restructuring of PAHs contaminated soil and sediments in coastal areas. Due to the environment heterogeneity, PAHs pollution in coastal areas remains as a daunting challenge. Therefore, new and suitable technologies are still needed to address the environmental issue.

Remediation of PAHs contaminated industrial soils by hypochlorous acid: performance and mechanisms

Polycyclic aromatic hydrocarbons (PAHs) mainly originate from incomplete combustion of organic substances and are carcinogenic, mutagenic and teratogenetic, posing a high risk to the ecosystem and human health. The remediation of soils contaminated with PAHs has aroused wide public concern. In this study, hypochlorous acid (HOCl) was applied to realize PAHs removal from industrial contaminated soil with an extremely high degradation efficiency of 93.33% when the initial chlorine concentration was 5000 mg L⁻¹. The degradation behavior of PAHs by HOCl oxidation was investigated in detail. Parameters including chlorine dosage, pH and temperature that had effects on the degradation process were evaluated systematically. The removal of PAHs was followed well with the pseudo-first-order kinetic model. It is found that HOCl and OH˙ were major contributors to the degradation products of chlorinated and oxygenated PAHs. This research provided an easy-operating and energy-saving way to realize the remediation of PAHs contaminated industrial soil practically with high efficiency.

An extremely high degradation efficiency of 93.3% was realized for PAHs in contaminated industrial soil by HOCl oxidation.

Effect of starch-derived organic acids on the removal of polycyclic aromatic hydrocarbons in an aquaculture-sediment microbial fuel cell

This study constructed sediment microbial fuel cells (SMFCs) for polycyclic aromatic hydrocarbons (PAHs) removal in contaminated aquaculture sediment. Starch, a waste deposited in aquaculture sediment, was employed as the co-substrate for electricity generation and PAHs removal, and the effect of starch-derived organic acids on SMFC performance was assessed. The results indicated that sufficient starch promoted PAHs removal (69.9% for naphthalene, 55.6% for acenaphthene, and 46.8% for pyrene) in dual-chamber SMFC, whereas excessive starch attenuated SMFC performance because the organic acids accumulation reduced anode pH, decreased species diversity, and changed the microbial communities. The electricity generation and PAHs removal were positively correlated (R > 0.96), and both of them were related to Macellibacteroides belonging to Bacteroidetes. However, a larger single-chamber SMFC device did not obtain enhanced PAHs removal owing to the restricted "effective range" of the anode. Hence, more challenges need to be addressed to realize the practical application of SMFC.

Insights into naphthalene degradation in aqueous solution and soil slurry medium: Performance and mechanisms

The performance of naphthalene (NAP) degradation in peroxodisulfate (PDS) and peroxymonosulfate (PMS) oxidation systems by nano zero valent iron (nZVI) combined with citric acid (CA) activation was reported in aqueous solution and soil slurry medium. The results in aqueous solution tests indicated that 98.1% and 98.9% of NAP were individually degraded in PDS/nZVI/CA and PMS/nZVI/CA systems within 2 h when the dosages of PDS, PMS, nZVI and CA were 1.0 mM, 0.1 mM, 0.2 mM and 0.1 mM, respectively. The consequences of scavenging tests and electron paramagnetic resonance detection demonstrated that HO• and SO₄^-• were the key factors on NAP removal. The presence of surfactants could consume ROSs and inhibit NAP removal. In addition, GC-MS was applied for the determination of NAP degradation intermediates, and three possible NAP degradation pathways were proposed in PDS oxidation process and two pathways in PMS oxidation process, respectively. The results in soil slurry medium showed that the presence of CA could promote the dissolution of soil minerals and the desorption of NAP from soil medium. 93.5% and 96.8% degradation of NAP were obtained in PDS/nZVI/CA and PMS/nZVI/CA systems within 24 h. Besides, the existence of DOM in soil could promote Fe(II)/Fe(III) cycle and NAP degradation through electron transfer. Based on the NAP degradation performance in the actual groundwater and soil medium, the above findings could provide basis and strong support for the potential application of PDS/nZVI/CA and PMS/nZVI/CA systems in the remediation of NAP contaminated sites.

Polycyclic aromatic hydrocarbon contamination in soils and sediments: Sustainable approaches for extraction and remediation

Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic environmental pollutants that are extremely hydrophobic in nature and resistant to biological degradation. Extraction of PAHs from environmental matrices is the first and most crucial step in PAH quantification. Extraction followed by quantification is essential to understand the extent of contamination prior to the application of remediation approaches. Due to their non-polar structures, PAHs can be adsorbed tightly to the organic matter in soils and sediments, making them more difficult to be extracted. Extraction of PAHs can be achieved by a variety of methods. Techniques such as supercritical and subcritical fluid extraction, microwave-assisted solvent extraction, plant oil-assisted extraction and some microextraction techniques provide faster PAH extraction using less organic solvents, while providing a more environmentally friendly and safer process with minimum matrix interferences. More recently, more environmentally friendly methods for soil and sediment remediation have been explored. This often involves using natural chemicals, such as biosurfactants, to solubilize PAHs in contaminated soils and sediments to allow subsequent microbial degradation. Vermiremediation and microbial enzyme-mediated remediation are emerging approaches, which require further development. The following summarises the existing literature on traditional PAH extraction and bioremediation methods and contrasts them to newer, more environmentally friendly ways.

Nanoparticled Titanium Dioxide to Remediate Crude Oil Exposure. An In Vivo Approach in Dicentrarchus labrax

The contamination of marine water bodies with petroleum hydrocarbons represents a threat to ecosystems and human health. In addition to the surface slick of crude oil, the water-soluble fraction of petroleum is responsible for the induction of severe toxic effects at different cellular and molecular levels. Some petroleum-derived hydrocarbons are classified as carcinogenic and mutagenic contaminants; therefore, the oil spill into the marine environment can have long term consequences to the biota. Therefore, new tools able to remediate crude oil water accommodation fraction pollution in marine water are highly recommended. Nanomaterials were recently proposed in environmental remediation processes. In the present in vivo study, the efficacy of pure anatase titanium nanoparticles (n-TiO2) was tested on Dicentrarchus labrax exposed to the accommodated fraction of crude oil. It was found that n-TiO2 nano-powders themselves were harmless in terms of DNA primary damage, and the capability of pure anatase n-TiO2 to lower the levels of DNA damage induced by a mixture of genotoxic pollutant was revealed. These preliminary results on a laboratory scale are the prerequisite for deepening this new technology for the abatement of the cellular effects related with oil spill pollutants released in marine environments.

Earthworm Eisenia fetida potential for sewage sludge amended soil valorization by heavy metal remediation and soil quality improvement

Sewage sludge reuse in agriculture is increasing, however it can be an important route for contaminants to enter the environment. The aim of this study was to evaluate earthworm Eisenia fetida capability to reduce heavy metal content in the sewage sludge (SS) amended soil and increase soil fertility in terms of soil nutrients content. Adult earthworms were introduced into aged SS amended soil (0-200 Mg ha^-1) and left for 65 days. Earthworms have stabilized soil pH and accelerated organic matter mineralization. The concentrations of most heavy metals during the vermiremediation sharply decreased, K and Mg decreased to a moderate extent, whereas Ca content has increased. The highest removal efficiency was detected for Ni, Co and Mn (> 80%), bioconcentration factors were as follows Zn > Co > Cu > Ni > Mn > Cr. The content of major nutrients (S, P) was substantially higher compared to the initial values. The most efficient remediation and soil quality improvement was achieved under the doses of 25-50 Mg ha^-1. Higher (≥ 100 Mg ha^-1) doses might restrict this technique application because of earthworm mortality and retarded growth. Overall, the study shows that vermiremediation might be a sustainable technique for ecological stabilization of SS amended soil and converting to usable for agricultural needs.

Bioaugmentation as a green technology for hydrocarbon pollution remediation. Problems and prospects

Environmental pollution mitigation measure involving bioremediation technology is a sustainable intervention for a greener ecosystem biorecovery, especially the obnoxious hydrocarbons, xenobiotics, and other environmental pollutants induced by anthropogenic stressors. Several successful case studies have provided evidence to this paradigm including the putative adoption that the technology is eco-friendly, cost-effective, and shows a high tendency for total contaminants mineralization into innocuous bye-products. The present review reports advances in bioremediation, types, and strategies conventionally adopted in contaminant clean-up. It identified that natural attenuation and biostimulation are faced with notable limitations including the poor remedial outcome under the natural attenuation system and the residual contamination occasion following a biostimulation operation. It remarks that the use of genetically engineered microorganisms shows a potentially promising insight as a prudent remedial approach but is currently challenged by few ethical restrictions and the rural unavailability of the technology. It underscores that bioaugmentation, particularly the use of high cell density assemblages referred to as microbial consortia possess promising remedial prospects thus offers a more sustainable environmental security. The authors, therefore, recommend bioaugmentation for large scale contaminated sites in regions where environmental degradation is commonplace.

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.