Abiotic factors controlling bioavailability and bioaccessibility of polycyclic aromatic hydrocarbons in soil: Putting together a bigger picture

Spatial variations and vertical migration potentials of petroleum hydrocarbons with varying chain lengths in soils of different depths: Roles of solid and dissolved organic matters and soil texture

Petroleum hydrocarbons (PHs) in contaminated sites may threaten human health and ecological safety, while the environmental behaviors of PHs with varying carbon chains and critical influencing factors need to be elucidated, thus facilitating efficient risk management. This study explored the occurrence characteristics and spatial variations of different PHs at the depths of 0-10 m in an abandoned industrial site, as well as evaluated the effects of solid organic matter (SOM), dissolved organic matter (DOM), and soil texture on the migration potentials of PHs with varying carbon chains. Furthermore, the leaching potentials of different PHs were integrated into their risk assessment. The total concentrations of PHs in contaminated soils ranged from 22.7 to 397 mg/kg in contaminated soils, and the long-chained PHs (C22-C40) represented the major components with an average contribution of 46.9 %, followed by short-chained PHs (C10-C12, 32.0 % average) and medium-chained PHs (C13-C21, 21.1 % average). As soil depth increased, a declining trend was observed in the proportions of long-chain PHs, with an augmentation in the relative abundance of short-chain PHs. The random forest model coupling redundancy analysis showed that SOM was the major contributor to the occurrence and vertical attenuation of PHs with longer carbon chains in underground soils, with clay component exerting a greater additional effect than silt and sand components. While DOM in soils exhibited a relatively high contribution to the retention of PHs with short carbon chains. These results demonstrated the significant influence of SOM and clay on the behavior of long-chained PHs and DOM on that of short-chained ones. Besides direct human exposure, the leaching potentials of PHs, particularly short-chained ones, in soils should be considered for a more comprehensive risk assessment. The findings of this study may assist with the behavior modelling and prediction of different PHs as well as the corresponding risk control.

Machine learning models to predict the bioaccessibility of parent and substituted polycyclic aromatic hydrocarbons (PAHs) in food: Impact on accurate health risk assessment

Food intake is the primary pathway for polycyclic aromatic hydrocarbons (PAHs) to enter the human body. Once ingested, PAHs tend to accumulate, posing health risks. To accurately assess the risk of PAHs from food, concentrations of 10 parent PAHs (PPAHs) and 15 substituted PAHs (SPAHs) were detected across 34 commonly consumed foods. Results indicated that SPAHs concentrations (3.89-11.6 ng/g dw) were higher than PPAH concentrations (1.66-3.43 ng/g dw) in shrimp and shellfish and freshwater fish. Four machine learning algorithms were used to predict the bioaccessibility of PAHs in foods, with the random forest model performing the best (R2 =0.987, RMSE=5.99). Feature variable importance analysis revealed that lipid and protein contents in food are critical variables influencing PAH bioaccessibility. Subsequently, the bioaccessibility of 25 PAHs in various foods was predicted to explore its impact on health risk assessment. Consequently, the carcinogenic risks considering bioaccessibility (5.62 ×10-5-7.12 ×10-5) was about an order of magnitude lower than that ignoring bioaccessibility (1.52 ×10-4-1.69 ×10-4), yet it still exceeded 10⁻6, indicating potential carcinogenic risks. Although PPAHs and alkylated PAHs were predominant in foods, 6-nitrochrysene was the main compound inducing both non-carcinogenic and carcinogenic risks owing to its high toxicity. This study developed a novel method for assessing pollutant bioaccessibility and evaluating its impact on health risk assessment, which provides a valuable model for managing massive hazardous pollutants and is essential for improving the accuracy of health risk assessment.

Dermal absorption of high molecular weight parent and alkylated polycyclic aromatic hydrocarbons from manufactured gas plant soils using in vitro assessment

An enhanced in vitro human dermal bioavailability method was developed to measure the release of twenty parent and seven alkylated high molecular weight (HMW) polycyclic aromatic hydrocarbons (PAHs) from contaminated soils collected from five former manufactured Gas Plants (MGP) in England. GC-MS/MS was used to quantify HMW PAHs in soil, Strat-M artificial membrane representing skin, and synthetic receptor solution (RS) representing systemic circulation at 1-h, 10-h, and 24-h timesteps. Fluoranthene and pyrene exhibited the highest fluxes from soils to membrane (ranging from 9.5 - 281 ng/cm2/h) and soil to RS ( Collapse

Key Words

• Bioavailability
• PAH
• Percutaneous
• Risk assessment

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MESH Headings

• Humans
• Polycyclic Aromatic Hydrocarbons/analysis
• Soil
• Tandem Mass Spectrometry
• Molecular Weight
• Soil Pollutants/analysis
• Pyrenes
• Environmental Monitoring/methods
• Fluorenes

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Grants Collapse

Affiliation(s)

Alison M Williams-Clayson British Geological Survey, Keyworth, Nottinghamshire, UK; University of Nottingham, Nottingham, UK
Christopher H Vane British Geological Survey, Keyworth, Nottinghamshire, UK
Matthew D Jones University of Nottingham, Nottingham, UK
Russell Thomas WSP UK, Bristol, UK
Christopher Taylor National Grid, Warwick, UK
Darren J Beriro British Geological Survey, Keyworth, Nottinghamshire, UK.

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Determining buffering capacity of polydimethylsiloxane-based passive dosing for hydrophobic organic compounds in large-volume bioassays

Polydimethylsiloxane (PDMS) is the most widely used material for passive dosing. However, the ability of PDMS to maintain constant water concentrations of chemicals in large-volume bioassays was insufficiently investigated. In this study, we proposed a kinetic-based method to determine the buffering capacity of PDMS for maintaining constant water concentrations of hydrophobic organic contaminants (HOCs) in large-volume bioassays. A good correlation between log Kow and PDMS-water partitioning coefficients (log KPW) was observed for HOCs with log Kow values ranging from 3.30 to 7.42. For low-molecular-weight HOCs, volatile loss was identified as the primary cause of unstable water concentrations in passive dosing systems. Slow desorption from PDMS resulted in a reduction of water concentrations for high-molecular-weight HOCs. The volume ratio of PDMS to water (RV) was the key factor controlling buffering capacity. As such, buffering capacity was defined as the minimum RV required to maintain 90% of the initial water concentration and was determined to be 0.0076-0.032 for six representative HOCs. Finally, passive dosing with an RV of 0.014 was validated to effectively maintain water concentrations of phenanthrene in 2-L and 96-h toxicity tests with adult mosquitofish. By determining buffering capacity of PDMS, this study recommended specific RV values for cost-efficient implementation of passive dosing approaches in aquatic toxicology, particularly in large-volume bioassays.

The high dose of biochar reduces polycyclic aromatic hydrocarbons losses during co-composting of sewage sludge and wheat straw

The aim of the study was to investigate the effect of the biochar (BC) dose on solvent extractable (Ctot) and freely dissolved (Cfree) polycyclic aromatic hydrocarbons (PAHs) content during co-composting. A significantly better reduction of Σ16 Ctot PAHs after 98 days occurred during composting with BC (for 1% of BC - 44% and for 5% of BC - 23%) than in the control (15%). Despite the relatively high reduction of Ctot PAHs in the experiment with 5% BC rate, the content of the PAHs was still the highest compared to other variants. Regarding Cfree PAHs, 5% rate of BC resulted in the best reduction of PAHs, while the 1% BC dose resulted in a lower reduction of Cfree than the control. For 1% BC, PAHs losses was more effective, and sequestration processes played a less significant role than in the experiment with 5% dose of BC. The total and dissolved organic carbon, and ash were predominantly responsible for Ctot and Cfree losses, and additionally pH for Cfree. The results of the experiment indicate that BC performs a crucial role in composting, affecting the Ctot and Cfree PAHs in the compost but the final effect strictly depends on the BC dose.

Characterisation of former manufactured gas plant soils using parent and alkylated polycyclic aromatic hydrocarbons and Rock-Eval(6) pyrolysis

Soils sampled from 10 former manufactured gas plants (MGP) in the UK were investigated using gas chromatography mass spectrometry (GC-MS/MS) and Rock-Eval (6) Pyrolysis (RE). RE is a screening tool used to characterise bulk organic matter in soils via the release of carbon compounds during pyrolysis and oxidation. Both the distributions and concentrations of 30 parent and 21 alkylated polycyclic aromatic hydrocarbons (PAHs) and the parameters of RE were analysed to establish relationships between soils and the MGP processes history. Principal component analysis (PCA) using the PAHs distributions and RE parameters can assist with differentiating between MGP processes. MGP processes utilizing oil provided the clearest results, attributed to petrogenic signatures with high proportions of low molecular weight PAHs. Processes using lower temperature processes were distinguished by higher proportions of high molecular weight PAHs. RE parameters alone were unable to distinguish MGP processes but showed potential in estimating the lability and thus the amount of PAH that could be released from soils. This research provides new insights that may be useful in understanding and characterising the risks posed to human health from PAHs in soils.

Distribution, sources, ecological risk and microbial response of polycyclic aromatic hydrocarbons in Qingdao bays, China

Bay ecosystem has garnered significant attention due to the severe threat posed by organic pollutants, particularly polycyclic aromatic hydrocarbons (PAHs). However, there is a dearth of information regarding the extent of PAHs pollutant risk and its impact on microbial communities and metabolism within this environment. In this study, the distribution, sources, ecological risk, and microbial community and metabolic response of PAHs in Jiaozhou Bay, Aoshan Bay, and Lingshan Bay in Qingdao, China were investigated. The results showed that the average concentration of ∑PAHs ranged from 120 to 614 ng/L across three bays, with Jiaozhou and Aoshan Bay exhibiting a higher risk than Lingshan Bay due to an increased concentration of high-molecular-weight PAHs. Further analysis revealed a negative correlation between dissolved organic carbon concentration and ∑PAHs concentration in water. Metagenomic analysis demonstrated that higher levels of PAHs can lead to decreased microbial diversity, while the abundance of PAHs-degrading bacteria is enhanced. Additionally, the Erythrobacter, Jannaschia and Ruegeria genera were found to have a significant correlation with low-molecular-weight PAH concentrations. In terms of microbial metabolism, higher PAH concentrations were beneficial for carbohydrate metabolic pathway but unfavorable for amino acid metabolic pathways and membrane transport pathways in natural bay environments. These findings provide a foundation for controlling PAHs pollution and offer insights into the impact of PAHs on bacterial communities and metabolism in natural bay environments.

Phytotoxicity of weathered petroleum hydrocarbons in soil to boreal plant species

Canada has extensive petroleum hydrocarbon (PHC) contamination in northern areas and the boreal forest region from historical oil and gas activities. Since the 2013 standardization of boreal forest species for plant toxicity testing in Canada, there has been a need to build the primary literature of the toxicity of weathered PHCs to these species. A series of toxicity experiments were carried out using fine-grained (<0.005-0.425 mm) background (100 total mg/kg total PHCs) and weathered contaminated soil (11,900 mg/kg total PHCs) collected from a contaminated site in northern Ontario, Canada. The PHC mixture in the contaminated site soil was characterized through Canadian Council of Ministers of the Environment Fractions, as indicated by the number equivalent normal straight-chain hydrocarbons (nC). The soil was highly contaminated with Fraction 2 (>nC10 to nC16) at 4790 mg/kg and Fraction 3 (>nC16 to nC34) at 4960 mg/kg. Five plant species (Elymus trachycaulus, Achillea millefolium, Picea mariana, Salix bebbiana, and Alnus viridis) were grown from seed in 0%, 25%, 50%, 75%, and 100% relative contamination mixtures of the PHC-contaminated and background soil from the site over 2-6 weeks. All five species showed significant inhibition in shoot length, shoot weight, root length, and/or root weight (Kruskal-Wallis Tests: p < 0.05, df = 4.0). Measurements of 25% inhibitory concentrations (IC25) following PHC toxicity experiments revealed that S. bebbiana was most significantly impaired by the PHC-contaminated soil (410-990 mg/kg total PHCs), where it showed <35% germination. This study indicates that natural weathering of Fraction 2- and Fraction 3-concentrated soil did not eliminate phytotoxicity to boreal plant species. Furthermore, it builds on the limited existing literature for toxicity of PHCs on boreal plants and supports site remediation to existing Canadian provincial PHC guidelines.

Effects of source materials on desorption kinetics of carcinogenic PAHs from contaminated soils

Research investigating the desorptive behaviour of PAHs from contaminated soils often overlooked the effects of source materials, especially coal tar and coal tar pitch and materials alike. In this study, a refined experimental approach was adopted to establish a simple-to-complex continuum of systems that allow the investigation of desorption kinetics of benzo(a)pyrene (BaP) and 3 other carcinogenic PAHs (cPAHs) over an incubation period of 48 d. By comparing the modelled desorption parameters, elucidation of the effects of PAH source materials on their desorptive behaviour was achieved. Desorption of cPAHs from coal tar and pitch was enhanced when they were added to soils, with rapidly desorbing fraction (F_rap) of BaP increased from 0.68% for pitch to 1.10% and 2.66% for pitch treated soils, and from 2.57% for coal tar to 6.24% for coal tar treated soil G and 8.76% for coal tar treated sand (1 d). At 1 d, desorption of target cPAHs from solvent and source material spiked soils generally followed the order of solvent > coal tar > pitch. Increases in F_rap of cPAHs were observed in coal tar-treated soils after 48 d soil incubation (0.33%-1.16% for soil M, p ≥ 0.05, 6.24%-9.21% for soil G, p < 0.05) and was attributed to the continuous migration of coal tar as a non-aqueous phase liquid (NAPL) into soil pore structures. Slow desorption was dominated by source materials, whereas the extents and rates of rapid desorption (F_rap and k_rap) were more controlled by the quantity of soil organic matter (SOM), rather than quality of SOM (as in solvent-spiked soils). The results of this study challenged the role of PAH source materials as 'sinks' and led to the proposed roles of coal tar and pitch and source materials alike as 'reservoirs' with a risk-driven perspective.

Bioaccessibility and bioavailability of NPAHs in soils using in vitro-in vivo assays: Comparison of laboratory and outdoor environmental aging effect

Aging process is one of the most important factors that markedly reduces bioaccessibility and bioavailability (bioac-bioav) of organic contaminants. However, only few data on comparison of the effects of laboratory artificial aging (LAA) and outdoor environmental aging (OEA) processes on nitrated polycyclic aromatic hydrocarbons (NPAHs) bioac-bioav are available. In the current study, oral bioac-bioav of NPAHs in LAA and OEA soils (aging time intervals: 0, 45, 90, 120 and 150 d) were measured by in vitro traditional Fed ORganic Estimation human Simulation Test (FOREhST) and Tenax improved FOREhST (TI-FOREhST) methods, and in vivo mouse model. Tenax significantly increased the bioaccessibility of NPAHs in freshly spiked and aging soils from 0.3-40.9 % to 15.6-95.3 %, and 0.3-40.9 % to 1.0-84.5 %, respectively. Aging significantly reduced the NPAHs bioaccessibility (from 36.5 % to 10.7 %, and 12.1 % to 5.1 % as measured by FOREhST and TI-FOREhST, respectively) and bioavailability (from 27.7 % to 9.9 %, as measured by mouse model). The changes in bioac-bioav were mainly observed within the first 120 d of aging. The statistical analyses of NPAHs bioac-bioav showed no significant difference (p > 0.05) among the aging time intervals in LAA and OEA soils, which demonstrated that the LAA can relatively represent the OEA. Determination of TOC content in LAA and OEA soil can intuitively reflect whether the difference of NPAHs bioac-bioav between two aging treatment groups is significant. The mean bioaccessibility of NPAHs in soil measured by TI-FOREhST (mean 20.6 %) is closer to the bioavailability measured by mouse model (mean 19.4 %), indicating that Tenax improved in vitro method is more reliable than traditional methods, to predict the bioavailability of NPAHs.

Impact of biochar coexistence with polar/nonpolar microplastics on phenanthrene sorption in soil

Microplastics and biochar normally coexist in soil. In this study, two microplastics of different polarities (nonpolar polyethylene (PE) and polar polybutylene adipate-co-terephthalate (PBAT)) and two wheat straw biochars produced at 400 (W4) and 700 °C (W7) were selected to investigate the sorption behaviors of phenanthrene in soil where microplastics and biochar coexisted. The results showed that the presence of PE more significantly weakened the adhesion of soil particles onto biochar than the presence of PBAT. Meanwhile, the presence of biochar enhanced the soil particle attachment on the microplastic surface. As a result, the sorption behavior of phenanthrene was significantly different in soil where biochar coexisted with microplastics of different polarities. The K_oc values of PE-biochar-soil mixtures at C_e= 0.005 C_s were up to 42 % lower than those of PBAT-biochar-soil mixtures, which is related to lower micropore area of particles isolated from the former. However, at C_e = 0.05 C_s and 0.5 C_s, the K_oc values of PE-biochar-soil mixtures were up to 1.4 times higher than those of PBAT-biochar-soil mixtures because of a more significant reduction in biochar surface polarity when it coexisted with nonpolar PE.

Novel insights into the predominant factors affecting the bioavailability of polycyclic aromatic hydrocarbons in industrial contaminated areas using PLS-developed model

Bioavailability is recognized as a useful technical standard for risk assessment and pollution rehabilitation. However, knowledge on the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in contaminated site soils is still limited, especially concerning the influential mechanism. With an abundance of soil collections from nine industrial areas in China, the bioavailabilities, as conceptually defined as bioconcentration factors (BCFs) of PAHs were analyzed using biomimetic extraction of hydroxypropyl-β-cyclodextrin (HPCD). Apart from the total content of PAHs varying with the different pyrogenic sources, the BCFs were greatly dependent on the soil physicochemical properties from the spatial scale and inversely proportional to the number of rings. Pearson correlation analysis indicated a weak relationship between bioavailability and the soil dissolved organic matter (DOM), pH and particle size. To incorporate the soil physicochemical properties and structural characteristics of PAHs determined by density functional theory (DFT), the optimum model for bioavailability was developed for BCFs by partial least square (PLS) analysis. The PLS-derived model was shown to be predictive within the applicability domain (AD). The structural characteristics, e.g., molecular polarizability and frontier orbital energy level that favor the soil adsorption of PAH isomers via dispersion interactions, and electron exchanges were indicated to be more impactful on bioavailability than soil environmental factors. However, soil factors should not be neglected, because the pH, DOM, etc. were significantly influential. It makes sense that the higher DOM causes greater bioavailability via increasing the free-dissolved fractions of PAHs. Interestingly, the effect of pH on bioavailability was spectrally validated by excitation-emission matrix (EEM) fluorescence, showing that the interaction between DOM and pyrene strengthened the fluorescence quenching of chromophores with the decline in pH.

Chemical properties of soil determine the persistence and bioavailability of polycyclic aromatic hydrocarbons in sewage sludge- or sewage sludge/biomass-derived biochar-amended soils

In this study the persistence (organic solvent extractable) and bioavailability (freely dissolved) of polycyclic aromatic hydrocarbons (PAHs) in soils with various properties amended with sewage sludge (BCSSL)- or sewage sludge/biomass (BCSSLW)-derived biochars was examined. Biochars produced at 600 °C were applied to soils (acidic, neutral, or alkaline) at a dose of 2% and subsequently incubated for 180 days. Here, the use of biochars regarding the soil's type was examined for the first time. Depending on the soil pH and the feedstock, the content of sum of 16 organic solvent extractable PAHs was found to decrease from 7.5 to 37% (soil + BCSSL) and from 24 to 40% (soil + BCSSLW). The decrease in the content of sum of 16 freely dissolved PAHs ranged from 18 to 36% (soil + BCSSL) and from 17 to 54% (soil + BCSSLW). In acidic BCSSL-amended soil and the alkaline BCSSLW-amended soil no statistically significant differences in the content of sum of 16 freely dissolved PAHs were noted between the beginning and end of the study. BCSSLW was characterized by a greater reduction content of organic solvent extractable PAHs in the acidic and alkaline soils, while in the neutral one - BCSSL. In turn, a larger reduction in freely dissolved PAH content in the acidic and neutral soils could be seen in the presence of BCSSLW, whereas in the alkaline soil in the presence of BCSSL. The persistence and bioavailability of PAHs in the biochar-amended soils were closely related to the chemical properties of these soils. This was confirmed by numerous statistically significant (P ≤ 0.05) relationships between organic solvent extractable PAHs and pH, cation exchange capacity, available magnesium, potassium and phosphorus, and dissolved organic carbon as well as between freely dissolved PAH and pH, dissolved organic carbon, available potassium and phosphorus content, and electrical conductivity.

Anthropogenic impact on soils of protected areas-example of PAHs

The following study presents the concentrations of 10 individual polycyclic aromatic hydrocarbons (PAHs10) and the total concentration of PAHs (ΣPAHs) determined in soils of protected areas in Southern Poland (EU). The protected areas discussed here include 5 landscape parks and 5 nature reserves located in the Małopolska region. Surface soil samples were collected at 39 locations characterised by different natural conditions and different human pressure level. The samples were used to determine the contents of anthracene, benzo[a]pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[g,h,i]perylene, chrysene, dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene and naphthalene. These concentrations of individual PAHs ranged from < 0.005 to 6.34 mg/kg. When considering the legal regulations currently in force, this meant that permissible values were exceeded in 23% of the samples analysed, while increased concentrations were found in another 26% of the samples. The limit values were exceeded most in the case of benzo[b]fluoranthene and benzo[a]pyrene. This occurred with samples collected in the vicinity of transportation routes, mainly local roads. In the case of naphthalene and anthracene, the limit values were not exceeded. Increased or excessive PAHs concentrations do not occur in the vicinity of major industrial plants located near the boundaries of individual landscape parks, which indicates how these pollutants spread. The paper also points to the need to develop new legal solutions to improve the method for assessing PAHs concentrations and their impact on valuable natural areas.

Bioaccessibility and Toxicity Assessment of Polycyclic Aromatic Hydrocarbons in Two Contaminated Sites

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous soil contaminants, and their bioaccessibility determines their environmental risks in contaminated land. In the present study, the residual concentrations of PAHs in the soils of two industrial sites were determined, and their bioaccessibility was estimated by the hydroxypropyl-β-cyclodextrin extraction (HPCD) extraction method. The results showed heavy PAH contamination at both site S1 (0.38-3342.5 mg kg^-1) and site S2 (0.2-138.18 mg kg^-1), of which high molecular weight (HMW) PAHs (4-, 5-, and 6-ring compounds) accounted for approximately 80%. The average bioaccessibility of PAHs at sites S1 and S2 was 52.02% and 29.28%, respectively. The bioaccessibility of certain PAH compounds decreased with increasing ring number of the molecule. Lower PAH bioaccessibility was detected in loamy and silty soil textures than in sandy soil. Moreover, among the soil properties, the dissolved organic matter, total organic carbon, total potassium, and total manganese concentrations had significant effects on the bioaccessibility of PAHs. The toxicity analysis showed that the composition and bioaccessibility of PAHs could affect their potential toxicity in soil. We suggest that bioaccessibility should be taken into consideration when assessing the toxicity of PAHs in soil, and more attention should be given to low-ring PAHs with high bioaccessibility.

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.

Thermally enhanced bioremediation: A review of the fundamentals and applications in soil and groundwater remediation

Thermally enhanced bioremediation (TEB), a new concept proposed in recent years, explores the combination of thermal treatment and bioremediation to address the challenges of the low efficiency and long duration of bioremediation. This study presented a comprehensive review regarding the fundamentals of TEB and its applications in soil and groundwater remediation. The temperature effects on the bioremediation of contaminants were systematically reviewed. The thermal effects on the physical, chemical and biological characteristics of soil, and the corresponding changes of contaminants bioavailability and microbial metabolic activities were summarized. Specifically, the increase in temperature within a suitable range can proliferate enzymes enrichment, extracellular polysaccharides and biosurfactants production, and further enhancing bioremediation. Furthermore, a systematic evaluation of TEB applications by utilizing traditional in situ heating technologies, as well as renewable energy (e.g., stored aquifer thermal energy and solar energy), was provided. Additionally, TEB has been applied as a biological polishing technology post thermal treatment, which can be a cost-effective method to address the contaminants rebounds in groundwater remediation. However, there are still various challenges to be addressed in TEB, and future research perspectives to further improve the basic understanding and applications of TEB for the remediation of contaminated soil and groundwater are presented.

Accumulation of high-molecular-weight polycyclic aromatic hydrocarbon impacted the performance and microbial ecology of bioretention systems

Bioretention has been considered as an effective management practice for urban stormwater in the removal of pollutants including polycyclic aromatic hydrocarbons (PAHs). However, the accumulation of high-molecular-weight (HMW) PAHs in bioretention systems and their potential impact on the pollutants removal performance and microbial ecology are still not fully understood. In this study, comparisons of treatment effectiveness, enzyme activity and microbial community in bioretention systems with different types of media amendments were carried out at different spiking levels of pyrene (PYR). The results showed that the removal efficiencies of chemical oxygen demand (COD) and total nitrogen in the bioretention systems were negatively impacted by the PYR levels. The relative activities of soil dehydrogenase and urease were increasingly inhibited by the elevated PYR level, indicating the declining microbial activity regarding organic matter decomposition. The spiking of PYR negatively affected microbial diversity, and distinct time- and influent-dependent changes in microbial communities were observed. The relative abundance of PAH-degrading microorganisms increased in PYR-spiked systems, while the abundance of nitrifiers decreased. The addition of media amendments was beneficial for the enrichment of microorganisms that are more resistant to PYR-related stress, therefore elevating the COD concentration removal rate by ∼50%. This study gives new insight into the multifaceted impacts of HMW PAH accumulation on microbial fingerprinting and enzyme activities, which may provide guidance on better stormwater management practices via bioretention in terms of improved system longevity and performance.

Biosurfactants and chemotaxis interplay in microbial consortium-based hydrocarbons degradation

Hydrocarbons are routinely detected at low concentrations, despite the degrading metabolic potential of ubiquitous microorganisms. The potential drivers of hydrocarbons persistence are lower bioavailability and mass transfer limitation. Recently, bioremediation strategies have developed rapidly, but still, the solution is not resilient. Biosurfactants, known to increase bioavailability and augment biodegradation, are tightly linked to bacterial surface motility and chemotaxis, while chemotaxis help bacteria to locate aromatic compounds and increase the mass transfer. Harassing the biosurfactant production and chemotaxis properties of degrading microorganisms could be a possible approach for the complete degradation of hydrocarbons. This review provides an overview of interplay between biosurfactants and chemotaxis in bioremediation. Besides, we discuss the chemical surfactants and biosurfactant-mediated biodegradation by microbial consortium.

Impact of lignocellulosic waste-immobilised white-rot fungi on enhancing the development of 14C-phenanthrene catabolism in soil

In this study, an investigation was carried out to explore the the impact of white-rot fungi (WRF) on enhancing the development of phenanthrene catabolism in soil over time (1, 25, 50, 75 and 100 d). The WRF were immobilised on spent brewery grains (SBG) prior to inoculation to the soil. The results showed that SBG-immobilised WRF-amended soils reduced the lag phases and increased the extents of ¹⁴C-phenanthrene mineralisation. Greater reductions in the lag phases and increases in the rates of mineralisation were observed in immobilised Trametes versicolor-amended soil compared to the other WRF-amendments. However, the presence of Pleurotus ostreatus and Phanerochaete chrysosporium influenced biodegradation more strongly than the other fungal species. In addition, fungal enzyme activities increased in the amended soils and positively correlated with the extents of ¹⁴C-phenanthrene mineralisation in all soil amendments. Maximum ligninolytic enzyme activities were observed in P. ostreatus-amended soil. Microbial populations increased in all amended soils while PAH-degrading fungal numbers increased with increased soil-PAH contact time and strongly positively correlated with fastest rates of mineralisation. The findings presented in this study demonstrate that inoculating the soil with these immobilised WRFs generally enhanced the mineralisation of the ¹⁴C-phenanthrene in soil. This has the potential to be used to stimulate or enhance PAH catabolism in field-contaminated soils.

Fate of PAHs in treated wastewater reused as irrigation water: Environmental risks in water-soil-ryegrass multimedia system

The main aim of this study was to determine the fate, bio-metabolism and environmental risk of low-ring and high-ring polycyclic aromatic hydrocarbons (PAHs) in a water-soil-ryegrass multi-media system, under long-term irrigation condition with micro-polluted treated wastewater. Field experiments were carried out to simulate garden irrigation using treated wastewater containing typical representative low-ring naphthalene (Nap) and high-ring benzo[a]pyrene (BaP). The results showed that BaP's vertical attenuation rate and adsorption accumulation rate were 1.7 and 1.2 times higher than Nap's, respectively. The adsorption, biodegradation, and the rhizosphere effect were responsible for 40.7%, 28.4%, 21.6%, and 30.5%, 36.6%, 17.7%, respectively, of the attenuation of BaP and Nap. The major metabolic pathways of Nap and BaP are hydroxylation, ring opening cleavage, and decarboxylation, with the metabolic chain of BaP being longer than that of Nap due to more ring cleaving reactions. Pseudomonas, Mycobacterium, and Sphingomonas were the functional microorganisms with PAHs degradation capacity that were positively correlated with PAHs degradation, particularly in the rhizosphere. After ten years of irrigation with treated wastewater, the prediction of environmental risk revealed that there were few potential risks. Thus, the results of this feasibility study demonstrated that using treated wastewater for garden irrigation was a relatively safe and effective strategy.

Evaluating the adsorption performance of Tenax TA® in different containers: An isolation tool to study the bioaccessibility of nitro-PAHs in spiked soil

The improved in vitro gastrointestinal simulation methods, with the addition of the adsorption sink, are considered as a promising tool for predicting the bioaccessibility of contaminants. However, the problem associated with the recovery of the adsorption sink from the complex matrix needs more understand. Although previous studies tried to solve this shortcoming by using the containers (a vessel to hold the adsorption sink), there is no systematic comparison study on the impact of containers on bioaccessibility till now, especially for nitro-polycyclic aromatic hydrocarbons (nitro-PAHs). In order to understand the problem, commonly used containers in previous studies (dialysis bags and stainless-steel screen) were selected and deployed in the Fed Organic Estimation Human Simulation Test (FOREhST) method to compare the effects of these containers on the bioaccessibility of nitro-PAHs desorbed from the five different types of soils into the gastrointestinal fluid (GIF). Results showed that in order to maintain a constant sorptive gradient for the high molecular weight (MW) nitro-PAHs, 0.25 g of Tenax TA® were required in FOREhST. Compared with Tenax TA® encapsulated in dialysis bag (Tenax-EDBG), the use of Tenax TA® encapsulated in dissolution basket (Tenax-EDBT) significantly increased the bioaccessibility of nitro-PAHs in the soil from 5.6-31.4% to 17.2-70.6%, due to the better diffusion performance. The bioaccessibility of nitro-PAHs by FOREhST extraction with Tenax-EDBT showed a significant negative correlation with soil total organic carbon (TOC), whereas a weak correlation with pH. This study provides the researchers with a more standardized in vitro method to quantify the bioaccessibility of PAHs and their derivatives in soil.

Active iron species driven hydroxyl radicals formation in oxygenation of different paddy soils: Implications to polycyclic aromatic hydrocarbons degradation

The frequently occurring redox fluctuations in paddy soil are critical to the cycling of redox-sensitive elements (e.g., iron (Fe) and carbon) due to the driving of microbial processes. However, the associated abiotic process, such as hydroxyl radical (^•OH) formation, was rarely investigated. Hence, we examined the under-appreciated role of ^•OH formation in driving polycyclic aromatic hydrocarbons (PAHs) degradation upon oxygenation of anoxic paddy slurries. Results showed that ^•OH production largely differed in different paddy slurries, in the range of 271.5-581.2 μmol kg^-1 soil after 12 h reaction. The ^•OH production was highly hinged on the contents of active Fe species, i.e., exchangeable, surface-bound Fe and Fe in low-crystalline phases rather than Fe in high-crystalline minerals or silicates. Besides, ^•OH production significantly decreased with increasing soil depth due to the declined active Fe species and abundance of functional microbes. Oxygenation also induced the transformation of these active Fe species into the low- and high-crystalline phases, which might affect the following redox process. The produced ^•OH can efficiently degrade PAHs with degradation extents depending on their physiochemical properties. Our findings highlight the key roles of active Fe species in driving ^•OH formation and organic contaminants degradation during redox fluctuations of paddy soils.

Mitigation of petroleum-hydrocarbon-contaminated hazardous soils using organic amendments: A review

The term "Total petroleum hydrocarbons" (TPH) is used to describe a complex mixture of petroleum-based hydrocarbons primarily derived from crude oil. Those compounds are considered as persistent organic pollutants in the terrestrial environment. A wide array of organic amendments is increasingly used for the remediation of TPH-contaminated soils. Organic amendments not only supply a source of carbon and nutrients but also add exogenous beneficial microorganisms to enhance the TPH degradation rate, thereby improving the soil health. Two fundamental approaches can be contemplated within the context of remediation of TPH-contaminated soils using organic amendments: (i) enhanced TPH sorption to the exogenous organic matter (immobilization) as it reduces the bioavailability of the contaminants, and (ii) increasing the solubility of the contaminants by supplying desorbing agents (mobilization) for enhancing the subsequent biodegradation. Net immobilization and mobilization of TPH have both been observed following the application of organic amendments to contaminated soils. This review examines the mechanisms for the enhanced remediation of TPH-contaminated soils by organic amendments and discusses the influencing factors in relation to sequestration, bioavailability, and subsequent biodegradation of TPH in soils. The uncertainty of mechanisms for various organic amendments in TPH remediation processes remains a critical area of future research.

Soil concentrations, compositional profiles, sources and bioavailability of polychlorinated dibenzo dioxins/furans, polychlorinated biphenyls and polycyclic aromatic hydrocarbons in open municipal dumpsites of Chennai city, India

Several studies have reported the release of halogenated persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons (PAHs) associated with open burning of municipal solid waste. Considering soil as a sink for such organic contaminants, we conducted an in-depth study on the surface soil concentrations of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs), polychlorinated biphenyls (PCBs) and sixteen USEPA enlisted PAHs collected from thirteen zones of the two major municipal dumpsites, Kodungaiyur dumpsite (KDS) and Perungudi dumpsite (PDS) of Chennai city. Indigenous microbes from dumpsite soil samples were isolated and identified based on 16S rRNA sequencing and phylogenetic analysis. Using indigenous microbes, we have elucidated the bioavailability of the targeted organic pollutants for each site.Range of Σ₁₇PCDD/Fs, Σ₂₅PCBs and ∑₁₆PAHs varied between 3.96-612 pg/g (96.0 pg/g; median), ND-182 ng/g (6.35 ng/g; median) and 0.62-3649 ng/g (64.3 ng/g; median), respectively. All the dumpsite samples showed bioavailability for POPs and PAHs. Toxicity equivalent values (TEQs) associated with dioxin-like PCBs and PCDD/Fs from the zones where dumped municipal solid wastes were collected from electronic-waste/IT-corridor/port areas and toxic PAHs from the zone receiving wastes from the industrial corridor of the city were higher than the soil permissible limit prescribed by the World Health Organization.

Past, present, and future perspectives on the assessment of bioavailability/bioaccessibility of polycyclic aromatic hydrocarbons: A 20-year systemic review based on scientific econometrics

Bioaccessibility/bioavailability (bioac-bioav) is an important criterion in the risk assessment of polycyclic aromatic hydrocarbons (PAHs), especially in the restoration of contaminated sites. Although, the bioac-bioav concept is widely employed in PAH risk assessment for both humans and wildlife, their growth and integration in risk assessment models are seldom discussed. Consequently, the relevant literature listed on Web of Science (WOS)™ was retrieved and analyzed using the bibliometric software Citespace in order to gain a comprehensive understanding of this issue. Due to the limitations of the literature search software, we manually searched the articles about PAHs bioac-bioav that were published before 2000. This stage focuses on research on the distribution coefficient of PAHs between different environmental phases and laid the foundation for the adsorption-desorption of PAHs in subsequent studies of the bioac-bioav of PAHs. The research progress on PAH bioac-bioav from 2000 to the present was evaluated using the Citespace software based on country- and discipline-wise publication volumes and research hotspots. The development stages of PAH bioac-bioav after 2000 were divided into four time segments. The first three segments (2000-2005, 2006-2010, and 2011-2015) focused on the degradation of PAHs and their in vivo (bioavailability)-in vitro (bioaccessibility) evaluation method and risk assessment. Meanwhile, the current (2016-present) research focuses on the establishment of analytical methods for assessing PAH derivatives at environmental concentrations and the optimization of various in vitro digestion methods, including chemical optimization (sorptive sink) and biological optimization (Caco-2 cell). The contents are aimed at supplying researchers with a deeper understanding of the development of PAH bioac-bioav.

Distribution and bioaccessibility of polycyclic aromatic hydrocarbons in industrially contaminated site soils as affected by thermal treatment

Thermal treatment can not only efficiently remove volatile pollutants but also distinctly alter the speciation of organic carbon (C) and the behaviors of residual pollutants in contaminated soils. Here we examined the distribution and bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in industrially contaminated site soils affected by thermal treatment (temperature ranging of 105-650 ℃) using synchrotron-based infrared microspectroscopy and n-butanol extraction (a mild solvent extractant). In the pristine soils, the sequestration and distribution of PAHs were simultaneously controlled by aromatic C, aliphatic C and clay minerals. Desorption efficiency of PAHs was substantially increased with increasing temperature, whereas the residual PAHs were strongly immobilized within their binding sites evidenced by their dramatically decreased bioaccessibility. Aliphatic and carboxylic C were gradually decomposed and/or carbonized with increasing temperature. In contrast, aromatic C remained relatively recalcitrant during the thermal treatment and was the key controlling factor for the desorption of residual PAHs in the soils with either thermal treatment or n-butanol extraction. This study is the first to visualize the changes in the binding sites and bioaccessibility of PAHs induced by thermal treatment, which have important implications for understanding the sequestration mechanisms of organic pollutants in soil and optimizing the remediation technique.

Do dissipation and transformation of γ-HCH and p,p'-DDT in soil respond to a proxy for climate change? Insights from a field study on the eastern Tibetan Plateau

While the influence of climate change on the fate of persistent organic pollutants (POPs) is becoming a topic of global concern, it has yet to be demonstrated how POPs and their transformation products in soil respond to a changing climate at the local scale. We conducted a year-long field experiment with spiked soils to investigate the impact of climate on the dissipation of γ-hexachlorocyclohexane (γ-HCH) and p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT) as well as the formation of their products. Four sites along an elevational gradient on the eastern Tibetan Plateau were selected to represent four scenarios ranging from a dry and cold to a warm and humid climate. Based on the measured concentrations of the two pesticides and their transformation products, we calculated the dissipation rates of γ-HCH and p,p'-DDT in soil using two biphasic kinetic models, and the formation rates of transformation products using a mid-point rectangular approximation method. The spiked γ-HCH generally showed the expected decrease in dissipation from soils with increasing altitudes, and therefore decreasing temperature and precipitation, whereas dissipation of p,p'-DDT was influenced more by photolysis and sequestration in soil. The formation rates of the primary products of γ-HCH (i.e. γ-HCH→PeCCH and γ-HCH→TeCCH) and p,p'-DDT (i.e. p,p'-DDT→p,p'-DDE and p,p'-DDT→p,p'-DDD) indicate that a warmer and wetter climate favors dechloroelimination (anaerobic biodegradation) over dehydrochlorination (aerobic biodegradation). The significantly longer dissipation half-lives of γ-HCH at the coldest site suggests that the fate of POPs in frozen regions (e.g. polar regions) needs more attention. Overall, the fate of more volatile chemicals (e.g. γ-HCH) might be more responsive to the climate change.

Targeting the right parameters in PAH remediation studies

Contaminated land burdens the economy of many countries and must be dealt with. Researchers have published thousands of documents studying and developing soil and sediment remediation treatments. Amongst the targeted pollutants are the polycyclic aromatic hydrocarbons (PAHs), described as a class of persistent organic compounds, potentially harmful to ecosystems and living organisms. The present paper reviews and discusses three scientific trends that are leading current PAH-contaminated soil/sediment remediation studies and management. First, the choice of compounds that are being studied and targeted in the scientific literature is discussed, and we suggest that the classical 16 US-EPA PAH compounds might no longer be sufficient to meet current environmental challenges. Second, we discuss the choice of experimental material in remediation studies. Using bibliometric measures, we show the lack of PAH remediation trials based on co-contaminated or aged-contaminated material. Finally, the systematic use of the recently validated bioavailability measurement protocol (ISO/TS 16751) in remediation trials is discussed, and we suggest it should be implemented as a tool to improve remediation processes and management strategies.

Nonlinear partition of nonionic organic compounds into humus-like substance humificated from lignin

Nonlinear sorption of nonionic organic compounds (NOCs) by soil organic matter (SOM) is a significant behaviour that affecting their distribution, transport and fate in the environment. Sorption of typical NOCs, including phenols, anilines, nitrobenzenes and polycyclic aromatic hydrocarbons (PAHs) by Lig48, a humus-like substance humificated from lignin (the principal component of plant precursors of SOM), is nonlinear and without desorption hysteresis, and interpreted by nonlinear partition mechanism in this study. The positively linear relationship between sorption capacity and water solubility of NOCs is a distinguish characteristic for their nonlinear partition into Lig48. Moreover, the nonlinear partition capacity of NOCs is mainly dependent on the aromaticity of humus-like substances with a positively linear relationship, while the nonlinear partition affinity is mainly dependent on the polarity of humus-like substances with a negatively linear relationship. Competition between phenols, anilines, nitrobenzenes and PAHs was observed for their nonlinear partition into Lig48. In addition to van der Waals force, specific interactions, i.e., hydrogen-bonding and π-π interactions are responsible for the nonlinear partitioning of NOCs into humus-like substances including Lig48. These novel observations are helpful for understanding the nonlinear sorption of NOCs by SOM and elucidating the migration and transport of NOCs in the environment.

How important is abiotic dissipation in natural attenuation of polycyclic aromatic hydrocarbons in soil?

Natural attenuation capacity, as one of the most important ecosystem functions in soil, plays a vital role in the detoxification of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). However, despite the role of biodegradation is established, the contribution of abiotic dissipation to natural attenuation has long been overlooked. Herein, the abiotic dissipations of 16 types of PAHs in a past coking site and of anthracene (ANT) in various cultivated soils were studied. Results showed that the contributions of abiotic dissipation to the total attenuation were in a wide range from 11.8 to 99.7% depending on the types of PAHs. Specifically, abiotic dissipation is higher for heavy PAHs (68.3-99.7%) than for light PAHs (11.8-71.5%), with the exception of ANT (80.7%). Similarly, the contribution of abiotic dissipation to ANT attenuation ranged from 30.7 to 68.6% in eight soils. The abiotic dissipation rate of ANT followed the order of lateritic-red earth > gray-desery soil > coastal solonchaks > cumulated-irrigated soil > cinnamon soil > fluvo-aquic soil > purplish soil ~ yellow-brown earth, which was positively correlated with transition metal contents in soils. These findings demonstrated that the abiotic dissipation of PAHs is determined by both molecule properties and soil types. Overall, this work provided valuable insights into clarifying the roles of abiotic dissipation in PAH attenuation in soil.

Municipal waste landfill as a source of polychlorinated biphenyls releases to the environment

This study aimed to investigate the impact of municipal waste landfill on polychlorinated biphenyls (PCBs) release to the environment concerning groundwater flow directions. The contents of polychlorinated biphenyls in soils, plants and water were analysed at various distances from the landfill. Thanks to low solubility PCBs in water groundwater flow direction, under the landfill, have an influence on PCBs concentration in groundwater. Strong PCBs' sorption to organic matter caused that no affect groundwater flow directions on PCB content in soils and plants' tissues was observed. The largest PCBs deposition zone was located 50 m from the contamination source (landfill). Tri-CB and tetra-CB homologues were capable of migration deep into the soil profile, which could be related to the geological material from which the soils under study were developed, as well as to the properties of the PCB homologues.

Soil-sebum partition coefficients for high molecular weight polycyclic aromatic hydrocarbons (HMW-PAH)

In vitro high molecular weight polycyclic aromatic hydrocarbons (HMW-PAH) soil-sebum partition coefficients (K_SS) were measured for twelve soils collected from former UK gasworks. Concentrations of ∑16 USEPA PAH in the soils ranged from 51 to 1440 mg/kg. Time series extractions (0.5, 1, 2, 4, 8 and 24 h) at skin temperature (32 °C) of HMW-PAH from sebum to soil for two samples were conducted to determine the maximum release time-step. The maximum HMW-PAH release time-step was determined as 4 h, which was subsequently used as the extraction time for the remaining samples. The 4 h extraction for all samples showed HMW-PAH K_SS ranging from 0.06 to 1.4 (median = 0.59; mean = 0.59; standard deviation = 0.27; median absolute deviation = 0.29). Evaluation of K_SS data for the 4 h extractions showed that soil type and selected HMW-PAH properties (literature based molecular weight and octanol-carbon partition coefficients) affect the amount of HMW-PAH released from soil into sebum.

The convertion of sewage sludge to biochar as a sustainable tool of PAHs exposure reduction during agricultural utilization of sewage sludges

There is a discussion in the literature whether PAHs introduced with biochar are safe and whether they are persistent in the environment. The persistence of PAHs (C_tot - total and C_free - freely dissolved) in sewage sludge (SSL) or SSL-derived biochar-amended soils was investigated. Biochar were produced at 500, 600 and 700 °C. We also compared the persistence of PAHs in these experimental treatments depending on the plants cultivated (grass, clover and thale cress). We showed that the C_tot PAHs in the biochar-amended soils exhibited higher persistence than in the SSL-amended soil. The opposite trend was observed for C_free PAHs. A higher reduction of C_free PAHs was noted in the biochar-amended soils than in SSL-amended soil. The persistence of both C_free and C_tot PAHs clearly varied between the biochars produced at different temperatures. It should be stated that despite that for biochar the persistence of C_tot PAHs is higher compared to SSL-amended soils, an opposite trend is observed for the fraction of C_free (which is directly responsible for the toxic effect), and this entails a lower risk to the environment (lower mobility and bioavailability). The plants had a significant impact on C_tot PAHs content depending on the number of PAH rings.

Bioaccessibility of polycyclic aromatic compounds (PAHs, PCBs) and trace elements: Influencing factors and determination in a river sediment core

Organic matter (OM), clays, sand or time are factors possibly influencing the bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyls (PCBs) from sediments. An experimental design was performed to monitor and quantify this process. The bioaccessible fraction, linked to the rapidly-desorbing fraction (F_rap) of contaminants, was assessed through a non-exhaustive extraction using a carboxymethyl-β-cyclodextrin polymer. OM content was the most influential factor as regards F_rap. Clay percentage was a slightly influential factor for PAHs while the interaction sand × OM was a slightly influential factor for PCBs. F_rap was also determined in a sediment core collected from Martot's Pond (France). The higher the PAH/PCB concentration in this sediment, the higher the bioaccessible fraction. The relationship between a lower bioaccessibility and a higher number of PAHs cycles or PCB chlorines was linear. OM content impacted on F_rap only for PAHs. Sequential extractions of some trace elements were also performed to evaluate their mobility. Cu, Cr, Pb, Ni were the less bioaccessible. A great part of As, Cd and Zn was found in the most bioaccessible sediment fractions. The 40-65 cm section might be considered as the most negatively impacting on the aquatic fauna, due to Cd and Zn high bioaccessible concentrations.

Derivation of site-specific remediation goals by incorporating the bioaccessibility of polycyclic aromatic hydrocarbons with the probabilistic analysis method

Incorporating bioaccessibility into human health risk assessment is recognized as a valid way to reduce the conservative properties of conventional results, where the total concentration of a contaminant analysed by exhaustive chemical extraction is applied. Taking a coke production site in Beijing as an example, a mild chemical extraction technology was employed to profile the bioaccessibility of benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (IcP) and dibenz[ah]anthracene (DBA) in soils. The results that were regressed using two bi-phase desorption models (Karickhoff and Weibull) revealed that the rapid desorption fractions of BaP, IcP and DBA, which are taken for bioaccessible fractions, were basically less than half of the total contents in the soils. Probabilistic analysis (PA) was carried out with pre-set distributions of the exposure parameters to characterize the uncertainty in the assessment. The results incorporating bioaccessibility and PA were several times higher than the generic remediation goals which equal to national screening levels, and orders of magnitude higher than the baselines of the region and nation. The results of the Weibull fit were finally recommended as site-specific remediation goals (SSRGs) (10.59 mg/kg, 95.48 mg/kg and 9.24 mg/kg). Over-remediation was avoided while contributing to considerable economic and environmental benefits.

Influence of pH, electrical conductivity and ageing on the extractability of benzo[a]pyrene in two contrasting soils

Higher soil pH and electrical conductivity (EC) were suspected to result in higher extractability and bioavailability of benzo[a]pyrene (B[a]P) in soils. In this study, we investigated the influence of pH, EC and ageing on the extractability of B[a]P in two contracting soils (varied largely in soil texture, clay mineralogy and organic carbon content) over 4 months. Dilute sodium hydroxide (0.2 mol L^-1) and sodium chloride (0.1 mol L^-1) solutions were used to adjust soil pH and EC either separately or simultaneously. Extractability of B[a]P in these soils was monitored using a mild solvent extraction using butanol (BuOH, end-over-end shake over 24 h), and an exhaustive mix-solvent extraction using dichloromethane/acetone (DCM/Ace, v:v = 1:1) facilitated by sonication and a subsequent NaOH saponification method following the DCM/Ace extraction. Results showed that increased pH and/or EC significantly increased the B[a]P extractability in the sandy soil (GIA). Variance analysis of contribution of pH and/or EC modification and ageing time on changes in B[a]P extractability indicated that in GIA > 55% and over 25% of the changes in B[a]P extractability was attributed to increased pH&EC and pH only respectively. While ageing resulted in >85% of the change in B[a]P extractability in the clayey soil (BDA), following by increased pH&EC (contribution < 15%). Large amount of non-extractable residue (NER) were formed over the ageing period, up to 95% and 79% in GIA/BDA and its modified soils, respectively. Significant correlations were observed between B[a]P BuOH extractability and the exhaustive sequential extraction using DCM/Ace followed by NaOH saponification for all soils (p < 0.001). With slopes of the correlations close to 1, our results indicated that the simple mild solvent BuOH extraction was equivalent to the complex sequential DCM/Ace and NaOH saponification extraction in these soils.

Effect of pyrolysis conditions on the total contents of polycyclic aromatic hydrocarbons in biochars produced from organic residues: Assessment of their hazard potential

The interest of using biochar, the solid byproduct from organic waste pyrolysis, as soil conditioner is significantly increasing. Nevertheless, persistent organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), are formed during pyrolysis due to the incomplete combustion of organic matter. Consequently, these pollutants may enter the environment when biochar is incorporated into soil and cause adverse ecological effects. In this study, we examined the content of the 16 United States Environmental Protection Agency (USEPA) PAHs in biochars produced from rice husk, wood, wheat and sewage sludge residues using three different pyrolytic reactors and temperatures (400, 500 and 600 °C). The total concentration of PAHs (∑PAH) ranged from 799 to 6364 μg kg^-1, being naphthalene, phenanthrene and anthracene the most abundant PAHs in all the biochars. The maximum amount of PAHs was observed for the rice husk biochar produced in the batch reactor at 400 °C, which decreased with increasing temperature. The ∑PAH value of the wood biochar produced via traditional kilns doubled compared with the wood biochar produced using the other pyrolytic reactors (5330 μg kg^-1 in Kiln; 2737 μg kg^-1 in batch and 1942 μg kg^-1 in the rotary reactor). Looking for a more reliable risk assessment of the potential exposure of PAHs in biochar, the total toxic equivalent concentrations (TTEC) of the 14 produced biochars were calculated. When comparing the same feedstock and temperature, TTEC values indicated that the rotary reactor produced the safest biochars. In contrast, the biochars produced using the batch reactor at 400 and 500 °C have the greatest hazard potential. Our results provide valuable information on the potential risk of biochar application for human and animal health, as well as for the environment due to PAHs contamination.

Influence of bacterial community composition and soil factors on the fate of phenanthrene and benzo[a]pyrene in three contrasting farmland soils

The fate of polycyclic aromatic hydrocarbons (PAHs) determines their potential risk in soil, which may be directly affected by abiotic conditions and indirectly through the changes in decomposer communities. In comparison, the indirect effects on the fate remain largely elusive. In this study, the fate of phenanthrene and benzo[a]pyrene and the corresponding bacterial changes were investigated in three contaminated farmland soils using a ¹⁴C tracer method and Miseq sequencing. The results showed that most benzo[a]pyrene was consistently extractable with dichloromethane (DCM) after the 60-day incubation (60.4%-78.2%), while phenanthrene was mainly mineralized to CO₂ during the 30-day incubation (40.4%-58.7%). Soils from Guangzhou (GZ) showed a different distribution pattern of ¹⁴C-PAHs exemplified by low mineralization and disparate bound residue formation. The PAH fate in the Shenyang (SY) and Nanjing (NJ) soils were similar to each other than to that in the GZ soil. The fate in the GZ soil seemed to be linked to the distinct edaphic properties, such as organic matter content, however soil microbial community could have influenced the distribution pattern of PAHs. This potential role of microorganisms was reflected by the unique changes in the copy numbers of Gram positive RHD_α gene, and by the distinct shifts in bacterial community composition during the incubation. A quite different shift in bacterial communities was found in the GZ microcosms which may influence PAH mineralization and non-extractable residue (NER) formation.

Changes in the pattern of polycyclic aromatic hydrocarbons in soil treated with biochar from a multiyear field experiment

The influence of biochar added to an agricultural soil on polycyclic aromatic hydrocarbon (PAH) levels, PAH diagnostic ratios and soil properties was investigated in a five-year field experiment. The experiment was carried out in an Italian vineyard and included two biochar treatments: 16.5 t ha^-1 of biochar applied in 2009 (soil B); 16.5 t ha^-1 in 2009 and further 16.5 t ha^-1 in 2010 (soil BB). A set of 75 samples that included five replicates and a control soil (untreated) was characterized in terms of organic carbon, pH, cation exchange capacity (CEC), bulk density and concentration of PAHs. Biochar addition to soil caused an increase in organic carbon, pH and CEC, and a decrease of bulk density. After almost two years the first application of biochar, PAH concentrations were higher in soil B (56 ng g^-1) and BB (153 ng g^-1) in comparison to control soil (24 ng g^-1). Thereafter, PAH concentrations decreased significantly, but the original PAHs levels were reached only in soil B after five years. The naphthalene/(naphthalene + phenanthrene) ratios were higher in the treated soils in accordance to the dominance of naphthalene in the original biochar. The cross plots naphthalene/(naphthalene + phenanthrene) vs. fluoranthene/(fluoranthene + pyrene) enabled to trace the signature of biochar PAHs up to five years after its first application. Diagnostic ratios can be a useful tool to study the persistence of PAHs introduced in soil by biochar when the pattern of these contaminants in biochar and original soil are different.

Effect of Tenax addition amount and desorption time on desorption behaviour for bioavailability prediction of polycyclic aromatic hydrocarbons

In this work, Tenax consecutive extractions of polycyclic aromatic hydrocarbons (PAHs) were conducted in two spiked sediments to investigate the influence of different Tenax addition amounts and desorption times on the rapidly desorbing fraction of PAHs, and to determine a reliable method for estimating PAHs bioavailability. The results indicated that a large Tenax addition amount has a positive effect on the desorption of PAHs from sediments. The desorption amounts of target PAHs compounds (3-ring phenanthrene and 4-ring fluoranthene) increased as the Tenax: sediment ratios increased from 0.25 to 2 in two spiked sediments. The highest desorption percentages of phenanthrene and fluoranthene were 48.91% and 34.70% for Jialing industrial park sediment, and 43.36% and 33.24% for Huanghuayuan bridge sediment, respectively. The results of desorption kinetics were suitably fitted with first order three-compartment model to estimate the rapidly desorbing fraction, Moreover, the Tenax: sediment ratio of 1 and desorption time of 24 h were found to be suitable for the desorption of phenanthrene and fluoranthene from sediments. The PAHs in sediments were biodegraded well by the bacterial strain J1-q. Comparing the maximum biodegraded amount of target PAHs in 30 days and the desorbed fraction over 400 h, the results showed that Tenax had better correlation with the high molecular weight fluoranthene than with the low molecular weight phenanthrene.

Enhanced Recovery of Nonextractable Benzo[ a]pyrene Residues in Contrasting Soils Using Exhaustive Methanolic and Nonmethanolic Alkaline Treatments

The fate, impacts, and significance of polycyclic aromatic hydrocarbon (PAH) nonextractable residues (NERs) in soils remain largely unexplored in risk-based contaminated land management. In this study, seven different methanolic and nonmethanolic alkaline treatments, and the conventional methanolic saponification, were used to extract benzo[ a]pyrene (B[ a]P) NERs that had been aged for 180 d from four contrasting soils. Up to 16% and 55% of the amount of B[ a]P spiked (50 mg/kg) into soils was nonextractable after 2 d and 180 of aging, respectively, indicating rapid and progressive B[ a]P sequestration in soils over time. The recovery of B[ a]P from soils after 180 d of aging was increased by up to 48% by the seven different alkaline extractions, although the extraction efficiencies of the different alkaline treatments did not differ significantly ( p > 0.05). Approximately 40% of B[ a]P NERs in the sandy-clay-loam organic matter-rich soil was recovered by the exhaustive alkaline extractions after 180 d of aging, compared to only 10% using conventional methanolic saponification. However, the amounts of B[ a]P NERs recovered depend on soil properties and the amounts of NERs in soils. A significant correlation ( R² = 0.69, p < 0.001) was also observed between the amounts of B[ a]P recovered by each of the seven alkaline extractions in the contrasting soils and corresponding NERs at 180 d of aging, indicating a potential association warranting further investigations. Extraction techniques that estimate the amounts of PAH NERs recoverable in soil can help give a better understanding of the fate of NERs in soil.

Comparative evaluation of influence of aging, soil properties and structural characteristics on bioaccessibility of polychlorinated biphenyls in soil

Though bioaccessibility commonly recognized as a guideline for risk assessment is closely related with pollution occurrence and chemical species of compounds, the mechanistic links are barely evaluated particularly for widespread polychlorinated biphenyls (PCBs) in soil. With the biomimetic extraction of hydroxypropyl-β-cyclodextrin (β-HPCD), the temporal and spatial influences of soil properties, aging and structural characteristics, e.g. polarity of PCB congeners on bioaccessibility were investigated for PCBs. Sensitive variation of bioaccessibility with aging, soil organic matter (SOM), particle size and soil moisture were clearly evidenced for different PCB congeners. Due to aging, the bioaccessibility decreased in the long term after stabilization for 36 h. In concert with the first-order kinetics, the decay rates of bioaccessibility were shown with congener-specificity and were well correlated with dipoles of PCBs. The increment of SOM diminished the bioaccessibility for the strengthened adsorption while the increased particle size and soil moisture elevated it possibly due to the less adsorption on soil particles and more accommodation of PCBs in soil pore water. Except the positive correlations with particle size, soil moisture and dipole moment, the greater dependency on aging and SOM was highlighted for bioaccessibility by partial least squares (PLS) analysis. The mutual relationship with influential factors was quantitatively formulated for accelerative prediction of bioaccessibility, and the comparative evaluation and detailed insights into the mechanistic links would thus help enhance the precise determination of bioaccessibility and risk assessment of PCBs in soil.

Assessing bioavailability of complex chemical mixtures in contaminated soils: Progress made and research needs

Understanding the distribution, behaviour and interactions of complex chemical mixtures is key for providing the evidence necessary to make informed decisions and implement robust remediation strategies. Much of the current risk assessment frameworks applied to manage land contamination are based on total contaminant concentrations and the exposure assessments embedded within them do not explicitly address the partitioning and bioavailability of chemical mixtures. These oversights may contribute to an overestimation of both the eco-toxicological effects of the fractions and the mobility of contaminants. In turn, this may limit the efficacy of risk frameworks to inform targeted and proportionate remediation strategies. In this review we analyse the science surrounding bioavailability, its regulatory inclusion and the challenges of incorporating bioavailability in decision making process. While a number of physical and chemical techniques have proven to be valuable tools for estimating bioavailability of organic and inorganic contaminants in soils, doubts have been cast on its implementation into risk management soil frameworks mainly due to a general disagreement on the interchangeable use of bioavailability and bioaccessibility, and the associated methods which are still not standardised. This review focuses on the role of biotic and abiotic factors affecting bioavailability along with soil physicochemical properties and contaminant composition. We also included advantages and disadvantages of different extraction techniques and their implications for bioavailability quantitative estimation. In order to move forward the integration of bioavailability into site-specific risk assessments we should (1) account for soil and contaminant physicochemical characteristics and their effect on bioavailability; (2) evaluate receptor's potential exposure and uptake based on mild-extraction; (3) adopt a combined approach where chemical-techniques are used along with biological methods; (4) consider a simplified and cost-effective methodology to apply at regulatory and industry setting; (5) use single-contaminant exposure assessments to inform and predict complex chemical mixture behaviour and bioavailability.