Protective role of fine silts for PAH in a former industrial soil

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.

Multistep thermodesorption coupled with molecular analyses as a quick, easy and environmentally friendly way to measure PAH availability in contaminated soils

Whether it is for risk assessment or for remediation purpose, contaminant availability in polluted soils is a key parameter to determine. Two methods were recently standardized for the estimation of the environmental available fraction of non-polar organics but, in some cases, their application on real historically contaminated soils does not provide satisfactory results. The present study aimed at proposing an alternative method for the estimation of PAH availability in soils, based on analytical thermal desorption and molecular analyses with the hypothesis that the binding strength between PAH and the solid matrix is linked to the desorption temperature. This hypothesis was validated by comparing the thermodesorption molecular distribution of different contaminated soils and of their respective extractable organic matter. Then, comparing the thermodesorption profiles of each studied PAH to the efficiency of biological and chemical remediation treatments through principal component analysis allowed obtaining the desorption temperature corresponding to PAH fractions available towards both treatments. This method was proven to effectively estimate the PAH fraction available towards biological (microbial incubation) and chemical (KMnO₄ oxidation) treatments and present multiple advantages such as being fast, easy to execute and solvent free.

Fenton-like and potassium permanganate oxidations of PAH-contaminated soils: Impact of oxidant doses on PAH and polar PAC (polycyclic aromatic compound) behavior

Potassium permanganate and Fenton-like oxidations were applied on two PAH-contaminated soils collected on former coking plant and gas plant sites. The impact of oxidant dose on the polycyclic aromatic compound (PAC) evolution, including 16 US-EPA PAHs, 11 oxygenated- and 4 nitrogen heterocyclic-PACs (O- and N-PACs) was studied for both treatments. The content of extractable organic matter and PACs was determined prior and after oxidation. Overall, permanganate treatment was more efficient than Fenton-like to decrease the PAH content, this latter being limited by the contamination availability. However, permanganate treatment resulted in incomplete PAH degradation, leading to the formation of O-PACs, that was limited with the application of higher dose. It underlines the importance of the dose and the oxidant type in the selection of oxidation parameters for remediation purpose, as improper use of oxidant can lead to the accumulation of oxidation by-products that could be as toxic as the parent compounds.

Aging as the main factor controlling PAH and polar-PAC (polycyclic aromatic compound) release mechanisms in historically coal-tar-contaminated soils

In industrial sites, historically contaminated by coal tar (abandoned coking and manufactured gas plants), other families of organic pollutants than the 16 PAHs (polycyclic aromatic hydrocarbons) classified by the US-EPA can occur and induce potential risk for groundwater resources. Polar PACs (polycyclic aromatic compounds), especially oxygenated and nitrogenated PACs (O-PACs and N-PACs), are present in the initial pollution and can also be generated over time (i.e., O-PACs). Their aqueous solubilities are much greater than those of the PAHs. For these reasons, we need to increase our knowledge on polar PACs in order to better predict their behavior and the potential on-site risk. Batch leaching tests were carried out under various conditions of temperature, ionic strength, and availability of pollutants to determine the mechanisms and key parameters controlling their release. The results show a release of low-molecular-weight PAHs and polar PACs mainly by dissolution, while higher molecular weight PAHs are mainly released in association with colloids. Aging mainly controls the former mechanism, and ionic strength mainly controls the latter. Temperature increased both dissolution and colloidal mobilization. The Raoult law predicts the PAC equilibrium concentration for soils presenting high pollutant availability, but this law overestimates PAC concentration in aged soils (low pollutant availability). This is mainly due to limitation of PAC diffusion within coal-tar particles with aging. The most soluble PACs (especially polar PACs) are the most sensitive to aging. For better prediction of the PAC behavior in soils and water resources management, aging needs to be taken into account.

Methodology to examine polycyclic aromatic hydrocarbons (PAHs) nitrated PAHs and oxygenated PAHs in sediments of the Paraguaçu River (Bahia, Brazil)

Conventional methods for determination of polycyclic aromatic compounds (PACs) in sediments usually require large sample sizes (grams) and solvent volumes (at least 100 mL) through the employment of Soxhlet extraction, which is both time (hours) and energy consuming, among other disadvantages. We developed a new analytical protocol for the determination of PACs in sediments using microextraction, which requires small sample masses (25 mg), 500 μL of acetonitrile-dichloromethane mix and sonication for 23 min, followed by GC-MS analysis. The method was validated using the certified reference material SRM 1941b - NIST organic marine sediment, as well as internal deuterated standards. Seventeen PAHs, seven nitro-PAHs and one quinone were detected and quantified. The mean concentrations were 90.4 ng g^-1 for PAHs, 179.2 ng g^-1 for nitro-PAHs and 822.5 ng g^-1 for quinones. The proposed method showed good sensitivity, linearity, precision and accuracy for the determination of PAC in sediments samples.

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

The bioavailability and bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in soil underpin the risk assessment of contaminated land with these contaminants. Despite a significant volume of research conducted in the past few decades, comprehensive understanding of the factors controlling the behaviour of soil PAHs and a set of descriptive soil parameters to explain variations in PAH bioavailability and bioaccessibility are still lacking. This review focuses on the role of source materials on bioavailability and bioaccessibility of soil PAHs, which is often overlooked, along with other abiotic factors including contaminant concentration and mixture, soil composition and properties, as well as environmental factors. It also takes into consideration the implications of different types of risk assessment (ecological and human health) on bioavailability and bioaccessibility of PAHs in soil. We recommend that future research should (1) account for the effects of source materials on bioavailability and bioaccessibility of soil PAHs; (2) adopt non-disruptive methods to analyse soil components controlling PAH sequestration; (3) integrate both natural organic matter (NOM) and xenobiotic organic matter (XOM) while evaluating the influences of soil organic matter (SOM) on the behaviour of PAHs; and (4) consider the dissimilar desorption scenarios in ecological risk assessment and human health risk assessment while assessing PAH bioavailability and bioaccessibility.

Bioremediation of PAH-contamined soils: Consequences on formation and degradation of polar-polycyclic aromatic compounds and microbial community abundance

A bioslurry batch experiment was carried out over five months on three polycyclic aromatic compound (PAC) contaminated soils to study the PAC (PAH and polar-PAC) behavior during soil incubation and to evaluate the impact of PAC contamination on the abundance of microbial communities and functional PAH-degrading populations. Organic matter characteristics and reactivity, assessed through solvent extractable organic matter and PAC contents, and soil organic matter mineralization were monitored during 5 months. Total bacteria and fungi, and PAH-ring hydroxylating dioxygenase genes were quantified. Results showed that PAHs and polar-PACs were degraded with different degradation dynamics. Differences in degradation rates were observed among the three soils depending on PAH distribution and availability. Overall, low molecular weight compounds were preferentially degraded. Degradation selectivity between isomers and structurally similar compounds was observed which could be used to check the efficiency of bioremediation processes. Bacterial communities were dominant over fungi and were most likely responsible for PAC degradation. Abundance of PAH-degrading bacteria increased during incubations, but their proportion in the bacterial communities tended to decrease. The accumulation of some oxygenated-PACs during the bioslurry experiment underlines the necessity to monitor these compounds during application of remediation treatment on PAH contaminated soils.

Comparison of the effectiveness of soil heating prior or during in situ chemical oxidation (ISCO) of aged PAH-contaminated soils

Thermal treatments prior or during chemical oxidation of aged polycyclic aromatic hydrocarbon (PAH)-contaminated soils have already shown their ability to increase oxidation effectiveness. However, they were never compared on the same soil. Furthermore, oxygenated polycyclic aromatic hydrocarbons (O-PACs), by-products of PAH oxidation which may be more toxic and mobile than the parent PAHs, were very little monitored. In this study, two aged PAH-contaminated soils were heated prior (60 or 90 °C under Ar for 1 week) or during oxidation (60 °C for 1 week) with permanganate and persulfate, and 11 O-PACs were monitored in addition to the 16 US Environmental Protection Agency (US EPA) PAHs. Oxidant doses were based on the stoichiometric oxidant demand of the extractable organic fraction of soils by using organic solvents, which is more representative of the actual contamination than only the 16 US EPA PAHs. Higher temperatures actually resulted in more pollutant degradation. Two treatments were about three times more effective than the others: soil heating to 60 °C during persulfate oxidation and soil preheating to 90 °C followed by permanganate oxidation. The results of this study showed that persulfate effectiveness was largely due to its thermal activation, whereas permanganate was more sensitive to PAH availability than persulfate. The technical feasibility of these two treatments will soon be field-tested in the unsaturated zone of one of the studied aged PAH-contaminated soils.

Validation and uncertainty estimation of UPLC-PDA method for the analysis of polycyclic aromatic hydrocarbons in concrete

Human exposure to persistent organic contaminants, from building materials, negatively affects people's health and overall quality of life. This paper presents the validation and uncertainty assessment of the analytical method, developed for the simultaneous determination of 16 EPA polycyclic aromatic hydrocarbons (PAHs) in solid-solid concrete by ultra-performance liquid chromatography with photo diode-array detector. Linearity of calibration curves was good over the whole range of calibration. Limits of detection varied between 0.2 and 2.9μgkg^-1. The accuracy in terms of recovery of the validated method is within the range from 54 to 106%. The developed method proved to be appropriate for analysis of PAHs and can be used for the quality control testing of concrete during the construction of new buildings, the old residences and related buildings associated with sick-building syndrome. In addition, this is the first reported method described for the evaluation of PAHs in solid-solid concrete.

Selection of oxidant doses for in situ chemical oxidation of soils contaminated by polycyclic aromatic hydrocarbons (PAHs): A review

In situ chemical oxidation (ISCO) is a promising alternative to thermal desorption for the remediation of soils contaminated with organic compounds such as polycyclic aromatic hydrocarbons (PAHs). For field application, one major issue is the selection of the optimal doses of the oxidizing solution, i.e. the oxidant and appropriate catalysts and/or additives. Despite an extensive scientific literature on ISCO, this choice is very difficult because many parameters differ from one study to another. The present review identifies the critical factors that must be taken into account to enable comparison of these various contributions. For example, spiked soils and aged, polluted soils cannot be compared; PAHs freshly spiked into a soil are fully available for degradation unlike a complex mixture of pollutants trapped in a soil for many years. Another notable example is the high diversity of oxidation conditions employed during batch experiments, although these affect the representativeness of the system. Finally, in this review a methodology is also proposed based on a combination of the stoichiometric oxidant demand of the organic pollutants and the design of experiments (DOE) in order to allow a better comparison of the various studies so far reported.

Effect of ageing on benzo[a]pyrene extractability in contrasting soils

Changes in benzo[a]pyrene (B[a]P) extractability over 160 days ageing in four contrasting soils varying in organic matter content and clay mineralogy were investigated using dichloromethane: acetone 1:1 (DCM/Ace), 60 mM hydroxypropyl-β-cyclodextrin (HPCD) solution, 1-butanol (BuOH) and Milli-Q water. The B[a]P extractability by the four methods decreased with ageing and a first-order exponential model could be used to describe the kinetics of release. Correlation of the kinetic rate constant with major soil properties showed a significant effect of clay and sand contents and pore volume fraction (<6 nm) on sequestration of the desorbable fraction (by HPCD) and the water-extractable fraction. Analysis of (14)C-B[a]P in soils after ageing showed a limited loss of B[a]P via degradation. Fractionation of B[a]P pools associated with the soil matrix was analysed according to extractability of B[a]P by the different extraction methods. A summary of the different fractions is proposed for the illustration of the effect of ageing on different B[a]P-bound fractions in soils. This study provides a better understanding of the B[a]P ageing process associated with different fractions and also emphasises the extraction capacity of the different methods employed.

Impact of clay mineral, wood sawdust or root organic matter on the bacterial and fungal community structures in two aged PAH-contaminated soils

The high organic pollutant concentration of aged polycyclic aromatic hydrocarbon (PAH)-contaminated wasteland soils is highly recalcitrant to biodegradation due to its very low bioavailability. In such soils, the microbial community is well adapted to the pollution, but the microbial activity is limited by nutrient availability. Management strategies could be applied to modify the soil microbial functioning as well as the PAH contamination through various amendment types. The impact of amendment with clay minerals (montmorillonite), wood sawdust and organic matter plant roots on microbial community structure was investigated on two aged PAH-contaminated soils both in laboratory and 1-year on-site pot experiments. Total PAH content (sum of 16 PAHs of the US-EPA list) and polar polycyclic aromatic compounds (pPAC) were monitored as well as the available PAH fraction using the Tenax method. The bacterial and fungal community structures were monitored using fingerprinting thermal gradient gel electrophoresis (TTGE) method. The abundance of bacteria (16S rRNA genes), fungi (18S rRNA genes) and PAH degraders (PAH-ring hydroxylating dioxygenase and catechol dioxygenase genes) was followed through qPCR assays. Although the treatments did not modify the total and available PAH content, the microbial community density, structure and the PAH degradation potential changed when fresh organic matter was provided as sawdust and under rhizosphere influence, while the clay mineral only increased the percentage of catechol-1,2-dioxygenase genes. The abundance of bacteria and fungi and the percentage of fungi relative to bacteria were enhanced in soil samples supplemented with wood sawdust and in the plant rhizospheric soils. Two distinct fungal populations developed in the two soils supplemented with sawdust, i.e. fungi related to Chaetomium and Neurospora genera and Brachyconidiellopsis and Pseudallescheria genera, in H and NM soils respectively. Wood sawdust amendment favoured the development of PAH-degrading bacteria holding Gram-negative PAH-ring hydroxylating dioxygenase, catechol-1,2-dioxygenase and catechol-2,3-dioxygenase genes. Regarding the total community structure, bacteria closely related to Thiobacillus (β-Proteobacteria) and Steroidobacter (γ-Proteobacteria) genera were favoured by wood sawdust amendment. In both soils, plant rhizospheres induced the development of fungi belonging to Ascomycota and related to Alternaria and Fusarium genera. Bacteria closely related to Luteolibacter (Verrucomicrobia) and Microbacterium (Actinobacteria) were favoured in alfalfa and ryegrass rhizosphere.

Effect of pre-heating on the chemical oxidation efficiency: implications for the PAH availability measurement in contaminated soils

Three chemical oxidation treatments (KMnO4, H2O2 and Fenton-like) were applied on three PAH-contaminated soils presenting different properties to determine the potential use of these treatments to evaluate the available PAH fraction. In order to increase the available fraction, a pre-heating (100 °C under N2 for one week) was also applied on the samples prior oxidant addition. PAH and extractable organic matter contents were determined before and after treatment applications. KMnO4 was efficient to degrade PAHs in all the soil samples and the pre-heating slightly improved its efficiency. H2O2 and Fenton-like treatments presented low efficiency to degrade PAH in the soil presenting poor PAH availability, however, the PAH degradation rates were improved with the pre-heating. Consequently H2O2-based treatments (including Fenton-like) are highly sensitive to contaminant availability and seem to be valid methods to estimate the available PAH fraction in contaminated soils.

Low-molecular-weight organic acids influence the sorption of phenanthrene by different soil particle size fractions

The impact of low-molecular-weight organic acids (LMWOAs) on the sorption of phenanthrene (a representative polycyclic aromatic hydrocarbon) by different particle size fractions of a soil was investigated using a batch technique. Citric and malic acids were used in experiments. Four soil fractions were fractionated: fine sand, silt, coarse clay, and fine clay. Laser granulometry confirmed the suitability of the fractionation method used for the particle size distributions in this investigation. The sorption of phenanthrene by the different soil fractions was described well using a linear distribution-type model, and the distribution coefficients () followed a descending order of fine sand > fine clay > coarse clay > silt, irrespective of the addition of organic acids. This order was significantly positively correlated with the organic carbon content of test soil solids. The values for phenanthrene sorption by soil fractions initially increased but then decreased as the concentrations of citric and malic acids increased (0-1000 mmol L). The presence of citric and malic acid at lower concentrations (<100 mmol L) generally promoted the sorption of phenanthrene, while higher concentrations (>100 mmol L) inhibited sorption irrespective of the soil fraction. The mechanism of the LMWOA-influenced sorption of phenanthrene by test solids is discussed based on the observed sorption of organic acid, the dissolution of metal cations and minerals in soil, and the competition from dissolved organic matter in solution that were released from soil solids.

Evolution of dissolved organic matter during abiotic oxidation of coal tar--comparison with contaminated soils under natural attenuation

In former coal transformation plants (coking and gas ones), the major organic contamination of soils is coal tar, mainly composed of polycyclic aromatic compounds (PACs). Air oxidation of a fresh coal tar was chosen to simulate the abiotic natural attenuation impact on PAC-contaminated soils. Water-leaching experiments were subsequently performed on fresh and oxidized coal tars to study the influence of oxidation on dissolved organic matter (DOM) quality and quantity. The characterization of the DOM was performed using a combination of molecular and spectroscopic techniques (high-performance liquid chromatography-size-exclusion chromatography (HPLC-SEC), 3D fluorescence, and gas chromatography coupled with mass spectrometry (GC-MS)) and compared with the DOM from contaminated soils sampled on the field exposed to natural attenuation for several decades. An increase in the oxygenated polycyclic aromatic compound concentrations was observed with abiotic oxidation both in the coal tar and the associated DOM. Polycyclic aromatic hydrocarbon concentrations in the leachates exceeded pure water solubility limits, suggesting that co-solvation with other soluble organic compounds occurred. Furthermore, emission excitation matrix analysis combined with synchronous fluorescence spectra interpretation and size-exclusion chromatography suggests that oxidation induced condensation reactions which were responsible for the formation of higher-molecular weight compounds and potentially mobilized by water. Thus, the current composition of the DOM in aged soils may at least partly result from (1) a depletion in lower-molecular weight compounds of the initial contamination stock and (2) an oxidative condensation leading to the formation of a higher-molecular weight fraction. Abiotic oxidation and water leaching may therefore be a significant combination contributing to the evolution of coal tar-contaminated soils under natural attenuation.

Impact of fresh organic matter incorporation on PAH fate in a contaminated industrial soil

The impacts of fresh organic matter (OM) incorporation in an industrial PAH-contaminated soil on its structure and contaminant concentrations (available and total) were monitored. A control soil and a soil amended with the equivalent of 10 years maize residue input were incubated in laboratory-controlled conditions over 15 months. The structure of the amended soil showed an aggregation process trend which is attributable to (i) the enhanced microbial activity resulting from fresh OM input itself and (ii) the fresh OM and its degradation products. Initially the added organic matter was evenly distributed among all granulodensimetric fractions, and then rapidly degraded in the sand fraction, while stabilizing and accumulating in the silts. PAH degradation remained slight, despite the enhanced microbial biomass activity, which was similar to kinetics of the turnover rate of OM in an uncontaminated soil. The silts stabilized the anthropogenic OM and associated PAH. The addition of fresh OM tended to contribute to this stabilization process. Thus, in a context of plant growth on this soil two opposing processes might occur: rhizodegradation of the available contaminant and enhanced stabilization of the less available fraction due to carbon input.

Effects of ageing and soil properties on the oral bioavailability of benzo[a]pyrene using a swine model

Oral bioavailability of benzo[a]pyrene (B[a]P) was studied in a swine model using eight spiked soil samples after incubation for 50 and/or 90 days. Silica sand was used as a reference material and the relative bioavailability (RB) of B[a]P in soils was calculated as the quotient of the area under the plasma B[a]P curve (AUC) for soil and AUC for the silica sand. Significantly reduced RB was observed in all study soils after 90 days ageing, ranging from 22.1±0.4% to 62.7±10.1%, except for one very sandy soil (sand content 87.6%) where RB was unchanged (108.1±8.0%). Apart from this, bioavailability decreased during ageing with the decrease (from day 50 to day 90) being only significant for a clayey soil containing expandable clay minerals. Statistical analyses of B[a]P RB at day 90 (eight soils) and soil properties showed no direct correlation between RB and specific soil properties such as total organic carbon (TOC) and clay content which were commonly linked to organic contaminant sequestration. However, strongly significant relationships (p<0.001) were found between RB and the fine particle associated carbon (FPAC) defined as (Silt+Clay)/TOC, and between RB and the soil mesopore (<6nm; p<0.001) fraction, after two samples with high pH and high EC being excluded from the analyses. The bioaccessibility estimated by four in vitro extraction methods: dichloromethane/acetone sonication (DCM/Ace), butanol vortex (BuOH), hydroxypropyl-β-cyclodextrin extraction (HPCD) and Milli Q water leaching methods at different sampling time (1 day, 50 days and 90 days after spiking) also showed a decreasing trend. Significant correlations were found between B[a]P RB and DCM/Ace (R(2)=0.67, p<0.05) extractable fraction and BuOH (R(2)=0.75, p<0.01) extractable fraction.

A Technosol as archives of organic matter related to past industrial activities

To better understand formation, functioning and evolution of a Technosol developing on a former settling pond of iron industry under forest cover, organic matter (OM) of layers along the soil profile was investigated. Spectroscopic and molecular analyses of extractable OM gave information on OM origin and state of preservation. In the surface layer, OM fingerprints indicated fresh input from vegetation while they revealed well preserved anthropogenic compounds related to industrial processes in deeper layers. OM variability and distribution according to the layers recorded deposition cycles of industrial effluents into the pond. Thus, the Technosol can be considered as archives of past industrial activities. The preservation of anthropogenic OM could be connected with mineralogy, high metal contents and particular physical properties of the Technosol.

Desorption kinetics of PAHs from aged industrial soils for availability assessment

Persistent organic pollutants (POPs), such as polycyclic aromatic hydrocarbons (PAHs), may be found in high concentrations in soils of former industrial sites including manufactured gas plants or coking plants. Techniques using moderate solvent extraction, biological tests or solid phase extraction have proved useful for pollution availability estimation. However, more accurate and reliable measurement tools specifically adapted to low concentrations are still needed. Based on a solid-liquid extraction using a Tenax® resin, we suggest a protocol to assess the bioavailability of PAHs, dedicated to aged industrial wasteland soils. Desorption kinetics were measured on three representative contaminated industrial soils. Results were modeled using a first order two-compartment model that provided an estimate of the rapidly desorbing fraction, which was considered to be available, over a 30 h extraction period. In conclusion, this method, allowing the measurement of the available fraction, might prove more relevant than the total concentration value when assessing soil contamination related risks. It may also predict achievable bioremediation performances.