Availability of polycyclic aromatic hydrocarbons to earthworms in urban soils and its implications for risk assessment

Accurate prediction bioaccessibility of PAHs in soil-earthworm system by novel magnetic solid phase extraction technique

Conventional chemical extraction methods may lead to overestimate or underestimate bioaccessibility due to their inability to provide realistic kinetic information regarding PAHs in soils. In this study, we propose the use of magnetic solid phase extraction (MSPE) technique for assessing the bioaccessibility of PAHs in the soil-earthworm system. Firstly, a novel polydopamine-coated magnetic core-shell microspheres (Fe3O4-C16@PDA) was developed by a one-pot sol-gel and self-polymerization method. The PDA coatings not only enhance the hydrophilicity of material surfaces but also exhibit excellent biocompatibility. The maximum adsorption capacity of Fe3O4-C16@PDA for 16 PAHs was 52.72 mg g-1, indicating that the proposed material fulfills the assessment requirements for highly contaminated soil. To compare the measurement of PAHs and their uptake by earthworms (Eisenia fetida), experiments were conducted using four different soils with varying properties. The desorption kinetics data obtained from these experiments demonstrated that the capability of the MSPE in accurately predicting the bioavailable portions of PAHs. After a 28-day exposure, the best predictor of bioavailable PAHs in earthworms was MSPE method exhibited the highest correlation coefficient (R2 > 0.90), and its slopes in the four soils were 0.972, 0.961, 1.012, and 0.962, respectively, all close to 1. These results demonstrate that the MSPE method successfully mimics the conditions encountered in soil-earthworm systems and effectively assess bioaccessibility of PAHs in soils.

Modelling polycyclic aromatic hydrocarbon bioavailability in historically contaminated soils with six in-vitro chemical extractions and three earthworm ecotypes

Accurate prediction of organic contaminant bioavailability for risk assessment in ecological applications is hindered by limited validation on relevant bioassay species. Here, six in-vitro chemical extraction methods (butanol, non-buffered and buffered hydroxypropyl-β-cyclodextrin (HPCD, Buf-HPCD), Tenax, potassium persulfate oxidation, polyoxymethylene solid phase extraction (POM)) were tested for PAH bioaccumulation prediction in three earthworm ecotypes with dissimilar exposures, Amynthas sp., Eisenia fetida, and Lumbricus terrestris, in historically contaminated soils from manufactured gas plant (MGP) sites. Extractions were compared directly and modelled in a calculation approach using equilibrium partitioning theory (EqPT) with a novel combination of different organic carbon/octanol-water partitioning parameters (K_OC and K_OW). In the direct comparison approach Buf-HPCD showed the closest prediction of accumulation for burrowing Amynthas sp. and L. terrestris (within 1.5 and 3.1, respectively), but Tenax and POM showed the closest approximation for E. fetida (within 1.1 and 0.9, respectively). The optimum method for predicting PAH bioaccumulation in the calculation approach depended on earthworm species and the partitioning parameters used in equations of the four models, but overall POM, which was independent of K_OC, showed the closest approximation of accumulation, within a factor of 2.5 across all species. This work effectively identifies the optimum in-vitro based approaches for PAH bioavailability prediction in earthworms as a model soil health indicator for ecological risk assessment within regulatory and remediation decision frameworks.

Comparison of fitness effects in the earthworm Eisenia fetida after exposure to single or multiple anthropogenic pollutants

Terrestrial ecosystems are exposed to many anthropogenic pollutants. Non-target effects of pesticides and fertilizers have put agricultural intensification in the focus as a driver for biodiversity loss. However, other pollutants, such as heavy metals, particulate matter, or microplastic also enter the environment, e.g. via traffic and industrial activities in urban areas. As soil acts as a potential sink for such pollutants, soil invertebrates like earthworms may be particularly affected by them. Under natural conditions soil invertebrates will likely be exposed to combinations of pollutants simultaneously, which may result in stronger negative effects if pollutants act synergistically. Within this work we study how multiple pollutants affect the soil-dwelling, substrate feeding earthworm Eisenia fetida. We compared the effects of the single stressors, polystyrene microplastic fragments, polystyrene fibers, brake dust and carbon black, with the combined effect of these pollutants when applied as a mixture. Endpoints measured were survival, increase in body weight, reproductive fitness, and changes in three oxidative stress markers (glutathione S-transferase, catalase and malondialdehyde). We found that among single pollutant treatments, brake dust imposed the strongest negative effects on earthworms in all measured endpoints including increased mortality rates. Sub-lethal effects were found for all pollutants. Exposing earthworms to all four pollutants simultaneously led to effects on mortality and oxidative stress markers that were smaller than expected by the respective null models. These antagonistic effects are likely a result of the adsorption of toxic substances found in brake dust to the other pollutants. With this study we show that effects of combinations of pollutants cannot necessarily be predicted from their individual effects and that combined effects will likely depend on identity and concentration of the pollutants.

Magnetic solid-phase extraction as a novel method for the prediction of the bioaccessibility of polycyclic aromatic hydrocarbons

Chemical methods used to predict the bioaccessibility of hydrophobic organic compounds (HOCs) still need further development and improvement. In this work, magnetic solid-phase extraction (MSPE) based on poly(β-cyclodextrin)-coated magnetic polydopamine (Fe₃O₄@PDA@PCD) was first introduced to assess the bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in soils. Due to its good hydrophilicity and submicrometer scale, Fe₃O₄@PDA@PCD displayed a higher extraction rate for PAHs in an aqueous solution (equilibrium time < 5 min) than Tenax resin, which had an equilibrium time longer than 30 min. The merits of Fe₃O₄@PDA@PCD are beneficial to accelerate the desorption of PAHs from soil, especially for high molecular weight PAHs, in which the amounts extracted by Fe₃O₄@PDA@PCD were 1.2-2.8 times higher than those extracted by Tenax resin. The desorption kinetics data were well fitted with a two- or three-fraction model. The fitting results indicated that the MSPE method can be used to predict the bioaccessible fractions of PAHs. By comparing the prediction results obtained from the MSPE method with bioassays using earthworms, a significant linear correlation (R² = 0.98) with a slope statistically close to 1 was obtained. These results suggested that the MSPE method can act as a simple and efficient method to measure the bioaccessibility of PAHs in soil.

The Impact of Organic Matter on Polycyclic Aromatic Hydrocarbon (PAH) Availability and Persistence in Soils

Polycyclic aromatic hydrocarbons (PAHs) exhibit persistence in soils, and most of them are potentially mutagenic/carcinogenic and teratogenic for human beings but also influence the growth and development of soil organisms. The PAHs emitted into the atmosphere are ultimately deposited (by dry or wet deposition processes) onto the soil surface where they tend to accumulate. Soil organic matter (SOM) plays an important role in the fate and transformation processes of PAHs, affecting their mobility, availability, and persistence. Therefore, the aim of this research was to investigate the influence of SOM fractional diversification (fulvic acids-FA, humic acids-HA, and humins-HN) on PAH availability and persistence in soils. Twenty soil samples (n = 20) were collected from upper horizons (0-30 cm) of agricultural soils exposed to anthropogenic emissions from industrial and domestic sources. The assessment of PAH concentrations included the determination of medium-molecular-weight compounds from the US EPA list: fluoranthene-FLA, pyrene-PYR, benz(a)anthracene-BaA, and chrysene-CHR. The assessment was conducted using the GC-MS/MS technique. Three operationally defined fractions were investigated: total extractable PAHs (TE-PAHs) fraction, available/bioavailable PAHs (PB-PAHs) fraction, and nonavailable/residual PAHs (RE-PAHs) fraction, which was calculated as the difference between total and available PAHs. TE-PAHs were analyzed by dichloromethane extraction, while PB-PAHs were analyzed with a hydrophobic β-cyclodextrin solution. SOM was characterized by total organic carbon content (Turin method) and organic carbon of humic substances including FA, HA, HN (IHSS method). Concentrations of PAHs differed between soils from 193.5 to 3169.5 µg kg^-1, 4.3 to 226.4 µg kg^-1, and 148.6 to 3164.7 µg kg^-1 for ∑4 TE-PAHs, ∑4 PB-PAHs, and ∑4 RE-PAHs, respectively. The ∑4 PB-PAHs fraction did not exceed 30% of ∑4 TE-PAHs. FLA was the most strongly bound in soil (highest content of RE-FLA), whereas PYR was the most available (highest content of PB-PYR). The soils were characterized by diversified total organic carbon (TOC) concentration (8.0-130.0 g kg^-1) and individual SOM fractions (FA = 0.4-7.5 g kg^-1, HA = 0.6-13.0 g kg^-1, HN = 0.9-122.9 g kg^-1). FA and HA as the labile fraction of SOM with short turnover time strongly positively influenced the potential ∑4 PAH availability (r = 0.56 and r = 0.52 for FA and HA, respectively). HN, which constitutes a stable fraction of organic matter with high hydrophobicity and poor degradability, was strongly correlated with ∑4 RE-PAHs (r = 0.75), affecting their persistence in soil.

Bioaccumulation of benzo[a]pyrene nonextractable residues in soil by Eisenia fetida and associated background-level sublethal genotoxicity (DNA single-strand breaks)

The potential for bioaccumulation and associated genotoxicity of nonextractable residues (NERs) of polycyclic aromatic hydrocarbon (PAHs) in long-term contaminated soils have not been investigated. Here we report research in which earthworms, Eisenia fetida, were exposed to a soil containing readily available benzo[a]pyrene (B[a]P) and highly sequestered B[a]P NERs aged in soil for 350 days. B[a]P bioaccumulation was assessed and DNA damage (as DNA single strand breaks) in earthworm coelomocytes were evaluated by comet assay. The concentrations of B[a]P in earthworm tissues were generally low, particularly when the soil contained highly sequestered B[a]P NERs, with biota-soil accumulation factors ranging from 0.6 to 0.8 kg_OC/kg_lipid. The measurements related to genotoxicity, that is percentage (%) of DNA in the tails and olive tail moments, were significantly greater (p < 0.05) in the spiked soil containing readily available B[a]P than in soil that did not have added B[a]P. For example, for the soil initially spiked at 10 mg/kg, the percentage of DNA in the tails (29.2%) of coelomocytes after exposure of earthworms to B[a]P-contaminated soils and olive tail moments (17.6) were significantly greater (p < 0.05) than those of unspiked soils (19.6% and 7.0, for percentage of DNA in tail and olive tail moment, respectively). There were no significant (p > 0.05) differences in effects over the range of B[a]P concentrations (10 and 50 mg/kg soil) investigated. In contrast, DNA damage after exposure of earthworms to B[a]P NERs in soil did not differ from background DNA damage in the unspiked soil. These findings are useful in risk assessments as they can be applied to minimise uncertainties associated with the ecological health risks from exposure to highly sequestered PAH residues in long-term contaminated soils.

Multivariate Analysis for Assessing Sources, and Potential Risks of Polycyclic Aromatic Hydrocarbons in Lisbon Urban Soils

Urban soils quality may be severely affected by polycyclic aromatic hydrocarbons (PAHs) contamination, as is the case of Lisbon (Portugal). However, to conduct a risk assessment analysis in an urban area can be a very difficult task due to the patchy nature and heterogeneity of these soils. Thus, the present study aims to provide an example on how to perform the first tier of a risk assessment plan in the case of urban soils using a simpler, cost effective, and reliable framework. Thus, a study was conducted in Lisbon to assess the levels of PAH, their potential risks to the environment and human health, and to identify their major sources. Source apportionment was performed by studying PAHs profiles, their relationship with potentially toxic elements, and general characteristics of soil using multivariate statistical methods. Results showed that geostatistical tools are useful for evaluating the spatial distribution and major inputs of PAHs in urban soils, as well as to identify areas of potential concern, showing their usefulness in risk assessment analysis and urban planning. Particularly, the prediction maps obtained allowed for a clear identification of areas with the highest levels of PAHs (close to the airport and in the city center). The high concentrations found in soils from the city center should be a result of long-term accumulation due to diffuse pollution mostly from traffic (through atmospheric emissions, tire debris and fuel exhaust, as well as pavement debris). Indeed, most of the sites sampled in the city center were historical gardens and parks. The calculation of potential risks based on different models showed that there is a high discrepancy among guidelines, and that risks will be extremely associated with the endpoint or parameters used in the different models. Nevertheless, this initial approach based on total levels was useful for identifying areas where a more detailed risk assessment is needed (close to the airport and in the city center). Therefore, the use of prediction maps can be very useful for urban planning, for example, by crossing information obtained with land uses, it is possible to define the most problematic areas (e.g., playgrounds and schools).