Multicomponent PFAS sorption and desorption in common commercial adsorbents: Kinetics, isotherm, adsorbent dose, pH, and index ion and ionic strength effects.

The adsorption and desorption of 9 PFAS, including 3 perfluoroalkyl sulphonic and 6 perfluoroalkyl carboxylic acids, in artificial groundwater was investigated using 3 commercial adsorbents that comprised a powdered activated carbon (PAC), a surface-modified organoclay (NMC+n), and a carbonaceous organic amendment (ROAC). Sorption kinetics and isotherms of PFAS, as well as the effects of adsorbent dose, pH, index ion and ionic strength on PFAS adsorption and desorption were investigated. Sorption of multicomponent PFAS in the adsorbents was rapid, especially for NMC+n and ROAC, regardless of PFAS chain length. The sorption and (and especially) desorption of PFAS in the adsorbents was impacted by the pH, index ion, and ionic strength of simulated groundwater, especially for the short chain PFAS, with only minimal impacts on NMC+n and PAC compared to ROAC. Although the potential mineral and charged constituents of the adsorbents contributed to the adsorption of short chain PFAS through electrostatic interactions, these interactions were susceptible to variable groundwater chemistry. Hydrophobic interactions also played a major role in facilitating and increasing PFAS sorption, especially in adsorbents with aliphatic functional groups. The desorption of PFAS from the adsorbents was below 8 % when the aqueous phase was deionised water, with no measurable desorption for NMC+n. In contrast, the desorption of short chain PFAS in simulated groundwater increased substantially (30-100 %) in the adsorbents, especially in ROAC and NMC+n, but more so with ROAC. In general, the three adsorbents exhibited strong stability for the long chain PFAS, especially the perfluoroalkyl sulphonic acids, with minimal to no sorption reversibility under different pH and ionic composition of simulated groundwater. This study highlights the importance of understanding not only the sorption of PFAS in groundwater using adsorbents, but also the desorption of PFAS, which may be useful for decision making during the ex-situ and in-situ treatment of PFAS-contaminated groundwater.

Sorption of PFOS in 114 Well-Characterized Tropical and Temperate Soils: Application of Multivariate and Artificial Neural Network Analyses.

The influence of soil properties on PFOS sorption are not fully understood, particularly for variable charge soils. PFOS batch sorption isotherms were conducted for 114 temperate and tropical soils from Australia and Fiji, that were well-characterized for their soil properties, including total organic carbon (TOC), anion exchange capacity, and surface charge. In most soils, PFOS sorption isotherms were nonlinear. PFOS sorption distribution coefficients (Kd) ranged from 5 to 229 mL/g (median: 28 mL/g), with 63% of the Fijian soils and 35% of the Australian soils showing Kd values that exceeded the observed median Kd. Multiple linear regression showed that TOC, amorphous aluminum and iron oxides contents, anion exchange capacity, pH, and silt content, jointly explained about 53% of the variance in PFOS Kd in soils. Variable charge soils with net positive surface charges, and moderate to elevated TOC content, generally displayed enhanced PFOS sorption than in temperate or tropical soils with TOC as the only sorbent phase, especially at acidic pH ranges. For the first time, two artificial neural networks were developed to predict the measured PFOS Kd (R2 = 0.80) in the soils. Overall, both TOC and surface charge characteristics of soils are important for describing PFOS sorption.

In vitro gastrointestinal mobilization and oral bioaccessibility of PAHs in contrasting soils and associated cancer risks: Focus on PAH nonextractable residues.

The gastrointestinal mobilization and oral bioaccessibility of polycyclic aromatic hydrocarbon (PAH) nonextractable residues (NERs) from soils remain unexplored, including associated incremental lifetime cancer risks. This study investigated the gastrointestinal mobilization of PAHs and their NERs from contrasting soils, using a physiologically based extraction test that incorporates a silicone-rod (Si-Org-PBET) as PAH sink. Associated cancer risks following soil ingestion were also evaluated. Four solvent-spiked and aged soils, and four long-term contaminated manufactured gas plant (MGP) soils, were utilized. Total-extractable PAH concentrations were measured after exhaustive solvent extractions of soils. We evaluated the PAH sorption efficiency of the silicone rods and associated sorption kinetics, using PAH-spiked silica sand as the contaminated matrix. We then assessed gastrointestinal mobilization of benzo[a]pyrene and benzo[a]pyrene NERs from the solvent-spiked soils, and mobilization of six PAHs and their NERs from the MGP soils. PAH oral bioaccessibility was determined. The incremental lifetime cancer risks (ILCRs), using Si-Org-PBET- and total-extractable PAH concentrations from the MGP soils, were calculated. Sorption kinetics modelling showed that 95% of mobilized PAHs sorbed to the silicone rods within 2-19 h, depending on PAH physico-chemical properties. Total-extractable and Si-Org-PBET extractable PAH concentrations exceeded health investigation levels (3 mg/kg based on benzo[a]pyrene toxic equivalent quotients) in soils. PAH oral bioaccessibility approached 100% for solvent-spiked soils, but only 24-36% for the MGP soils. Associated ILCRs exceeded target levels (10-5) for one MGP soil, particularly for 2-3 year olds, despite oral bioaccessibility considerations. In contrast, mobilized PAH NERs did not exceed health investigation and ILCR levels, as the NERs were highly sequestered, especially in the MGP soils. PAH nonextractable residues in long-term contaminated soils are unlikely to be mobilized in concentrations that pose cancer risks to humans following soil ingestion, and do not need to be considered in risk assessments.

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 kgOC/kglipid. 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.

Extremely small amounts of B[a]P residues remobilised in long-term contaminated soils: A strong case for greater focus on readily available and not total-extractable fractions in risk assessment.

There is a lack of understanding about the potential for remobilisation of polycyclic aromatic hydrocarbons (PAHs) residues in soils, specifically after the removal of readily available fractions, and the likelihood to cause harm to human and environmental health. Sequential solvent extractions, using butanol (BuOH), dichloromethane/acetone, and methanolic saponification were used to investigate the time-dependent remobilisation of B[a]P residues in aged soils, after removal of readily available or total-extractable fractions. After 120 d of aging, BuOH-remobilised B[a]P were small or extremely small ranging from 2.3 ± 0.1 mg/kg to 4.5 ± 0.5 mg/kg and from 0.9 ± 0.0 mg/kg to 1.0 ± 0.1 mg/kg, after removal of readily available and total-extractable fractions, respectively. After removal of readily available fractions, the remobilisation rates of B[a]P residues were constant over 5 re-equilibration times, as shown by first-order kinetics. The amounts of B[a]P remobilised significantly (p < 0.05) decreased with aging, particularly in hard organic carbon-rich soils. After 4 years of aging, BuOH- and total-remobilised B[a]P were generally < 5% of the initially spiked 50 mg/kg. Based on the findings of this study, the potential or significant potential for B[a]P NERs in soils to cause significant harm to human and environmental health are minimal.

Time-Dependent Remobilization of Nonextractable Benzo[a]pyrene Residues in Contrasting Soils: Effects of Aging, Spiked Concentration, and Soil Properties.

The environmental and health risks associated with "nonextractable" residues (NERs) of polycyclic aromatic hydrocarbons in soils and their potential for remobilization remain largely unexplored. In this novel study, sequential solvent extractions were employed to interrogate time-dependent remobilization of benzo[a]pyrene (B[a]P) NERs and associated kinetics after re-equilibration (REQ) periods lasting 30 d in four artificially spiked soils aged for up to 200 days. Following sequential extractions of the re-equilibrated soils, remobilization of B[a]P NERs was observed and further confirmed by decreases in the absolute amounts of B[a]P recovered following methanolic saponification after REQ. Remobilization may occur through slow intercompartmental partitioning of more sequestered into less sequestered B[a]P fractions in soils. The amounts of B[a]P remobilized in soils decreased throughout aging following first-order kinetics, and the rates of decrease were slow but 2 to 4 times faster than those of extractable B[a]P before re-equilibration. Sandy-clay-loam soils with large amounts of hard organic carbon exhibited less NER remobilization compared to sandy soils. The amounts of remobilized B[a]P decreased significantly ( p < 0.05) with aging. Specifically, butanol-remobilized B[a]P in soils spiked at 10 mg/kg and 50 mg/kg B[a]P ranged from 0.15 to 0.39 mg/kg and 0.67 to 2.30 mg/kg, respectively, after 200 d of aging.

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.

Comparison of Single- and Sequential-Solvent Extractions of Total Extractable Benzo[ a]pyrene Fractions in Contrasting Soils.

The fate and behavior of polycyclic aromatic hydrocarbons (PAHs) in soil are of interest in the risk assessment of contaminated land and are usually based on determinations of fractions extracted from soil. For decades, either single- or sequential-solvent extractions have been used to determine PAH extractability in soils; however, there is a lack of certainty as to which fractions are being extracted by these techniques. This study is the first report of differences and similarities in the extractability of benzo[ a]pyrene (B[ a]P) in four contrasting soils (sandy loam, loamy sand, clayey loam, and sandy) when determined using both single-solvent (dichloromethane/acetone (DCM/Ace) mixture) and sequential-solvent (butanol followed by DCM/Ace) extraction. Residues after extraction were subjected to methanolic saponification (MeKOH). Butanol (BuOH) extractability and total extractability of B[ a]P following sequential-solvent extraction decreased significantly ( p < 0.05) with time after addition of B[ a]P. The decrease in BuOH extractability was particularly marked in the organic-matter-rich clayey-loam soil, which also had the largest (>40%) amounts of nonextractable residues. The cumulative amounts of B[ a]P extracted in each soil by single- and sequential-solvent extractions were similar ( p > 0.05) at each aging period, which indicated access to similar B[ a]P fractions in soil by both solvent extractions. The similarities in the amounts of B[ a]P nonextractable residues recovered by MeKOH from pre-extracted soils, through either of the extraction methods, confirms that the total extractable B[ a]P fractions from both methods are similar.

Residual hydrophobic organic contaminants in soil: Are they a barrier to risk-based approaches for managing contaminated land?

Risk-based approaches to managing contaminated land, rather than approaches based on complete contaminant removal, have gained acceptance as they are likely to be more feasible and cost effective. Risk-based approaches aim to minimise risks of exposure of a specified contaminant to humans. However, adopting a risk-based approach over alternative overly-conservative approaches requires that associated uncertainties in decision making are understood and minimised. Irrespective of the nature of contaminants, a critical uncertainty is whether there are potential risks associated with exposure to the residual contaminant fractions in soil to humans and other ecological receptors, and how they should be considered in the risk assessment process. This review focusing on hydrophobic organic contaminants (HOCs), especially polycyclic aromatic hydrocarbons (PAHs), suggests that there is significant uncertainty on the residual fractions of contaminants from risk perspectives. This is because very few studies have focused on understanding the desorption behaviour of HOCs, with few or no studies considering the influence of exposure-specific factors. In particular, it is not clear whether the exposure of soil-associated HOCs to gastrointestinal fluids and enzyme processes release bound residues. Although, in vitro models have been used to predict PAH bioaccessibility, and chemical extractions have been used to determine residual fractions in various soils, there are still doubts about what is actually being measured. Therefore it is not certain which bioaccessibility method currently represents the best choice, or provides the best estimate, of in vivo PAH bioavailability. It is suggested that the fate and behaviour of HOCs in a wide range of soils, and that consider exposure-specific scenarios, be investigated. Exposure-specific scenarios are important for validation purposes, which may be useful for the development of standardised methods and procedures for HOC bioaccessibility determinations. Research is needed to propose the most appropriate testing methods and for assessing potential risks posed by residual fractions of HOCs. Such investigations may be useful for minimising uncertainties associated with a risk-based approach, so that consideration may then be given to its adoption on a global scale. This review critically appraises existing information on the bioavailability of HOC residues in soil to establish whether there may be risks from highly sequestered contaminant residues.