- Title
- Bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in soil: investigations into the potential risks associated with exposure to residual PAHs in soil
- Creator
- Umeh, Anthony
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The presence, fate, impact, potential health risks, and significance of highly sequestered nonextractable residues (NERs) of polycyclic aromatic hydrocarbons (PAHs) in soils have long been speculated. This represents a critical uncertainty for risk-based management of contaminated lands. This thesis contains a series of first-time investigations of the solvent-extractability, remobilisation, bioaccumulation and sublethal genotoxicity, gastrointestinal mobilisation and oral bioaccessibility, as well as potential cancer risks of PAH NERs, from the perspectives of human and/or environmental health risk assessments. The changes and similarities in extractability of benzo[a]pyrene (B[a]P) in soils with contrasting properties were determined, simultaneously using both single (dichloromethane-DCM/acetone-Ace mixture) and sequential solvent (butanol followed by DCM/Ace) extractions. Residues after extraction were subjected to methanolic saponification (MeKOH). Butanol (BuOH)- 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 non-extractable 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 indicate access to similar B[a]P fractions in soil by both solvent extractions. The similarity in the amounts of B[a]P non-extractable residues recovered by MeKOH of pre-extracted soils, through either of the extraction methods, confirms that the total-extractable B[a]P fractions from both methods are similar. Seven different methanolic and non-methanolic alkaline treatments, and the conventional methanolic saponification, were used to extract 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 non-extractable 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 7 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 7 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. Also, sequential solvent extractions were employed to interrogate time-dependent remobilisation of 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, remobilisation 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. Remobilisation may occur through slow intercompartmental partitioning of more sequestered into less sequestered B[a]P fractions in soils. The amounts of B[a]P remobilised 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 remobilisation compared to sandy soils. The amounts of remobilised B[a]P decreased significantly (p < 0.05) with aging. Specifically, butanol-remobilised 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. Again, sequential solvent extractions, using BuOH, DCM/Ace, 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. Further, the potential for bioaccumulation and associated genotoxicity of PAH NERs in long-term contaminated soil was also investigated in this thesis. 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 – 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. There were no 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. In addition, the potential gastrointestinal mobilisation and oral bioaccessibility of carcinogenic PAH NERs from soils following ingestion were also studied. Residual PAH mobilization was investigated in solvent-spiked soils aged for 500 days and 4 years, and manufactured gas plant (MGP) soils. A physiologically based extraction test incorporating a silicone-rod (Si-Org-PBET) as a sink for PAHs was the mobilization medium utilized. Sorption kinetics modelling showed that 95% of mobilized PAHs sorbed to the silicone rods within 2 – 19 h of incubating spiked silica sand in Si-Org-PBET, depending on PAH physico-chemical properties. PAH oral bioaccessibility approached 100% and 24 – 36% for the solvent-spiked and MGP soils, respectively. Associated cancer risks exceeded target levels for one MGP soil, particularly for 2 – 3 year olds. In contrast, the mobilized PAH NERs did not exceed health investigation and cancer risk target levels, as the NERs were highly sequestered, particularly in the MGP soils. The direct tripartite evidence from the extremely small solvent extractability and remobilisation, earthworm bioaccumulation and sublethal toxicity, and human gastrointestinal mobilisation and oral bioaccessibility of PAH NERs from long-term contaminated soils are useful in risk assessments as they can be applied to minimise the uncertainties associated with the ecological and human health risks from exposure to highly sequestered PAH residues in long-term contaminated soils. Overall, the information in this thesis will help stakeholders within the contaminated land to make better decisions towards realistic and cost-effective management of PAH contaminated soils.
- Subject
- polycyclic aromatic hydrocarbons; nonextractable residues; earthworms; bioavailability; human health risk assessment; thesis by publication; long-term contaminated soils; risk-based contaminated land management; cancer risk assessment; remobilisation; PAHs; NERs; ecological health risk assessment; bioaccessibility
- Identifier
- http://hdl.handle.net/1959.13/1408744
- Identifier
- uon:35880
- Rights
- Copyright 2019 Anthony Umeh
- Language
- eng
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