- Title
- Effects of source materials on bioavailability and bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in soils
- Creator
- Yu, Linbo
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- PAHs are of great environmental concerns due to their toxicity, mutagenicity, carcinogenicity that causes significant harms to both ecological and human receptors. These contaminants are produced and released to the environment, via a wide range of anthropogenic and industrial activities, and able to persist in soil and sediment for prolonged period of time from years to decades due to their hydrophobicity and resistance to degradation. Risk assessment of contaminated land with PAHs in recent decades has moved towards a bioavailability- and bioaccessibility-based practice for forensic characterisation of the hazards from these contaminants. Several controlling factors of bioavailability and bioaccessibility of PAHs in soil, including soil composition and properties, PAH concentration and co-contaminants, and environmental conditions, have been identified. Unlike other hydrophobic organic contaminants (HOCs), such as pesticides and polychlorinated biphenyls (PCBs) that are synthesised and applied for certain purposes in the form of ‘pure chemicals’, PAHs are unintentionally produced and are released to the environment carried by source materials that contain complex mixtures of both organic and inorganic contaminants. These materials could be in states of solid, semi-solid, or non-aqueous phase liquids (NAPLs). Importantly, source materials also interact with soil matrices over the period of contamination and may consequently dominantly affect the bioavailability and bioaccessibility of PAHs. However, the majority of PAH bioavailability studies used highly lab-controlled ‘solvent spiking’ procedures which overlooked the effects of source materials. This put the knowledge and theories about the fate and behaviour of soil PAHs to question regarding their relevance to the reality. With the wide knowledge gap identified, this thesis underpins and updates the scientific basis of the knowledge and principles of PAH bioavailability and bioaccessibility. Desorption kinetics study using sequential Tenax extractions of an index carcinogenic PAH, benzo(a)pyrene (BaP), from 2 Australian soils and a silica sand treated respectively with solvent, pitch, and tar revealed that the release of BaP was enhanced when source materials were mixed with soil compared to results from source materials alone. Such enhancement was suggested to be induced through the disruption of source materials surfaces and the dispersion of tar as a NAPL in soil and sand particles. The desorption of BaP from source material contaminated soils and sand was found to deviate from that of solvent spiked soils and sand, following the order solvent > tar > pitch at 1 day after spiking. Importantly, over an ageing period of 48 days, the extents of rapid BaP desorption from tar amended soils, captured by two-compartment desorption model parameter Frap, were found to increase and even exceeded the Frap obtained from solvent treatment in 1 soil significantly (p < 0.05). Given that such increase of Frap was not found in tar treated sand, it was suggested that the abundant pore structures, which were present in soils but absent in sand, have driven the continuous expansion of tar surfaces under capillary actions causing enhanced release of BaP. Kinetic studies also revealed that the slow desorption of BaP in tar treated soil could also take place at higher rate constants (kslow) if the soil does not contain sufficient SOM sink for BaP. The values of Frap from two-compartment desorption model fitting were converted to concentrations of BaP to represent bioaccessibility and compared against Australian regulatory threshold for BaP (3 mg/kg). It was found that at 1 d of soil incubation only solvent spiked soil presented unacceptable risks, which was quickly mediated by ageing effects, while risks of BaP in 1 tar treated soil increased over the period of soil incubation and became unacceptable at 24 d. The comparison between Frap and model calculated desorption after 24 and 48 h (F24 and F48) indicated that a single point Tenax extraction of 24 h may not cover the rapidly desorbing BaP from pitch treated soils. Empirical study on the extractability of 6 carcinogenic PAHs (cPAHs) by organic solvent, Tenax (single point extraction of 48 h), and passive sampling from 6 Australian soils treated respectively with solvent, pitch, and tar was launched to evaluate the risks of these chemicals represented by total extractable, bioaccessible, and bioavailable concentrations. The role of soil organic matter in determination of bioaccessibility and bioavailability of PAHs in contaminated soils was also investigated. The result showed that the total extractability of cPAHs in soils spiked by solvent was evidently reduced after initial soil incubation of 17 days, while that in pitch and tar amended soils did not show extensive declines. Concentrations of cPAHs recovered by Tenax extractions also demonstrated more prominent ageing effects in solvent spiked soils than pitch and tar treated soils. Further validating the findings from the kinetic study, Tenax extractability of cPAHs in tar treated soils increased after initial ageing of 17 days and exceeded the Tenax extractability of cPAHs in certain solvent spiked soils. Moreover, the role played by soil organic matter in determining the results of Tenax extractions and passive sampling was also reflected through simple linear regressions. It was found that total soil organic matter (SOM) and organic carbon (TOC) in the investigated soils could explain the variations of the apparent soil-water partition coefficients (Kd) for Σ6cPAHs calculated from the results of passive sampling. TOC were also correlated to the concentrations of Σ6cPAHs obtained from Tenax extractions of solvent and tar spiked soils but not pitch spiked soils, while better correlations were achieved between soil fine particle associated carbon (FPAC) and Tenax extracted concentrations of Σ6cPAHs from solvent spiked soils. It was suggested that the dissolution of PAHs, represented by the results of passive sampling, is largely controlled by the total SOM or OC, while the rapid desorption, represented by the results of Tenax extractions, is controlled by both quantity and quality of SOM, latter of which was expressed by FPAC. However, the importance of SOM quality was greatly weakened when source materials of PAHs are present, given the weak or no correlations between Tenax extractable concentrations and FPAC. Moreover, the positive correlations between Tenax extracted Σ6cPAHs and FPAC for solvent and tar treated soils were opposite to the findings in published papers and called for an empirical investigation on the roles of SOM in soil fine particles in determination of PAH bioaccessibility. Lastly, the significance of source materials in risk assessment was characterised in by comparing the total BaP equivalent concentrations of 6 investigated cPAHs against the Australian regulatory thresholds. The results further confirmed the overlooked effects of source materials identified in the kinetic study as 3 out of 6 tar treated soils presented unacceptable bioaccessible BaP-equivalent concentrations. The findings from the empirical study of 6 cPAHs in soils with different source materials were further interrogated by an extended research on the distribution and extractability of cPAHs in the size fractions of the same soils. For this purpose, soils contaminated respectively with solvent, pitch, and tar at the different stages of ageing was fractionated to coarse (> 63 μm) and fine (<63 μm) fractions and extracted by organic solvent and single point 48 h Tenax extractions. The results demonstrated very different distribution and extractability of cPAHs in size fractions of soils with different source materials. Both coarse and fine fractions of solvent spiked soils demonstrated sequestration of cPAHs and formation bound residues over the ageing period of 70 days reflected by concentrations as well as total masses of cPAHs obtained by solvent extraction from these fractions. In contrast, the soils treated with pitch and tar did not show such typical patterns of ageing. Further analysis attributed these findings to the redistribution of cPAHs between size fractions of pitch and tar treated soils during soil incubation. SOM in fine fractions displaced a part of cPAHs from pitch, which mainly resided in the coarse fractions, and resulted in higher solvent extractability of cPAHs in the fine fractions. The migration of tar as a NAPL into the fine fractions also led to larger amounts of cPAHs recovered by solvent from the fine particles in tar treated soils. Coarse fractions were found to dominate the distribution of bioaccessible cPAHs in solvent spiked soils, while for pitch and tar treated soils more bioaccessible cPAHs were extracted by Tenax extractions from fine fractions, which is another indication of the re-redistribution of cPAHs from coarse to fine fractions with the presence of source materials. The importance of SOM in PAH distribution between soil size fractions was reflected by regression analyses. The contribution of SOM from fine fractions of soils were found to significantly impact the concentrations and masses of total extractable and bioaccessible cPAHs in both fractions of solvent and tar treated soils and fine fractions of pitch treated soils. Extractability and plant accumulation of the investigated cPAHs in 3 contrasting soils respectively contaminated by solvent, pitch, and tar after 300 d of ageing was surveyed to characterise the long-term risks of these soils and evaluate the predictive performance of well-established techniques in soils with different source materials. The solvent extractability of cPAHs was found to be significantly lower in solvent spiked soils than pitch and tar spiked soils. In contrast, Tenax extracted concentrations of cPAHs were the highest in solvent spiked soils followed by tar amended soils and pitch amended soils, while the dissolved concentrations of cPAHs (modelled by passive sampling) followed the order of tar > solvent > pitch. Given that solvent extractable PAHs represent the pools of readily dissolved, rapidly desorbing, and slowly + very slowly desorbing fractions, Tenax (infinite sink) extractable PAHs represent the pools of dissolved and rapidly desorbing fractions, while aqueous concentrations obtained via passive sampling represent the dissolved concentrations, following conclusions were obtained: (a) after extensive ageing, pitch and tar continued to be the sources of slow and very slow release of PAHs in soils, which is an important role played by these source materials unable to be captured by solvent spiking procedures; (b) the rapidly desorbing fractions of PAHs were largely dissipated via sequestration or biodegradation and with the extra sequestration domains provided by pitch and tar such rapid release of PAHs were further restrained; (c) although the rapidly desorbing fractions were of smaller sizes in pitch and tar treated soils than in solvent spiked soils, the dissolved PAHs may still pose higher imminent risks in tar treated soils than solvent spiked soils. These findings provide new insights to the risk assessment of PAH-contaminated soils with the consideration of the long-term hazards of soils contaminated with source materials of PAHs. Furthermore, the results from plant accumulation assays generally followed the same order as Tenax extractions regarding cPAH concentrations in roots and root lipid contents (solvent > tar > pitch). The accumulation of cPAHs was also largely determined by levels of SOM and soil texture, as well as the water solubility and bioavailability of individual cPAHs. The performance of chemical assays in prediction of bioaccumulation was the best by passive samplers due to the strong correlations between passive sampling modelled and measured root ant root lipid concentrations of cPAHs, as well as the closeness to 1: 1 relationship between these two pairs. However, passive sampling is not suggested for the assessment of PAH associated risks because of the occasional underestimation of accumulated cPAHs. Instead, Tenax extractions which overestimated the accumulation results by 12 – 18 times were considered more appropriate as it provided reasonably conservative estimations which is much more desired in risk assessment – more reasonable than solvent extractions (130 – 300 times overestimation), better precaution than passive samplers (up to 8 times underestimation).
- Subject
- PAHs; bioavailability; bioaccessibility; source materials
- Identifier
- http://hdl.handle.net/1959.13/1488953
- Identifier
- uon:52585
- Rights
- Copyright 2020 Linbo Yu
- Language
- eng
- Full Text
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