- Title
- Lead bioavailability as influenced by its sources, speciation and soil properties
- Creator
- Yan, Kaihong
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Lead has been of particular concern as a neurotoxin since the 1970s due to its permanent adverse effects on human health. People can be exposed to Pb by ingestion (either through accidental oral ingestion or through food or drinking), inhalation (e.g. fine Pb particles in dust) and dermal uptake. Ingestion of Pb contaminated soils poses a significant risk to humans, especially children and babies due to their behaviors including crawling and hand-to-mouth activities, fast metabolic rates and rapidly developing neuronal systems. Thus, determining the bioavailability of Pb (Pb-BA) in soils is critical in human health risk assessment. However, it remains a serious challenge due to measurement uncertainties and the lack of information on the influences of sources of Pb contamination, Pb speciation and soil properties to Pb-BA. Consequently, this thesis focuses on the following issues: 1) validation of a reliable model to measure Pb bioaccessibility (Pb-BAc) and minimization of associated uncertainties; 2) prediction of Pb-BAc using soil properties from various sources of Pb contaminated soils; 3) investigating the contribution of different Pb speciation existing in various sources of Pb contaminated soils; and 4) transformation of Pb speciation during in vivo and in vitro assays. A total of 40 soils and 5 house/roof dusts were collected from various Pb contaminated sites throughout Australia. Soil properties were investigated for both soil particle sizes of < 2 mm and < 250 μm using established methods, including pH, electrical conductivity (EC), cation exchange capacity (CEC), total organic carbon (TOC), clay content, total Pb and other metals (Cd, Zn, As, Cu). Pb bioaccessibility (Pb-BAc) was measured using the in vitro models reflecting the Relative Bioavailability Leaching Procedure (RBALP) and the unified BARGE method (UBM). Pb’s relative bioavailability (Pb-RBA) was measured using an in vivo mice model. The Scanning Electron Microscope (SEM) (Zeiss Sigma 300 VP-FESEM), X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) were applied to investigate Pb speciation in soils, dusts and mice excreta after a 10-day in vivo mice study. Blank samples and three replications were conducted for both UBM and RBALP assays. Continuing calibration verification (CCV) served to determine Pb by ICPMS. The amount recovered was 100.6% ± 6.1% with a detection limit of 0.1 μg/L. All the statistical analyses of the data, including parameter inferences, hypotheses testing, and linear regression were conducted using Excel, Origin and Statistical Package for the Social Sciences (SPSS) software (version 19.0). Quantitative comparisons of Pb-BAc data were done via analysis of variance (ANOVA) and standard t-tests. Validation of in vitro models (RBALP and UBM) using the in vivo (mice) model were applied to 9 top soils (0-20 cm depth). Transformation of Pb speciation during in vitro and in vitro assays were investigated on selected soil samples and mice excreta using SEM, XRD and XANES analyses. Both the RBALP and UBM models (gastric phase) were well correlated with in vivo bioavailability, while the UBM model may not be reliable for soils that contain high soluble Pb and total Pb exceeding 10,000 mg/kg due to its lower solid:liquid ratio (1:37.5). No differences in the Pb release were observed between the UBM and RBALP models in XANEs analysis. The free Pb2+ was released from Pb minerals with relatively high solubility products (Ksp), including PbO2, PbSO4 and MgO Pb, combined with free Cl- and PO43- in solution. Smaller amounts of Ksp Pb minerals such as Pb5(PO4)3Cl and organicallycomplexed Pb were identified in mice excreta, which is largely because a portion of free Pb2+ was combined with food and humic acid. To investigate the influence of soil properties on Pb-BAc and generate a potential predictive model, the soil properties between soil particle sizes of < 2 mm and < 250 μm were compared for various sources of Pb contaminated soils; the Pb-BAc were measured using the RBALP model. Results demonstrated that: 1) CEC, TOC, sand and silt content, and total Pb were significantly different (p < 0.05) between the two particle size fractions of < 2 mm and < 250 μm; and 2) EC, CEC and total Pb were significantly correlated to Pb-BAc for soils that particle size of < 250 μm (p < 0.05). Moreover, the potential relationships between soil properties and Pb-BAc were investigated using the RBALP model for 31 soils originating various sources of Pb contamination. Soil analyses based on source of Pb contamination demonstrated a strongly significant relationship between Pb-BAc and soil properties (CEC, EC, clay content and total Pb) for mining Pb contaminated soils from Broken Hill (r2 = 0.86, p < 0.05, n = 18). These results confirmed the influence of sources of Pb contamination, soil properties and particle size fractions on Pb-BAc as well as the prediction of Pb-BAc using soil properties. The impact of sources of Pb contamination on Pb-RBA was investigated by comparing the correlations between in vivo and in vitro models (IVIVCs) on both mining Pb contaminated soils and all sources of Pb contaminated soils. The increase in slope and r2 of IVIVCs with the increase in sources of Pb contamination indicated that IVIVC is more representative of all sources of Pb contamination compared to a single source of Pb contamination. The SEM, XRD and XANES results demonstrated that the Pb mineral forms and binding status varied among various sources of Pb contamination, even for the soils/dust contaminated by the same source of Pb contamination. One possible explanation is that the conversion of Pb mineral forms occurred during the weathering and deposition processes. The Pb speciation may vary among various sources of Pb contamination, and then influence Pb-RBA. Measuring Pb-RBA should in fact consider the source of Pb contamination and Pb speciation. In summary, this study contributed to minimizing uncertainties in Pb-BA assessment, described and explained the influence of soil properties and sources of Pb contamination on Pb-BA, investigated the changes in Pb speciation during both in vivo and in vitro assessments, and generated a potential predictive tool of soil properties to Pb-BA. The findings are fundamentally useful for the measurement of Pb-BA and risk assessment practices. Further research activities are expected to: 1) improve the intestinal phase of the UBM model to indicate Pb-RBA; 2) minimize uncertainties in measurement of Pb-BA in both in vivo and in vitro assays; and 3) generate a model that can potentially utilize soil properties to predict Pb-RBA on various sources of Pb contamination.
- Subject
- soil; uncertainties; in vivo; in vitro; Pb; bioavailability; bioaccessibility; soil properties; prediction; speciation
- Identifier
- http://hdl.handle.net/1959.13/1408761
- Identifier
- uon:35882
- Rights
- Copyright 2019 Kaihong Yan
- Language
- eng
- Full Text
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