Long-term low-level ingestion of cadmium (Cd) causes human health problems, and in Australia, vegetables supply ~40% of the Cd in the typical diet. Plants take up Cd from the soil; however, the uptake is poorly predicted by simple soil tests, such as the total concentration of Cd (Cdt). Therefore, a greater understanding of Cd behaviour in soils is needed to improve the prediction of Cd uptake by plants and open a new path to minimise the risks for human health. The objectives of the research in this thesis were to: identify key soil properties affecting Cd behaviour, identify/develop selective methods to measure them, and to formulate a conceptual model of Cd partitioning. These objectives were based on the hypothesis that empirical modelling informed by a better understanding of Cd chemistry would accurately describe Cd partitioning in soil. To test the hypothesis, the key properties were measured on soils from the peri-urban fringe of Greater Sydney (n = 41) and a series of models of increasing complexity were fitted to the data. A model with three explanatory variables— log10 Cdt, pH and log10 ECEC (effective cation exchange capacity)—explained 94.6% of variation in log10 CdCa (the concentration of Cd in solution in a suspension of soil in 10 mM CaCl2), which strongly supported the hypothesis. The study also indicated that the explanatory variables, Cdt, pH and ECEC, may describe Cd behaviour in many soils, and that for these general models, partition coefficients, such as log10 (Cdt/CdCa), are unsuitable dependent variables. The preceding model used Cdt as an explanatory variable, notwithstanding that labile Cd (CdE) was mechanistically preferable. However, CdE can only be measured using isotopic techniques: a requirement that has constrained the evaluation of CdE as an index of Cd behaviour and bioavailability. Therefore, a simple proxy measure of CdE was investigated. The literature indicated that solutions of chloride salts might selectively extract CdE, and Cd extracted into 1 M NH4Cl (CdNH4Cl) was compared with CdE measured by stable isotope dilution ICPMS. For 23 soils from the partitioning study, 1 M NH4Cl failed to completely extract CdE, unless the pH was less than 5. The cause(s) of this effect will be investigated with the aim of developing a universally applicable measure of CdE that does not require isotopic measurements. All models of Cd uptake by plants rely on soil properties measured on homogenised samples, although the distribution and bioavailability of Cd vary spatially in the field. Were such variability to increase at the micro-scale, its effects could erode the accuracy with which models could predict Cd behaviour and uptake. Consequently, I tested whether the distribution of Cd could be mapped by using synchrotron micro-x-ray fluorescence spectroscopy (micro- XRFS): the most sensitive method of observation. The soils examined contained 0.3–6.4 mg Cd/kg, i.e. were typical agricultural soils, and one was spiked to ~100 mg Cd/kg. Micro-XRFS mapped the Cd in the spiked soil, and in one particle in the other soils. For typical agricultural soils, the sensitivity realised in this study would have been sufficient to characterise the average Cd binding site, but fell at least 10-fold below that needed to map the Cd distribution in them. The research satisfied the objectives, advanced knowledge of Cd behaviour in soils, and provided new research leads. These leads include the possibility of developing general models of Cd partitioning in soils, derivatives of which may predict Cd uptake by plants. The accuracy of these models may be strengthened by the use of CdE as an explanatory variable, but may be weakened by the effects of in situ variation in the distribution of Cd. The benefits to human health of agricultural practices that decrease dietary Cd justify continuation of research to develop models that accurately predict Cd uptake by plants. / Doctor of Philosophy (PhD)

Bibliography: leaves 202-223.

Heavy metal bioaccumulation in aquatic organisms may occur through direct or indirect uptake routes. Research indicates that the significance of uptake route varies with contaminant and organism exposed. The relative importance of different metal sources in aquatic systems was investigated by exposing freshwater snails to dietary or dissolved sources of cadmium. Snails were exposed to control, contaminated food only, contaminated water only, and contaminated food and water treatments. During the 15-day exposure, samples were taken to determine Cd concentration in snail soft tissue, snail shell, algal food, and overlying water. Analyses of snail soft tissue and shells indicate that exposure route significantly affects Cd concentrations in the tissues. In both cases, dissolved Cd is the primary contributor to metal body burden.

Nonradiative decay of excitons is a competing process to Multi-Exciton Generation (MEG) in nanoparticles. Nonradiative decay of single excitons with sufficient energy to generate bi-excitons in Cd₂₀ Se₁₉ and Cd₈₃ Se₈₁ nanoparticles was studied using Tully's Molecular Dynamics with Quantum Transitions (MDQT) method and a CdSe pseudopo- tential. Exciton decay rates increase with increases in nanoparticle temperature and density of lower-lying excitonic states. There did not appear a significant effect of size on energy decay rates. The decay dynamics generally follow a gradual decay with transitions between nearby states. This is punctuated by periodic, short-lived periods of rapid downhill tran- sitions that result in a large proportion of excess exciton energy being transferred to the vibrational motion of the nanoparticle. The time for relaxation to below the 2.0E[subscript g] cutoff was on the order of 1ps. / text

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