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Title: Environmental risk assessment of heavy metals in urban street dust
Author: Wang, Wan Hsuan
Awarding Body: University of Greenwich
Current Institution: University of Greenwich
Date of Award: 1999
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Environment risk assessment of heavy metals has been widely used as a scientific method for evaluating possible environmental hazards presented in a product, a system and/or an industrial process. The results have been used for setting occupational standards and environmental regulations. However, in practice, environmental risk assessment generally does not take into account the bioavailability/mobility of chemicals in different environmental pathways and attention has rarely been paid to widely spread and un-regulated diffuse sources in the urban environment. This thesis employs several practical approaches (chemical, biological, kinetics) together with a theoretical model to assess the potential risk of diffuse sources of cadmium, copper, lead and zinc in urban street dust in two pathways, namely, dust ingestion and the impact of heavy metals on soil adjacent to a road. Dust samples were collected from four sampling sites (two in Hong Kong and the other two in London) during a period of nineteen months. Tessier's sequential extraction schemes were used to assess the geochemical phases/bioavailability of mobilisation potential of heavy metals in the dust. A series of single leaching methods were also used and time dependent metal release kinetics were carried out for evaluating the potential metal release under simulated different environmental pathways. These experiments showed that the partitioning and distribution of cadmium, copper, lead and zinc in urban street dust, regardless of locations, is relatively consistent. On the other hand, the different leaching methods gave rise to different results while pH was identified as a main factor influencing the metal release. Under acidic conditions, metal release generally followed a fast-slow release pattern. Gastric conditions were also simulated in the laboratory to study the metal release mechanism of urban street dust to the risk of dust ingestion by human beings, especially children. High percentages of heavy metal release under simulated gastric conditions suggested the use of the total metal concentration in dust for risk assessment. A theoretical model based on the assumption of a worst case scenario was developed and applied to evaluate the exposure between heavy metals in urban road dust and in soil adjacent to a road. The partition coefficient describing the tendency of heavy metals distributed between solid and liquid phases showed equal importance as physical factors such as rainfall intensity, traffic flow and road geometry in the risk model. Root elongation, algal growth and light emission of luminescent bacteria were employed for assessing the toxic effect of heavy metals in urban dust. All test organisms showed suppressed response under different concentrations of dust elutriates, while no consistent correlation was found between the chemical speciation tests and the biological toxicity tests. The results of the above operationally defined approaches to the risk assessment of heavy metal in urban street dust suggest that there is no simple and quick way for assessing their risk in different environmental pathways. An integrated approach based on the pH dependent chemical leaching method, sequential extraction, together with a modelling strategy in risk assessment may provide a more comprehensive view of the pathways between the source and receptor as well as an estimate of the most important factors.
Supervisor: Fisher, Bernard E. A. Sponsor: University of Greenwich ; Hong Kong Baptist University ; British Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology ; TD Environmental technology. Sanitary engineering