Every day, thousands of tons of wastes from businesses, industries and residences need to be disposed of in a safe, efficient and environmental-acceptable manner. In 1994, a daily average of about 24,000 tons of solid waste in Hong Kong was delivered to different waste facilities for disposal (EPD, 1995) . The industrial wastes and sewage sludge are among the major types of wastes produced. In order to reduce the harmful effects and environmental risks, certain treatment processes are required prior to the waste disposa l. Stabilization is one of the major treatment methods in dealing with hazardous wastes and sewage sludge( Poon et al, 1989)
Stabilization is a process employing additives (reagents ) to reduce the hazardous nature of waste by converting the waste and its hazardous constituents into a more stable form (Culliane et al, 1986; Conner, 1990). Stabilization of hazardous wastes frequently employs cement and clay minerals as the principal reagents (Poon et al, 1985; Lo, 1996). This process will minimize the rate of contaminants\rquote migration into the environment and reduce the level of toxicity. The stabilized waste materials can then be disposed in landfill sites. Although stabilization technology has been used for a long period of time, the physical and chemical cha nges that take place as a result of interaction of contaminants with cement and other reagent have not been fully characterized. A waste chemical component may chemisorb, precipitate, form surface compounds, form inclusions or be chemically incorporated i nto the cement structures (Conner, 1990; Space, 1993).
The mechanisms of the stabilization process and the potential for contaminant loss from a stabilized mass required further detailed studies to assess the long term environmental impacts. This is usuall y determined by chemical leaching tests and particle characterization techniques (LaGrega et al, 1994; space, 1993). Leaching is the process by which contaminants are transferred from a stabilized matrix to liquid medium such as water or other solutions. H owever, most of the leaching procedures are concentrated on one or two steps without a detailed analysis of the binding mechanisms and chemical forms of the contaminants within the stabilized materials. Particle characterization involves determination the microscopic and morphological structure of the stabilized waste materials and X-ray diffraction (SRD). SEM with X-ray analysis can provide very valuable insight into physical and chemical identification of particles in waste materials. SRD can give the be s t structure look at the solid system, particularly the major crystalline phases in waste materials. These two methods are very useful tools in determination of the associations of metal contaminants with main cement components. There has been lack of stud y to combine the sequential chemical extraction with particle characterization methods to obtain detailed information of binding mechanisms and chemical forms of heavy metals in stabilized waste materials.
The binding mechanisms of hazardous constituents are particularly important in assessing the long tem environmental effects of stabilized waste materials. There is a need to study the chemical forms of the hazardous contaminants by the sequential chemical extraction and particle characterization methods in order to address the issues.
Sequential chemical extraction method has been used in study of heavy metal contamination in soils and sediments (Forstner, 1985; Tessier and Campbell, 1988). The method has been successfully used by the principal investig ator in the study of the fate and transport of metal contaminants in polluted soil. It seems that it would be appropriate to apply this method in the study of the binding mechanisms of the different solidification and stabilization techniques. The initial results from application of the sequential extraction in waste study show that it can be an important and useful tool.
The aims of this project are to study the binding mechanisms and chemical species of metal contaminant in stabilized waste materials u sing sequential chemical extraction, scanning electron microscope (SEM) and X-ray diffraction (XRD). The effectiveness and environmental impacts of different solidification and stabilization techniques will be assessed based on the results obtained from the sequential extraction, and SEM and XRD analysis.
The research program will focus on:
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