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The community of North Lake Macquarie (NLM), NSW has been affected since the 1890s by the presence of the Cockle Creek lead (Pb) zinc (Zn) base metal smelter in close proximity to what are now populated areas. Like many similar facilities in Australia and internationally the presence of this smelter has resulted in significant pollution of the surrounding suburbs, the consequence of a combination of poor internal pollutant control, poor waste management and lack of appropriate emission regulation. The impact of the pollutants on the local population is most clearly demonstrated by high community child blood Pb levels, which only decreased significantly after strict controls were placed on stack emissions in the 1990s and with further decreases following the smelter closure in late 2003. On a number of occasions since the 1990s ineffective attempts at remediation have been made in the surrounding community with the aim of decreasing child blood Pb levels and lowering community soil pollutant levels to meet legislative guidelines.The Imperial Smelting Furnace (ISF) developed during the 1950s and capable of smelting mixed Pb-Zn concentrate feeds was introduced at Cockle Creek, beginning production in 1961 to replace other smelting methods. Along with significant emissions to the atmosphere and surrounding waterways, the smelting process resulted in the production of large tonnages of glassy slag waste. The ISF slag has a size consistency similar to coarse sand and contains significant levels of heavy metal(loid)s, notably Pb (~1%), Zn (~7 %), arsenic (As), antimony(Sb), copper (Cu) and cadmium (Cd). The ISF process developers envisaged the slag as 'a safe disposal medium' for unwanted heavy metal(loid)s. As a result of this perception the Cockle Creek smelter operators promoted this material for reuse, and its utilisation ranged from minor domestic landscaping to major engineering projects. It is estimated that this practice resulted in the expatriation to the community of at least 2.1 million tonnes of slag, externalising significant potential costs which may have been incurred by the smelter operator for safe storage. Research detailed in this thesis has shown that rather than being a 'safe repository, the Pb and a group of associated metal(loids) (Ag, As, Cd, Sb) in the ISF slag are accessible to infiltrating fluids. A comprehensive examination of the morphology of the slags was carried out using optical and scanning electron microscopy (SEM). This examination determined that the Pb and associated elements exist in the slags as discrete nodular entities and are not (unlike Zn) incorporated within the glass phase of the slag. Phase analysis using electron andwavelength dispersive and Raman spectroscopy (EDS, WDS and Raman) of individual slag components suggests that the entities containing Pb and other environmentally undesirable metal(loid)s are principally Pb oxides with occasional residual cores of Pb metal.Characteristically these nodules have an outer PbO2 rim which follows any crack or exposed surface and surrounds a mixed Pb oxide, or occasionally Pb metal core. Quantitative mapping of large numbers of slag particles with QEMSCAN (an automated SEM system) was used to determine the overall mineralogical distribution of slag components and their accessibility. Lead phases in the slags were found to have a high degree of liberation and accessibility, only approximately 30% of the available Pb phases in the bulk slag were determined to be inaccessible (or "locked in") as a consequence the potential for reaction and dissolution of the Pb phases should not be significantly structurally inhibited The potential for reaction and dissolution dictated by the slag morphology was investigated from the chemistry of different size fractions, bioaccessibility in artificial gastric fluids and sequential leaching of the slags. For comparative purposes chemistry and bioaccessibility determinations were also made on soil and dust samples collected within NLM residential areas. These investigations showed that the levels of pollutants in the slags, soils and dusts were significantly elevated.The chemistry by sizing information revealed that fine material containing high levels of Pb(6,490-41,400 ppm) along with other contaminants such as Cd and As are present in the slags, with the potential for these metal(loid)s to be transferred to children's blood by hand-to-mouth behaviour and possibly inhalation. It was established that smelter slag bioaccessibility had a marked dependence on particle size, with almost 100% of the Pb being bioaccessible in the smallest particles (-20 μm), bioaccessibility for the -250 μm fraction averaged 45% (range 29-63%) and was slightly lower than for -250 μm sized soil and dust samples, which averaged 62% (range 46-75%). For a finer (-53+32 μm) size fraction the Pb bioaccessibility results were comparable for all material types: slags ranged from 66-86% and soils and dusts 60-76 %. The chemistry by sizing and bioaccessibility results suggest the slags have a potential for harm, given the high levels of contained pollutants and their relative availability and as such should be treated by regulators with the same caution as soils or dusts that are similarly contaminated. The potential for in-situ leaching from the ISF slags was ascertained by sequential leaching of "naturally" sized material taken from slag emplacements in NLM. The sequequential leach testing allowed discrimination between elemental species which were labile and those which were structurally bound. These investigations provided an insight into the existing elemental partitioning and possible redistribution of metal(loid)s which has occurred, or is or may occur in the future. The results indicate that about 30% of the Pb contained in the slags is in a mobile or easily dissolvable form and provides a significant indicator that Pb release from the slag emplacements is likely to be ongoing. It is suggested that the slag emplacements represent a large reservoir of potential pollutants within the community and that these pollutants are likely to be released over time. Conversely, elements like Zn, Cr and Cu, encapsulated within the glassy slag component were shown to have a low level of lability. These elements will leach at a much lower rate and are relatively, but not completely, protected from infiltrating leaching solutions and redistribution to the surrounding environment will be limited.Given the potential hazard to the community and the environment identified by the research and the large prospective costs for slag removal and storage in a controlled landfill, an examination was made of alternative methods which might be employed to mitigate the risk posed by existing slag emplacements. These included processes to lower pollutant levels, transformation of labile elements to allow in-situ sequestration and isolation. Of the strategies examined none were applicable in all situations. For small occurrences such as in residential backyards, removal to a controlled landfill will be permanent and cost effective. For larger emplacements in public open space, in-situ sequestration by the use of soil amendments,containment and ongoing monitoring may be more appropriate. A need for further research was identified given a lack of knowledge of effective amendments for the transformation of poly-elemental materials (like the ISF slags) to ensure permanent in-situ sequestration of all labile species. Publicity of the results contained in this thesis has had the effect of informing the public and assisting in mobilising them to demand action from their elected representatives. The uncontrolled release of the ISF slag for widespread use in the community has been recognised as inappropriate and the research described has clearly contributed to attitudinal change byregulators, local government and within the community. The ISF slag is no longer seen as a cheap and benign sand substitute but rather as a toxic waste that needs to be handled with caution and isolated from the community and the surrounding environment. |
