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Elemental characteristics of organic deposits from an area surrounding a lead-zinc smelter : concentration, distribution, mode of occurrence and mobility Hawke, Michelle Irene

Abstract

Monitoring and remediation of anthropogenic trace elements requires knowledge of the magnitude of emissions and their fate in the environment. This study examines the role of organic sediments in the partitioning of Sb, As, Cd, Cu, Ni, Pb and Zn in the environment surrounding a lead-zinc smelter. Trace element concentrations in peat profiles were highest near the smelter and decreased with distance. Peat concentrations were compared to mineral soil concentrations and to atmospheric deposition rates. Anomalies between peat concentrations and deposition rates identify locations of secondary source impact. Concentrations are higher in impacted peats than in corresponding soils, indicating preferential sequestration. Assigning a geochemical baseline and differentiating between trace element sources is difficult in areas with heterogeneous geochemistry. Two geochemical fingerprinting methods were applied to estimate smelter impact: normalisation to "conservative elements" and rare earth element patterns. Interpretation of "conservative element" ratios proved difficult, due to concentration variability. It was concluded that conservative elements may not represent a geochemical baseline. Light rare earth element enrichment was noted in peats and other sampling media closest to the smelter, but not in smelter feedstock or wastes, suggesting that LREE enrichment is overprinted from background geochemistry. Sources of trace element-rich particles include stack emissions and fugitive and geogenic dust. Peat ash was examined by SEM-EDX to determine the morphology and elemental composition of the particulates. Rounded smelter-emitted particles are present in peats sampled close to the smelter. Weathering and mobilisation are indicated by changes in chemistry between fresh and weathered particles. Angular fugitive dust sulphide particles also occur. Sequential leaching of peats and soils illustrates that trace element speciation is controlled by environmental conditions and by source. In smelter-impacted peats, elements are sequestered as exchangeable, carbonate, Fe-oxide or organic species, indicating precipitation from solution. Non-impacted peats contain higher proportions of residual and sulphide species. Environmental parameters impact peat diagenesis and influence element behaviour. An organic petrographic evaluation of smelter-impacted peats aids in determining past and current conditions. A well-humified profile, which indicates aerobic conditions, contained abundant Fe-oxide species. Profiles containing maceral assemblages that indicate anaerobic conditions contained sulphide and organic species.

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