School of Earth Sciences - Theses
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ItemInvestigating potential pathways to remediate thiocyanate-contaminated groundwater and wastewater at a Victorian gold mine( 2014)Thiocyanate (SCN-) is toxic to higher organisms, affecting the central nervous system, to cause irritability, nervousness, hallucinations, psychosis, mania, delirium and convulsions. This contaminant is commonly associated with gold mining activities and forms when cyanide, used for extraction of gold, reacts with reduced sulphur species in wastewater. Significant concentrations of thiocyanate have been detected in groundwater bores around the No. 2 Tailings Dam at Stawell Gold Mine in Victoria, with concentrations steadily increasing over time. Thiocyanate can be eliminated from groundwater and wastewater by oxidation to less harmful products such as ammonia and sulphate through chemical oxidation and biodegradation. Certain bacterial strains indigenous to thiocyanate-contaminated sites have demonstrated the ability to degrade thiocyanate and can be used to remediate contaminated land and water. Chemical and biological oxidation of thiocyanate was explored in this study through synthetic abiotic laboratory redox optimisation experiments and field-based injection experiments to determine the controls on thiocyanate degradation and potential pathways which could be implemented to remediate thiocyanate-contaminated groundwater and wastewater at the site. Analysis of site groundwater chemistry and aquifer properties revealed the presence of a plume of thiocyanate in acidic to near-neutral groundwater outside the tailings dam wall at monitoring bores SE12 and SE14 confined to the upper unconsolidated aquifer with low hydraulic conductivity (0.001-0.004 m/d) and low transmissivity (0.01-0.05 m2/d). The source of thiocyanate at the site was determined as the tailings dam. Inconclusive results from the nitrate reduction-thiocyanate oxidation experiments in synthetic and actual groundwater and wastewater suggests that nitrate may not form a redox couple with thiocyanate. However, in acidic solutions (pH 2) of Fe-EDTA and thiocyanate heated to 80°C, the Fe2+ was generated from thermal degradation of the Fe-EDTA compound, while thiocyanate was completely hydrolysed within 22 days. Catalysis of thiocyanate hydrolysis by iron reduction was not determined. However, solution pH and temperature were important factors, as thiocyanate hydrolysis did not proceed at pH 5.5 and 80°C. At pH 2, the hydrolysis of thiocyanate was faster at 80°C compared to 70°C. Thiocyanate-degrading microorganisms were not successfully stimulated in simulated injection (push-pull) experiments, as no appreciable decrease in thiocyanate concentrations was observed in groundwater or wastewater replicates.