Characterisation of acidic and metalliferous drainage precipitates using transformed iron oxides

Precipitates arising from the oxidation of acidic and metalliferous drainage (AMD) detrimentally impact aquatic ecosystems and industrial operations. In acidic environments (pH < 3) biological and abiotic processes are involved in the precipitation of nanophases including Fe oxyhydroxides and Fe hydroxysulfates. Metastable properties and nanoparticulate nature of these precipitates challenge detection limits of many analytical systems, resulting in characterisation of AMD precipitates reliant on multiple and specialised instrumental techniques. The main objective of this thesis was to examine the use of thermal treatments as solid phase derivatisation technique, to alter nanoparticle structures through crystallisation and impart measurable properties in transformed Fe oxides that reflect constituents in the original AMD precipitate. Material from AMD formed in the Mt Lyell and Queen River system, Queenstown (Tasmania) was investigated. Specimens were sampled from six sites (n=15), and represented typical precipitates from AMD waters, including clogged pipes, adits and the river. Samples contained minor amounts of inherent magnetic and/or coloured (purple) fractions that represented flow conditions and water content of sample, and assessed not to be representative of bulk AMD precipitate. Multiple techniques (including neutron activation analysis, CHNS elemental analysis and X-ray fluorescence, phospholipid fatty acid analysis, X-ray powder diffraction, and electron microscopy) were used characterise the elemental, biological and mineral composition, particle size distribution and shape of AMD precipitates. Multivariate analysis of the elemental and biogenic signatures was used to assess the extent of inter-and intra-site variation. AMD precipitates contained Fe (36−45 wt%) and S (2–9% wt) with 14 elements in minor (0.01−2.0 wt%) and 25 elements in trace concentration (<0.01 wt%). Multivariate analysis demonstrated site specific elemental signatures in the minor and trace elements, with relationships established between some sites. Bacterial biomarkers were present in all AMD precipitates and varied in abundance according to site. AMD precipitates were mainly goethite and XRD-amorphous phases (presumed to be Fe nanophases including schwertmannite and ferrihydrite), with minor concentrations of phyllosilicates and quartz, and some site-specific phases. The biomass abundance was found to be positively correlated to XRD-amorphous content and negatively correlated to goethite concentration. Comprehensive biogeochemical profiles of AMD precipitates were established. Four experiments were conducted to study thermal treatments against AMD precipitate compositions. The first thermal experiment studied the effect of different atmospheres (air and N2) on precursors (n=3) heated to 300, 600 and 1000 °C. Magnetometry (vibrating sample magnetometry) and minerology (X-ray diffraction) were used to quantify changes in magnetic parameters and phase compositions of the precursors and derivatives, with supporting morphology (scanning and transmission electron microscopy) and thermal gravimetric analysis in N2. As temperature increased, nanoparticles crystallised and formed porous aggregates with colour and magnetic properties typical of Fe oxides hematite (Fe2O3) and magnetite (Fe3O4). Heating in both atmospheres dehydrated and dehydroxylated precursors, and produced structural rearrangements of Fe mineral phases via an intermediate hematite structure, referred to as protohematite. The air atmosphere resulted in hematite-rich derivatives with enhanced coercivity (HC) from precursors with goethite-rich content. The N2 atmosphere resulted in greater variation of magnetic properties and colour as a function of temperature compared to derivatives transformed in air. These distinguishing ferromagnetic properties were found to be precursor specific at 1000 °C/N2, and depended on at least C and phyllosilicate content in the precursor. Application of 1000 C/N2 to a broader set of precursors provided further details of the constituents associated with three types of distinguishing ferromagnetic properties. A black, magnetite-rich derivative with magnetic enhancement transformed from precursors with goethite (>62 wt%) and C (>1.1 wt%) through a reductive process (Type 1). A red−purple, hematite-rich derivative with magnetic enhancement transformed from precursors with elevated quartz/phyllosilicates, Al, Si, Cu, Ba, Ti, and trace elements (Type 2). Purple, hematite-rich derivatives that lacked magnetic enhancement transformed from precursors with least C, Al, Si, Cu, Ba and Ti and generally contained elevated S and biomass (Type 3). Transformations in oxidative conditions were undertaken to further study the relationship between goethite/XRD-amorphous and colour of transformed hematite. Precursors (n=3) and a synthetic goethite pigment were transformed at 550, 700, 800, 900 and 1000 °C in air. Analysis of mineral composition, hematite microstructure (XRD), pH slurry and colour coordinates, demonstrated a maximum red colour at 800 °C, which decreased in redness at 900 °C depending on precursors’ composition of goethite, Si, Al, P and S. The use of thermal treatments in tandem offers a new approach to study minor constituents, including C, Si, Al, P, Cu, Ba, Ti and phyllosilicates/quartz in the presence goethite and XRD-amorphous-rich AMD precipitates. The chemical reactions and pathways offered are useful to elucidate the composition of AMD precipitates in terms of mineral and elements, and offers insight to potential ways to enhance properties of thermal derivatives, as new products, for different application (Fe oxides), which are of relevance to mineral processing, environmental and material fields. Two instrument-free techniques were developed in conjunction with quantitative analysis. A dichotomous key was proposed for the classification of Types 1,2,3 ferromagnetic properties transformed in Thermal Treatment 1. A novel approach was developed and used to collect and compare colour data (based on CIE-L*a*b* colour system principles) via application as a painted mixture with subsequent digital acquisition. These techniques have potential to be used as an archival or screening tool to monitor changes in AMD precipitate compositions in lieu of access to technical resources