Investigations into the effects of birnessite-like material structures on their effectiveness for metal dissolution

Author: Jennie Bartle

Bartle, Jennie, 2020 Investigations into the effects of birnessite-like material structures on their effectiveness for metal dissolution, Flinders University, College of Science and Engineering

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Birnessite-like manganese oxides (BLMOs) are useful in a wide variety of applications, including in the oxidation of organic molecules and heavy metal cations. However, many of the application studies utilise poorly characterised materials, creating difficulties for researchers wishing to expand on prior research. There is also significant variation in the materials prepared for these applications.

This study synthesised BLMO materials utilising a variety of methods, and the resulting materials were characterised using x-ray diffraction and subsequent Rietveld refinements, scanning electron microscopy, and determination of the average manganese oxidation state. It was found that the materials prepared had different unit cell parameters, levels of stacking order and morphologies. Further investigations into the reproducibility of material synthesis between batches showed that even when conducting the synthesis identically, slight differences in the purity and unit cell parameters occur.

This thesis demonstrates, for the first time, that BLMO materials are able to oxidise elemental vanadium (V(0)), gold (Au(0)) and other metals when the materials are placed in an acidic saline solution. The conditions utilised are significantly less hazardous than most existing metal dissolution methodologies. Triclinic BLMO materials oxidised V(0) inconsistently and hexagonal BLMO materials had greater consistency. The variability observed in the triclinic BLMO materials was found to be caused by the stabilising effect of K+ cations on the layered structure of triclinic BLMOs. It was shown that the dissolved V can be incorporated into the interlayer space and octahedral sheets of BLMO. Long term experiments observed the formation of the manganese vanadate mineral ansermetite, which is likely formed from the coprecipitation of the dissolved Mn2+ and V5+ generated by this reaction system.

The study was extended to tunnel structured manganese oxide materials in the form of cryptomelane (2x2), todorokite (3x3) and pyrolusite (1x1). It was found that the reactivity of these tunnelled materials towards oxidation of V followed the trend of 1x1>2x2>3x3. The variability found in the oxidation of V led to a preliminary investigation into the oxidation of elemental Au by a selection of layered and tunnelled manganese oxide materials. A general trend where a material that achieved high levels of oxidised V also resulted in high levels of oxidised Au. There were two exceptions to this trend, pyrolusite and a disordered BLMO material, which could be due to surface passivation and a greater affinity for absorption, however, further investigations into this area are required.

Further investigations showed that BLMOs can oxidise elemental palladium (Pd), in addition to vanadium and gold at higher levels than an acidic saline control. Contrastingly, copper, lead, nickel and iron were generated at higher amounts in the acidic controls than the BLMO test, suggesting that the BLMO materials inhibits the oxidation of these metals. The trend observed with the metals able to be oxidised by BLMOs follows the trend observed by the corresponding metal chloride salt solubility, with a more soluble chloride salt resulting in more metal oxidised. This work led to a preliminary investigation into assessing the ability of BLMO materials to recover metals from electronic waste. It was found that this system can oxidise metals from CPUs and RAM pins however further work is required to adapt this reaction system for the efficient extraction of metals.

Keywords: Birnessite-Like Manganese Oxides, Vanadium Oxidation, E-waste reclaimation.

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Professor Claire Lenehan