published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

Low temperature (165°C) Wet Oxidation (WO) and Catalytic Wet Oxidation (CWO) of 12 organic compounds has been studied in highly alkaline, high ionic strength solution (simulating that encountered in the Bayer process used to refine alumina) for the first time. Most (11 out of 12) of the 12 organic compounds studied (formic, acetic, propionic, butyric, oxalic, malonic, succinic, glutaric, citric, lactic, malic and tartaric acids) have been identified in various worldwide Bayer liquors. The various aspects of WO and CWO studied for each of the above-mentioned compounds were as follows; -Extent of complete oxidation to carbonate (i.e. extent of removal of organic compound) -Extent of overall oxidation (i.e. extent of complete oxidation and partial oxidation to stable products) -The product(s) formed from partial (incomplete) oxidation -The reaction mechanism occurring -Why certain compounds undergo low temperature WO and/or CWO in highly alkaline, high ionic strength solution -The ability of various transition metal oxides to catalyse the WO of the selected organic compounds Of the 12 organic compounds studied only six (formic, malonic, citric, lactic, malic and tartaric acids) underwent appreciable (>2% overall oxidation) WO in isolation under the reaction conditions used (4.4 -7.0 M NaOH, 165°C, 500 kPa Po₂, 2 hours). Each of these six compounds underwent some complete oxidation and therefore can be partly removed from highly alkaline, high ionic strength solution using low temperature WO. The order of extent of complete oxidation determined was as follows tartaric> citric> malonic> formic> lactic> malic. All of these compounds also underwent some partial oxidation under the reaction conditions used, excluding formic acid, which only underwent complete oxidation. Oxalic acid was a major product of partial oxidation of all of the above-mentioned compounds (excluding formic acid), while acetic acid was a major product of partial oxidation of citric, lactic, malic and tartaric acids. The WO of formic, malonic, citric, lactic, malic and tartaric acids varied considerably with NaOH concentration over the NaOH concentration range studied (4.4 - 7.0 M). The extent of overall oxidation undergone by each of these compounds increased significantly with increasing NaOH concentration. All of the compounds that underwent appreciable WO under the reaction conditions studied contained hydrogen(s) significantly more acidic then the compounds that did not undergo appreciable WO, thus indicating that only organic compounds that contain acidic (albeit weakly acidic) hydrogens undergo low temperature (165°C) WO in highly alkaline, high ionic strength solution. Two different reaction mechanisms were identified to occur during low temperature WO in highly alkaline, high ionic strength solution. Malonic and formic acids underwent WO predominantly via a free radical based reaction mechanism, while citric, lactic, malic and tartaric acids underwent WO predominantly via an ionic based reaction mechanism. The six organic compounds that did not undergo appreciable WO in isolation (acetic, propionic, butyric, oxalic, succinic and glutaric acids) all underwent appreciable WO when in the presence of malonic acid undergoing low temperature WO. Hence, low temperature WO of all of the above-mentioned compounds can be initiated by free radical intermediates produced by malonic acid undergoing WO in highly alkaline, high ionic strength solution. The ability of several transition metal oxides to catalyse the WO of the chosen 12 organic compounds was investigated. Of the transition metal oxides studied CuO was clearly the most active. Five of the organic compounds studied (malonic, citric, lactic, malic and tartaric acids) were catalytically wet oxidised by CuO in highly alkaline, high ionic strength solution in isolation. The order of catalytic activity observed was malonic > tartaric> lactic> malic> citric. Two different catalytic reaction mechanisms were identified for CuO catalysed WO in highly alkaline solution for the organic compounds studied. CuO catalysed the WO of malonic acid predominantly by catalysing the formation of free radical intermediates. CuO catalysed the WO of citric, lactic, malic and tartaric acids predominantly via a complexation-based reaction mechanism.

Surface modification is a process that is applied to the surfaces of titanium substrates in order to improve the biocompatibility after implanting in the body. Two methods were used in the present work: Anodisation and gel oxidation. Anodisation was performed at room temperature in strong mineral acids (sulphuric acid (H2SO4) and phosphoric acid (H3PO4)), an oxidising agent (hydrogen peroxide (H2O2)), mixed solutions of the preceding three, and a weak organic acid mixture (β-glycerophosphate + calcium acetate). The parameters used in anodisation were: Concentrations of the electrolytes, applied voltage, current density, and anodisation time. Gel oxidation was carried out by soaking titanium substrates in sodium hydroxide (NaOH) aqueous solutions at different concentrations (0.5 M, 1.0 M, 5.0 M, and 10.0 M) at 60??C for 24 h, followed by oxidation at 400??, 600??, and 800??C for 1 h. Conceptual models representing changes in the microstructure as a function of the experimental parameters were developed using the anodisation data. The relevant parameters were: Applied voltage, current density, acid concentration, and anodisation time: ?? The model for anodisation using the strong acid (H2SO4) illustrates the growth rate of the film, identification of the threshold for the establishment of a consistent microstructure, and prediction of the properties of the film. ?? For the oxidising agent (H2O2), two models were developed: Current-control and voltage-control, the applicability of which depends on the scale of the current density (high or low, respectively). These models are interpreted in terms of the coherency/incoherency of the corrosion gel, arcing, and porosity. ?? The model for the strongest acid (H3PO4) is similar to that of H2O2 in current-control mode, although this system showed the greatest intensity of arcing and consequent pore size. ?? Anodisation in mixed solutions uses Ohm??s law to explain four stages of film growth in current-control mode. These stages describe the thickness of the gel, its recrystallisation, and the achievement of a consistent microstructure. ?? Anodisation in weaker organic acids allows the most detailed examination of the anodisation process. Both current density and voltage as a function time reveal the nature of the process in six stages: (1) instrumental response, (2 and 3) gel thickening, (4) transformation of the amorphous gel to amorphous titania, (5) recrystallisation of the amorphous titania, and (6) subsurface pore generation upon establishment of a consistent microstructure. Gel oxidation was done at low and high NaOH concentrations followed by oxidation. Three models were developed to represent the gel oxidation process: (1) Low concentration, (0.5 M and 1.0 M NaOH), (2) Medium concentration (5.0 M NaOH), and (3) high concentration (10.0 M NaOH). For the low concentrations with increasing temperature, the model involves: (1) amorphous sodium titanate forms over a layer of amorphous anatase and (2) a dense layer of rutile forms. For the high concentrations with increasing temperature, the model involves: (1) amorphous sodium titanate forms over a layer of amorphous anatase, (2) a dense layer of anatase forms and raises up the existing porous anatase layer, and (3) the dense and porous anatase layers transform to dense and porous rutile layers, respectively. The main difference between the two is the retention of crystalline sodium titanate in the higher NaOH concentration. Anodised and gel oxidised samples subsequently were soaked in simulated body fluid in order to study the precipitation of hydroxyapatite in the absence and presence of long UV irradiation, which has not been investigated before. With the anodised surfaces, the porous and rough titania coating facilitated both the precipitation of hydroxyapatite and the attachment of bone-like cells. UV irradiation showed greatly enhanced hydroxyapatite precipitation, which is attributed to its photocatalytic properties. With the gel oxidised surfaces, the greatest amount of hydroxyapatite precipitation occurred with the presence of both anatase and amorphous sodium titanate. Rutile suppressed precipitation.

Surface modification is a process that is applied to the surfaces of titanium substrates in order to improve the biocompatibility after implanting in the body. Two methods were used in the present work: Anodisation and gel oxidation. Anodisation was performed at room temperature in strong mineral acids (sulphuric acid (H2SO4) and phosphoric acid (H3PO4)), an oxidising agent (hydrogen peroxide (H2O2)), mixed solutions of the preceding three, and a weak organic acid mixture (β-glycerophosphate + calcium acetate). The parameters used in anodisation were: Concentrations of the electrolytes, applied voltage, current density, and anodisation time. Gel oxidation was carried out by soaking titanium substrates in sodium hydroxide (NaOH) aqueous solutions at different concentrations (0.5 M, 1.0 M, 5.0 M, and 10.0 M) at 60??C for 24 h, followed by oxidation at 400??, 600??, and 800??C for 1 h. Conceptual models representing changes in the microstructure as a function of the experimental parameters were developed using the anodisation data. The relevant parameters were: Applied voltage, current density, acid concentration, and anodisation time: ?? The model for anodisation using the strong acid (H2SO4) illustrates the growth rate of the film, identification of the threshold for the establishment of a consistent microstructure, and prediction of the properties of the film. ?? For the oxidising agent (H2O2), two models were developed: Current-control and voltage-control, the applicability of which depends on the scale of the current density (high or low, respectively). These models are interpreted in terms of the coherency/incoherency of the corrosion gel, arcing, and porosity. ?? The model for the strongest acid (H3PO4) is similar to that of H2O2 in current-control mode, although this system showed the greatest intensity of arcing and consequent pore size. ?? Anodisation in mixed solutions uses Ohm??s law to explain four stages of film growth in current-control mode. These stages describe the thickness of the gel, its recrystallisation, and the achievement of a consistent microstructure. ?? Anodisation in weaker organic acids allows the most detailed examination of the anodisation process. Both current density and voltage as a function time reveal the nature of the process in six stages: (1) instrumental response, (2 and 3) gel thickening, (4) transformation of the amorphous gel to amorphous titania, (5) recrystallisation of the amorphous titania, and (6) subsurface pore generation upon establishment of a consistent microstructure. Gel oxidation was done at low and high NaOH concentrations followed by oxidation. Three models were developed to represent the gel oxidation process: (1) Low concentration, (0.5 M and 1.0 M NaOH), (2) Medium concentration (5.0 M NaOH), and (3) high concentration (10.0 M NaOH). For the low concentrations with increasing temperature, the model involves: (1) amorphous sodium titanate forms over a layer of amorphous anatase and (2) a dense layer of rutile forms. For the high concentrations with increasing temperature, the model involves: (1) amorphous sodium titanate forms over a layer of amorphous anatase, (2) a dense layer of anatase forms and raises up the existing porous anatase layer, and (3) the dense and porous anatase layers transform to dense and porous rutile layers, respectively. The main difference between the two is the retention of crystalline sodium titanate in the higher NaOH concentration. Anodised and gel oxidised samples subsequently were soaked in simulated body fluid in order to study the precipitation of hydroxyapatite in the absence and presence of long UV irradiation, which has not been investigated before. With the anodised surfaces, the porous and rough titania coating facilitated both the precipitation of hydroxyapatite and the attachment of bone-like cells. UV irradiation showed greatly enhanced hydroxyapatite precipitation, which is attributed to its photocatalytic properties. With the gel oxidised surfaces, the greatest amount of hydroxyapatite precipitation occurred with the presence of both anatase and amorphous sodium titanate. Rutile suppressed precipitation.

Techn. Hochsch., Diss., 2003--Aachen.

M.Sc. (Chemistry) / Cobalt is mainly associated with copper, both in the primary ores and in the oxidation zone. In Southern Africa cobalt metal is produced as a by-product of the extraction of copper, nickel and platinum group metals. The hydrometallurgical route is commonly used, since cobalt bearing materials are acid leached prior to the clarification and impurity removal process preceding the electrowinning of the value. In order to understand the dissolution behaviour of cobalt and copper bearing minerals from Co-Cu oxide ores, the relationship between and the impact of physical, chemical and mineralogical properties of the materials and the dissolution behaviour of cobalt and copper bearing minerals contained in the feed materials was studied. Four Co-Cu oxide ore samples namely; high cobalt ore (HCo), high copper ore (HCu), low cobalt ore 1 (LCo1) and low cobalt ore 2 (LCo2) were characterised in terms of elemental composition, cobalt and copper species, functional groups, mineral phases, mineral abundances, mineral/ore grains size distribution, particle specific surface area, particle ore density and porosity, in order to elucidate the impact of these properties on the dissolution rate and percentage recovery of cobalt and copper in the acid generated leachate...

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Acid mine drainage is a major environmental pollution concern associated with the mining of sulphide-containing ore bodies. Both physicochemical and biological options have been investigated for the treatment of acid mine drainage with recent interest in biological processes targeting low-cost and passive treatment applications. All acid mine drainage biological treatment processes are based to some extent on the activity of sulphate reducing bacteria, and their ability to reduce sulphate to sulphide in the presence of a range of carbon and electron donor sources. A portion of the sulphide produced may be consumed in the precipitation of heavy metals present in the mine drainage. Residual sulphide must be removed, not only due to its toxicity, but especially to prevent its reoxidation to sulphate where salinity reduction is a target of the treatment process. The partial oxidation of sulphide to elemental sulphur is an option that has received considerable attention and both physicochemical and biological options have been investigated. Biological processes have substantial potential cost advantages and run at ambient temperatures and pressures. However, the oxidation of sulphide to elemental sulphur is poised over a narrow redox range and process control to maintain optimum conditions remains a serious problem. In addition little has been reported in the literature on process control of sulphide oxidation to elemental sulphur, in the heterotrophic conditions prevailing in the reaction environment following sulphate reduction. This study undertook an investigation of biological sulphide oxidation under heterotrophic conditions in order to establish the effect of organic compounds on biological sulphide oxidation, and to determine whether the presence of organics, and associated heterotrophic oxygen consumption, may be manipulated to maintain the defined redox conditions required for the production of elemental sulphur. Biological sulphide oxidation under heterotrophic conditions was investigated in a series of flask experiments. Based on these results three different reactor configurations, a Fixed-Film Trickle Filter Reactor, Submerged Fixed-Film Reactor and a Silicone Tubular Reactor were used to investigate sulphur production. The flask studies indicated that organics, and associated heterotrophic metabolism in the presence of excess oxygen in the sulphide oxidation reaction environment, did contribute to the poising of redox conditions and thereby enabling the production of elemental sulphur. While the Fixed-Film Trickle Filter Reactor was found to be redox unstable, probably due to excess oxygen ingress to the system, a reduced oxygen challenge in the Submerged Fixed-Film Reactor configuration was found to be more successful for production of elemental sulphur. However, due to the production of a predominantly filamentous sulphur producing microbial population, recovery of sulphur from the column was intermittent and unpredictable. Extended residence times for produced sulphur on the column increased the likelihood for its eventual oxidation to sulphate. The Silicone Tubular Reactor was found to support a vigorous sulphide oxidising biofilm and produced elemental sulphur effectively. Electron microscopic studies showed that this occurred as both biologically produced sulphur and, probably mainly, as crystalline sulphur in the ortho-rhomic form. Given the linear extension of the sulphur production reaction environment it is was possible to investigate the sequence of the reaction mechanism in grater detail than is possible in mixed systems. Based on these findings a model explaining sulphur production under heterotrophic conditions has been proposed and is presented. The commercial implications of the development have also been noted.

Hydrophobicity is the generalized characteristic of non-polar substances that brings about their exclusion from aqueous phases. This property, entropic in its nature, drives key self-assembly and phase separation processes in water. Protein folding, the formation of DNA double helix, the existence of lipid bilayers and the wetting properties of leaf surfaces are all due to hydrophobic interactions. Inspired by Nature, we aimed to use hydrophobicity for creating novel and improved catalytic systems. (I) A number of fluorous amphiphilic star block-copolymers containing a tris(benzyltriazolylmethyl)amine motif have been prepared. These polymers assembled into well-defined nanostructures in water, and their mode of assembly could be controlled by changing the composition of the polymer. The polymers were used for enzyme-inspired catalysis of alcohol oxidation. (II) An enzyme-inspired catalytic system based on a rationally designed multifunctional surfactant was developed. The resulting micelles feature metal-binding sites and stable free radical moieties as well as fluorous pockets that attract and preconcentrate molecular oxygen. In the presence of copper ions, the micelles effect chemoselective aerobic alcohol oxidation under ambient conditions in water, a transformation that is challenging to achieve nonenzymatically. (III) Development of a facile means of photo/electrocatalytic water splitting is one of the main barriers to establishing of a solar hydrogen economy. Of the two half-reactions involved in splitting water into O2 and H2, water oxidation presents the most challenge due to its mechanistic complexity. A practical water oxidation catalyst must be highly active, yet inexpensive and indefinitely stable under harsh oxidative conditions. Here, I shall describe the synthesis of a library of molecular water oxidation catalysts based on the Co complex of tris(2-benzimidazolylmethyl)amine, (BimH)3. A wide range of catalysts differing in their electronic properties, surface affinity, and steric bulk was explored. We identified hydrophobicity as the key variable in mediating the catalytic competence of Co-(BimH)3 complexes. The change in this parameter correlates both with the conformational mobility of the ligand core and the structural changes in the local solvent environment around the catalytic metal site. The optimal ligand identified is superhydrophobic due to three fluorinated side chains. The corresponding Co complex catalyzes water electrooxidation efficiently, with an onset potential equal to that for the well-established CoPi heterogeneous system, albeit with a dramatically higher turnover frequency (TOF) and in the absence of soluble Co salts. As an added benefit, the hydrophobic catalyst can be immobilized through physisorption, and remains stable after prolonged controlled-potential electrolysis. A DFT calculation was also performed to understand the catalytic pathway.