K.R. Blight

Pyrite surfaces after bio-leaching : a mechanism for bio-oxidation

A pyrite mineral surface was exposed to a mixed culture of the lithotrophic bacteria Thiobacillus ferrooxidans (Tf). The surface structure of this sample was compared to surfaces exposed to uninoculated solutions of either sulphuric acid or ferric and ferrous sulfate dissolved in sulfuric acid solution at equivalent concentrations as the inoculated solution. All surfaces were examined using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A chemical profile of the surfaces, as a function of depth, was obtained by surface ablation. The colonised surface produced an oxidised phase extending to a depth of more than 4 μm. A carbonaceous phase was not observed within the oxide layer. No oxide layers of significant depth were observed for samples exposed to uninoculated solutions. The importance of the sessile cells in bio-oxidation of pyrite is demonstrated and a qualitative model is proposed.

Surface photovoltage studies of n-type and p-type InP

Surface photovoltage spectroscopy (SPV) was used to study the initial stages of oxidation of single crystal InP(110) in an attempt to understand the nature and origin of the surface states that develop. Distinct surface states were seen to develop on n-type as the surface was exposed to oxygen. A surface state, associated with cleavage damage, was also observed on p-type. A detailed fit to the experimental data was made by using a model of the dependence of surface charge on photon energy. This was used to unfold the position and intensity of the states. States trailing into the band gap from the bulk bands were seen on both n- and p-types. The analysis also indicated that pairs of isolated states, a donor and an acceptor state, were produced. On p-type, these were present on the clean, cleaved surface while they developed with oxygen exposure on n-type. These states are consistent with the point defect states proposed by the unified defect model. The time response of the SPV signal was also recorded for these surfaces. They were analysed by careful fitting to a model describing the charging and discharging characteristics. This revealed that the midgap state on n-type had a fast and a slow component.

Analysis of the bacterial sulphur system

Heterogeneous bacterial sulphur systems are inherently complicated. However, developing an understanding of the influence of environmental factors such as pH, I and PCO2 is important for a number of fields. Examples of these include minimising acid mine drainage and maximising metal recovery from low-grade sulphide minerals. Measuring the effect of these factors on the extent and rate of sulphur (S) oxidation is complicated by the presence and nature of solid phase elemental S. The rate and extent of S oxidation can be determined indirectly via the reaction product, H2SO4, which was quantified using pH measurements in this study. The method was critically dependent on the quality of pH data but proved effective in providing rate constants for the catalysed S oxidation reaction and yield (biomass/substrate) estimates in the range pH > 1.5. Increasing I over the range 0.176 0.367 mol L-1 decreased bacterial cell yields but increased the rate of sulphur oxidation significantly. Partial pressures of CO2 in the range of 0.039 1.18% v/v produced no significant effect on the rates of S oxidation or bacterial cell yields. Bacterial cell yields were not affected in the pH range 1.5 2.5, however the rate of S oxidation increased significantly from pH 2.0 2.5. In the range pH < 1.5 the batch cultures progressed and although no reliable rate data was recorded cell yields decreased from 7.43 to 2.05 (× 1012 cells mol-1) at pH 1.5 to 1.0 respectively.