| AVS 60th International Symposium and Exhibition | |
| Applied Surface Science | Tuesday Sessions |
| Session AS-TuM |
| Session: | Developments in Electron Spectroscopies for Non-Ideal Samples |
| Presenter: | M. Fantauzzi, Università degli Studi di Cagliari, Italy |
| Authors: | M. Fantauzzi, Università degli Studi di Cagliari, Italy B. Elsener, Università degli Studi di Cagliari, Italy G. Rossi, Università degli Studi di Cagliari, Italy A. Rossi, Università degli Studi di Cagliari, Italy |
| Correspondent: | Click to Email |
Sulfide minerals biooxidation processes (oxidation mediated by microorganisms) are particularly suitable for gold recovery from low-grade ores and are more environmentally friendly compared to conventional physicochemical mineral beneficiation processes. Oxidative dissolution mediated by microorganisms besides being of economic interest is environmentally significant since the latter usually occur in AMD (Acid Mine Drainage) and play a decisive role in the release of toxic elements such as Arsenic into the environment. According to the integral model for bioleaching [1], Fe (III) and H+ ions control sulfides dissolution and the role of bacteria is to catalyze Fe (III) and H+ regeneration and to concentrate those oxidants at the mineral surfaces by extra-cellular polymeric substances (EPS). EPS is produced by the living bacteria and forms kind of a bridge between the cell and the mineral surface. As the EPS layer is able to complex iron (III) ions into glucuronic acid-iron complexes, the actual concentration of iron (III) ions on the mineral surface may be higher than their concentration in the surrounding solution. This work aims to provide surface analytical and electrochemical evidences of the dissolution mechanism of enargite (Cu3AsS4). All experiments were carried out in presence of a strain of Acidithiobacillus ferrooxidans adapted to arsenic and of iron ranging from 10-5 M to 10-3 M at pH 2. The electrochemical potential was found to increase explaining the higher dissolution rate observed in the presence of bacteria. The enargite solubility was strongly enhanced by A. ferrooxidans: solution analyses showed that Cu and As dissolved stoichiometrically with a dissolution rate of about 3 - 5 times higher compared to the abiotic control.
The XPS C1s, O1s, N1s and P2p regions acquired on bacterial cells and on bioleached mineral grains were processed so that the surface composition of the EPS layer was obtained. The fraction of proteins, hydrocarbon and polysaccharides of the EPS was estimated and monitored for a period of time up to 157 days. Also the iron signal initially absent was observed after bioleaching.
The average thickness of the EPS layer at the mineral surfaces, as well as the protein content, increases with bioleaching time from 1 nm (after 44 days) to 2.5 nm (after 157 days) whereas the hydrocarbons and polysaccharides amount decrease with leaching time. These results are thus demonstrating the role of EPS in the sulphide dissolution and substantiate the model for bioleaching.
1. W. Sand, T. Gehrke, P. Josza, A. Schipper, Hydrometallurgy 59 (2001) 159 - 175