Surface Studies Relevant to Microbial Adhesion and Bioflotation of Sulphide Minerals Prashant K. Sharma Division of Mineral Processing Luleå University of Technology SE-97187, Luleå Sweden November, 2001 SUMMARY Biomineral beneficiation concerns the manner in which different microorganisms bring about the enrichment of an ore matrix. It involves the selective removal of undesirable mineral constituents from an ore through microbe-mineral interactions in the processes such as selective flotation and flocculation. The adhesion of microorganisms to minerals result in alteration of surface chemistry of minerals relevant to beneficiation process due to a consequence of the formation of a biofilm on the surface or biocatalysed surface oxidation or reduction products. Physico-chemical properties of microbial cell surface influence their adhesion behaviour, therefore the physico-chemical characterisation of microbial cell is essential in order to fully understand and control the biomineral beneficiation process. Different bacteria have different surface properties and adaptation of bacteria by growing them in presence of minerals introduces further changes in their surface properties. The pure strains of Thiobacillus thiooxidans, Thiobacillus ferrooxidans (T.f.) along with sulphur grown T.f. and Paenibacillus polymyxa (P.p.) along with chalcopyrite, pyrite, galena and sphalerite adapted strains are used in the study. The charge characteristics of mineral and bacterial cell surfaces are determined using electrokinetic measurements and the chemical composition is determined using FT-IR, FT-Raman and XPS spectroscopy. Hydrophobicity of the mineral and bacterial cell surface is evaluated using contact angle measurements, adhesion to organic solvents and surface energy evaluation. The surface energy of bacterial cells has been evaluated using different physico-chemical approaches - Fowkes, Equation of state, Geometric mean and Lifshitz-van der Waals/Acid-Base (LW-AB) approach. A detailed analysis of the physico-chemical approaches available to evaluate the bacterial cells surface energy from liquid contact angles is performed using literature data on 147 different microbial cells. It has been concluded that Geometric mean and Equation of state approaches evaluate similar surface energy values. But both of them give inconsistent results as the surface energy values change with the use of different liquid contact angles. Surface energy evaluated using LW-AB approach gives most detailed information of the bacterial cell surface. This approach is effected by mathematical instability but contact angle with the three liquids - Water, Formamide and Methyleneiodide/α-Bromonapthelene evaluates the most consistent results. The electron donor characteristics evaluated by LW-AB approach can differentiate between the gram-negative and gram-positive bacterial species. The other advantage of LW-AB approach is the fact that extended DLVO approach could be used to study the adhesion of bacterial cells on mineral surfaces. The polar components of the surface energy of mineral were lower than bacterial cells. Bacterial cells had a very high electron donating characteristics but for mineral surface it was on the lower side. Adhesion of microbial cells on mineral surface has been studied by constructing adsorption isotherms and indirectly by FT-IR spectroscopy and electrokinetic studies. Adsorption studies, electro-kinetic studies, IR-spectroscopic I studies and change in the flotation behaviour of the minerals experimentally showed that the adhesion of bacteria is taking place on the mineral surface. The bacterial adhesion is theoretically assessed by thermodynamic and extended DLVO calculations. The extended DLVO approach is found to be more effective in predicting the adhesion behaviour than the expectations from the thermodynamic approach. The thermodynamic approach yields no bacterial adhesion on minerals and this discrepancy could be the result of inadequate description of the phenomena, which strongly depends on the distance of separation between bacterial cell and mineral surface. The adhesion predictions by the DLVO approach are able to partially explain the adhesion and hence the bioflotation results of pyrite and chalcopyrite. Extended DLVO also shows that on account of high bacterial surface energy their aggregation is not feasible. But due to the hydrophobicity of pyrite and chalcopyrite, their aggregation is possible. Mineral-adapted bacterial cells showed marked differences in their surface properties from the unadapted ones. Paenibacillus polymyxa cells became more hydrophilic and gained the electron-accepting characteristic after adaptation, where as for Thiobacillus ferrooxidans the IEP shifted to higher pH value. Single mineral Hallimond flotation is performed for chalcopyrite and pyrite after interaction with microbial cells and in presence and absence of collector. Microbial cells were able to successfully depress pyrite flotation and not chalcopyrite. Hence, a separation among pyrite and chalcopyrite is possible by bioflotation. Keywords: Bacterial adhesion, Bioprocessing, Biobeneficiation, bioflotation, microorganisms, sulphide minerals, Zeta-potential, Contact angle, surface energy, DLVO, XPS, FT-IR, FT-Raman II ACKNOWLEDGEMENTS First of all, I would like to express my sincere gratitude to my supervisor, Associate Professor K. Hanumantha Rao for all his help and patience. I would like to thank Prof. Eric Forssberg for making it possible for me to come here and pursue my doctoral studies. Sincere thanks to Prof. K.A. Natarajan, Indian Institute of Science, Bangalore, India for introducing me to the field of bio-processing and his continued help, fruitful discussions and suggestions. I would also like to acknowledge The Swedish Foundation for International Co- operation in Research and Higher Education (STINT), Sweden for financial support for the project “Bio-mineral and bio-hydrometallurgical processing”. I would also like to acknowledge the financial support from the KKS (Stiftelsen för Kunskaps- och Kompetensutveckling) Företagsforskarskola inom berg- och mineralteknik in the later period of my studies. I would like to thank Ulf Nordström, Maine Ranheimer, Birgitta Nyberg and Björg Tangen Lundmark for their help in the lab. Thanks to all my friends and colleagues (alphabetically)- Andreas Krig, Aruna Thakur, Hamid-Reza Manouchehri, Maneesh Singh, Nourreddine Menad, Philippe Lingois, Tarun Kundu and Vidyadhar Ari for the time spent together. Last but most important, I would like to thank my wife, Aradhana, for her patience with me especially during the preparation of this thesis. I would also like to thank my parents in India for their continued support. Prashant K. Sharma November 2001 III CONTENTS Contents Page Summary I Acknowledgements III 1. Introduction 1 2. Minerals and Microbes 5 2.1 The Cell 5 2.2 Bacterial Strains: Their Growth and Adaptation 8 2.3 Minerals and Reagents 12 Paper I 13 3. Surface Characterisation 23 3.1 Bacterial Cell Surface Structure 23 3.2 Bacterial Cell Surface Properties and its Characterisation 30 Paper II 53 Paper III 77 Paper IV 89 4. Microbial Adhesion and Adsorption on Mineral Surfaces 201 4.1 Microbial Adhesion Mechanisms 201 4.2 Factors Effecting Adhesion 202 4.3 Thermodynamic Aspects of Adhesion 204 4.4 Colloidal Aspects of Adhesion 205 4.5 Mineral Microbe Interaction: Experimental Studies 209 Paper V 213 Paper VI 233 5. Biobeneficiation: Bioflotation 245 Paper VII 251 Paper VIII 259 Paper IX 279 6. Conclusions 289 7. References 291 List of Publications 297 IV Chapter 1: Introduction 1. Introduction Surface Studies Relevant to Microbial Adhesion and Bioflotation of Sulphide Minerals, P.K. Sharma, 2001 Minerals exist in nature abundantly in the earth's crust in the form of ore bodies, i.e., in association with other minerals. In order to extract metals from minerals by hydro- and pyro-metallurgical methods, it is very important to concentrate the mineral from the ore. Mineral processing is the branch of science, which concerns itself with the processes of mineral separation from ore. Conventionally, physico-chemical methods are used in mineral processing, but now a days, biotechnological processing routes are sought to solve the problems associated with lean grade ores and where the traditional methods fail to separate the minerals from complex ores. Two major areas, which are making advances in minerals bioprocessing, are bio-leaching and bio-beneficiation (Fig. 1.1). Bio-leaching can be defined as a hydrometallurgical dissolution process assisted by microorganisms for the recovery of metals from their ores/concentrates. Major activity has been in bio-leaching of sulphide minerals and chemolithoautotrophic bacteria have been used for the bio- leaching process. Over the past three decades bio-leaching has come a long way and is now economically competitive, many processes have been commercialised and are in use. Whereas, bio-beneficiation is relatively a new area and a new term which, has recently has been defined as “bio-beneficiation involves the selective removal of undesired mineral constituents from an ore through interaction with microorganisms, enriching the solid residue with respect to the desired mineral phase” (Natarajan, 1998). The subject of this thesis lies in the broad area of minerals bio-processing and more specifically in bio-beneficiation. Bio-beneficiation uses the conventional wet mineral processing methods, which separate minerals based on their surface property differences,