Adhesion forces between cells of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans or Leptospirillum ferrooxidans and chalcopyrite

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Abstract

The efficiency of copper leaching is improved by bacteria attached to chalcopyrite. Therefore, the study of the attachment mechanism to control leaching is important. The adhesion of three species of leaching microorganisms including Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans to chalcopyrite was investigated by using atomic force microscopy (AFM). The forces were measured with tip-immobilized cells approached to and retracted from the mineral. The results show that both the surface charge and the hydrophobicity of bacteria cells influence the adhesion force. Furthermore, the adhesion force decreased in case the extracellular polymeric substances (EPS) had been removed. In addition, the data indicate that the amount of attached cells increased with increasing adhesion force.

Highlights

► Measuring adhesive forces between Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans and chalcopyrite by AFM. ► Surface charges and hydrophobicity of bacteria influence the adhesion force. ► The adhesion force decreased markedly while the EPS were removed. ► The amount of attached cells increased with the growth of the adhesion force.

Introduction

It is difficult and expensive to extract copper through chalcopyrite by using traditional methods because it is often associated with other minerals such as pyrite, calcite, etc. [1]. Bioleaching may be the most cost-effective and environmentally friendly alternative process. The mobilization of metals from ores by biological oxidation is referred to as bioleaching [2]. Acidophilic leaching bacteria are vital to the extraction of metals from sulfidic ores through bioleaching. Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans are the most common mesophilic species [3].

Bacteria adsorbing on mineral surfaces enhance bioleaching of chalcopyrite and other minerals. The bacterial EPS mediate this contact and are the prerequisite for initial attachment of bacterial cells [2], [3]. However, the interfacial mechanism is complicated and not fully understood [4]. Some authors report that removing EPS would result in a decrease of initial attachment to chalcopyrite [5], [6]. Nevertheless, no significant experiments have been done to explain this.

To measure the interaction force might be a direct way to gain some more insight on the interface-action and the bacterial attachment to mineral. Atomic force microscopy (AFM) is a useful tool for imaging microbial cells as well as for force measurement. Functionalized probes were used to measure the force between bacteria cells and a solid material [7], [8], [9], but only a few were dealing with bioleaching. One author theoretically modeled the force curves of leaching bacteria–mineral attachment [10]. In terms of laboratory experiments, however, little work has been done to probe the adhesion forces.

Thus, here the adhesion forces between chalcopyrite and cells of three bacteria A. ferrooxidans, A. thiooxidans and L. ferrooxidans were measured by AFM. Furthermore, we quantified the interaction forces between EPS-deficient cells and minerals. The results evidently indicate the importance of EPS for attachment.

Section snippets

Strains and growth conditions

A. ferrooxidans strain ATCC23270, which was purchased from the American Type Culture Collection (ATCC), USA, A. thiooxidans strain DSM622 and L. ferrooxidans strain DSM2391, both of which were obtained from Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ), Germany, were tested. A. ferrooxidans ATCC23270 and L. ferrooxidans DSM2391 were cultured in 9 K medium: (NH4)2SO4 3 g/L, KCl 0.1 g/L, K2HPO4·3H2O 0.5 g/L, MgSO4·7H2O 0.5 g/L, Ca(NO3)2 0.01 g/L. 4.47% FeSO4 were added for energy. A.

Typical force curve analysis

Fig. 2 shows a typical AFM force–separation curve of an A. thiooxidans-coated tip approaching the chalcopyrite. As expected, at large distances from the surface, no interaction was detected. As the tip approached the chalcopyrite surface, there was a gradual increase in the repulsion between cells and minerals until contact. This could be attributed to both long-range repulsive forces between the cell-coated tip and chalcopyrite and the repulsive interactions between mineral and the EPS of the

Conclusions

The forces between three types of leaching microorganisms and chalcopyrite have been directly quantified by Atomic Force Microscopy using force–separation curves. Following results were gotten:

  • The interaction forces between cells and chalcopyrite surface are controlled by electrostatic forces and hydrophobic forces. The electric charges of the surfaces control the electrostatic force, while the contact angles indicate the hydrophobic forces.

  • L. ferrooxidans exhibits the strongest force of

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 30700008, 50974134, 51174239) and the National Basic Research Program of China (No. 2010CB630905).

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