Enhanced “contact mechanism” for interaction of extracellular polymeric substances with low-grade copper-bearing sulfide ore in bioleaching by moderately thermophilic Acidithiobacillus caldus

Abstract

In order to enhance the “contact mechanism” governing the interaction of extracellular polymeric substances (EPS) with low-grade copper-bearing sulfide ore for the bioleaching of copper, moderately thermophilic Acidithiobacillus caldus was subjected to exogenous intervention with iron and sulfur. The enhancement of the contact mechanism was systematically investigated by evaluating the attached cells/EPS dynamics, intracellular adenosine triphosphate (ATP), cell functional groups, gene transcriptional level, and ore characteristics. Confocal laser scanning microscopy (CLSM) revealed that exogenous intervention with iron and sulfur led to the production of a denser EPS layer and faster adsorption of the attached cells to the ore based on differential fluorescence staining, which indicated enhancement of the “contact mechanism”. The increased intracellular ATP content of the attached cells in the exogenous substrate system provided the required energy for the adsorption processes associated with the “contact mechanism”. Fourier-transform infrared spectroscopic (FTIR) analysis of the attached cells and the ore showed a dramatic shift of the NH and COS peaks (associated with EPS formation), whereas the FTIR peaks of SO and SO42− associated with sulfur metabolism were also significantly influenced. Moreover, reverse transcription polymerase chain reaction (RT-PCR) analysis revealed that the expression of genes related to cellular energy metabolism (nuoB, nuoC, atpE, atpF), sulfur metabolism (sor, sqr, sdo, soxA), biofilm formation (pgaA, pgaB), and cell colonization (acfA, acfB, acfC, acfD) was up-regulated after exogenous intervention, verifying enhancement of the “contact mechanism” at the transcriptional level. In addition, scanning electron microscopy (SEM) indicated more obvious adsorption traces on the ore surface. X-ray diffraction (XRD) indicated the presence of more complex derivatives, such as Fe3(SO4)4, FeSO4, Fe2(SO4)3, and Cu2S, which is suggestive of more active iron/sulfur metabolism with addition of the exogenous iron and sulfur. Overall, a model for bioleaching of low-grade copper-bearing sulfide ore by moderately thermophilic A. caldus was constructed. The results of this investigation should provide a guide for similar industrial bioleaching processes.