Bioelectrochemical and Spectroscopic Study during Interfacial Biooxidation Process of Chalcopyrite Mediated By Sulfur and Iron Oxidizing Microorganisms

Abstract

The chalcopyrite is a sulfide mineral (SM), that is the main copper source, which is extracted by oxidative leaching. As initial reaction products, secondary phases of sulfur, such as Sn 2- and S0 are formed [1], which are sulfur chains and sulfur rings [2], respectively. These phases tend to deposited on chalcopyrite surface, limiting ion and electron transferences between mineral and solution, so the reaction kinetic results decresed [1] . One of the options suggested to avoid this condition, is to remove sulfur phases by biooxidation with Acidithiobacillus microorganisms, such as Acidithiobacillus thiooxidans (sulfurooxidizing microorganism SOM) and Leptospirillum sp. (ironoxidizing microorganisms, IOM) [3,4]. It has been found that direct contact mechanisms of chalcopyrite bioleaching occurs by attachment of microorganisms, which is improved by the initial presence of sulfur phases, mainly for Acidithiobacillus thiooxidans. In addition, a different behavior has been observed as a function of kind of sulfur phase, which influences on the physicochemical and biological characteristics of the biofilm developed during the biooxidation [5,6]. Hence, for SOM the hydrophobic composition of EPS (extracellullar polymeric substances) and cell density of the biolm is improved by presence of S0 on mineral surface, affecting the biooxidation process of chalcopyrite. However, the above has been only evaluated for SOM systems, hence, that would be important analyze the effect of sulfur phases on the performance of IOM systems, due to it will keep the presence of an oxidant agent (ferric iron), which will affect improves the electrochemical process, such as would occur in the SOM+IOM consortia used in the industrial bioleaching process. So, in this work the objective is analyze the electrochemical process of system bacteria - electrolyte – mineral on modified chalcopyrite surface exhibting different sulfur species exposed to SOM and SOM+IOM cultures. For this the chemical changes in electrolyte, and morphological, composition and hydrophobic modifications on surface exposed to microorganisms are correlated with Electrochemical impedance Electrochemical (EIS) and Electrochemical Noise (EN) data obtained for each evaluated condition. Global analysis of the results shows that the presence of microorganisms improves the ionic and electronic activity of interface, due to the initial sulfur phases (Sn 2- and S0) generated by potentiostic oxidation were removed. Then, a secuencial and cyclic transformation between several kinds of reactive sulfur species, determined the electroactivity and hydrophobicity of chalcopyrite surface as a function of bioleaching time, which influences the oxidation capacity of microorganisms. In addition, it was found that on Sn 2- rich surfaces, oxidation was improved with SOM, by an uniform mechanism of acidic dissolution and through indirect contact, these associated with SOM membrane enzymatic characteristics, according to [2,7]. However, on S0 rich surfaces, the bacterial activity occurred by direct contact, but was slower than on Sn 2-, due to generation of hydrophobic and inactive (passive) sulfur species. Such kind of phases were result of a low amount of EPS-Fe3+ complexes that limited the ring sulfur activation, based in the reported in [2], and therefore the charge transfer and diffusion processes. On the other hand, SOM+IOM showed higher activity by an uniform mechanism of ferric dissolution on both sulfur phases, but it was mainly by direct contact on initial S0 surface, generating predominantly hydrophilic and reactive surfaces species at early times, which was associated to higher amount of EPS-Fe3+, that contributed with either fast activation of initial ring S0 [2] and mass transfer.