Electrochemical Study Of The Oxidation Mechanism And Passivation Behavior Of Pyrite

Pyrite (FeS2) is a common iron sulfide mineral in tailings and waste rock dumps; it is also present in many valuable mineral raw materials. The oxidation of pyrite can produce acid mine drainage, which is then a cause of pollution by heavy metals and acidity. For the control of acid mine drainage at source, it's essential to understand the oxidation mechanism of pyrite. Then we could find an effective passivation method to suppression the oxidation process of pyrite.Electrochemical analysis methods, such as open circuit potential (OCP), cyclic voltammetry (CV), Tafel polarization curves and electrochemical impedance spectroscopy (EIS) were used to investigate electrochemical behavior of pyrite in systems with and without bacteria, revealing the mechanism of pyrite's chemical and biological oxidation. On these bases, the feasibility of triethylenetetramine (TETA) to inhibit the chemical and biological oxidation of pyrite had also been investigated. The major conclusions are given below:The oxidation process of pyrite in acid solution is via a two-step reaction occurring at the surface of pyrite: the first step is the dissolution of iron moiety and that a passivation film composed of elemental sulphur,polysulfides and metal-deficient sulfide is formed during the process of the first-step reaction. The second step is the further oxidation of these intermediate products to SO₄^2-. In sterile solution, Ferric iron plays an important role in the dissolution of pyrite by enhancing the direct oxidation. The Tafel polarization curves indicate that the polarization current of the pyrite electrode increases with an increase in Fe³⁺ concentration. The results of polarization curves and EIS had also been shown that the higher concentration of Fe³⁺, the more easily the pyrite can be transformed into the passivation region.The results of bioleaching tests showed that Acidithiobacillus ferrooxidans (A.f) can significantly improved the leaching rate of pyrite. At the end of the experiment, the concentration of total soluble iron in bioleaching solution was 8 times of that in sterile solution. Moreover, the leaching efficient of A.f bacteria after domestication was increased compared with that before domestication. The redox potential in the system with bacteria was increased much faster than that in sterile solution, which resulted from the fast oxidation of ferrous ions to ferric ions. The leached Fe³⁺ could in turn accelerate the oxidation of the pyrite. In addition, the pH of bioleaching solution was decreased with times. This may also be an important canse of the fast dissolution of pyrite in bioleaching system.The electrochemical behavior of a pyrite-carbon paste electrode in system with and without Acidithiobacillus ferrooxidans was also investigated. The results showed that the addition of A.f bacteria could enhance the corrosion current of pyrite electrode greatly. The corrosion of pyrite electrode became more serious with the increase of bioleaching time. The EIS responses were different with time in both inoculated and sterile solution, which suggested the kinetic processes occurring in the pyrite–solution interface were changed during the leaching process. In the initial stage of pyrite oxidation, the corrosion rate of the pyrite was mainly controlled by the process of iron moiety dissolution in both of the systems with and without bacteria, which results in the formation of some intermediate products such as elemental sulfur and polysulfide on the surface of pyrite. In the presence of bacteria, these intermediate products could be continuously oxidized to SO₄^2-. But in the sterile solution, the further oxidation of S was difficult to be detected by EIS measurement. These results were also confirmed by surface analysis and which showed that the principal advantage of the presence of microorganisms was to continuously remove the elemental sulfur from the surface of pyrite.The potential of triethylenetetramine (TETA) to inhibit the oxidation of pyrite acid solution has been investigated using the open circuit potential (OCP), cyclic voltammetry (CV), potentiodynamic polarization and electrochemical impedance (EIS) respectively. Experimental results indicate that TETA is an efficient coating agent in preventing the oxidation of pyrite and the inhibition efficiency is more pronounced with the increase of TETA. The data from potentiodynamic polarization show that the inhibition efficiency (?%) increases from 42.08% to 80.98% with the concentration of TETA increasing from 1% to 5%. These results are consistent with the measurement of EIS (43.09% to 82.55%). The information obtained from potentiodynamic polarization also display the TETA is a kind of mixed type inhibitor, both cathodic and anodic process can be inhibited by the coating of TETA. The coating treatment of pyrite with TETA can be described as chemisorption of TETA molecules on the surface of sample particles and lead to the formation of a protective film, and this film prevent from the oxidation of pyrite.At the end of this paper, the feasibility of using triethylenetetramine (TETA) as protecting agent to reduce the biological oxidation of pyrite had also been studied by methods of electrochemical techniques. A pyrite-carbon paste electrode was utilized to monitor the oxidation rate of pristine and TETA coated pyrite oxidation in the presence of Acidithiobacillus ferroxidans bacterium. By comparing the changes of the electrochemical activity for the pyrite-carbon paste electrode, reflected by the measurements of open circuit potential (OCP), cyclic voltammetry(CV), Tafel polarization analysis and electrochemical impedance spectroscopy (EIS), in different bioreactor with pyrite samples coated by various concentrations of TETA, we could find that TETA is an efficient coating agent in preventing the oxidation of pyrite in the presence of bacteria. Compared to other coatingt reatments, the coating by TETA can limit both the chemical and biological oxidation of pyrite; meanwhile, it could represent a simple, cost-eficient method to control the oxidation of pyrite.