The results of studies to ameliorate acid mine drainage by retarding the rate of oxidation of pyrite at source are discussed. In the transpassive region, pyrite oxidizes by reactive dissolution, mass transfer through the product layer and diffusion in the bulk solution. One can decrease the rate of pyrite oxidation by inhibiting either oxidation reactions or cathodic reactions. The extension to higher potentials occurs by inhibition of the anodic dissolution process whereas to lower cathodic potentials through decrease in the rate of reduction of molecular oxygen and/or ferric ions. To retard the oxidation of pyrite by ferric ions, which is considered to be the dominant oxidant under conditions of acid mine drainage, it would be necessary to restrict mass transfer or promote complexation of ferric ions at the surface. Imidazoline based reagents were found to be most effective in meeting this goal.
The mechanism of pyrite oxidation has been studied extensively over the years. The overall oxidation of pyrite could be written by the following reactions:
Although anodic oxidation of pyrite has been studied by many investigators, the cathodic reactions have been studied in less detail. Since ferric ions and oxygen act as prime oxidants of pyrite, it is important to study the kinetics of their reduction on pyrite. Reduction of molecular oxygen has been studied extensively but, the importance of reduction of ferric ions on pyrite has been addressed to by a few researchers only.
Materials and Results
The electrodes used for voltammetric studies were made from relatively pure, natural crystals of pyrite. For cyclic voltammetry study a standard three electrode system was used in which the working electrode was a rotating disk of polished sample of pyrite, mounted in teflon. A saturated calomel electrode (SCE) was used as the reference electrode. However, all the potentials in the text are given in the standard hydrogen scale.
The potential scan was started in the cathodic direction from the rest potential (500 mV) of the electrode. However, this portion of the scan has not been shown in the figure. Reduction of surface species and molecular oxygen occurred in the cathodic region whereas oxidation of pyrite took place in the anodic region of the scan. The anodic current increased substantially when the potential was greater than 750 mV. This anodic current represents the surface oxidation of pyrite leading to the formation of a sulfur layer at the electrode surface and the sulfate ions which dissolve in the solution.
Researchers have also suggested that formation of sulfate instead of sulfur becomes the dominating reaction at high anodic potentials. To confirm this hypothesis, sulfur to sulfate ratio was calculated by subjecting the electrode to anodic treatment at various anodic potentials. Charge passed during anodic oxidation could be taken as a measure of the total amount of sulfur and sulfate produced. Since the reduction of oxygen and sulfur occurs together, and the oxygen reduction current is a function of the coverage of the electrode surface by sulfur, it is difficult to get an exact estimate of the amount of charge passed for sulfur reduction.
The ferrous ions could hydrolyze to form ferrous hydroxides depending upon the pH. Also they could oxidize to form ferric ions and ferric hydroxide. The sulfur layer could also oxidize to sulfate at higher oxidation potentials. Therefore the thickness and porosity of the sulfur layer will depend on the potential (driving force) and extent of oxidation. However, this study is inadequate to predict the exact composition and morphology of sulfur layer.
The cathodic current in the potential range of 450 mV to -310 mV represents reduction of molecular oxygen on pyrite surface. The region of activation control lies between 450 mV to 300 mV on bare pyrite surface. With further decrease in potential, the current goes through a mixed control of charge and mass transfer before it attains a mass transfer limiting value.
The breakdown of sulfur film during cathodic scan is marked by a sharp increase in the cathodic current at 0 mV. Since sulfur is present on the surface, a current peak is observed when the reduction takes place. The reduction of sulfur could take place through the following reaction
S° + H+ +2e- → HS-
The presence of sulfur layer also makes the oxygen reduction on pyrite more sluggish. On the bare pyrite surface, the Tafel slope for oxygen reduction was 100 mV per decade change in current, whereas in the presence of sulfur layer, it became 256 mV per decade change in current.
Cyclic voltammetry was carried out on pyrite electrode in the presence of ferric ions. Figure 5 shows such a voltammogram in a solution containing 4×10 -4 M Fe³+ ions. The cathodic current in the range of 420 mV to 640 mV is mainly due to ferric reduction. Ferric reduction current attained mass transfer limiting value without showing any region of mixed control of charge and mass transfer.