Table of Contents
The Mechanism of Sulfur Oxidation in Pyrite is base on what is generally believed as anodic oxidation of pyrite occurs according to the reactions to form S° and SO4=.
FeS2 = Fe++ + 2 S° + 2 e-…………………………………………………………………….(1)
FeS2 + 8 H2O = Fe+++ + 2 SO4= + 16 H+ + 15 e-…………………………………(2)
Pyrite crystals were cut to size, and the exposed surface of the sample was polished on 600-grit silicon carbide paper. The samples were polished wet under distilled water and then transferred to the cell quickly after washing. Samples were handled only by gloves during the entire polishing procedure. Although pyrite oxidation could not be prevented by this procedure, alternate chemical treatments were found to be unsatisfactory.
Cyclic Voltammetry
Successive cycling of the electrode potential in the range of interest is expected to provide information regarding accumulation of intermediate species. For the anodic peak, βA was 0.06 V for the first two cycles whereas it was 0.03 V for all cycles greater than 5. The change in electrochemical behavior on repeated scanning is associated with formation of intermediates during oxidation. For the cathodic peak, the parameter βC remained constant at 0.044 V, except for the first cycle. In the first negative-going scan, the cathodic process is controlled by the type and amount of the oxidized products which might have formed during polishing was 0.06 V.
In the case of pyrite, the total ferrous ions available for complexation are the ions added to the solution and the ions released from the lattice. Therefore, when compared to platinum, the oxidation peak for pyrite shifted to lower potentials. The 60 mV difference between oxidation and reduction peaks is indicative of a one electron transfer process.
In a separate set of measurements, it was observed that the current for peaks A1, and AII increased with EDTA concentration. Based on these observations, we postulate that the decomposition of pyrite occurs through reaction of the type:
FeS2(s) + 3 HS- + EDTA -4 + 3 O2 = FeEDTA-² + 2 S2O3-² + (S)(lattice) + 3 H+ + 4 e-…………………….(4)
Impedance Spectroscopy
To determine the effect of anodic oxidation on impedance of pyrite/solution interface, measurements were made after various quantities of charge had been passed at different current densities. With increase in the rate of oxidation, the surface film becomes more porous and less protective. In the presence of EDTA, the iron layer dissolves and the extent of dissolution depends on EDTA concentration and potential.
