Studies Of Sulfur Transformation In Oxidation Of Pyrite And Surface Electrochemistry Of Pyrite Oxidation

Pyrite?FeS₂?is the most abundant metal sulfide mineral in the earth's crust,and it is easy to be oxidized and generated acid mine drainage?AMD?when exposed to circumstance.Surface oxidation of pyrite is very important in extracting valuable metal from its structure,in control of acid mine drainage,and in global cycling of iron and sulfur.As a result,the oxidation of pyrite has been the subject of extensive research.However,to date,the detailed mechanism remains incompletely understood.The oxidation of S₂^2-in pyrite to SO₄^2-is a multistep process,in which some intermediate sulfur species inevitably formed.However,many studies have focused on the factors influencing the oxidation rate of pyrite,very little has been known previously about the intermediate sulfur species and their transformation pathways during the pyrite oxidation process.In addition,pyrite is a natural semiconductor,and its surface oxidation has often been described as an electrochemical oxidation for involving transfer of electrons.Therefore,electrochemical methods have been productively employed in investigation of the behavior and mechanism of pyrite electrochemical oxidation.However,most such studies have been carried out using pyrite carbon paste electrode in combination with Eh-pH diagrams and thermodynamic data to speculate on the electrochemical oxidation mechanism of pyrite involved.Few works were focused on the detection of the products covered electrode surfaces.In fact,classic thermodynamic parameters and the Eh-pH diagrams often assume a pure reaction system and ideal conditions,which is not always the case.For some reactions,although thermodynamically possible,reactions still may not occur due to the extremely slow kinetics.Thus,it is necessary to carry out some surface analysis techniques to detect the actual products on the electrode surface to further determine whether the predicted reactions actually occur or not.In the study,shake flask leaching experiments in combination with high performance liquid chromatography?HPLC?,ion chromatography?IC?,X-ray powder diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?and scanning electron microscopy-Energy Dispersive Spectrometer?SEM-EDS?were carried out to comprehensively investigate and analysis intermediate sulfur species both in solution and on the surfaces of pyrite residues during pyrite chemical and biological oxidation process.Besides,experiments were employed to further investigate the re-oxidation process of some intermediate sulfur species which have been detected.The results indicated that thionate(S₂O₃^2-),trithionate(S₃O₆^2-),tetrathionate(S₄O₆^2-),pentathionate(S₅O₆^2-)and hexathionate(S₆O₆^2-)have been found,and concentrations of these polythionates(S_nO₆^2-,n?3)increased over the time during pyrite biological oxidation process.Furthermore,jarosite?KFe₃?SO₄?₂?OH?₆?,iron?III?oxide-hydroxide?FeOOH?,elemental sulfur?S⁰?,monosulfide(S^2-)and polysulfide(S_n^2-)also have been detected on pyrite residues surfaces.During the pyrite chemical oxidation process,only a small amount of S₃O₆^2-,S₄O₆^2-and S₅O₆^2-have been detected at the end of the experimental period.Dominant products on pyrite residues surfaces were also S⁰,S^2-and S_n^2-.Several kinds of S_nO₆^2-and S₂O₃^2-have been detected during pyrite oxidation process indicated that there is indeed the presence of“thiosulfate pathway”,that is,the disulfide in the pyrite lattice is oxidized to S₂O₃^2-,followed by further oxidation resulting in the generation of S_nO₆^2-.The results of re-oxidation of intermediate sulfur species showed that two reactions further occurred following the formation of S₂O₃^2-during the pyrite chemical oxidation process.First,most of S₂O₃^2-was oxidized to S₄O₆^2-by the dissolved oxygen through a catalytic reaction on the pyrite surface.Then,the S₄O₆^2-was subject to a rearrangement reaction that generated both S₃O₆^2-and S₅O₆^2-in acidic solution.Second,S₂O₃^2-is unstable and is easily disproportionated to S⁰ and HSO₃^-.However,in the biological oxidation process,almost all the formed S₂O₃^2-was rapidly oxidized to S₄O₆^2-by the high concentrations of Fe³⁺in solution.After that,the presence of microorganism resulted in most of the S₄O₆^2-being involved in the enzyme hydrolysis reaction and producing the intermediate species,disulfane-monosulfonic acid(S₃O₃^2-).The highly reactive S₃O₃^2-may trigger several reactions and yield several compounds;e.g.,S⁰,SO₃^2-,S₃O₆^2-,S₅O₆^2-and S₆O₆^2-.Certainly,all the S_nO₆^2-are metastable and would eventually convert to SO₄^2-.Experimental results also revealed that the pyrite oxidation was inhomogenous,occurring first at some specific sites with high surface energy,such as particle edges,corners,surface impurities,and crystal defects.Acidithiobacillus ferrooxidans?A.ferrooxidans?has been shown to remarkably accelerate pyrite oxidation and also can promote the formation of secondary minerals such as jarosite?KFe₃?SO₄?₂?OH?₆?and iron?III?oxide-hydroxide?FeOOH?.Electrochemical techniques such as open circuit potential(E_ocp),cyclic voltammetry?CV?,linear sweep voltammetry?LSV?,current-time?i-t?and electrochemical impedance spectroscopy?EIS?were used to investigate the behavior and mechanism of pyrite electrochemical oxidation.Meanwhile,the morphological changes and oxidation products after pyrite electrodes electrochemical oxidation at different anodic potentials of 0.50 V,0.60 V,0.70 V and 0.80 V were further characterized and analyzed by atomic force microscopy?AFM?,Raman spectroscopy?Raman?and XPS.The results indicated that electrochemical oxidation of pyrite occurred via two reaction pathways.The first reaction occurred at low potential of 0.50 V and was characterized by the formation of Fe?OH?₃ and a sulfur rich layer?S_L?composed of S⁰,iron deficiency disulfide(Fe_1-xS₂)and S_n^2-.This layer,especially the presence of S⁰ covered the pyrite surface partially hinders the diffusion of the reactants to the electrode surface and the product diffuses from the electrode surface into the solution resulted in the reaction becomes diffusion limited at this low potential.As the potential increased to 0.60 V,the diffusion-limitation ceased due to the conversion of amorphous S⁰ was slowly converted into crystalline S⁰,which re-exposed previously blocked active sites on the pyrite surface and allowed for the continued oxidation of pyrite.At potentials of 0.70 V and 0.80 V,the second reaction pathway of pyrite oxidation resulting in the formation of Fe?OH?₃ and SO₄^2-occurred.Furthermore,more S⁰ and S_n^2-,together with an iron-rich layer composed of Fe?OH?₃,Fe O and Fe₂O₃,formed and accumulated on the pyrite surface.These products led to a decreased rate of oxidation rather than a complete passivation of the surface.Resultsalso indicated that once S⁰ formed on the pyrite electrode surface it is difficult to be further electrochemically oxidized even at a high potential.At the end of this paper,electrochemical techniques including E_ocp,CV,Tafel polarization?Tafel?curves,EIS were carried out to investigate the effects of SO₄^2-on the electrochemical oxidation rate of pyrite.The results showed that E_ocp and the corrosion potential(E_corr)of the pyrite electrode decreased with the increase of the concentration of SO₄^2-.CV and EIS curves showed that the increase of SO₄^2-concentrations did not change the surface electrochemical oxidation mechanism of pyrite,but the oxidation rate decreased by the increase of the electron transfer resistance.The corrosion current density(j_corr)from Tafel curves also indicated that surface oxidation rates of pyrite were about 52%,43%,28%and 26%of the control when the SO₄^2-concentrations were 0.25 M,0.50 M,0.75 M and 1 M,respectively.