Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Synergistic Effect of Cu2+−Fe2+−Fe3+ Acidic System on the Oxidation Kinetics of Ag-Doped Pyrite Lin Li* and Ahmad Ghahreman*
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The Robert M. Buchan Department of Mining Engineering, Queen’s University, 25 Union Street, Kingston, ON, Canada K7L 3N6 ABSTRACT: The impurities contained in the structure of natural pyrite have been reported to introduce variation in its semiconducting properties and oxidation behavior. However, there is no direct study about the effect of specific impurities on the pyrite electrodissolution process kinetics. Among the impurity elements, silver is of particular interest and is often found in natural pyrite. Silver is shown to be capable of catalysis of sulfide mineral dissolution in an acidic environment. In the present study, Ag-doped pyrite was fabricated by a hydrothermal method. X-ray powder diffraction and X-ray photoelectron spectroscopy analyses of the pyrite showed a homogeneous silver distribution in the pyrite. The electrochemical dissolution kinetics of the Ag-doped pyrite were studied in different sulfuric acid electrolytes, with or without the addition of Fe and Cu cations. A variety of electrochemical techniques, including linear sweep voltammetry, cyclic voltammetry, chronoamperometry, and impedance analysis were applied to evaluate the electrochemical response of the pyrite minerals. The experimental results showed a reduced pyrite electrochemical reactivity with increasing Ag content of the Ag-doped pyrites. The exchange current density of the pyrite electrodes in the sulfuric acid electrolytes decreased from 4.0 × 10−6 to 1.0 × 10−6 A/cm2 by increasing the silver content of the pyrites from 138 to 3820 ppm. However, the anodic dissolution of the Ag-doped pyrite electrode showed a substantial increase in the Cu−Fe-containing sulfuric acid solution. For instance, i0 of a pyrite electrode with 138 ppm Ag increased from 9.6 × 10−6 A/cm2 in the Fe-containing sulfuric acid solution to 2.3 × 10−5 A/cm2 in the Fe−Cu-containing sulfuric acid solution. In the electrochemical impedance spectroscopy analysis, the Ag-doped pyrite with a higher concentration of Ag showed smaller charge-transfer resistances.
1. INTRODUCTION Pyrite is usually considered an undesired sulfide mineral associated with other valuable minerals, and it significantly contributes to acid rock drainage. Pyrite oxidative dissolution has been intensively studied under a wide range of conditions.1,2 However, the fact that the dissolution of pyrite is highly dependent on its geographical location presents an obstacle to natural pyrite oxidation mechanism studies.3 This difference was initially connected to the stoichiometric ratios of Fe and S, because the ideal stoichiometric ratio of 1:2 rarely occurs for a natural pyrite.4 The reason for nonstoichiometric ratios of Fe and S in natural pyrite is believed to be related to the impurities within the pyrite. The impurity elements substituting for Fe and S in the pyrite lattice lead to significant variation of the pyrite electrical and semiconducting properties. A natural pyrite sample often contains several impurity elements, such as As, Pb, Sb, Bi, Cu, Co, Ni, Ag, and Au. Among all these impurities, As, Ni, and Co are usually considered closely connected with the semiconductor properties of pyrite, and their effects have been systematically studied.5,6 Lehner et al. found that the impurity atoms in the pyrite crystal could introduce defect states at the surface of the pyrite particles and mediate the charge-transfer process at the interface.7 A kinetics study comparing the oxidation behavior of a natural pyrite and the As, Co and Ni doped pyrite in sulfuric acid solution had reported that the As, Co and Ni © XXXX American Chemical Society
impurities increase the oxidation rate of pyrite, and the Asdoped pyrite had shown the highest electrochemical activity.8,9 The high electrochemical activity for As-doped pyrite was connected to the p-type behavior of pyrite, while the Co- and Ni-doped pyrite samples showed n-type semiconducting properties.8,9 However, the effect of impurity silver on the semiconducting properties of pyrite and the electrodissolution of pyrite has not been studied before. Silver often is present in natural pyrite in various ranges, from
