Anoxic dissolution of troilite in acidic media
Graphical abstract
The integral form of rate law that can describe within experimental uncertainties the troilite anoxic dissolution is: and .
Introduction
The dissolution reaction of iron sulfide is of high importance in various natural or industrial processes (acid mine drainage, remediation of mineral wastes, or metal extraction). There are two important categories of natural iron sulfide: iron disulfide (FeS2), such as pyrite and marcasite, and iron monosulfide (Fe1−xS), such as troilite, mackinawite and pyrrhotite. The second category encompasses a series of nonstoichiometric members (), on the whole called pyrrhotite, and a stoichiometric endmember called troilite (). However, a limited iron deficiency is allowed even for troilite [1]. Furthermore, an important characteristic of troilite and pyrrhotite consists in the presence of various imperfections on their surface, such as truncated FeS bonds, iron deficiency, or the presence of oxidized species of iron and sulfur [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].
In contrast to the high interest paid to the dissolution process of iron disulfide [12], [13], [14], [15], [16], [17], [18], little attention was given to the dissolution process of iron monosulfide. Only a few studies were dedicated to troilite [10], [19] and pyrrhotite dissolution [8], [9], [10], [19], [20], [21], [22] and their results are not always in good agreement. Unlike iron disulfides, which dissolve only in presence of an oxidizing agent, troilite and pyrrhotite can undergo both oxidative and nonoxidative dissolution processes [23]. Both dissolution mechanisms proceed nonstoichiometrically, with a preferential release of iron [8], [9], [10], [21], [22]. Oxidative dissolution releases ferrous and ferric iron, and sulfate [22], [24]. Additionally, a partial oxidation of FeS by ferric iron or oxygen may produce elemental sulfur [8], [9], [22] according to:FeS(s) = S0(s) + Fe2+(aq) + 2e− accompanied by2Fe3+(aq) + 2e− = 2Fe2+(aq) or1/2O2(aq) + 2H+(aq) + 2e− = H2O. It must be noted that during air oxidation, the iron monosulfide surface undergoes sulfur enrichment [2], [5]. The sulfur-rich layer (SRL) forming on the iron monosulfide surface consists of elemental sulfur and various polysulfide ions [2], [5], [7].
The stability diagram of FeS system (calculated for ) [23] predicts that troilite is thermodynamically unstable in acidic medium (pH < 5). This diagram also shows that the main sulfur-bearing product of troilite dissolution in anoxic medium should be hydrogen sulfide. The overall mass balance for acidic dissolution of iron monosulfide can be simply written:FeS(s) + 2H+(aq) = Fe2+(aq) + H2S(aq).
Laboratory investigations however showed that the dissolution mechanism is more complex than would be inferred from this simple mass balance relation. This complexity can be illustrated by the four stages reported during pyrrhotite dissolution [8], [9], [10], [19]:
- (1)
the dissolution of outermost layer consisting from oxidized species of iron and sulfur;
- (2)
the inhibition of dissolution as a result of the formation/presence of a SRL;
- (3)
the “reduction” of the SRL, concomitant with proton attack on S2− groups resulting in a rapid release of iron and sulfur;
- (4)
the “oxidative” dissolution, characterized by formation of a new sulfur-rich phase on the surface and a slow release of iron.
It is assumed that at temperatures ⩽40 °C, the SRL strongly hinders the attack of protons on surface S2− groups [8]. In such conditions the inhibition periods are long and the pyrrhotite dissolution is controlled by diffusion. However, when temperature exceeds 40 °C, the SRL can be destroyed as a result of complex reactions that, on the whole, consist in sulfur reduction into hydrogen sulfide by the electrons accumulated on the pyrrhotite surface [8], [9], according to:S2−n(s) + 2(n−1)e− = nS2−(aq). Therefore, at high temperatures the solid dissolution is kinetically controlled by a surface reaction [19].
The proposed mechanism explains many features of FeS anoxic dissolution, but some aspects of this reaction are still unclear. Perhaps, the main remaining question is that the anoxic dissolution is depicted as a mixture of reductive, oxidative, and nonredox processes.
In this work we examine the kinetics of the anoxic dissolution of synthetic troilite as a function of perchloric acid concentration [HClO4] at 50 °C by monitoring dissolved Fe () and released H2S (). The variation in concentration of both reaction products will be investigated to clarify the mechanism of troilite dissolution, and discuss the various different mechanisms for iron and sulfur release from iron monosulfide proposed in earlier studies.
Section snippets
Experimental
Synthetic troilite obtained from Rieden-De-Haen was used in this study. The troilite was prepared into a nitrogen-filled glove bag. The material was crushed under ethanol in a ceramic mortar. After crushing, the supernatant suspension was decanted and the solid material was rinsed with ethanol. The cleaning procedure was repeated until the supernatant was relatively clear. The troilite sample was transferred under vacuum, where it was dried. The particles below 63 μm were further separated by
Effect of perchloric acid concentration on troilite anoxic dissolution
The temporal evolution of dissolved iron and formed hydrogen sulfide as a function of perchloric acid concentration ([HClO4]) is illustrated in Figs. 2a and 2b. Both figures show that the rate of anoxic troilite dissolution increases with [HClO4]. Practically, when the [HClO4] increases from 0.04 to 0.2 mol L−1, and the cumulative amount of released H2S () increase from 7 to 19 and from 5 to 15 mmol m−2, respectively, over a time span of 1200 s.
Also visible in Fig. 2a is the decreasing
Discussion
Our experiments clearly demonstrate that global troilite dissolution is an incongruent process, the rate controlling step of which is a diffusion process of reaction products across an obstructive layer existing between troilite surface and acidic solution. However, these experiments also point to distinct rate coefficients for Fe and H2S, presumably because the formed H2S undergoes a series of side reactions that modify . In the next sections one will discuss the manner by which the iron
Conclusions and implication for the anoxic dissolution of pyrrhotite
Troilite anoxic dissolution is a process that can be described by the diffusion of reaction products (i.e., iron and hydrogen sulfide) across an obstructive layer with increasing thickness. Diffusion through the forming layer causes the apparent dissolution rate to show a factionary dependence on hydrogen ions concentration. The accumulation of H2S between the surface and the SRL, eventually leads to the mechanical destruction of this outer layer, a process that results an increased flux of
Acknowledgements
We are grateful to Dr. Michel Schlegel (CEA) and an anonymous reviewer for valuable comments and suggestions of the manuscript.
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