An electrochemical study of the oxidative dissolution of iron monosulfide (FeS) in air-equilibrated solutionsby Cătălina E. Bădică, Paul Chiriță

Electrochimica Acta

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Year
2015
DOI
10.1016/j.electacta.2015.08.093
Subject
Electrochemistry / Chemical Engineering (all)

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e u an f F ic a om

Electrochimica Acta 178 (2015) 786–796

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FeS oxidation 1,10-phenanthroline potentiodynamic polarization electrochemical impedance spectroscopy phenanthroline, the oxidation current density (jox) decreases when pH increases from 2.5 to 4. A further increase of pH to 5 causes a slight increase of jox. Most probably, the rapid precipitation of Fe(III) oxyhydroxides at pH > 4 induces a local decrease of pH and promotes the oxidative dissolution of FeS. The current density decreases when 1,10-phenanthroline is added in the system, but after a moderate decrease, in the range of 0.1–0.5 mmol dm3 1,10-phenanthroline, it slightly increases. The oxidation potential of FeS electrode increases when 1,10-phenanthroline is added to the solutions. This behavior can be explained by the mixed potential theory. The largest shift of Eox is registered at pH 2.5 (303.2 mV) and it decreases when pH increases up to 5 (48.8 mV).

The analysis of experimental data highlights the importance of the surface concentration of S-Fe bonds polarized by H+ attack ({>Fe2+}) in the oxidative dissolution of FeS and confirms that the reaction rate is controlled by a mix regime of electron transfer from surface Fe2+ to the cathodic centers and diffusion. We expect that the organic ligands which increase the lability of S-Fe bonds and stabilize the dissolved ferrous iron will control {>Fe2+} and implicitly the oxidative dissolution of FeS. ã 2015 Elsevier Ltd. All rights reserved. 1. Introduction

It is known that oxidative dissolution of iron sulfides is involved in many natural and industrial processes (acid mine drainage, biogeochemical cycles of S, Fe and other metals, metal extraction and coal cleaning). Whereas the oxidative dissolution of iron disulfides (pyrite and marcasite) has been fairly well characterized [1–9], the reactivity of iron monosulfides is still poorly known.

Among the iron monosulfides, pyrrhotite (Fe1-xS, 0.125x0), troilite (FeS) and mackinawite (Fe1.000.01S, according to Rickard et al. [10]) are the most important. Iron monosulfide phases occur in the waste rock dumps at base and precious metal mine sites [11– 13]. Reactive iron monosulfide phases may result from the interaction of iron corrosion products with H2S in permeable reactive barriers [14] and in the nuclear industry [15]. They easily react with the aerated solutions that reach them and produce ferric oxyhydroxides, elemental sulfur and H2SO4 [11,13].

The oxidative dissolution of iron monosulfides starts with the proton adsorption on the mineral surface [16]. The adsorbed protons polarize the S-Fe bond and promote the separation of Fe2+ from iron monosulfide surface. The most likely reaction proceeds by electrophilic substitution mechanism. This scenario is in good agreement with the observed preferential dissolution of iron relative to sulfur from iron monosulfide surface [11,12,16–20]. Once the S-Fe bond is polarized (or broken), both iron and sulfur can easily oxidize [18,20,21]. Thus, the surface of iron monosulfide minerals accumulates polysulfide species, elemental sulfur and Fe (III) oxyhydroxides [11,12,16–20,22–28]. The question is whether the presence of other species in the reaction system influences the oxidative dissolution. A chemical species which can weaken the SFe bond, stabilize aqueous Fe2+ or both can alter the oxidation of

FeS. Basically, the ligands of Fe2+ meet these conditions. Moreover, organic ligands might be adsorbed on the surface of FeS because they contain different functional groups, multiple bounds and donor atoms. Chirita et al. [29] have shown that 2,2'-bipyridine (a ligand of Fe2+) exerts a complex effect on the oxidative dissolution of FeS at 25 C and pH 5. At low concentrations of 2,2'-bipyridine the dominant reaction is the adsorption of the ligand on the FeS* Corresponding author. Tel.: +40251597048; fax: +40251597048.

E-mail address: paulxchirita@gmail.com (P. Chirit????¸aI). http://dx.doi.org/10.1016/j.electacta.2015.08.093 0013-4686/ã 2015 Elsevier Ltd. All rights reserved.An electrochemical study of the oxidativ monosulfide (FeS) in air-equilibrated sol

Catalina E. Badica, Paul Chirit¸aI*

Department of Chemistry, University of Craiova, Calea Bucureşti 107I, 200478 Craiova, Rom

A R T I C L E I N F O

Article history:

Received 5 April 2015

Received in revised form 16 August 2015

Accepted 17 August 2015

Available online 21 August 2015

A B S T R A C T

The oxidative dissolution o investigated by electrochem by H+ attack in the overall re at initial concentrations fr dissolution of iron tions ia eS in air-equilibrated solutions at 25 C and pH between 2.5 and 5 was al methods. In order to understand the role played by S-Fe bonds polarized ction, we added 1,10-phenanthroline (a Fe2+ ligand) to the reaction system 0 to 1 mmol dm3. It was found that, at all concentrations of 1,10ica Acta v ier .com/locate /e lectacta

C.E. Badica, P. ChirițaI / Electrochimica Acta 178 (2015) 786–796 787surface, and the mineral oxidative dissolution is inhibited.

Increasing the concentration of 2,2'-bipyridine causes the rate of oxidative dissolution of FeS to increase because the prevalent reaction becomes the breaking of S-Fe bonds. Tests with 1,10phenanthroline indicated a similar effect of Fe2+ ligand on the oxidative dissolution of FeS [30]. Shu et al. [31] reported an inhibition of metal-sulfides oxidative dissolution in the presence of sodium triethylenetetramine-bisdithiocarbamate (DTC-TETA).

They explained the observed behavior by the formation of an organic layer by coordination of DTC-TETA to mineral surfaces.

These findings suggest that knowing the effect of Fe2+ ligands on the oxidative dissolution of FeS is a means to understand the role played by polarization (weakening)/breaking of the S-Fe bond in the overall reaction.

The aim of this work was to extend the preliminary investigation of the effect of 1,10-phenanthroline on the oxidative dissolution of FeS in air-equilibrated solutions [30]. The electrochemical experiments were carried out at 25 C and pH in the range of 2.5–5. The electrochemical kinetic parameters of the oxidative dissolution of FeS were determined using potentiodynamic polarization method. The properties of the FeS/water interface were measured by using Electrochemical Impedance Spectroscopy (EIS). The rate of FeS oxidative dissolution evaluated by electrochemical methods reflects the amount of the transferred electrons during mineral oxidation, i.e. the reaction progress variable is the transferred electron. This is an important advantage over the methods where the reaction progress variables are the released iron and sulfur, because the latter are accompanied by an overestimation (due to nonoxidative release of iron) or underestimation (due to incomplete oxidation of sulfur to soluble species) of oxidative dissolution rate. The interaction between FeS and 1,10phenanthroline was analyzed using Fourier transform infrared spectroscopy (FTIR). 2. Experimental procedure 2.1. Materials