Inhibition of troilite (FeS) oxidative dissolution in air-saturated acidic solutions by O-ethyl-S-2-(2-hydroxy-3,5-diiodophenyl)-2-oxoethylxantogenate
Graphical abstract
Introduction
Iron sulfide minerals are found in various geological environments together with other base metal sulfides, coal and gold. When iron sulfides are exposed to oxygen-bearing aqueous solutions they can be easily oxidized causing severe environmental problems such as acid mine drainage (AMD). AMD is characterized by low pH and high concentrations of sulfate, ferric iron and toxic impurities, such as As, Cd, Pb, Hg or Cu. The formation of H2SO4 induces H2S release by non-oxidative dissolution of iron monosulfide minerals (pyrrhotite and troilite), which can explain their self-heating [1]. The self-heating of iron monosulfides leads to difficulties in the safe mining of valuable metals [2].
Iron sulfide phases are produced during sulfate reduction and zero valent iron oxidation in permeable reactive barriers [3], in the energy industry (in oil and gas pipelines) [4] and in the nuclear industry [5]. The main iron sulfide phases formed on iron/steel surfaces are mackinawite (Fe1.00±0.01S), pyrrhotite (Fe1-xS), troilite (FeS), greigite (Fe3S4) and pyrite (FeS2) [6]. The iron sulfide films interposed between iron/steel and corrosive media break down in the presence of oxygen and can affect the localized corrosion of the metallic phase. It has been reported that depending on the corrosion conditions iron sulfide films can either accelerate [7], [8] or inhibit [9], [10] the corrosion process. Although many studies have been devoted to the study of pyrite, pyrrhotite and mackinawite oxidative dissolution in aerated solutions [11], [12], [13], [14], [15], [16], [17], [18], [19], less is known about troilite (FeS) dissolution in aerated solutions and the effect of organic molecules on its oxidative dissolution.
It is well known that oxidative dissolution of iron monosulfide minerals is controlled by the surface layer developed on the mineral [20], [21], [22]. The surface layer results from preferential release of iron relative to sulfur and contains polysulfide species, elemental sulfur, ferrous and ferric oxyhydroxides [20], [21], [22]. It was observed that the rate of oxidative dissolution can be reduced by the pretreatment of sulfide minerals with organic compounds [23], [24], [25], [26]. The inhibiting action of these compounds is usually attributed to their interaction with the mineral surface. Interaction is possible by means of different functional groups, multiple bounds and donor atoms contained by organic compounds.
Here, we present an electrochemical study that investigates the inhibiting effect of O-ethyl-S-2-(2-hydroxy-3,5-diiodophenyl)-2-oxoethylxantogenate (EHDO) on the oxidative dissolution of synthetic troilite. Potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS) were employed. The uninhibited and inhibited surfaces were characterized using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and Raman spectroscopy.
Section snippets
Materials
Synthetic iron(II) sulfide obtained from Merck was used in the study. Wet chemical analysis [27] and X-ray diffraction (Philips PW3710 diffractometer, CuKα) analysis showed that the iron(II) sulfide (identified as troilite (FeS)) had no impurities excepting 4% mass of metallic iron.
All solutions were prepared with distilled water and chemicals of reagent grade purity. EHDO (molecular structure given in Fig. 1) was synthesized according to the procedure described below. To a solution of 2.34 g
Electrochemical polarization studies
The inhibition efficiency (IEi) for different times of immersion of the FeS electrode in 1 mM ethanolic solution of EHDO is presented in Fig. 2. IEi was computed using the following expression [28].where icorr and icorr0 are the current densities of corrosion in the presence and absence of the inhibitor, respectively. Since corrosion is generally used to mean the electrochemical oxidation of metals, for the dissolution of FeS (which is not a metal) in the presence of
Conclusions
EHDO has been evaluated as an inhibitor of oxidative dissolution of FeS in air-saturated HCl solutions at pH 1.3 and 25 °C. The experimental results indicate that EHDO acts as an anodic inhibitor. EHDO shifts the corrosion potentials in the positive direction. Results of SEM/EDX analysis, Raman spectroscopy and UV–vis absorption spectroscopy indicated the development of particular iron oxyhydroxides phases (maghemite and Fe3O4) and adsorption of EHDO on the surface of the electrode immersed in
Acknowledgments
This work was supported by a grant of the Romanian National Authority for Scientific Research (51/2012), CNDI– UEFISCDI, project number 51/2012.
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