Surface Oxidation of Pyrite as a Function of pH

Abstract

Pyrite, the most abundant metal sulfide at the surface of Earth, plays a key role in many processes such as acid mine drainage, redox cycling of metals at oxic-anoxic boundaries of lake bottom, and degradation of pollutants. The oxidation of pyrite was studied in batch experiments over a large range of pH (2.5-12), with trace oxygen. Surface analysis of the samples was performed using X-ray photoelectron spectroscopy (XPS). Speciation of the aqueous species was investigated by inductively coupled plasma atomic emission spectrometry (ICP-AES), ionic chromatography, and UV-vis spectrophotometry. The pyrite surface can drastically change with the pH, which was never at steady state and tended to reach an acidic value whatever the initial pH. For pH <4, Fe(II) and SO 4 2were released into solution; from XPS analyses, the pyrite surface presented O-H groups, an Fe-deficient composition Fe 1-x S 2 , and iron(III) (hydr)oxide traces. Whatever the pH, the sulfur of the FeS 2 surface was mainly under the (-I) state oxidation. When the pH increased, Fe(II) disappeared and the surface was covered with iron(III) (hydr)oxides. This overlayer did not passivate the sample against further oxidation, and a decrease in pH was still observed.

Novel Adsorbent-Reactants for Treatment of Ash and Scrubber Pond Effluents

“…Knipe et al (1995) said that the anomalous peak near 540 eV might be caused by adsorption of electrically isolated water clusters through hydrogen bonding. Bonnissel-Gissinger et al (1998) reported that extra peaks in O 1s spectra were observed at 535 ± 0.1 eV or 537.6 ± 0.1 eV as pyrite was oxidized by oxygen and formed islands of iron (hydr)oxides on the pyrite surface. In this study, however, we did not find evidence of iron (hydr)oxides, in spite of observing the oxidation of surface Fe(II).…”

“…Longer contact times resulted in larger peaks for Fe(III)-S and Fe(III)-O. In particular, the Fe(III)-O peaks at 710.3~712.3 eV are attributed to Fe(III) hydr(oxides) and the peak at 713.3 eV may be due to an Fe(III) multiplet structure (Bonnissel-Gissinger et al, 1998 Figure 51 shows the S 2p XPS spectra for mackinawite reacted with As(III) for various times. The major component of the S 2p spectra is located at ~161.9 eV and this peak is assigned to sulfide (S 2-) in mackinawite.…”

“…The experiment conducted at pH 10 gave results that are similar to those that would be predicted by a BET isotherm ( Figure 80), which can be interpreted as being the result of adsorption followed by surface precipitation. Behra et al (2001) and Bonnissel-Gissinger et al (1998) observed that the surface of pyrite was oxidized under basic conditions (pH ≥ 10) and was covered by Fe(III) oxyhydroxides that can also serve sorption sites for Hg(II). Similar behavior could occur on the surface of FeS at pH 10.…”

Comparison of rate laws for the oxidation of five pyrites by dissolved oxygen in acidic solution

“…O 2 is an effective oxidant over a range of pH conditions; however, Fe 3+ is the dominant oxidant (reaction 1) at low pH (< 3.5) (Bonnissel Gissinger et al, 1998). Reactions 2 and 3 predominate at pH > 3.5, and reaction 3 dominates generally than reaction 2 at higher pH.…”

An improved pyrite pretreatment protocol for kinetic and isotopic studies

“…Due to its diverse roles in the natural and anthropogenic sulfur cycle, pyrite has been extensively studied in various experimental investigations of the kinetics of its dissolution and oxidation [9][10][11][12][13][14][15][16], the isotopic fractionations associated with these reactions [17][18][19][20][21][22], the microbiological processes involved [23][24][25], and the effect of pyrite surface reactivity and hydroxyl radical formation on human health [26][27][28][29]. Common to many of these studies is the pretreatment of pyrite to prevent experimental artifacts and inaccuracies.…”