A Review on Pyrrhotite Oxidation

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A review on pyrrhotite oxidation

Nelson Belzile*, Yu-Wei Chen, Mei-Fang Cai, Yuerong Li

Department of Chemistry and Biochemistry, Laurentian University, Ramsey Lake Road, Sudbury, Ontario, Canada P3E 2C6

Received 21 July 2003; accepted 29 March 2004

Available online 14 May 2004

Abstract

The non-stoichiometric compounds of iron sulphide named pyrrhotite (Fe 1 x S) are often associated with pyrite (FeS2) in

sulphidic ores and their waste products. The factors affecting pyrite and pyrrhotite oxidation are similar but the latter has

received much less attention. As it is the case for pyrite, an increase in temperature has an increasing effect on the oxidation of

pyrrhotite and the process follows the Arrhenius behaviour. Both ferric iron and bacteria act as catalysts in the oxidation

reactions and play a significant role in the oxidation kinetics. Several spectroscopic techniques are used to study the oxidation

mechanisms and products of pyrrhotite oxidation. This paper presents and discusses the main factors controlling pyrrhotite

oxidation and the proposed mechanisms of oxidation under normal conditions of temperature and pressure.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Pyrrhotite; Oxidation rate; Iron sulphide; Acid generation; Oxidation mechanism

1. Introduction

After pyrite, pyrrhotite is the most common iron

sulphide in nature. A better understanding of the

reactivity and oxidation of pyrrhotite is needed for

improved mineral processing and recovery and to

prevent the production of acid mine drainage (AMD)

from iron sulphides in mine waste. AMD is a major

environmental problem causing acidification, ferric

iron precipitation and an increased mobility of trace

metals in drainage waters (Alpers and Blowes, 1994;

Belzile et al., 1997a; Johnson et al., 2000). This paper

reviews the recent literature on the oxidation of pyr-

rhotite, presents the main factors affecting the process

and discusses the mechanisms proposed for the oxi-

dation of pyrrhotite. When possible, a comparison is

made with pyrite.

2. Structure and solubility

Pyrrhotite is found in a wide range of hydrothermal

deposits. It is often associated with mafic intrusive

and volcanic rocks and found in massive sulphide

deposits associated with pyrite, sphalerite, galena and

chalcopyrite.The name pyrrhotite covers a wide range

of minerals and synthetic iron–sulphur phases based

on the NiAs structure (Becker et al., 1997). Pyrrhotite

0375-6742/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.gexplo.2004.03.003

Abbreviations: AES, Auger electron spectroscopy; AMD, acid

mine drainage; EPR, electron paramagnetic resonance; FTIR,

Fourier transform infrared; NL, non-equilibrium layer; SEM,

secondary electron microscopy; SLS, sequential layer sputtering;

ToF SIMS, time of flight-secondary electron mass spectrometry;

XES, X-ray electron spectrometry; XRD, X-ray diffraction; XPS,

X-ray photoelectron spectrometry.

* Corresponding author. Tel.: +1-705-675-1151x2114; fax: +1-

705-675-4844.

E-mail address: [email protected] (N. Belzile).

www.elsevier.com/locate/jgeoexp

Journal of Geochemical Exploration 84 (2004) 65–76



shows a non-stoichiometric composition as Fe1 x S,

where x varies from 0 (FeS) to 0.125 (Fe7S8). The

non-stoichiometry is due to a system of ordered

vacancies within the Fe lattice (Vaughan and Craig,1978; Pofsai and Dodonay, 1990; Thomas et al., 2000,

2001). The formula can also be expressed as Fen 1S

n

with nz 8 to give structures from Fe7S8 to Fe11S12.

The least Fe-deficient forms have hexagonal and

orthorhombic (FeS) structures whereas those with

greater iron deficiency have monoclinic symmetry

(Arnold, 1967; Janzen et al., 2000; Thomas et al.,

2001). The Fe–S distance is in the range of 0.237–

0.272 nm with an average of 0.250 nm (Vaughan and

Craig, 1978). The iron content ranges between 46.5%

and 46.8% in Fe (on a mole basis) in monoclinic

pyrrhotite and between 47.4% and 48.3% in hexago-

nal forms (Ward, 1970). Monoclinic pyrrhotite is

ferromagnetic at room temperature (Becker et al.,

1997). Pyrrhotite is an important iron sulphide waste

mineral in many mining environments. It is often

found as a gangue mineral in Cu–Ni deposits, asso-

ciated with valuable minerals. Massive sulphide

deposits containing pyrrhotite can be found all around

the world, more specifically in Russia, China, Aus-

tralia and Canada.

Because of its sulphidic nature, pyrrhotite can be

dissolved rapidly under acidic conditions and generateFe2 + and H2S according to reaction (1) or (2) or more

slowly via oxidative dissolution according to reactions

(3) and (4):

Fe1 xS þ 2Hþ Z ð1 xÞFe2þ þ H2S ðfor x ¼ 0Þ

ð1Þ

Surface > S2 þ 2Hþ Z H2S ð2Þ

Surface > S2 þ 4H2O Z SO24 þ 8Hþ þ 8e ð3Þ

together with: 2O2 þ 4H2O þ 8e Z 8OH or 8Fe3þ

þ 8e Z 8Fe2þ ð4Þ

It should be mentioned that Eq. (4) will neutralize acid

produced in Eq. (3).

However, Thomas et al. (1998) report that an

induction period during which a slow release of Fe

and little or no production of H2S is observed before

the rapid dissolution of pyrrhotite can occur under anoxic acidic (1 M HClO4) and acid-consuming

conditions. The onset of non-oxidative dissolution

and production of H2S (reaction 1) can increase the

rate of oxidative dissolution by three orders of mag-

nitude from 10 8 to 10 5 mol m 2 s 1. The main

factors controlling the length of the induction period

were identified as the amount of surface oxidation

products on the mineral surface, the acid strength (no

apparent induction in 0.05 M acid) and the tempera-

ture. The authors present a four-step dissolution

process and use X-ray photoelectron spectroscopy

(XPS) to study the initially oxidized surface of pyr-

rhotite samples and identify changing sulphur (e.g.

S2 , S2

2 , Sn

2 ) and iron (e.g. Fe(III)S, Fe(II)S,

Fe(III)O) species at the different stages of dissolution.

In a more recent paper, Thomas et al. (2001) refine the

model by proposing the trapping of electrons in

metastable chemical states (e.g. polysulfides) during

the oxidative dissolution of pyrrhotite before the

production of H2S according to:

S

2

n þ 2ðn 1Þe

Z

nS

2

ð5Þ

It is then proposed that not all electrons are lost to

an oxidizing agent and that some accumulation of

electrons in a crystal-wide surface space-charge sur-

face region of pyrrhotite may occur particularly at

temperature of 40 jC and above. The overall mech-

anism is a non-oxidative dissolution with Fe2 + leav-

ing the surface without the release of electrons,

followed by the reduction of polysulphides to sul-

phides according to reaction (5) and then production

of negative charges as HS

.The true oxidative dissolution of pyrrhotite under

acidic conditions does not involve the production of

H2S. At low pH, the rate of acidic dissolution will

compete with the oxidative dissolution.

3. Oxidation of pyrrhotite and oxidation products

Oxygen is the ultimate oxidant of sulphide miner-

als in natural surface waters and the direct oxidant at

N. Belzile et al. / Journal of Geochemical Exploration 84 (2004) 65–76 66



pH>4 (Nordstrom and Alpers, 1999). At pH below 4,

sulphides are oxidized by ferric iron. In most mine

wastes, oxygen is the primary oxidizing agent of

ferrous iron to ferric, which implies that sulphideoxidation generally occurs only in ar eas where dis-

solved or gaseous oxygen is present (Benner et al.,

2000).

As is the case for pyrite (Lowson, 1982; Bierens de

Haan, 1991; Belzile et al., 1997a), oxygen and ferric

iron are generally the two important oxidants for

pyrrhotite. When O2 is the primary oxidant, the

oxidation reaction can proceed as follows (Nicholson

and Scharer, 1994):

Fe1 xS þ ð2 ð1=2Þ xÞO2 þ xH2O Z ð1 xÞFe2þ

þ SO24 þ 2 xHþ ð6Þ

The oxidation of ferrous iron produces ferric ions

that can precipitate out of solution to form ferric

hydroxide, if pH is not too low. Fe2 + is oxidized

and precipitated as ferric oxyhydroxides, principally

ferrihydrite and goethite.

Fe2þ

þ 1=4O2 þ 2Hþ Z

Fe3þ

þ 1=2H2O ð7Þ

Fe3þ þ 3H2O Z FeðOHÞ3ðsÞ þ 3Hþ ð8Þ

Ferric iron can in turn oxidize more pyrrhotite and

generates more acidity in the system according to the

following reaction:

Fe1 xS þ ð8 2 xÞFe3þ þ 4H2O Z ð9 3 xÞFe2þ

þ SO24 þ 8Hþ ð9Þ

If reaction (7) occurs under acidic conditions, a

significant quantity of Fe3 + will remain in solution

and maintain a cyclic reaction with reaction (9) where

ferric iron is the oxidant.

There is evidence from field and laboratory studies

that the oxidation may not be complete and instead

generates elemental sulphur (e.g. Steger, 1982; Jam-

bor, 1986; Ahonen and Tuovinen, 1994) according to

an acid-consuming reaction:

Fe1 xS þ 1=2ð1 xÞO2 þ 2ð1 xÞHþ Z ð1 xÞFe2þ

þ S0 þ ð1 xÞH2O ð10Þ

Fe1 xS þ ð2 2 xÞFe3þ Z ð3 3 xÞFe2þ þ S0 ð11Þ

Steger and Desjardins (1978) report the predomi-

nant oxidation products of pyrrhotite to be goethite

and elemental sulphur (reactions (8), (10) and (11)) as

well as smaller amounts of ferric sulphate and various

sulphooxyanions. Using XPS, Buckley and Woods

(1985a) show that the exposure of pyrrhotite to air

leads to the consecutive formation of iron(II) oxide, an

iron(III) hydroxy-oxide or hydrated iron(III) oxide.

This is confirmed by Fe(2p) and Fe(3p) spectra that

indicate the diffusion of iron from the outermost

layers of the mineral lattices to form the observed

oxidation products. Based on an S(2p) spectrum,

Buckley and Woods (1985b) detect the presence of

elemental sulphur together with sulphate and iron-

deficient sulphide when a hydrogen peroxide solution

was used as an oxidant. They suggest that the oxida-tion of pyrrhotite proceeds via a series of iron-defi-

cient sulphides and possibly polysulphides through to

elemental sulphur, these steps being acid-consuming.

Metastable intermediates of sulphate including thio-

sulphate (Steger and Desjardins, 1977; 1978), poly-

thionate and S4O6

2 are also reported (Plysunin et al.,

1990; Mikhlin et al., 2002), the presence of these

intermediates being likely related to the conditions of

oxidation. The suggested mechanisms will be dis-

cussed later.

The biological leaching of pyrrhotite by Thioba-cillus ferrooxidans leads to an acid-consuming step

with the formation of elemental sulphur, followed by

an acid-producing phase with the formation of K-

jarosite [KFe3(SO4)2(OH)6], goethite (a-FeOOH), and

schwertmannite [(Fe8O8(OH)6SO4] as solid products

(Bhatti et al., 1993). The evolution of H2S gas could

be noticed during the initial stages of pyrrhotite

bioleaching (Ahonen and Tuovinen, 1994), likely

indicative of a non-oxidative dissolution process as

shown by reaction (1). Elemental sulphur could be




detected with pyrrhotite dissolution in t he presence or

absence of bacteria (Bhatti et al., 1994). The presence

of jarosite and iron oxyhydroxides occurring as goe-

thite with traces of ferrihydrite was confirmed in anOntario mine tailings impoundment (Johnson et al.,

2000). Wetting–drying treatment of synthetic hexag-

onal pyrrhotite results in the formation of a-FeOOH

(goethite) and elemental sulphur on the surface of

mineral grains as confirmed by diffuse reflectance

FTIR (Kalinkin et al., 2000). A more detailed dis-

cussion of the mechanism that could explain the

oxidation and the occurrence of oxidation products

is presented in Section 4.

3.1. Factors affecting oxidation

As suggested by reactions (6) to (11), the oxidation

of pyrrhotite is affected by several factors that deter-

mine the rate of the process. Some of the factors

presented below can play a determining role in the

nature of the oxidation products.

3.1.1. Crystal structure

Various studies indicate that specific surface val-

ues of various size fractions of museum-grade pyr-

rhotite samples (determined by BET surface area

analysis) are f2 –10 times higher than those ob-served for crystalline pyrite and 6–40 times greater

than the theoretical values based on spherical geom-

etry (Kwong, 1995; Janzen et al., 2000). This differ-

ence can partially be attributed to surface roughness

and extended fractures into pyrrhotite grains. The

modification of mineral structures due to the pres-

sures and stresses associated with the milling of ore

can increase the reactivity of pyrrhotite in mine

tailings and its susceptibility to oxidation processes

(Pratt et al., 1996). Compared to pyrite, 20– 100

times faster oxidation rate of pyrrhotite can also berelated to lower crystal symmetry due to the vacancy

of iron atoms in the crystal structure (Nicholson and

Scharer, 1994). Although it has been suggested that

the crystal structure of pyrrhotite can affect its

oxidation rate (Yakhontova et al., 1983; Orlova et

al., 1989), the results of the study by Janzen et al.

(2000) report no apparent correlation between pyr-

rhotite oxidation by oxygen or ferric iron and crystal

structure. More recent results from our research group

suggest that monoclinic pyrrhotite could be less

reactive than the hexagonal form (Belzile et al.,

unpublished).

3.1.2. Oxygen and moistureReaction (1) indicates the required presence of

water when pyrrhotite is oxidized by oxygen (air).

The effect of humidity was studied by Steger (1982)

and his results indicate a direct increase of the

oxidation rate with increasing relative humidit y (Table

1). A study using XPS (Knipe et al., 1995) confirm

that oxygen is the primary oxidant as no evidence of

oxidation could be shown when pyrrhotite is exposed

to deoxygenated water. No systematic studies relate

the effect of oxygen content to pyrrhotite oxidation

but the oxidation rate is expected to be related to the

concentration or partial pressure of O2. The rate is

shown to be proportional to the square root of the O2

partial pressure for pyrite by McKibben and Barnes

(1986) and Williamson and Rimstidt (1994) in Eqs.

(12) and (13), respectively, but different coefficients

for pyrrhotite are expected:

r ¼ 106:77½O20:5 ð12Þ

r ¼ 10

8:19

½O2

0:5

=½H

þ

0:11

ð13Þ

Rates are expressed in mol of pyrite cm 2 min 1 for

Eq. (12) and in mol m 2 s 1 for Eq. (13).

3.1.3. Ferric iron

Oxidation rates are significantly increased when

Fe3 + is present (Janzen, 1996; Janzen et al., 2000).

The oxidation of pyrrhotite by ferric iron differs under

oxic and anoxic conditions ranging from zero order

for anoxic to half order under oxic conditions

(Kwong, 1995). In the presence of oxygen, the reac-tion was first order with respect to Fe3 + concentration.

The same author reports both molecular oxygen and

ferric iron to be inefficient in oxidizing sulphide

sulphur to sulphate. A linear effect of ferric iron

concentration expressed on a log scale could be

observed on the reaction rates also expressed as log

values (Janzen et al., 2000). They suggest an adsorp-

tion-type mechanism with a Freundlich- or Langmuir-

type isotherm similar to that proposed by Nicholson et

al. (1988) and Williamson and Rimstidt (1994) or the




Table 1

Oxidation rate and activation energy of pyrrhotite

Samples Oxidant Conditions Rate (mol m 2 s 1)

based on the release of

Activation energy

(kJ mol 1)

Reference

Iron Sulphate

Monoclinic

(Falconbridge, Ontario)

Air pH not given,

68% r.h.a , 52 jC

1.5 10 8 6.0 10

10 Steger and

Desjardins, 1978 b

Monoclinic

(Wards Scientific)

Air pH not given,

62% r.h.

Steger, 1982 b

28 jC 3.9 10 9 6.5 10 10

35 jC 5.0 10 9 7.1 10 10

43 jC 6.3 10 9 7.8 10 10

50 jC 8.9 10 9 8.4 10 10

50 jC

37% r.h. 3.2 10 9 8.0 10 10

50% r.h. 4.4 10 9 9.1 10 10

55% r.h. 5.2 10 9 9.1 10 10

75% r.h. 1.1 10 8 9.0 10 10

Monoclinic and

hexagonal

Eletrochemical pH= 1,

25–80 jC

50.21 (mono),

46.23 (hexa),

34.48 ([Fe] = 0.54 M),

22.97 ([Fe] = 0.90 M)

Orlova et al., 1989

Museum-grade Air flow reactor pH = 2.0–6.0 Nicholson and

10 jC 3.1 10 9 58.1 (pH = 2) Scharer, 1994c

22 jC 8.5 10 9 52.4 (pH = 4)

33 jC 3.3 10 8 100.4 (pH = 6)

Museum-grade Fe3 +f 10 3 M 22 jC, pH= 2.0 1.44 10 8 2.05 10 9 same as Nicholson Kwong, 1995

samples from Air flow reactor A biotic 22 jC and Scharer, 1994

N. America pH = 2.0 2.28 10 9 8.51 10 10

Abiotic 35 jC

pH = 2.0 2.87 10 9

pH = 3.0 3.86 10 9

pH = 4.0 2.51 10 9

Biotic 22 jC 6.74 10 9 6.97 10 9 13.3

Biotic 35 jC

pH = 2.0 8.76 10 9

pH = 4.0 6.91 10 9

Several locations Air flow reactor pH = 2.75;

25 jC

4 10 9 2 10 10 47.7–62.5d,

79.1–106.0e

Janzen et al., 2000

in N. America

(12 samples) Fe3 + pH = 2.75;

Fe3 + = 0.2 mM

3.5 10 8 22.8–63.0d

pH = 2.50;

Fe3 + = 0.2 mM

3.1 10 8

pH = 2.50;

Fe3 + =1.0 M

6.8 10 8

Pyrrhotite ore, Bolivia Chemical and

bioleaching

pH = 2.0

30 jC

Fe3 +

0– 200 mM

f 100 Beolchini and Veglio,

1999; Veglio et al., 2000

a r.h. for relative humidity. b Estimated mean using an average particle specific surface area from Janzen et al. (2000).c Average value of oxidation rates.d Based on iron release.e Based on sulfate release.




dual-site adsorption model of Zheng et al. (1986) for

pyrite oxidation. The rate limiting sequence is electron

transfer reactions from ferric iron to an activated

complex of Fe3 +

, decomposition to a Fe2 +

activatedcomplex, decomposition to a Fe2 + ion and desorption

of the ferrous ion from the surface. Janzen (1996)

proposes a multi-solute Fritz and Schluender (1974)

sorption isotherm to describe the dissolution of pyr-

rhotite by ferric iron (Eq. (14)):

r Feð1 xÞS ¼ k 1½Fe3þ þ k 2 10a pH

1 þ k 1½Fe3þ þ k 2 10a pH þ k 3½Fe2þ1=2

ð14Þ

where k represents adsorption equilibrium constants

and a the activity.

With either mechanism, the apparent rate of solid

dissolution at low Fe3 + concentration and constant

Fe2 + and pH level is close to half order with respect to

Fe3 +. At an Fe3 + concentration of 0.2 mM, the author

reports fractional order of dependence ranging from

0.45 to 0.66 but at a higher initial Fe3 + concentration

of 10 mM, a zero order dependence on ferric concen-

tration was observed suggesting a more complex rate

dependence.

Recent studies (Beolchini and Veglio, 1999; Veglioet al., 2000) on the chemical leaching of pyrrhotite ore

by ferric iron show that sulphur present in pyrrhotite is

only partially oxidized to elemental sulphur. The

authors also demonstrate that the process kinetics

are controlled by the chemical reaction with an

activation energy of 99 kJ mol 1 and they present a

mathematical model which takes into account both

direct (bacteria as the primary oxidant) and indirect

(bacteria regenerating ferric iron through the oxidation

of Fe2 +) mechanisms to describe the kinetics as a

differential equation of the following type (Beolchiniand Veglio, 1999; Veglio et al., 2000):

da

dt ¼ 1440:0 104e

99 103

R

1

T

1

298

þ 27 103

a

RT

C Fe3þ

1000

0:47

ð1 aÞ2=3 ð15Þ

where a is the pyrrhotite conversion, t the time (days),

C Fe3+ the concentration of ferric iron in solution (mol

m 3), R the universal gas constant (J mol 1 K 1),

and T the temperature (K).

3.1.4. pH Very few studies mention the pH effect although it

appears clearly that low pH is required to prevent the

precipitation of Fe3 + and maintain the activity of the

Thiobacilli-type acidophilic bacteria. Nicholson and

Scharer (1994) report an inconsistent effect of pH

between 2.0 and 6.0 at three different temperatures. A

similar observation is made by Kwong (1995) who

does not observe a real trend in the abiotic oxidation

of pyrrhotite associated to pH and the major pH effect

being on the control of bacterial activity below 4.5. A

study of the pH influence on oxidation of pyrrhotite

by ferric iron also shows inconsistent results and

suggests a competing effect of the proton with Fe3 +

and possibly Fe2 + for adsorption sites on the surface

(Janzen, 1996) as suggested by Eq. (14) representing a

model for the reaction rate of oxidation. As it has been

mentioned in the introduction, the non-oxidative dis-

solution of pyrrhotite can occur at low pH and

liberates H2S.

3.1.5. Temperature

A significant increase in the oxidation rate of

pyrr hotite with increasing temperature is presented by Steger (1982); the effect is more obvious when

the rate is calculated on the basis of iron release

(Table 1). The effect of temperature is reported to be

more significant than that of pH with the reaction

rate doubling between 25 and 35 jC (Kwong, 1995)

or increasing by 3–5 times for a 20 jC increase with

oxygen as oxidant and by 2–11 times for a 30 jC

increase with ferric iron as the oxidant (Janzen,

1996). In both studies, the oxidation process follows

Arrhenius behaviour. Abiotic activation energy be-

tween 58.1 kJ mol

1

(pH = 2.0) and 100.4 kJ mol

1

(pH = 6.0) and biotic activation energy at pH 2.0 of

only 13.3 kJ mol 1 are reported (Kwong, 1995).

Activation energies for pyrrhotite oxidation by oxy-

gen range from 48 to 63 kJ mol 1 based on iron

release and from 79 to 124 kJ mol 1 based on

sulphate release (Janzen, 1996; Janzen et al., 2000).

Similar values are presented by the same authors for

oxidation by ferric iron (Table 1). For comparison,

values of activation energy originating from various

studies on the order of 50–80 kJ mol 1 are reported




by Williamson and Rimstidt (1994) and a tempera-

ture dependence following Arrhenius behaviour with

an activation energy of 88 kJ mol 1 is given in

Nicholson et al. (1988). A study on the oxidation of sulphide minerals at high temperature (120 jC) and

high oxygen pressure (855 kPa) showed that pyrrho-

tite could oxidize much more rapidly at pH 2.7 than

all the other sulphides considered in the study

(Majima and Peters, 1966). The same study indicates

that the oxidation of pyrrhotite is considerably re-

duced at neutral and alkaline pH values. Steger

(1982) also reports that an increase in temperature

can enhance the rate of O2 diffusion and therefore

the formation of ferric oxide and ferric sulphate

products. It can be speculated that the mechanism

of ferric oxide formation as well as the mineralogical

composition of these oxides will be different under

different temperatures.

3.1.6. Trace metal content

A trend of decreasing reaction rat es with increas-

ing trace metal content is noticed (Kwong, 1995;

Janzen et al., 2000) but no statistically significant

effect on the oxidation rate can be established for

individual trace metal (Co, Cu, Mn, Ni) or total trace

metal content. More research needs to be done to

clarify the effect of trace metal on the oxidation of pyrrhotite.

3.1.7. Bacteria

Thiobacilli can oxidize ferrous to ferric iron and

inorganic sulphur compounds to sulphuric acid

(Suzuki, 1974; Suzuki et al., 1994). The autotrophic

bacteria T. ferrooxidans play a catalytic role in the

oxidative dissolution of sulphide minerals and the

effect is generally attributed to the increased supply

of Fe3 + in solution resulting from bacterial activity

(Nordstrom and Southam, 1997). The essential role of bacteria in the oxidation of pyrite and pyrrhotite,

especially in coal, is well established and the bacterial

oxidation of pyrrhotite is more effective than that of

pyrite (Pinka, 1991). The biotic oxidation of pyrrho-

tite appears to be three times faster than the abiotic

one at pH 2 and 4 (Kwong, 1995). It seems more

difficult to put an exact number on the influence of

bacteria on pyrite oxidation but most studies refer to a

significant catalytic effect of bacteria (Nordstrom and

Southam, 1997). Thiobacilli are most active in tem-

peratures ranging from 20 to 55 jC and T. ferroox-

idans is the dominant organism at temperatures below

40 jC. Most strains have optimum growth at 25–35

jC (Tuovinen and Kelly, 1972) with some Canadian

strains showing optimum strength at 20 jC (Mason

and Rice, 2002). T. ferrooxidans is most active in the

pH range 1.0 – 2.5 deriving its energy from redox

reactions where Fe2 + or reduced sulphur compounds

serve as electron donor and oxygen as electron

acceptor.

The intermediate steps of oxidation (from S2 to

S, SO3

2 and SO4

2 including thionates and poly-

thionates) along with the enzymes responsible for the

individual reactions are presented by Suzuki et al.

(1994). Other microorganisms are associated with the

oxidation of sulphides and the generation of acid

mine drainage (Gould et al., 1994; Benner et al.,

2000).

4. Oxidation mechanisms

A relatively abundant literature exists on the

mechanisms involved during the alteration or oxida-

tion of the pyrrhotite surface. Information has been

obtained using X-ray techniques such as electron

(XES), photoelectron (XPS) and Auger electronspectroscopy (AES), X-ray powder diffraction

(XRD) as well as from Mossbauer spectroscopy

and secondary electron microscopy (SEM). Although

a clear mechanism has yet to be defined, many

studies agree on a progressive enrichment of the

surface with respect to sulphur when pyrrhotite is

oxidized. Taylor (1970) and Taylor and Mao (1971)

use XRD and optical microscopy to detect the

conversion of pyrrhotite into a more S-rich phase

named smythite (Fe3.25S4). The electrochemical ox-

idation of pyrrhotite at pH 4.6, 9.2 and 13.0 produ-ces mainly sulphur and significant quantities of

sulphate in the alkaline solutions and the oxidation

of pyrrhotite is strongly inhibited under alkaline

conditions due to the passivation of the surface by

ferric oxides (Hamilton and Woods, 1981).

In an oxidation study of pyrrhotite by air using

XPS, Buckley and Woods (1985a) propose the diffu-

sion of iron from the outermost layers of the solid

lattice to form, via an iron(II) oxide, an iron(III)

hydroxyl-oxide or hydrated oxide at the air/solid




interface based on the interpretation of Fe(2p), Fe(3p)

and S(2p) spectra.

4Fe1 xS þ 3 yO2 þ 2 yH2O Z 2 yFe2O3 H2O

ðor 4 yFeOOHÞ þ 4Fe1 x yS

with y < (1 x).

When a H2O2 solution is used, the presence of

elemental sulphur and sulphate can be detected using

XPS (Buckley and Woods, 1985b). Further studies

using voltammetry and XPS show that the initial

oxidation products for pyrrhotite and pyrite oxidation

include metal-deficient sulphides rather than elemen-

tal sulphur (Buckley et al., 1988). The presence of

sulphate species, iron(III) oxide/hydroxides and iron-

deficient sulphide species including a tetragonal Fe2S3

reaction product is also noticed when ground pyrrho-

tite is oxidized by air and water as confirmed by XPS

and XRD (Jones et al., 1992). An extensive study of

the mechanism of air oxidation based on XPS and

AES (Pratt et al., 1994a) identifies for the first time

Fe(III) bonded to sulphur in fresh pyrrhotite where the

latter is present as monosulphide (S2 ), along with

minor amounts of disulphide (S2

2 ) and polysulphide

(Sn

2 ). After 6.5 h of air exposure, Fe(III) is bonded to

oxygen and most of the remaining Fe(II) remains bonded to sulphur with some of the initial monosul-

phide being converted into a range of compounds

including sulphite, elemental sulphur, polysulphides

and predominantly disulphides. The authors propose a

sequential layer sputtering (SLS) model with whichAES depth profiles can be calculated. After 50 h of air

oxidation, the model suggests an oxygen-rich and

sulphur-depleted outermost layer of less than 10 A

(1 nm), covering an Fe-deficient, S-rich layer display-

ing a continuous and gradual decrease in S/Fe from

the outer layer to the unaltered inner layers of pyr-

rhotite with a sequential composition of FeO1.5, FeS2,

Fe2S3 and Fe7S8 (Fig. 1). They propose a mechanism

where molecular oxygen is adsorbed onto the surface

of pyrrhotite and reduced to O2 and reacts with

Fe(III) bonded to sulphur to form Fe(III)–O bonds

and Fe(III) – oxyhydroxides. The establishment of

subsequent ferric oxyhydroxide layers is realized

through the diffusion of Fe from pyrrhotite, which

causes a depletion of iron in the zones immediately

below the oxyhydroxide layer and a rise of S/Fe

ratios. A similar chemical stratification can also be

obtained when pyrrhotite is oxidized in sulphuric acid

solutions at pH 3.0 (Pratt et al., 1994b).

The model pro posed by Pratt et al. (1994a) was

further refined by Mycroft et al. (1995) who confirm

by angle-resolved XPS that ferric oxyhydroxides

requiring the presence of water are the only speciesto form during the initial stages of air oxidation of

Fig. 1. Model illustrating the sequence of oxidation products at the surface of pyrrhotite.




pyrrhotite. The air oxidation of pyrrhotite forces Fe to

diffuse from the interior to the surface and to combine

with oxygen. This removal of Fe results in an S

enrichment in the zone below and promotes theformation of disulphide bonds and the reorganization

of the pyrrhotite structure towards a marcasite (FeS2)

structure. This last step requires oxidation of mono-

sulphide ions to disulphides and polysulphides. An

increase in S–S bonding is also observed after the

oxidation of pyrrhotite when using time of flight-

secondary ion mass spectrometry ToF-SIMS (Smart

et al., 2000). In another study based on XPS, Knipe et

al. (1995) show no evidence of oxidation when

pyrrhotite and pyrite are exposed to deoxygenated

water.

The formation of a non-stoichiometric non-equi-

librium or metastable layer (NL) as a result of

preferential release of iron relative to sulphur in the

oxidation or dissolution of pyrr hotite has been further

studied by Russian researchers (Mikhlin, 2000; Mikh-

lin et al., 1998, 2000, 2001, 2002; Kuklinskii et al.,

2001) through a combination of spectroscopic tech-

niques (XRD, FTIR, XPS, XES, EPR, Mossbauer,

etc). These studies provide more information on the

composition and reactivity of the metal-deficient

layer and on the mechanisms involved in the complex

transformations of sulphide species (formation of elemental sulphur and sulphoxy compounds) and

alterations of the oxidation and spin state of iron

(Mikhlin et al., 2002). The oxidation kinetics and

products depend on the environment humidity and on

the crystalline structure of pyrrhotite. The same

authors suggest that the partially oxidized, disordered

NL may be passive.

5. Prevention and remediation

Pyrrhotite is oxidized by O2 and the oxidation

products are generally protons, ferrous ions, sulphate

ions and free sulphur, the oxidation to elemental

sulphur being acid-consuming. Several engineered

devices have been proposed for the remediation and

prevention of acid drainage and metal solubilization.

One logical approach consists in preventing or limit-

ing the contact between pyrrhotite and oxygen (air). It

can be done by applying different types of dry and

organic covers (clay, soil, peat, hay, straw, sawdust,

sludge or compost) that could prevent the diffusion of

oxygen and also maintain some reducing conditions

when organic amendments are used (e.g. Nicholson et

al., 1989; Blowes et al., 1994; Belzile et al., 1997a ).Wet covers are also efficient in reducing the oxidation

of sulphidic wastes because of the lower concentration

and lower diffusion rate of oxygen in water compared

to those in air (Belzile et al., 1997a). For preventing

AMD, a technology by which sulphidic minerals are

coated with ferric phosphate in presence of H2O2 can

be used to form a stable mineral at the surface of

pyrrhotite (Evangelou, 1994; Georgopoulou et al.,

1996). Natural phosphate rock can be used and a

biofilm induced by the presence of microbes feeding

on phosphate can be formed (Ueshima et al., 2003).

Fe1 xS þ 3:5H2O2 þ ð1 xÞH2PO4

! ð1 xÞFePO4 þ SO24 þ ð3 xÞHþ þ 3H2O

ð17Þ

This iron phosphate coating can control the oxi-

dation process of pyrrhotite by stabilizing pH around

4 and by reducing iron generation. The long-term

resistance of coating agent remains to be tested.

Pyrrhotite can be stabilized in combination with a

binder (such as cement), neutralizers, bactericides or surfactants to form agglomerates (Amaratunga, 1991;

Hmidi and Amaratunga, 1998). It is also suspected

that the oxidation of pyrrhotite could be partially or

totally inhibited through surface treatments (coating

agents) similar to those suggested for pyrite (Lalvani

et al., 1990, 1991; Belzile et al., 1997b; Lan et al.,

2002).

6. Concluding remarks

The oxidation of pyrrhotite depends on several

biotic and abiotic factors. Most studies confirm the

importance of surficial and adsorptive reactions in the

oxidation mechanisms, which could explain why

crystal structure and surface defects, the presence of

catalysts such as bacteria, and to a lesser extent,

temperature can be considered as the most important

factors in pyrrhotite oxidation studies. Two of these

factors are indirectly controlled by pH as the presence

of high Fe3 + concentrations and acidophilic bacteria




both require low pH environments. Taking into ac-

count the variable nature of pyrrhotite compounds and

the various experimental designs, it is not surprising

to find variable results for oxidation rates and prod-ucts of oxidation. However, the few studies reporting

oxidation rate of pyrrhotite seem to agree on values on

the order of 10 8 – 10 9 mol m 2 s 1 (Table 1),

generally based on iron release. However, a reliable

rate law similar to those presented for pyrite still needs

to be defined. The oxidation rates based on sulphate

production clearly indicate an incomplete oxidation as

elemental sulphur and sulphoxy intermediates are

formed and detected. Since oxidation to polysulphides

and elemental sulphur is acid-consuming, this repre-

sents a major difference to pyrite oxidation which

produces sulphate and is always acid producing. In

recent years, the combination of X-ray (e.g. XRD) and

spectroscopic (e.g. XPS, FTIR) techniques has been

particularly useful in improving our understanding the

mechanism of oxidation at the molecular level and the

chemical bonds formed at the surface of oxidized

pyrrhotite. More systematic studies are required to

better understand and control the oxidation of this

reactive mineral so as to improve both mineral ex-

traction and environmental protection.

Acknowledgements

Financial support from the Natural Sciences and

Engineering Research Council of Canada and from the

Materials and Manufacturing Ontario Centre of

Excellence is acknowledged. Constructive comments

from J.E. Thomas and R.B. Herbert Jr. greatly

improved the paper.

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