Efecto del Cl y Fe+3 en FeS2

8/9/2019 Efecto del Cl y Fe+3 en FeS2

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Pergamon

Geochimica et Cosmochimica Acta. Vol. 59, No. 15, pp. 3155-3158, 1995

Copyright 0 1995 Elsevier cience Ltd

Printed n the USA.

All rights reserved

00 I6-7037/95 9.50 + . I0

0016-7037 95)00203-O

Confirmation of a sulfur-rich layer on pyrite after oxidative dissolution

by Fe II1) ions around pH 2

KEIKO SASAKI,

MASAMI TSUNEKAWA,

TOSHIAKI

OHTSUK A

and

HIDETAKA

KONNO

Faculty of Engineering, Hokkaido University, Sapporo 060, Japan

*Department of Applied Chemistry, Nagoya Institute of Technology, Nagoya 466, Japan

(Received December 29, 1994; accepted in revised form Apri l 24, 1995)

Abstract-The

stoichiometry of pyrite dissolution by Fe( III) ions was studied in chloride media aroun d

pH 2. Pyrite wa s found to dissolve nonstoichiometrically during th e initial tens of hours and a S-rich layer

was formed on the pyrite due to preferential dissolution of iron. The major constituent of the layer was

elemental S, identified by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

1. INTRODUCTION

Pyrite is abundant in nature and the most common sulfide

mineral. The oxidation of pyrite in aqueou s systems is closely

related to environmental and geological problem s, like the

formation of acid mine drainage, th e supergen e alteration o f

ore deposits, and others. The important oxidants in the natural

environment are Fe(II1) ions and oxygen, the forme r being

more reactive than the latter (e.g., McKibben and Barnes,

1986 ). Th e overall reaction of pyrite w ith the Fe(II1) ion is

conventionally expre ssed as:

Fe & + 1 4Fe+ + 8H20 = 15 Fe*+ + 2SOi- + 16 Hf.

(1)

The kinetics for the above reaction around p H 2 have been

studied by measuring: (a) the total dissolved Fe ions and

redox potential, E (Smith and Shumate, 1970; Rimstidt and

Newcomb , 1993); (b) Fe( II) ions (McKibben and Barnes,

1986);

(c)

Fe( III) ions and E (Wiersm a and Rimstidt, 1984) ;

(d) Fe( III) ions (Mathew s and Robbins, 1972) ; (e) E (Gar-

rels and Thompson, 1960), and (f) dissolved S species

(Moses et al., 1987). Only a few papers presented experi-

mental dissolution curves fo r both Fe and S species measu red

simultaneously. The above dissolution schem e, Eqn. 1, is gen-

erally acce pted as correct, but it has not been possible to locate

stoichiometric dissolution data. Some results suggest nonstoi-

chiometric dissolution of pyrite on the basis of surface anal-

ysis (Buckley and Woods, 1987) or electrochemical mea-

surements (My croft et al., 1990), and an analysis of the dis-

solution data reported by McKibben and Barnes ( 1986) may

also suggest nonstoichiometric dissolution. In a previous pa -

per, we repor ted that during nonstoichiometric pyrite disso-

lution by oxygen, Fe species dissolved preferentially leaving

S species on the surface (Sasaki, 1994).

To understand better the mechanism of pyrite oxidation, it

is important to know w hether py rite dissolves stoichiometri-

tally or nonstoichiometrically (Luther , 1987; Mo ses et al.,

1987 ). In the present w ork, oxidative dissolution of pyrite by

Fe( III) ions in chloride solutions is investigated. The stoichi-

ometry is discussed based on dissolution data for Fe and S

species measu red simultaneously and the results of XP S and

Raman spectroscopy.

2.1. Pyrite

2.

EXPERIMENTAL

Pyrite was supplied from the Yanahara mines (Japan) and its com-

position was listed in Table 1. The major impurity is SiOz and its

3155

mass percent was estimated to be around 0.25 in the sample. The

pyrite was ground using a tungsten carbide planetary-type ball mill

(Fritsch Co. Ltd., P-5) and sieved. The 200-400 mesh size fraction

was used in the experiments. The X-ray powder diffraction lines were

measured with NaCl as the standard and were found to be identical

with that of FeS, (Sasaki et al., 1994a). There were no diffraction

lines other than FeS, except a very weak single line of SiO* (d

= 0.338 nm), which took no part in the dissolution experiments (Sa-

saki, 1994).

The samples were pretreated to establish a well-defined surface of

stoichiometric composition. Oxidized surface species were removed

according to a previous method (McKibben and Barnes, 1986; Sasaki

et al., 1994b): the pyrite was ultrasonically cleaned in ethanol for 30 s

with adhering mineral powder decanted, and then the pyrite was

rinsed with 1 mol/L HNO, for one mitt, followed by rinsing in triply

distilled water, before dehydrating in acetone, and finally it was vac-

uum-dried using an aspirator. The pretreated surface was compared

with a cleaved one and reproducibly verified to be stoichiometric by

XPS (Sasaki et al., 1994b).

The specific surface area of the pretreated pyrite was determined

by the NZ gas adsorption BET method on a Quantasorb QS- 13 (Yuasa

Ionics Co. Ltd.) with a QS-300 cell for low value measurements.

Each result was in good agreement with the data obtained by the Nz

gas adsorption seven points BET method on an Autosorb AS-l

(Yuasa Ionics Co. Ltd.). The average of twelve experiments was 0.40

? 0.060 (~frlcr,,, n = 12) mlg.

2.2.

Dissolution Experiments

Dissolution experiments were carried out using a batch reactor

with an impeller and a G2 glass ball filter for Nz bubbling. The so-

lution was sufficiently stirred to suspend pyrite particles. All assem-

blies were made of glass and rinsed with a diluted HNO? solution

before use to avoid Fe contamination. As the oxidant, 15 mmol/L

FeCl

6Hz0 was used. The initial pH was adjusted to 2.0 with super

special grade (SSG) HCl. To avoid comnlications bv a two oxidant

.

system,dissolved oxygen was removed to the extent that the oxida-

tion of pyrite and dissolved Fe(B) ions by dissolved oxygen was

insignificant. Prior to each dissolution experiment, 200 cm of each

solution was de-aerated by bubbling Nz gas (299.999 ) for 1 h: the

dissolved oxygen in the de-aerated solution was under 1 ppm. Then,

2.00 g of the just pretreated pyrite powder was added to the solution.

The dissolution experiment was carried out for 72 h under Nz bub-

bling at room temperature (not controlled). Solution pH was also not

controlled, since pH does not affect the dissolution rate in the range

between 0 and 2 (Wiersma and Rimstidt, 1984). At appropriate in-

tervals, 1 cm of solution was sampled and filtrated with a 0.20 pm

pore membrane filter for solution analysis, and pH and E measure-

ments. The total Fe and Fe(B) concentrations were determined by

the 1, IO-phenanthroline method and the total S concentration by

ICP-AES (SEIKO Co. Ltd., SPS 1200). Dilution of the sampled

solution was with a 0.01 mol/L SSG HCl solution. After the disso-

lution experiment, the residue was separated by filtration and stored

in a Nz-purged desiccator for XPS and Raman spectroscopic analysis.


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3 56

K. Sasaki et al.

Table 1 Composition (mg gt) of the pyrite from the Yanahara mines (Japan)*

Na Al

Si P K Ca h4n Fe Cu Zn

Unkown residues

0.046 1.38 12.0 0 .806 1.05 1.36

0.055 458 2.10 2.22 520.983

* Owina to the decomposition of ovrite. an accurate value of sulfur was not obtained

(Naka&ra, 1991). - -

2.3. XPS

Measurement

The chemical species on the surface of the pyrite were analyzed

by XPS, with an ESCA LAB MkII (V. G. Scientific). The samples

were pressed o nto Cu foil on a holder and introduced into the spec-

trometer. After evacuating to better than 10 - Pa within 15 min, the

sample was transferred into an analyzer chamb er of better than 5

X lo- Pa and cooled to below 150 K, then irradiated with Al Ka

X-ray ( 15 kV, 10 mA). The binding energies, Ear were calibrated

with &[Au 4hi2] = 84.0 eV. Intensity by area was measured after

drawing the background by the Shirley method ( 1972 ). Details of

the data analysis are described elsewhere (Konno et al., 199 I ),

2.4. Raman Spectroscopy

For Raman spectroscopy, excitation w as accomplished by a single

line 488. 0 nm wavelength light from an Ar ion laser. The power of

incidence was about 50 mW at the sample point. The Raman scattered

light was detected by a triple type monochromator (JASCO R-NO T)

equipped with a photomultiplier (Hama matsu R-464) and a photon

counter. Th e band path of the monochromator was about 8 cm- The

pyrite sample was diluted to 5 wt% with KBr powder and 0.30 g of

the mixture was compressed to form a 10 mm ~#~isk for measurement

by a rotating a pparatus. There is a possibility that heating by laser

irradiation causes some change in spectral quality, so that the above

measures were taken. The laser light was polarized parallel to the

plane of incidence and the incidence angle was 80 degrees. The Ra-

man scattered light was collected in the incidence plane in the direc-

tion normal to the incident laser light.

3. RESULTS

AND DISCUSSION

Typical dissolution curves of pyrite in 0.01 mol/L HCl

containing 15 mmo l/L Fe( III) ions are shown in Fig. 1 . From

the net amount of dissolved iron in Fig. 1, the average thick-

ness of the pyrite dissolved is calculated to be 6.2 nm. It

corresponds to an average of more than 10 lattice layers of

pyrite crystal, thoug h uniform dissolution actually does not

take place in acid solutions (M cKibben and Barnes, 1986 ).

The total amount of species

i

in the solution per unit initial

surface area of pyrite at time

t

is expressed as xi

(t)

in mm01

mm*, where

i

indicates S, Fe( II), or Fe( III). Assuming that

pyrite is dissolved with Fe( III) ions accordin g to Eqn. 1, the

_

20 40 60 80

Time,

t /

h

FIG. 1. Dissolution curves of pyrite by Fe(II1) ions in a HCI soh-

tion at pH 2.0.

stoichiometry at t, n(t)

= xs ( t) lxFe t)

, can be exp ressed the-

oretically using x~~;,(, (

t)

and the initial amount of Fe( III) ions

per unit initial surface area of pyrite, xFe(rll( 0), as:

nthco(f) = xS(t)kk,Ill)(f) + - c(ll,(t))

= 2xFe~l~(~)~o/(1~xFe~ll~

(0) + -+edt)). (2)

In Fig. 2, the theoretical stoichiometric ratio, &w (t), is com-

pared with the observed stoichiometric ratio, nabs(t), obtained

from the experimental data, showing that nthco(

)

is larger than

nobs( ) , and that the difference between &h._(t) and noha t)

increases with time. This indicates that Fe dissolves more eas-

ily than sulfur from pyrite. The values of &.o( t) can also be

calculated by using x,(t) or xFe

t)

, and though the values are

less accurate than with

+(,,)(t),

the results were similar to

Fig. 2.

The XP-spectra for (a) the cleaved pyrite, (b) the pre-

treated pyrite, and (c) the sample after dissolution experiment

are shown in Figs. 3 and 4. The surface com position, [Fe],/

[S], mole ratios, was calculated according to Eqn. 3:

[FeldlISh = (kJ~Fe)I(~s~ss),

(3)

whe re s is a relative sensitivity factor including an escap e

depth of electrons, and

I

an intensity by area in which contri-

bution from X-ray satellite peaks was subtracted by computer

calculation. The relative sensitivity factors were experimen-

tally determined and ss/sFc =

0.14 + 0.00, ( + 1c)

;

the relative

standard deviation is 5% (Konno et al., 1991; Sasaki et al.,

199413). The compositions, [Feh/[ S],, of both the cleaved

surface and pretreated one were determined to be 0.50. After

72 hour dissolution by Fe( III) ions, the Fe 2p spectrum was

different from that of pure pyrite as shown in Fig. 4, where a

broken line indicates th e envelope for pure pyrite. The S 2p

spectrum (c) was separated into three components, each com-

posed of a set of 2p,,, and 2p,,, peaks (Fig. 3). Pea k (I) is

assigned to pure pyrite at &[ S 2p,,,] = 162.2 eV, peak (II)

at Es[S 2~?,~] = 164.25 eV to elemental S, and peak (III) at

&[ S 2p,,,] = 166.4 eV to oxidized S species such as sulfite.

Time,

t /

h

FIG.

2. Theoretical and observed ratios, n(t) = n,(t)lx,,(t).


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Stoichiometry of pyrite dissolution 315 7

I

I

s 2P

L

L

a>

160 170

Binding energy, I?& eV

FIG. 3. The XP-spectra of S 2p for pyrite. Vertical bars indicate

500 cps. (a) cleaved pyrite, (b) pretreated pyrite and (c) after disso-

lution by Fe(III) ions. Component (I) is pyrite S, (II) elemental S,

and (III) oxidized S species.

The mole ratio, [Fe]r/ [ S], , after dissolution was determined

to be 0.34, excluding the oxidized components at peak (III)

in Fig. 3 and arrows in Fig. 4. It indicates that nonstoichiomet-

tic dissolution resulted in the sulfur-rich layer on the surface,

in agreement with the dissolution data. The accuracy of the

semiquantitative analysis by XPS is not very high, but the

difference established here is much larger than the relative

standard deviation of relative sensitivity factors, as indicated

above. Distinguishing among sulfide, elemental sulfur, and

polysulfide is difficult only by XP-sp ectra, as the binding en-

ergy,

EB

for pyrite is 162.2 -163.0 eV (e.g., Brian, 1980 ;

Konno et al., 1991; Peisert et al., 1994; Nesbitt and Muir,

1994), Es for elemental S is 163.7 -164.3 eV (e.g., Brion,

1980 ; Hyland and Bancroft, 1989 ; Peisert et al., 1994) , a nd

EB or polysulfides is 161.7- 163.4 eV (e.g., Buckley et al.,

1988).

Hamilton and Woods (1980) and Buckley and Woods

( 198 7) reported that a metal-deficient sulfide was formed by

electrochemical oxidation of sulfides in acidic solutions (pH

4.6), and polysulfide was detected by XPS, mass spectros-

copy, and voltammetry. Mycroft et al. (1990 ) reported that

after anodic oxidation at 700 mV (SCE) in near-neutral aque-

ous chloride solutions for more than one hour, polysulfide was

detected on pyrite by Rama n spectroscopy. The Rama n spec-

trum was measured to elucidate further the composition of the

S-rich layer. As shown in Fig. 5a, the Ram an spectrum of the

same sample with (c) in Figs. 3 and 4 has five Raman bands:

at 150, 220, 357, 387, and 470 cm-. The three of them at

150, 220, and 470 cm- are assigned to S-S vibration modes

Fe 2p

h

oxidized species

700

710

720

730

Binding energy, &I eV

FIG. 4. The XP-spectra of Fe 2p for pyrite. Vertical bars indicate

1 kcps. (a) cleaved pyrite, (b) pretreated pyrite and (c) after disso-

lution by Fe(II1) ions. A profile by dashed line in (c) is for pure pyrite.

of elemental S and the remaining two, at 353 and 387 cm-,

to vibration modes of pyrite, agreeing with previous studies

(Mycroft et al., 1990 ; Mem agh and Truda , 1993 ; Li et al.,

1993 ). According to Buckley and Woods ( 1987 ), elemental

S dissolves moderately in cyclohexane, but polysulfide does

not. After the sample was immersed in cyclohexane for five

minutes, the three bands at 150, 220, and 470 cm-, corre-

sponding to the vibration modes of the elemental S disap-

peared, as shown in Fig. 5b. This is an additional evidence

600

400

200

Raman shift, Av

I

cm-

FIG. 5. The Raman spectra for pyrite. A vertical bar indicates a

scattering light intensity of 1 kcps. (a) after dissolution by Fe(II1)

ions and (b) after immersion of the oxidized pyrite in cyclohexane

for five minutes.


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3158

K. Sasaki et al.

that the S-rich layer form ed on pyrite is composed of elemen-

tal S and not of polysulfide. There seems to be a weak Raman

band around 460 cm- in Fig. 5a, but it disappe ared after

immersion in cyclohexan e, as shown in Fig. 5b. It indicates

that the band was not due to polysulfide but a noise. Thu s, it

is concluded that the reaction of pyrite with Fe(III) ions in

chloride solutions around pH 2 proce eds non stoichiometri-

tally during t he initial tens of hours, resulting in the formation

of an elemental S-rich lay er on the pyrite.

As described above and also reported previously (Sasaki,

1994; Sasaki et al., 1994b), experimental results sho wed that

dissolved amounts of sulfur species were lacking O-50 %

compared with the stoichiometric values. According to the

oxidation mechanism of pyrite by Fe(II1) ions based on the

molecular orbital theory (Luthe r, 1987 ), the reaction is ini-

tiated by attack of Fe( III) ions to S s- and finally one pyrite

molecule is released as S,O:- and Fe+ ions. This, how ever,

is not consistent with the above results. F or the present, it is

uncertain that the prop osed m echanism can explain the overall

oxidation reaction of pyrite by Fe(II1) ions. Further investi-

gation is necessary considering recent reports on the detailed

band structure of Fe& and related sulfides (Sato, 1985; Tem-

merman et al., 1993).

In the natural environment, nonstoichiometric dissolution

is not considered to continue indefinitely. The formation of

an elemental S-rich layer may reduce the dissolution rate and

finally stop the dissolution, or oxidation of the S-rich layer

may occur by proce sses like those by S-oxidizing bacteria

such as Thiobacillus thiooxidans. It has been considere d that

pyrite weathering is enhanced mainly by Thiobacillus fer-

rooxidans, which can oxidize Fe(I1) ions in acidic environ-

ments. M uch attention has not been paid to T. thiooxidans,

since it mainly oxidizes elemental sulfur to sulfate but rarely

does sulfide to sulfate, with no ability to oxidize Fe( II) ions

to Fe(II1) ions. The present data, howeve r, su ggests th at T.

thiooxidans

might also play an important role in pyrite we ath-

ering in coexistence with T. errooxidans. Synergetic effect

by two species o f bacteria on the pyrite w eathering is an in-

teresting subject and under investigation in our laboratory.

Acknowledgments-We wish to express appreciation to Fritsch Japan

Co. Ltd. for carrying out the grinding experiments and to the Hok-

kaido Industrial Research Institute where the ICP-AES measurements

were carried out by courtesy of Mr. K. To&a. The authors also thank

reviewers, Dr. Kevin Rosso, Dr. Allen Pratt, and the anonymous third

person, for their critical and constructive review.

Editotial handling: M. F. Hochella Jr.

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Efecto del Cl y Fe+3 en FeS2