Transcript of Thermochemical reduction and sulfur isotopic behavior of ...
Thermochemical reduction
the presence of native sulfur
YASUHIRO KIYOSU' and H. Roy KROUSE2
Department of Earth and Planetary Sciences, Nagoya University,
Chikusa, Nagoya 464, Japan' and Department of Physics and
Astronomy, The University of Calgary, Calgary, Alberta T2N 1N4,
Canada2
(Received July 15, 1992; Accepted February 19, 1993)
Reaction between sodium sulfate and acetic acid under hydrothermal
conditions has been in
vestigated in order to demonstrate the thermochemical reduction of
seawater sulfate by organic matter
at moderate temperatures during diagenesis.
Although no reduction of sodium sulfate occurred directly, the
sulfate was reduced by acetic acid
to hydrogen sulfide in the presence of native sulfur over the
temperature range of 200-270°C. The sulfur
isotopic behavior of sulfur compounds suggests that the isotope
fractionations are not controlled by
kinetic isotope effects but rather by the isotope exchange reaction
between the sulfate and sulfide.
INTRODUCTION
In lithospheric sulfur cycles, it is of great im
portance that the mechanism of seawater sulfate reduction by
interactions between seawater and sediments or sedimentary rocks is
clarified. Dur ing sedimentation, the seawater sulfate is general
ly preserved as sulfide minerals such as pyrite through bacterial
reduction (e.g., see Kemp and Thode, 1968). However, during the
process of diagenesis, the possibility of thermochemical sulfate
reduction by organic material has been
proposed (Orr, 1974, 1977; Powell and Ma queen, 1984; Machell,
1987; Krouse et al., 1988; Tasse and Schrijiver, 1989; Leventhal,
1990). Taking this into consideration, a study of
thermochemical sulfate reduction by organic acids derived from
kerogen that exists in sedimentary rocks was conducted (Kiyosu and
Krouse, 1990). The study determined the reduc tion rate of the
sulfuric acid by acetic acid and the sulfur isotope effects.
However, since the rates of the direct chemical reduction of
sulfate are significantly dependent on the temperature and pH of
solution, the abiogenic reduction of seawater sulfate at low
temperatures and at high
pHs would be difficult (e.g., see Kiyosu, 1980;
Ohmoto and Lasaga, 1982). On the other hand, it has been
experimentally demonstrated that the thermochemical reduction of
sulfate would oc cur by reaction with organic matter and H2 S ac
ting as an initiator (e.g., Toland, 1960; Orr, 1982). Furthermore,
Toland (1960) suggested that elemental sulfur plays an important
role in the sulfate reduction system containing hydrogen sulfide.
However, these processes have not been sufficiently examined. This
paper conducts an experimental in
vestigation of the role of native sulfur in the ther
mochemical reduction of seawater sulfate by
organic matter, and the sulfur isotope behavior
of oxidized and reduced sulfur species during the
reduction.
EXPERIMENTAL PROCEDURES
The reagents Nat SO4 and acetic acid were used as the seawater
sulfate and organic acid, re spectively. The sulfate solution at
0.188 M
(mole/1) was produced from acetic acid at 5.24 M by the addition of
sodium sulfate. The pH of the solution was about 2.00.
A volume of 5 ml of the mixed solution of
Na2SO4 and CH3COOH was injected into a Pyrex
49
50 Y. Kiyosu and H. R. Krouse
glass tube which contained 25 mg of elemental sulfur. After
evacuation to 10-3 Torr, the tube
was sealed with a hand burner. The glass tube
was then inserted into a stainless steel autoclave.
The autoclave was placed in an electric furnance
and heated to temperatures ranging from 200 to
270°C for 12 to 140 hours. After the reaction oc
curred, the autoclave was quenched with cooled
water.
The produced concentrations of the residual sulfate and sulfide
were measured according to the analytical method described in
Kiyosu (1980). The samples were converted to BaSO4 and Age S, then
to S02 by the method discussed in Ueda and Krouse (1986). The
sulfur isotope ratios of sulfur dioxide were measured with use of a
mass spectrometer. The overall reproducibil ty of the 834S
determinations was within ± 0.2%o.
RESULTS
acid in the presence of native sulfur are listed in
Table 1.
As the reaction proceeded, decreases in sulfate and increases in
sulfide concentrations were observed. That is, the thermochemical
reduction of sulfate was found in the
e temperature range of 200 to 270°C. This result is consis tent
with that of the abiogenic reduction of sulfuric acid by dextrose
as reported in Kiyosu
(1980), except for the temperature range. However, when sulfuric
acid is thermochemical ly reduced by acetic acid at temperatures
above 250°C (Kiyosu and Krouse, 1990), hydrogen
sulfide is not produced until the sulfate disap
pears. The sulfur isotope compositions of the
sulfate and sulfide increased as the reduction
reaction proceeded. As will be described below,
Table 1. Experimental results of sulfate reduction by acetic
acid
Run No. Temp. oC
-3 .9 -4 .1 -4 .0
-5 .7 -6 .7 -5 .1 -4 .0 -2 .7 -2 .3
-9 .6 -8 .1 -5 .7 -3 .6
-2 .4 -0 .7
0.188MNa2SO4+5.24MA.A. Sml; S25.2mg (dMS=0.1%o)
Thermochemical reduction of sulfate 51
this suggests an isotope effect or isotope ex change reaction of
the sulfate with the sulfide. In addition, this trend is consistent
with the results of abiogenic reduction of sulfate reported by
Kiyosu (1980) and Kiyosu and Krouse (1990). However, the O34S
values of the sulfur com
pounds are fairly variable compared with those previously
reported.
DISCUSSION
Reduction rate
Figure 1 shows a plot representative of the first-order reaction in
the sodium sulfate reduc tion by acetic acid containing elemental
sulfur. The experimental data at each temperature are fitted with a
straight line. The slope of each straight line indicates the
reduction rate. The rate constants (per minute) for the temperature
of 223 to 270°C vary from 5.4x 10-5 to 6.8 x10-4 (mina-'),
respectively. Kiyosu and Krouse (1990) reported first-order
constants for the reduction of sulfuric acid by acetic acid over
the same temperature range of 2.2 x 10-6 to 1.4 x l0-4 (min.-'),
which are slightly smaller than the values obtained for the present
experi ment.
A typical Arrhenius plot of the rate constants
for the reduction of sodium sulfate by acetic acid
in the presence of sulfur is shown in Fig. 2. The
temperature dependence of the rate constants in
the present system can be fitted to the line
In K= -14.5 X103/ T+ 19.4, (1)
and yields an activation energy of 120.6 kJ /mole
(28.8 kcal mole-'), a low value compared with those in other
abiogenic sulfate reductions (e.g., see Kiyosu and Krouse, 1990).
The activation energy in sulfuric acid-acetic acid systems has been
described in detail by Kiyosu and Krouse
(1990). The relatively low activation energy in dicates that the
abiogenic reduction of the sulfate by organic matter in the
presence of free sulfur may occur even at temperatures below 200°C.
The rates of reduction at 100 and 200°C
calculated with the use of equation (1) are very rapid. At a
temperature of 100°C, for example, half the concentration of the
sulfate in a solution will be reduced to H2S in only about 395 y;
at 200°C the reduction occurs in 0.11 y (40 days). Therefore, the
supposed reaction rates are so rapid that significant quantities of
seawater sulfate would be reduced in reasonable geologic time if
native sulfur is present in sedimentary basins. Native sulfur
commonly occurs in sour
gas (e.g., C02, H2S) reservoirs which consist of sulfate-bearing
carbonate host rocks associated
0.8
1.0
233-C
47 ~
0 20 40 60 80 100 120 140 160 180 200 Time (h)
First-order plot of the reduction for sodium sulfate by acetic acid
in the presence of elemental sulfur.
52 Y. Kiyosu and H. R. Krouse
300 250 220 `C
1.7 1.8 s 1.9 2.0 2.1 10IT
Fig. 2. An Arrhenius plot of the temperature dependence in the
reaction of sodium sulfate reduc tion. The dashed line indicates
the reduction of sulfuric acid by acetic acid after Kiyosu and
Krouse (1990).
with hydrocarbon (Machel, 1987; Krouse et al., 1988). That is,
during the diagenesis of marine sediments, the reduction of
seawater sulfate by organic acids becomes very much accelerated in
the presence of native sulfur.
The mechanism of the abiogenic sulfate reduc
tion
The thermochemical reduction of sulfate by
organic compounds in aqueous solutions occurs
at temperatures above 250°C under acidic condi tions (Kiyosu, 1980;
Nikolayeva et al., 1982;
Kiyosu and Krouse, 1990). However, when native sulfur is not
initially present in a sodium sulfate reduction system by acetic
acid, no ther mochemical sulfate reduction occurs (Table 2). The
rates of reduction in a system of sodium sulfate-acetic acid
containing native sulfur were calculated. The rate increases from
4.2 x10-4 to 9.2 x10-4 (min.-') as the amount of sulfur in creases
from 25 to 60 mg. That is, the role of native sulfur is to increase
the rate of reduction of sodium sulfate. Thus, native sulfur does
not work as a catalyst, but is related to the reduction reaction,
since the reduction rate depends on the amount of native sulfur
present.
In general, the hydrolysis reaction of native
sulfur is well known and can be described as
follows (e.g., Davis et al., 1970):
4S+4H2O-+3H2S+HSO4 +H+. (2)
Reaction (2) shifts to the right when the pH is high. This reverse
reaction has been proposed as
the genesis of sulfur from hydrogen sulfide in the
salt-dome sulfur deposits. However, it should be
clarified that the oxidation of hydrogen sulfide to
native sulfur by sulfate ions does not occur at
temperatures below 100°C (Davis et al., 1970).
Alternatively, native sulfur can be formed from
hydrogen sulfide, which is oxidized by dissolved
oxygen of groundwater or seawater as
2H2 S + 02--) 2S + 2H20. (3)
This suggests that the reaction shifts to the left if
the oxygen concentration is low. That is, only
hydrogen sulfide is produced from elemental
sulfur; no sulfate as in reaction (2) is formed.
Therefore, the abiogenic reduction of sulfate by H2 S indicates
that reaction (2) shifts to the left.
Table 2. Effect of free sulfur in an Na2SO4-acetic acid
system
Run No. Time
h free sulfur
54.6 87.3
Temp. =252°C; n.d.: not detected. 0.188 M Na2SO4+5.24 M A.A. 5 ml;
is initial; f: final.
Thermochemical reduction of sulfate 53
In addition, native sulfur is reduced to H2 S ac cording to
reaction (3), under the conditions of low oxygen concentration.
From reactions (2) and (3), it is thought that the sulfate would be
reduced to hydrogen sulfide. Furthermore, the effect of . acetic
acid was examined as the substance responsible for the lowering of
oxygen concentration. As listed in Table 3, no sulfate reduction
oc
curs in the absence of organic acid, since reac tion (2) shifts to
the right only, and hydrogen sulfide is formed with sulfate.
Alternatively, in the presence of organic acid, the sulfate reduc
tion occurs by the reverse reactions in reactions
(2) and (3) (i.e., to the left). To make sure that the reverse
reaction occurs in reaction (3), the behavior of sulfur compounds
produced in the system, in which native sulfur has been added to
organic acid solution initially containing no sulfate, is examined.
As shown by the results listed in Table 4, it is clear that all the
native sulfur added to the solution is reduced to hydrogen sulfide.
This result is consistent with those reported by Toland (1960) and
Pryor
(1962). Furthermore, Douglas and Mair (1964) also concluded that
hydrogen sulfide is produced by the reaction between sulfur and
organic mat ter, suggesting that the thermal reaction of
sulfur with organic compounds plays an impor
tant role in the genesis of petroleum. These reac
tions can be written, that when
4S+CH3000H+2H20-*4H2S+2CO2 (4)
then,
This suggests that seawater sulfate is ther
mochemically reduced by acetic acid in the
presence of native sulfur. During sulfate reduction, no organic
sulfur may be produced since only sulfide was found in the results
listed in Table 1. As was shown in reaction (2), it is very
difficult that reaction (5)
proceeds to the right if the pH of the solution is high. However,
the hydrogen ion dissociated from acetic acid reacts with the OH in
reaction
(5) as the concentration of the acetic acid is high. That is, as
shown in reaction (6), seawater sulfate is reduced by organic
acid.
Sulfur isotopic behavior during abiogenic sulfate reduction
Figure 3 shows the O34S values of the sulfur
Table 3. Effect of organic acid in an Nat SO4-S system
Run No. Time
25
50
24
48
48
72
48
72
0
0
5
5
31.3
31.3
31.3
31.3
36.9
37.1
7.9
4.3
18.8
17.4
33.6
50.8
Temp. =252°C; 0.188M Na2SO4+5.24 M A.A. 5 ml; S=25.2 mg.
Table 4. Results of the free sulfur-acetic acid system
Run No. Time
54 Y. Kiyosu and H. R. Krouse
b3S
20
10
0
-10
0
0
A
M •
04 08 1.2 1.6 2.0 -1n F
Fig. 3. Plot of the 8 34S of sulfur species and the isotope
fractionation as a function of a fraction of the remain ing sulfate
(F). Dotted circle represents the initial 834S value of sulfate.
Open and closed circles represent the value of a 34S of the sulfate
at 241 and 252°C, respectively, while the open and closed triangles
are those for the sulfide at 241 and 252°C, respectively.
compounds and the isotope fractionations be
tween sulfate and sulfide versus reaction time.
The 634S and isotope fractionation of the sulfur
species increase as the reduction proceeds. That
is, since 32S is easily reduced compared with 34S in
order to maintain the isotope effect during
sulfate reduction, the isotope composition of residual sulfate
increases. In a closed system, the
634S value of the sulfate is considerably increas
ed by the Rayleigh distillation process. Conse
quently, the 634S value of the sulfide approaches the initial value
of the sulfate. In this study, this
value of sulfide is consistent with the 634S of the
total sulfur estimated by the mass balance of sulfate sulfur and
elemental sulfur.
Concerning the kinetic isotope effect, the frac tionation between
sulfate and sulfide should decrease with an increase in
temperature. Therefore, the &34S value of the sulfate
will
gradually increase and, on the contrary, the & 34S of the
sulfide will decrease (e.g., see Kiyosu and Krouse, 1990). However,
the opposite trend is observed in Fig. 3. This suggests that the
isotope exchange equilibrium rather than the kinetic isotope effect
may have occurred during the proc ess of sulfate reduction. That
is, as the isotope ex
change reactions proceed with an increase in tem
perature, the isotope fractionation between the sulfate and sulfide
increases.
Furthermore, the equilibrium fractionation
over this temperature range.
due to the isotope effect, but rather mainly the
result of the isotopic exchange equilibrium in
Fig. 4, the 34S enrichment of the sulfate is plotted
versus the ratio of the sulfate reduction. If the
kinetic isotope effect is associated with ther mochemical
sulfate-reduction, the experimental
data can be plotted on a straight line with the
slope indicating the kinetic isotope fractiona tion. For
comparison, the kinetic isotope frac
tionation estimated by Kiyosu and Krouse (1990) is also shown.
However, all of the data deviate
from the straight line that indicates the kinetic
fractionation. Furthermore, the 834S value of the sulfate increases
with an increase in temperature.
On the basis of Kiyosu's data, Kaiser (1988)
re-examined the isotope effect in the abiogenic
sulfate reduction by dextrose, theoretically ex
plaining the difference of the isotope effect and isotope exchange
between sulfate and sulfide dur
25
20
0 0.5 1.0 1.5 2.0 -l n f
Fig. 4. Changes in the isotopic composition of sulfate that has
been subjected to reduction by acetic acid. The straight lines of
241 and 252°C are the kinetic isotope effects estimated by the
equation of Kiyosu and Krouse (1990).
ing unidirectional sulfate reduction. That is, the
fractionation is controlled by the isotopic ex
change between the two sulfur compounds and
deviates from kinetic fractionation since the
isotopic exhange reaction proceeds with an in
Table 5. Effect of the sulfur isotope composition of elemental
sulfur
of sulfate 55
crease in temperature.
Judging from these facts, the isotopic com
positions of the sulfate and sulfide in this study may be affected
by the isotope exchange
equilibrium. Since the a34S value of the sulfate in
creases over a very short reaction time, the
isotopic exchange equilibrium between the two
sulfur species should occur as in the following
equation
32SO2 + H234S = 34SO24 + H232S (7)
The equilibrium fractionation factors of the isotopic exchange
reactions at 241 and 252°C are 25.0 and 24.0%o, respectively
(Ohmoto and Lasaga, 1982). The latter value may explain the
difference between the sulfate (+23.0%o) and sulfide (-0.7%o) after
72 hours at 252°C in the
present study, indicating the establishment of isotopic
equilibrium.
In order to clarify that the 634S value of
sulfate is controlled by the isotope exchange with
hydrogen sulfide, the sulfate was reduced by
organic acid, using elemental sulfur with
different &34S values (Table 5). The 534S values
of the sulfate. and sulfide increase with an in
crease in the S34S value of the native sulfur. This
trend is observed when the sulfate reduction only
slightly occurs. Thus, since the sulfur isotope compositions of
sulfur compounds are controll
ed by isotope exchange reaction during reduc
tion, the fractionation between sulfate and
sulfide is not due to kinetic isotope effect.
Temp. Run No .
Time h mg
CONCLUSIONS
The sodium sulfate used as seawater sulfate could not be directly
reduced by acetic acid over the temperature range of 200 to 270°C.
However, when native sulfur was added to this system,
thermochemical reduction of the sulfate
occurred. The reduction rate of the system con taining sulfur was
found to depend on the amount of native sulfur present. Since all
of the sulfur reacting with acetic acid was converted to hydrogen
sulfide, the sulfate may have been indi rectly reduced by hydrogen
sulfide derived from native sulfur, as was reported by Toland
(1960) and Orr (1982). The variation in the sulfur isotopes of
the
sulfur species in the sodium sulfate reduction system containing
native sulfur, controlled by equilibrium fractionation between the
two sulfur species rather than by kinetic isotope effect, is
similar to that found in submarine hydrothermal systems (e.g.,
Peter and Shanks, 1992). Therefore, on the basis of the isotopic
behavior, it may be easy to distinguish between biogenic and
abiogenic reduction of seawater sulfate if native sulfur coexists
with sulfate. Native sulfur may be formed by redox reac tions in
bacterial and/or volcanic activity. In par ticular, since the
lithospheric sulfur cycle in a subduction zone such as an island
arc can be in fluenced by volcanic and/or the subvolcanic marine
enviroment, the genesis of native sulfur
based on the later activity is very important.
Acknowledgments-The authors are greatly indebed to M. Kusakabe and
Y. Kajiwara for critical reading the manuscript. This study was
funded by the Shell Canada Ltd. and an operating grant from the
Natural Sciences and Engineering Research Council of Canada.
REFERENCES
Davis, J. B., Stanley, J. P. and Custard, H. G. (1970) Evidence
against oxidation of hydrogen sulfide by
sulfate ions to produce elemental sulfur in salt
domes. Amer. Assoc. Petrol. Geol. Bull. 54, 2444
2447.
Douglas, A. G. and Mair, B. J. (1964) Sulfur: role in
genesis of peteroleium. Science 147, 499-501. Kaiser, C. J. (1988)
Chemical and isotopic kinetics of
sulfate reduction by organic matter under
hydrothermal conditions. Ph. D. Thesis. The Penn
sylvania State University.
Kemp, A. L. W. and Thode, H. G. (1968) The mechanism of the
bacterial reduction of sulphate and of sulphide from isotope
fractionation studies. Geochim. Cosmochim. Acta 32, 71-91.
Kiyosu, Y. (1980) Chemical reduction and sulfur isotope effects of
sulfate by organic matter under hydrothermal conditions. Chem.
Geol. 30, 47-56.
Kiyosu, Y. and Krouse, H. R. (1990) The role of organic acid in the
abiogenic reduction of sulfate and the sulfur isotope effect.
Geochem. J. 24, 21 27.
Krouse, H. R., Viau, C. A., Eliuk, L. S., Ueda, A. and Halas, S.
(1988) Chemical and isotopic evidence of thermochemical sulfate
reduction by light hydrocarbon `gases in deep carbonate reser voir.
Nature 333, 415-419.
Leventhal, J. S. (1990) Organic matter and ther mochemical sulfate
reduction in the Viburnum Trend, Southeast Missouri. Econ. Geol.
85, 622 632.
Machel, H. G. (1987) Saddle dolomite as a by-product of chemical
composition and thermochemical sulfate reduction. Geology 15,
936-940.
Nikolayeya, O. V., Ryzhenko, B. N. and Germanov, A. I. (1982)
Reduction of sulfate sulfur by hydrocar bons and alcohols in
aqueous solution at 200 300°C. Geokhimiya 5, 726-744.
Ohmoto, H. and Lasaga, A. C. (1982) Kinetics of reac tions between
aqueous sulfates and sulfides in hydrothermal systems. Geochim.
Cosmochim Acta 46, 1727-1745.
Orr, W. L. (1974) Changes in sulfur content and isotopic ratios of
sulfur during petroleum-study of
Big Horn Basin paleozoic oils. Amer. Assoc. Petrol. Geol. Bull. 58,
2295-2318.
Orr, W. L. (1977) Geologic and geochemical controls on the
distribution of hydrogen sulfide in natural gas. Advances in
Organic Geochemistry 1975. (R. Campos and J. Goni. eds.) Enadisma,
Madrid. Spain. 571-597.
Orr, W. L. (1982) Rate and mechanism of non microbial sulfate
reduction. Geological Society of America, Annual Meeting, Abstracts
with pro
grams 14, 580. Peter, J. M. and Shanks, W. C. 111(1992) Sulfur,
car
bon, and oxygen isotope variations in submarine hydrothermal
deposits of Guay mas Basin, Gulf of California, U.S.A. Geochim.
Cosmochim. Acta 56, 2025-2040.
Thermochemical reduction of sulfate 57
Powell, T. G. and Macqueen, R. W. (1984) Precipita tion of sulfide
ores and organic matter: Sulfate reac
tions at Pine Point Canada. Science 224, 63-66.
Pryor, W. A. (1962) Mechanism of sulfur reactions. McGraw-Hill Book
Co., New York, NY, 241p. Tasse, N. and Schrijve, K. (1989)
Formation of ac cessory sphalerite by thermochemical sulphate
reduction in Lower Paleozoic carbonate rocks (St.
Lawrence Lowlands, Quebec, Canada). Chem. Geol. (Isot. Geosci.
Sect. Section) 80, 55-70.