Thermochemical reduction and sulfur isotopic behavior of ...
1
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.
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.
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