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Chemical reduction and sulfur-isotope effects of sulfate by organic matter under hydrothermal conditions
Chemical Geology, 30 (1980) 47--56
47
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CHEMICAL REDUCTION AND SULFUR-ISOTOPE EFFECTS OF SULFATE BY ORGANIC MATTER UNDER HYDROTHERMAL CONDITIONS
YASUHIRO KIYOSU
Department of Earth Sciences, Faculty of Science, Ngagoya University, Chikusa, Nagoya 464 (Japan) (Received February 1, 1980; accepted for publication April 16, 1980)
ABSTRACT Kiyosu, Y., 1980. Chemical reduction and sulfur-isotope effects of sulfate by organic matter under hydrothermal conditions. Chem. Geol., 30: 47--56. Under hydrothermal conditions sulfuric acid, sodium bisulfate and sodium sulfate solutions were reduced by dextrose to hydrogen sulfide in order to clarify the origin of sulfide species in hot-springs, geothermal water and ore-forming fluids. At temperatures above 250 ° C, reduction of sulfuric acid and at above 300°C of sodium bisulfate and sodium sulfate was observed. The reduction rate depends fairly well on the temperatures, pH and sulfate species. The reduction of sulfate seems to be a first-order reaction. Sulfur-isotope compositions of sulfate and hydrogen sulfide were measured in order to disclose isotope effects in the reduction of sulfate. The reduction of sulfuric acid and sodium bisulfate solution results in enrichment of 32S in the hydrogen sulfide and of the heavy isotope into residual sulfate. The fractionation factor in the reduction is independent of the temperature and is seen to be 1.007 to 1.009, in agreement with previously published values.
INTRODUCTION
It has been commonly accepted that variation of the sulfur-isotopic composition in natural sulfate and sulfide species is mainly controlled by the extent of isotopic fractionation in the reduction of sulfate by anaerobic bacteria. In thermal water, both the disproportionation reaction of sulfurous acid and the reaction of sulfur with water have been proposed as mechanisms for the formation of sulfate and sulfide species and for the isotopic variation of both sulfur species (e.g., Iwasaki and Ozawa, 1960; Oana and Ishikawa, 1966). Many dissolved sulfide species in hot-spring and geothermal water are generally believed to have been produced with both such reactions. On the otherhand, it has been demonstrated from available geochemical data that hydrothermal sulfide--sulfate deposits such as the Kuroko type and Red sea type were formed by reduction of seawater sulfate (e.g., Kajiwara, 1973; Shanks and Bischoff, 0009-2541/80/0000--0000/$02.25 © 1980 Elsevier Scientific Publishing Company
48
1977 ). Although one of the sulfide species in natural hydrothermal systems may be produced as by-product in the inorganic reduction of dissolved sulfate by ferrous iron in the rocks, it is uncertain whether such reduction is possible. Malinin and Khitarov (1969) found that reduction of zinc sulfate by H: gas under hydrothermal conditions began at ~ 250°C and was very intensive at 300 ° C and higher. Because of a wide distribution and abundance of carbonaceous matter in hot-springs, geothermal areas and hydrothermal deposits, it seems to be likely that sulfate ions can be reduced by thermal decomposition of organic matter in sedimentary rocks under hydrothermal conditions. If the reduction of sulfate occurred at different temperatures in hydrothermal solutions, either a unidirectional or an equilibrium isotope effect may be expected. The kinetic isotope effect in the chemical reduction of sulfate to sulfide was first reported by Harrison and Thode (1957), and investigated over the temperature range from 18 ° to 50°C. Grinenko et al. (1969) determined the isotope separation factor in the reduction of sulfuric acid to sulfurous acid with atomic hydrogen at temperatures from 138 ° to 235 ° C, and with molecular hydrogen at 290 ° C. However, S-isotope separation during chemical reduction of sulfates has never been examined in detail. The purpose of this study is to attempt the chemical reduction of sulfate by organic matter under hydrothermal conditions and to define the S-isotope effect during the process of reduction. EXPERIMENTAL
Solutions of H2SO4, NaHSO4 and Na2SO4, and dextrose were used as sulfate species and organic matter, respectively. The amounts used and pH of each solution are summarized in Table I. Dextrose and 4 ml of sulfate solution were put in Pyrex ® glass tubes, 18 mm in inner side diameter and 150 mm in length. The tubes were sealed after evacuation and put into an autoclave and heated in an electric furnace at the desired temperature. For quenching, the autoclave was cooled in water. The H2S produced and other gaseous components were condensed into the tube at liquid nitrogen temperature. After opening the tubes, the products were added
TABLE I S t a r t i n g materials Experiment
Solution
Sulfate (tool/l)
pH
Volume (ml)
Dextrose (rag)
1 2 3
H2SO 4 NaHSO 4 Na~SO4
0.187 0.177 0.179
0.90 1.35 7.02
5 5 5
600 600 600
49 to 6 N HCh The H2S liberated was carried b y a stream of N2 gas to be absorbed in a solution of cadmium acetate, precipitated as cadmium sulfide, converted to silver sulfide and weighed. The sulfate remaining was determined gravimetrically as barium sulfate. Other gas components were analyzed b y gas chromatography. The precipitate produced was identified with an X-ray powder diffractometer. Barium sulfate was converted to silver sulfide by the graphite-reduction method. All the silver sulfides were combusted with cuprous oxide to obtain SO2 which was analyzed for 34S/32S ratios on a double-collector mass spectrometer. The isotopic data obtained were given in terms of the ~ 34S-value relative to troilite sulfur (Canyon Diablo):
1"¢34S/32S'~ ~348 ( % 0 ) = [~ ! Jsample /[34S/32S~ t Jstandard - - 1 ] X 10 3 The reproducibility of isotopic measurements was -+ 0.2 °/oo. RESULTS AND DISCUSSION Our experiments were made to determine the conditions of the reduction of sulfate-sulfur species at the temperature range of 200 ° - 3 4 0 ° C. However, signs of reduction of sulfate appeared above 250 ° C as listed in Table II. In experiment 1, H2SO4 was reduced to H2S at temperatures higher than 250°C while reduction of sulfate in experiments 2 and 3 occurred at temperatures above 300 ° C. SO2 was absent in all experiments. In the experiments of the three series, black precipitates appeared. These precipitates were identified as amorphous C b y X-ray powder diffraction. Other gases produced (in all experiments) were recognized as CO2, CH4 and H2 by gas chromatography.
Reduction o f sulfuric acid solution The time variation of the observed reduction of sulfuric acid is plotted in Fig. 1. The reduction of sulfate is fairly slow at 250 ° C, whereas at temperatures above 280°C reduction occurs rather rapidly. It is clear that the reduction rate of sulfate depends on temperature. Previous studies on the reduction of sulfate under hydrothermal conditions have been carried o u t in connection with the problem of the S-isotope effect (Harrison and Thode, 1957; Grinenko et al., 1969). However, the reducing agents used are n o t likely to be present under natural conditions. In the present study, the H2SO4 solution was reduced b y dextrose to H2S. As the atomic and/or molecular hydrogen produced by the reaction of dextrose with water m a y reduce H2SO4 to H2S under similar conditions as in nature. Judging from the amounts of sulfate and sulfide in this experiment, it is found that the loss of sulfur exceeds 40% at above 280 ° C. This fact suggests that H2 S may have reacted with sulfate to form elemental S in the acidic region as follows:
50
3H2S + H + + HSO4- -~ 4S + 4H2S The black precipitates were oxidized with concentrated HNO3 and Br2, and after evaporation, the residue was tested for the presence of other S species by addition of dilute HC1 and BaC12 solution. Other S species were detected. However, it may have been organic S since no elemental S was identified by the X-ray powder diffraction of the black precipitates. n~S
|
1
i
o"
0
301 O
25'-
-
201~°
15
I0 t~>0
i ">
0
HSO~
o 2~ "c o 280 "C
°,,
<>340"(: 0 300 "C
l
i
I0
20
I0
l
l
l
k
40
50
60
70
hours Fig. 1. Variation with time of products (sulfide) and reactants (sulfuric acid) at various temperatures.
Reduction of NariS04 and Na2S04 Fig. 2 shows the relationship between the reduction rate of sodium bisulfate and reaction time. Results of experiment 1 at 300 ° and 340°C are included for comparison. At temperature below 300 ° C reduction of sulfate is extremely difficult. The reduction in this experiment t o o k place at 300°C but is fairly slow in comparison with that of experiment 1. It is found that small amounts of H2 S are formed by reduction of sodium bisulfate even after 100 hr. Substantial reduction begins at 340 ° C. The results suggest that the reduction rate of both experiments is influenced by temperature and sulfate species rather than by pH, since the solutions of both experiments are acidic hydrothermal conditions. On the other hand, it is evident that the reduction rate of sodium sulfate solution is very low even above 340 ° C, as listed in Table I. Although molecular H2 was formed by the thermal decomposition of dextrose at temperatures lower than 300 ° C, reduction of sodium sulfate solution did not occur.
51
mgS
30o
300"C
20-
o
o~ .\
.
%\
~H~
%so~
30-
""340 "C
\ o
3
i
Fig. 2. R e d u c t i o n
4
5
6
7
8
9
10 days
o f sulfate to sulfide for e x p e r i m e n t 2 at 3 0 0 ° and
340°C.
If the reduction of sulfate is assumed to be a first-order reaction, the rate o f change in the concentration of sulfate in reaction at time t may be expressed as follows: in ( C i / C f ) = k t
where C i and Cf are the initial and final concentrations of sulfate, respectively; h denotes the rate constant; and t is the reaction time. Fig. 3 is a plot of ln(Ci/Cf) vs. time in hours for the experiments 1 and 2. The relationship is essentially a straight line, thus indicating a constant k-value, and since h is constant, it is suggested that the reaction in the experiments 1 and 2 is a firstorder one. From this figure, it is also found that the reduction rate of experiment 1 is higher than that of experiment 2.
28
• 250"C
o 241-/ o /
/
o 30o'c Hsq-
L / / /
2.0
Q8 0
. 10
Fig. 3. Ratios
20
o 300~
4 310 410
-
-
~ 50
610
I
710 810 90
,I~
I
100 hours
o f rate c o n s t a n t s during reduction at various temperatures.
52 Much or all of the organic matter present in sediments or sedimentary rocks may be lipids, humic acids and kerogen except for hydrocarbons accumulated in natural gases and associated petroleum deposits. Therefore, the present study is a laboratory simulation experiment of sulfate reduction by organic matter under hydrothermal conditions. However, hydrocarbons, lipids, humic acids and kerogen are insoluble in water, reduction of sulfate may be fairly difficult and probably not realized in nature by these organic materials. It was demonstrated in the investigation of Toland {1959) that under hydrothermal conditions sulfate reduction was not caused by hydrocarbon. However, the examination on sulfate reduction by the thermal decomposition of lipids, humic acids and kerogen is necessary to resolve the origin of sulfide species in hydrothermal solutions. All the available data now suggest that the reduction rate of sulfate is dependent on pH, temperature and dissolved sulfate species such as HSO4-, N a S O ~ and SO42-. Judging from the result of experiment 3, reduction of seawater sulfate by organic matter may be extremely difficult under hydrothermal conditions.
Kinetic isotope effect in reduction of sulfate The question of whether sulfide minerals in the Kuroko- and Red Sea-type deposits can form as a result of reduction of seawater sulfate under hydrothermal conditions must be answered not only from the viewpoint of reduction of sulfate, b u t also from an isotope effect in the reduction. The isotopic fractionation in the chemical reduction of sulfuric acid solution by dextrose was indeed observed. As shown in Table I, the S-isotope compositions of sulfate and hydrogen sulfide in experiments 1 and 2 changed during the process of reduction. In experiment 3, however, the 534S-values of sulfate do not vary with the time of reaction. Since the reduction rate of sulfate is very low in this experiment and amounts of hydrogen sulfide produced are t o o small, although the reduction occurred, the 534S of sulfate may be constant. The relationship between 534S of sulfur species in experiments 1 and 2 and reaction time are shown in Fig. 4. The S-isotopic composition of residual sulfate in experiment 1 increases from --4.4 to +11.0 %0. On the other hand, 5 34S of the hydrogen sulfide produced changes from --12.5 to - 6 . 0 %o 280 ° C. The fractionation values between sulfate and hydrogen sulfide can be seen to increase with the reaction time. At 300 ° and 340°C the 534S-value of sulfate increases rapidly and the isotopic composition of hydrogen sulfide approaches to 5 34S-value of the initial sulfate. On the other hand, the isotopic composition of sulfate increases slightly throughout the reduction at 250 ° C. Fig. 4 demonstrates that below 250°C the reduction is very slow. In experiment 2, a similar result was obtained and the 5 34S of sulfate increased during the process of reduction. However, the isotopic composition of hydrogen sulfide decreased.
53
%. 16-
0 250"C o 280"C ¢' 300"C 0340"(:
12-
¢'340"C
84-
~¢--_
r/
/
0-
J
H50~-
-8-12-
4~
Us
0 hours
Fig. 4. Changes in
6
s4S-values of sulfide and sulfate vs. reaction time.
3~
~S-~
15
I0
/
50 "c
I
0.4
0.8
1.2
I
1.6
26
2B
I
3.2 - Inf
Fig. 5. Relation between reduction.
6
s4S-w 'les and the fraction of sulfate remaining in the sulfate
54 Our results dent
suggest
on the reduction
that
S-isotopic
ratio
compositions
of sulfate
of sulfate
as in the following
remained
depen-
equation:
6--60=103(a-1)lnf where sulfate
5 0 is t h e when
fractionation
6 34S-value of the original
the fraction factor
of the sulfate
for this process.
sulfate; remaining
When
6 is the 6 34S-value of the is f; and a is the kinetic
(6 - - 6 0 ) - v a l u e s a r e p l o t t e d
T A B L E II Results from reduction experiments Temperature (o C)
Time (hr.)
SO4 ~( m g S)
~ 34S
H2S ( m g S)
(%0)
Experiment 1 : 250 0 250 24 250 48 250 70 280 6 280 12 280 16 280 22 300 4 300 6 300 7 300 8.5 340 3 340 4
28.5 25.0 22.8 22.4 18.5 10.8 5.5 1.4 17.5 8.2 2.7 2.2 9.4 1.2
-- 4.4 - - 3.5 -- 2.6 - - 2.3 -- 0.59 + 5.2 +11.0 n.d. -- 0.1 + 6.6 +14.2 +14.4 + 4.7 n.d.
--
E x p e r i m e n t 2: 300 0 300 50 300 96 300 144 300 180 340 8 340 48 340 72
28.3 26.9 26.5 25.4 19.4 24.4 17.7 12.5
-----+ + +
2.8 2.2 2.2 1.1 0.14 1.1 1.6 3.4
--
Experiment 300 300 300 340 340 340
28.6 27.9 28.1 28.2 26.5 27.0
+ + + + + +
2.7 2.2 2.5 2.1 2.6 2.3
--
-
-
5 34S (%o)
1.4 2.8 3.3 4.5 7.3 10.0 12.4 4.4 5.9 11.8 12.0 6.0 11.9
0.2 0.5 2.9 4.9 1.2 6.1 7.5
-n.d. n.d. --14.2 --12.5 -- 9.6 -- 7.5 -- 6.0 --13.0 -- 7.9 -- 5.3 -- 5.0 -- 9.1 -- 4.7
-n.d. n.d. -- 3.0 -- 8.6 n.d. -- 8.2 --12.4
3: 0 168 340 48 170 236
= n o t a n a l y s e d ;'n.d. = n o t d e t e r m i n e d .
0.2 0.1 0.2 1.9 1.1
-n.d. n.d. n.d. n.d. n.d.
as in
55 Fig. 5 against t h e f r a c t i o n o f t h e sulfate r e m a i n i n g (f), we see t h a t t h e f a c t o r 103(a - - 1) is i n d e p e n d e n t o f t h e t e m p e r a t u r e in t h e range o f 2 5 0 o - - 3 4 0 ° C. F r o m t h e d a t a in Table II, a n d using this e q u a t i o n , t h e f a c t o r 103(a - - 1) has b e e n c a l c u l a t e d f o r t h e S-isotopic variations o f e x p e r i m e n t s 1 a n d 2. These values are s h o w n in Table III. N o d e p e n d e n c e o f f r a c t i o n a t i o n o n t e m p e r a -
TABLE III Isotope effect in reduction of sulfate f
--In f
103 (~ -- 1)
+ 0 + 0.9 + 1.8 + 2.1 + 3.8 + 9.6 +15.4 + 4.3 +11.0 +18.6 +18.8 + 9.1
1.000 0.877 0.800 0.786 0.649 0.379 0.193 0.614 0.288 0.095 0.077 0.330
0 0.133 0.222 0.241 0.433 0.972 1.64 0.488 1.25 2.35 2.56 1.11
-6.77 8.11 8.71 8.78 9.88 9.39 8.81 8.80 7.91 7.34 8.27
0 0.6 0.6 1.7 2.7 1.7 4.4 6.6
1.000 0.951 0.936 0.898 0.686 0.862 0.625 0.442
0 0.05 0.06 0.108 0.377 0.148 0.469 0.817
Temperature
Time
~ 34Sso42-*
(°C)
(hr.)
(%o)
Experiment 250 250 250 250 280 280 280 300 300. 300 300 340
1:
0 24 48 70 6 12 16 4 6 7 8.5 3
Experiment 2: 300 0 300 50 300 96 300 144 300 180 340 8 340 48 340 72
+ + + + + + +
-
-
12.0 10.0 15.7 7.16 11.5 9.38 8.08
"5 S4Ss04 2 - - - ~ 3 4 8 f - ~34Si; f = final; i = initi~.
t u r e was d e t e c t e d . T h e s e p a r a t i o n f a c t o r is 1 . 0 0 8 - - 1 . 0 0 9 f o r t h e t e m p e r a t u r e interval f r o m 2 5 0 ° t o 3 4 0 ° C . This indicates t h a t t h e light i s o t o p e o f t h e sulfate ion (32SO42-) reacts 8 . 0 - 9 . 0 °/00 faster t h a n t h e h e a v y i s o t o p e in this t e m p e r a t u r e range. A t t e m p e r a t u r e s a b o v e 3 0 0 ° C, the kinetic r e a c t i o n , w h i c h favours t h e h e a v y i s o t o p e in sulfate and the light entering t h e h y d r o g e n sulfide, p r o c e e d s rapidly. In t h e e x p e r i m e n t s o f s o d i u m bisulfate r e d u c t i o n , it is f o u n d t h a t t h e s e p a r a t i o n o f i s o t o p e s t h a t o c c u r r e d at t e m p e r a t u r e s a b o v e 3 0 0 ° C is slightly higher t h a n t h a t o f e x p e r i m e n t I . T h e kinetic f r a c t i o n a t i o n in r e d u c t i o n o f s u l f a t e m a y d e p e n d o n t h e p H and sulfate species.
56 Fig. 6 gives the kinetic fractionations and their correlation with temperature, together with other data (Harrison and Thode, 1957; Grinenko et al., 1969). The results obtained in the reduction of sulfuric acid to hydrogen sulfide and to SO: at different temperatures by Grinenko et al. (1969) fall on the same curve. This fact indicates that reduction is a multistage reaction and that the main stage of kinetic effect is reduction of sulfate to sulfite. In the experiment 3, no isotopic fractionation in the chemical reduction was observed, even at temperatures above 300 ° C. Therefore, re-examination on the reduction of sodium sulfate solution is necessary to obtain the separation factor during the process of reduction.
b H~rlson and Thode() 957) o Gr irenkoet a1.(1969)
20 I-
~ Th~ work (Exp't) (Exp.:~
o~\ o,
]4 12
~:~o
~o
o
8 4 I
0
1oo
200
300
400
"C
Fig. 6. Dependence of kinetic isotope effect on temperature.
REFERENCES Grinenko, V.A., Grinenko, L.N. and Zagryazhskaya, G.D., 1969. Kinetic isotope effect in high temperature reduction of sulfate. Geokhimya, 4: 484--491. Harrison, A.G. and Thode, H.G., 1957. The kinetic isotope effect in the chemical reduction of sulphate. Trans. Faraday Soc., 53: 1648--1651. Iwasaki, I. and Ozawa, T., 1960. Genesis of sulfate in acid hot spring. Bull. Chem. Soc. Jpn., 33: 1018--1019. Kajiwara, Y., 1973. A simulation of the Kuroko type mineralization in Japan. Geochem. J., 6: 193--209. Malinin, S.D. and Khitarov, N.I., 1969. Reduction of sulfate sulfur by hydrogen under hydrothermal conditions. Geokhimya, 11: 1312--1318. Oana, S. and Ishikawa, H., 1966. Sulfur isotopic fractionation between sulfur and sulfuric acid in the hydrothermal solution of sulfur dioxide. Geochem. J., 1 : 45--50. Shanks, W.C. and Bischoff, J.L., 1977. Ore transport and deposition in the Red Sea geothermal system; a geochemical model. Geochim. Cosmochim. Acta, 41: 1507--1519. Toland, W.G., 1959. Oxidation of organic compounds with aqueous sulfate. J. Am. Chem. Soc., 82: 1911--1916.
47
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CHEMICAL REDUCTION AND SULFUR-ISOTOPE EFFECTS OF SULFATE BY ORGANIC MATTER UNDER HYDROTHERMAL CONDITIONS
YASUHIRO KIYOSU
Department of Earth Sciences, Faculty of Science, Ngagoya University, Chikusa, Nagoya 464 (Japan) (Received February 1, 1980; accepted for publication April 16, 1980)
ABSTRACT Kiyosu, Y., 1980. Chemical reduction and sulfur-isotope effects of sulfate by organic matter under hydrothermal conditions. Chem. Geol., 30: 47--56. Under hydrothermal conditions sulfuric acid, sodium bisulfate and sodium sulfate solutions were reduced by dextrose to hydrogen sulfide in order to clarify the origin of sulfide species in hot-springs, geothermal water and ore-forming fluids. At temperatures above 250 ° C, reduction of sulfuric acid and at above 300°C of sodium bisulfate and sodium sulfate was observed. The reduction rate depends fairly well on the temperatures, pH and sulfate species. The reduction of sulfate seems to be a first-order reaction. Sulfur-isotope compositions of sulfate and hydrogen sulfide were measured in order to disclose isotope effects in the reduction of sulfate. The reduction of sulfuric acid and sodium bisulfate solution results in enrichment of 32S in the hydrogen sulfide and of the heavy isotope into residual sulfate. The fractionation factor in the reduction is independent of the temperature and is seen to be 1.007 to 1.009, in agreement with previously published values.
INTRODUCTION
It has been commonly accepted that variation of the sulfur-isotopic composition in natural sulfate and sulfide species is mainly controlled by the extent of isotopic fractionation in the reduction of sulfate by anaerobic bacteria. In thermal water, both the disproportionation reaction of sulfurous acid and the reaction of sulfur with water have been proposed as mechanisms for the formation of sulfate and sulfide species and for the isotopic variation of both sulfur species (e.g., Iwasaki and Ozawa, 1960; Oana and Ishikawa, 1966). Many dissolved sulfide species in hot-spring and geothermal water are generally believed to have been produced with both such reactions. On the otherhand, it has been demonstrated from available geochemical data that hydrothermal sulfide--sulfate deposits such as the Kuroko type and Red sea type were formed by reduction of seawater sulfate (e.g., Kajiwara, 1973; Shanks and Bischoff, 0009-2541/80/0000--0000/$02.25 © 1980 Elsevier Scientific Publishing Company
48
1977 ). Although one of the sulfide species in natural hydrothermal systems may be produced as by-product in the inorganic reduction of dissolved sulfate by ferrous iron in the rocks, it is uncertain whether such reduction is possible. Malinin and Khitarov (1969) found that reduction of zinc sulfate by H: gas under hydrothermal conditions began at ~ 250°C and was very intensive at 300 ° C and higher. Because of a wide distribution and abundance of carbonaceous matter in hot-springs, geothermal areas and hydrothermal deposits, it seems to be likely that sulfate ions can be reduced by thermal decomposition of organic matter in sedimentary rocks under hydrothermal conditions. If the reduction of sulfate occurred at different temperatures in hydrothermal solutions, either a unidirectional or an equilibrium isotope effect may be expected. The kinetic isotope effect in the chemical reduction of sulfate to sulfide was first reported by Harrison and Thode (1957), and investigated over the temperature range from 18 ° to 50°C. Grinenko et al. (1969) determined the isotope separation factor in the reduction of sulfuric acid to sulfurous acid with atomic hydrogen at temperatures from 138 ° to 235 ° C, and with molecular hydrogen at 290 ° C. However, S-isotope separation during chemical reduction of sulfates has never been examined in detail. The purpose of this study is to attempt the chemical reduction of sulfate by organic matter under hydrothermal conditions and to define the S-isotope effect during the process of reduction. EXPERIMENTAL
Solutions of H2SO4, NaHSO4 and Na2SO4, and dextrose were used as sulfate species and organic matter, respectively. The amounts used and pH of each solution are summarized in Table I. Dextrose and 4 ml of sulfate solution were put in Pyrex ® glass tubes, 18 mm in inner side diameter and 150 mm in length. The tubes were sealed after evacuation and put into an autoclave and heated in an electric furnace at the desired temperature. For quenching, the autoclave was cooled in water. The H2S produced and other gaseous components were condensed into the tube at liquid nitrogen temperature. After opening the tubes, the products were added
TABLE I S t a r t i n g materials Experiment
Solution
Sulfate (tool/l)
pH
Volume (ml)
Dextrose (rag)
1 2 3
H2SO 4 NaHSO 4 Na~SO4
0.187 0.177 0.179
0.90 1.35 7.02
5 5 5
600 600 600
49 to 6 N HCh The H2S liberated was carried b y a stream of N2 gas to be absorbed in a solution of cadmium acetate, precipitated as cadmium sulfide, converted to silver sulfide and weighed. The sulfate remaining was determined gravimetrically as barium sulfate. Other gas components were analyzed b y gas chromatography. The precipitate produced was identified with an X-ray powder diffractometer. Barium sulfate was converted to silver sulfide by the graphite-reduction method. All the silver sulfides were combusted with cuprous oxide to obtain SO2 which was analyzed for 34S/32S ratios on a double-collector mass spectrometer. The isotopic data obtained were given in terms of the ~ 34S-value relative to troilite sulfur (Canyon Diablo):
1"¢34S/32S'~ ~348 ( % 0 ) = [~ ! Jsample /[34S/32S~ t Jstandard - - 1 ] X 10 3 The reproducibility of isotopic measurements was -+ 0.2 °/oo. RESULTS AND DISCUSSION Our experiments were made to determine the conditions of the reduction of sulfate-sulfur species at the temperature range of 200 ° - 3 4 0 ° C. However, signs of reduction of sulfate appeared above 250 ° C as listed in Table II. In experiment 1, H2SO4 was reduced to H2S at temperatures higher than 250°C while reduction of sulfate in experiments 2 and 3 occurred at temperatures above 300 ° C. SO2 was absent in all experiments. In the experiments of the three series, black precipitates appeared. These precipitates were identified as amorphous C b y X-ray powder diffraction. Other gases produced (in all experiments) were recognized as CO2, CH4 and H2 by gas chromatography.
Reduction o f sulfuric acid solution The time variation of the observed reduction of sulfuric acid is plotted in Fig. 1. The reduction of sulfate is fairly slow at 250 ° C, whereas at temperatures above 280°C reduction occurs rather rapidly. It is clear that the reduction rate of sulfate depends on temperature. Previous studies on the reduction of sulfate under hydrothermal conditions have been carried o u t in connection with the problem of the S-isotope effect (Harrison and Thode, 1957; Grinenko et al., 1969). However, the reducing agents used are n o t likely to be present under natural conditions. In the present study, the H2SO4 solution was reduced b y dextrose to H2S. As the atomic and/or molecular hydrogen produced by the reaction of dextrose with water m a y reduce H2SO4 to H2S under similar conditions as in nature. Judging from the amounts of sulfate and sulfide in this experiment, it is found that the loss of sulfur exceeds 40% at above 280 ° C. This fact suggests that H2 S may have reacted with sulfate to form elemental S in the acidic region as follows:
50
3H2S + H + + HSO4- -~ 4S + 4H2S The black precipitates were oxidized with concentrated HNO3 and Br2, and after evaporation, the residue was tested for the presence of other S species by addition of dilute HC1 and BaC12 solution. Other S species were detected. However, it may have been organic S since no elemental S was identified by the X-ray powder diffraction of the black precipitates. n~S
|
1
i
o"
0
301 O
25'-
-
201~°
15
I0 t~>0
i ">
0
HSO~
o 2~ "c o 280 "C
°,,
<>340"(: 0 300 "C
l
i
I0
20
I0
l
l
l
k
40
50
60
70
hours Fig. 1. Variation with time of products (sulfide) and reactants (sulfuric acid) at various temperatures.
Reduction of NariS04 and Na2S04 Fig. 2 shows the relationship between the reduction rate of sodium bisulfate and reaction time. Results of experiment 1 at 300 ° and 340°C are included for comparison. At temperature below 300 ° C reduction of sulfate is extremely difficult. The reduction in this experiment t o o k place at 300°C but is fairly slow in comparison with that of experiment 1. It is found that small amounts of H2 S are formed by reduction of sodium bisulfate even after 100 hr. Substantial reduction begins at 340 ° C. The results suggest that the reduction rate of both experiments is influenced by temperature and sulfate species rather than by pH, since the solutions of both experiments are acidic hydrothermal conditions. On the other hand, it is evident that the reduction rate of sodium sulfate solution is very low even above 340 ° C, as listed in Table I. Although molecular H2 was formed by the thermal decomposition of dextrose at temperatures lower than 300 ° C, reduction of sodium sulfate solution did not occur.
51
mgS
30o
300"C
20-
o
o~ .\
.
%\
~H~
%so~
30-
""340 "C
\ o
3
i
Fig. 2. R e d u c t i o n
4
5
6
7
8
9
10 days
o f sulfate to sulfide for e x p e r i m e n t 2 at 3 0 0 ° and
340°C.
If the reduction of sulfate is assumed to be a first-order reaction, the rate o f change in the concentration of sulfate in reaction at time t may be expressed as follows: in ( C i / C f ) = k t
where C i and Cf are the initial and final concentrations of sulfate, respectively; h denotes the rate constant; and t is the reaction time. Fig. 3 is a plot of ln(Ci/Cf) vs. time in hours for the experiments 1 and 2. The relationship is essentially a straight line, thus indicating a constant k-value, and since h is constant, it is suggested that the reaction in the experiments 1 and 2 is a firstorder one. From this figure, it is also found that the reduction rate of experiment 1 is higher than that of experiment 2.
28
• 250"C
o 241-/ o /
/
o 30o'c Hsq-
L / / /
2.0
Q8 0
. 10
Fig. 3. Ratios
20
o 300~
4 310 410
-
-
~ 50
610
I
710 810 90
,I~
I
100 hours
o f rate c o n s t a n t s during reduction at various temperatures.
52 Much or all of the organic matter present in sediments or sedimentary rocks may be lipids, humic acids and kerogen except for hydrocarbons accumulated in natural gases and associated petroleum deposits. Therefore, the present study is a laboratory simulation experiment of sulfate reduction by organic matter under hydrothermal conditions. However, hydrocarbons, lipids, humic acids and kerogen are insoluble in water, reduction of sulfate may be fairly difficult and probably not realized in nature by these organic materials. It was demonstrated in the investigation of Toland {1959) that under hydrothermal conditions sulfate reduction was not caused by hydrocarbon. However, the examination on sulfate reduction by the thermal decomposition of lipids, humic acids and kerogen is necessary to resolve the origin of sulfide species in hydrothermal solutions. All the available data now suggest that the reduction rate of sulfate is dependent on pH, temperature and dissolved sulfate species such as HSO4-, N a S O ~ and SO42-. Judging from the result of experiment 3, reduction of seawater sulfate by organic matter may be extremely difficult under hydrothermal conditions.
Kinetic isotope effect in reduction of sulfate The question of whether sulfide minerals in the Kuroko- and Red Sea-type deposits can form as a result of reduction of seawater sulfate under hydrothermal conditions must be answered not only from the viewpoint of reduction of sulfate, b u t also from an isotope effect in the reduction. The isotopic fractionation in the chemical reduction of sulfuric acid solution by dextrose was indeed observed. As shown in Table I, the S-isotope compositions of sulfate and hydrogen sulfide in experiments 1 and 2 changed during the process of reduction. In experiment 3, however, the 534S-values of sulfate do not vary with the time of reaction. Since the reduction rate of sulfate is very low in this experiment and amounts of hydrogen sulfide produced are t o o small, although the reduction occurred, the 534S of sulfate may be constant. The relationship between 534S of sulfur species in experiments 1 and 2 and reaction time are shown in Fig. 4. The S-isotopic composition of residual sulfate in experiment 1 increases from --4.4 to +11.0 %0. On the other hand, 5 34S of the hydrogen sulfide produced changes from --12.5 to - 6 . 0 %o 280 ° C. The fractionation values between sulfate and hydrogen sulfide can be seen to increase with the reaction time. At 300 ° and 340°C the 534S-value of sulfate increases rapidly and the isotopic composition of hydrogen sulfide approaches to 5 34S-value of the initial sulfate. On the other hand, the isotopic composition of sulfate increases slightly throughout the reduction at 250 ° C. Fig. 4 demonstrates that below 250°C the reduction is very slow. In experiment 2, a similar result was obtained and the 5 34S of sulfate increased during the process of reduction. However, the isotopic composition of hydrogen sulfide decreased.
53
%. 16-
0 250"C o 280"C ¢' 300"C 0340"(:
12-
¢'340"C
84-
~¢--_
r/
/
0-
J
H50~-
-8-12-
4~
Us
0 hours
Fig. 4. Changes in
6
s4S-values of sulfide and sulfate vs. reaction time.
3~
~S-~
15
I0
/
50 "c
I
0.4
0.8
1.2
I
1.6
26
2B
I
3.2 - Inf
Fig. 5. Relation between reduction.
6
s4S-w 'les and the fraction of sulfate remaining in the sulfate
54 Our results dent
suggest
on the reduction
that
S-isotopic
ratio
compositions
of sulfate
of sulfate
as in the following
remained
depen-
equation:
6--60=103(a-1)lnf where sulfate
5 0 is t h e when
fractionation
6 34S-value of the original
the fraction factor
of the sulfate
for this process.
sulfate; remaining
When
6 is the 6 34S-value of the is f; and a is the kinetic
(6 - - 6 0 ) - v a l u e s a r e p l o t t e d
T A B L E II Results from reduction experiments Temperature (o C)
Time (hr.)
SO4 ~( m g S)
~ 34S
H2S ( m g S)
(%0)
Experiment 1 : 250 0 250 24 250 48 250 70 280 6 280 12 280 16 280 22 300 4 300 6 300 7 300 8.5 340 3 340 4
28.5 25.0 22.8 22.4 18.5 10.8 5.5 1.4 17.5 8.2 2.7 2.2 9.4 1.2
-- 4.4 - - 3.5 -- 2.6 - - 2.3 -- 0.59 + 5.2 +11.0 n.d. -- 0.1 + 6.6 +14.2 +14.4 + 4.7 n.d.
--
E x p e r i m e n t 2: 300 0 300 50 300 96 300 144 300 180 340 8 340 48 340 72
28.3 26.9 26.5 25.4 19.4 24.4 17.7 12.5
-----+ + +
2.8 2.2 2.2 1.1 0.14 1.1 1.6 3.4
--
Experiment 300 300 300 340 340 340
28.6 27.9 28.1 28.2 26.5 27.0
+ + + + + +
2.7 2.2 2.5 2.1 2.6 2.3
--
-
-
5 34S (%o)
1.4 2.8 3.3 4.5 7.3 10.0 12.4 4.4 5.9 11.8 12.0 6.0 11.9
0.2 0.5 2.9 4.9 1.2 6.1 7.5
-n.d. n.d. --14.2 --12.5 -- 9.6 -- 7.5 -- 6.0 --13.0 -- 7.9 -- 5.3 -- 5.0 -- 9.1 -- 4.7
-n.d. n.d. -- 3.0 -- 8.6 n.d. -- 8.2 --12.4
3: 0 168 340 48 170 236
= n o t a n a l y s e d ;'n.d. = n o t d e t e r m i n e d .
0.2 0.1 0.2 1.9 1.1
-n.d. n.d. n.d. n.d. n.d.
as in
55 Fig. 5 against t h e f r a c t i o n o f t h e sulfate r e m a i n i n g (f), we see t h a t t h e f a c t o r 103(a - - 1) is i n d e p e n d e n t o f t h e t e m p e r a t u r e in t h e range o f 2 5 0 o - - 3 4 0 ° C. F r o m t h e d a t a in Table II, a n d using this e q u a t i o n , t h e f a c t o r 103(a - - 1) has b e e n c a l c u l a t e d f o r t h e S-isotopic variations o f e x p e r i m e n t s 1 a n d 2. These values are s h o w n in Table III. N o d e p e n d e n c e o f f r a c t i o n a t i o n o n t e m p e r a -
TABLE III Isotope effect in reduction of sulfate f
--In f
103 (~ -- 1)
+ 0 + 0.9 + 1.8 + 2.1 + 3.8 + 9.6 +15.4 + 4.3 +11.0 +18.6 +18.8 + 9.1
1.000 0.877 0.800 0.786 0.649 0.379 0.193 0.614 0.288 0.095 0.077 0.330
0 0.133 0.222 0.241 0.433 0.972 1.64 0.488 1.25 2.35 2.56 1.11
-6.77 8.11 8.71 8.78 9.88 9.39 8.81 8.80 7.91 7.34 8.27
0 0.6 0.6 1.7 2.7 1.7 4.4 6.6
1.000 0.951 0.936 0.898 0.686 0.862 0.625 0.442
0 0.05 0.06 0.108 0.377 0.148 0.469 0.817
Temperature
Time
~ 34Sso42-*
(°C)
(hr.)
(%o)
Experiment 250 250 250 250 280 280 280 300 300. 300 300 340
1:
0 24 48 70 6 12 16 4 6 7 8.5 3
Experiment 2: 300 0 300 50 300 96 300 144 300 180 340 8 340 48 340 72
+ + + + + + +
-
-
12.0 10.0 15.7 7.16 11.5 9.38 8.08
"5 S4Ss04 2 - - - ~ 3 4 8 f - ~34Si; f = final; i = initi~.
t u r e was d e t e c t e d . T h e s e p a r a t i o n f a c t o r is 1 . 0 0 8 - - 1 . 0 0 9 f o r t h e t e m p e r a t u r e interval f r o m 2 5 0 ° t o 3 4 0 ° C . This indicates t h a t t h e light i s o t o p e o f t h e sulfate ion (32SO42-) reacts 8 . 0 - 9 . 0 °/00 faster t h a n t h e h e a v y i s o t o p e in this t e m p e r a t u r e range. A t t e m p e r a t u r e s a b o v e 3 0 0 ° C, the kinetic r e a c t i o n , w h i c h favours t h e h e a v y i s o t o p e in sulfate and the light entering t h e h y d r o g e n sulfide, p r o c e e d s rapidly. In t h e e x p e r i m e n t s o f s o d i u m bisulfate r e d u c t i o n , it is f o u n d t h a t t h e s e p a r a t i o n o f i s o t o p e s t h a t o c c u r r e d at t e m p e r a t u r e s a b o v e 3 0 0 ° C is slightly higher t h a n t h a t o f e x p e r i m e n t I . T h e kinetic f r a c t i o n a t i o n in r e d u c t i o n o f s u l f a t e m a y d e p e n d o n t h e p H and sulfate species.
56 Fig. 6 gives the kinetic fractionations and their correlation with temperature, together with other data (Harrison and Thode, 1957; Grinenko et al., 1969). The results obtained in the reduction of sulfuric acid to hydrogen sulfide and to SO: at different temperatures by Grinenko et al. (1969) fall on the same curve. This fact indicates that reduction is a multistage reaction and that the main stage of kinetic effect is reduction of sulfate to sulfite. In the experiment 3, no isotopic fractionation in the chemical reduction was observed, even at temperatures above 300 ° C. Therefore, re-examination on the reduction of sodium sulfate solution is necessary to obtain the separation factor during the process of reduction.
b H~rlson and Thode() 957) o Gr irenkoet a1.(1969)
20 I-
~ Th~ work (Exp't) (Exp.:~
o~\ o,
]4 12
~:~o
~o
o
8 4 I
0
1oo
200
300
400
"C
Fig. 6. Dependence of kinetic isotope effect on temperature.
REFERENCES Grinenko, V.A., Grinenko, L.N. and Zagryazhskaya, G.D., 1969. Kinetic isotope effect in high temperature reduction of sulfate. Geokhimya, 4: 484--491. Harrison, A.G. and Thode, H.G., 1957. The kinetic isotope effect in the chemical reduction of sulphate. Trans. Faraday Soc., 53: 1648--1651. Iwasaki, I. and Ozawa, T., 1960. Genesis of sulfate in acid hot spring. Bull. Chem. Soc. Jpn., 33: 1018--1019. Kajiwara, Y., 1973. A simulation of the Kuroko type mineralization in Japan. Geochem. J., 6: 193--209. Malinin, S.D. and Khitarov, N.I., 1969. Reduction of sulfate sulfur by hydrogen under hydrothermal conditions. Geokhimya, 11: 1312--1318. Oana, S. and Ishikawa, H., 1966. Sulfur isotopic fractionation between sulfur and sulfuric acid in the hydrothermal solution of sulfur dioxide. Geochem. J., 1 : 45--50. Shanks, W.C. and Bischoff, J.L., 1977. Ore transport and deposition in the Red Sea geothermal system; a geochemical model. Geochim. Cosmochim. Acta, 41: 1507--1519. Toland, W.G., 1959. Oxidation of organic compounds with aqueous sulfate. J. Am. Chem. Soc., 82: 1911--1916.