Effects of Metals and Ni3S2 on Reactions of Sulfur Species (HS–, S

Article pubs.acs.org/IECR

Effects of Metals and Ni3S2 on Reactions of Sulfur Species (HS−, S, and S2O32−) under Alkaline Hydrothermal Conditions Yuanqing Wang,† Fengwen Wang,†,‡ Fangming Jin,*,§ and Zhenzi Jing∥ †

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China ‡ School of Resources and Environment, Anhui Agricultural University, 130 West Changjiang Road, Hefei 230036, China § School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China ∥ School of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China S Supporting Information *

ABSTRACT: The reactions of sulfur species (HS−, S, and S2O32−) under hydrothermal conditions are an important scientific subject due to their implications in biology and geology and potential application for hydrogen production as well. In this paper, effects of representative metals and alloy such as Ni, Fe, Co, W, Hastelloy C-276 alloy, SUS 316 alloy, and sulfide mineral Ni3S2 on reactions of sulfur species under alkaline hydrothermal conditions were studied. The results showed that there are mainly three types of reactions among sulfur species, H2O, and metal (or Ni3S2): (i) sulfidation of metals and Ni3S2 by sulfur species; (ii) disproportionation of sulfur species; (iii) oxidation of sulfur species by water to produce hydrogen. Co and Ni can be sulfided, and W remains unchanged in the presence of HS−. The presence of Ni and Ni3S2 would significantly enhance the transformation of S2O32− into SO32− and SO42−. This study would give clues to sulfur chemistry within hydrothermal waters and guide the selection of the reaction pathway of sulfur species by adding metals or Ni3S2. changes of valence state.4,5,14−18 Sulfide (HS− or S2−) can be oxidized readily in air particularly in the presence of heavy metals or on exposure to light, with the dominant product being elemental sulfur (S) and a small portion of sulfur oxyanions such as SO42−.19 HS− or S2− may also react with S to form polysulfide (Sn2−), which was dark brown in aqueous solution and plays a crucial role in pulp industry.20 Recent research21 showed that the hydrolysis of S was dependent on the reaction temperature and pH. Under alkaline conditions at a reaction temperature of 170 °C, the main disproportionation mechanism takes place according to eqs 2 and 3.21 At the neutral pH region, H2S and H2SO4 are mainly formed.22

1. INTRODUCTION The reactions of sulfur species under hydrothermal conditions, such as H2S, HS−, S2−, S22−, S, S2O32−, SO32−, SO42−, etc., have attracted a lot of attention in recent years in biochemical, geochemical, energy, and environmental research. Hydrogen sulfide (H2S) has been reported to take part in the reduction of CO2 into an organism with a flow of volcanic exhalations, which was related to the origins of life.1−3 Thiosulfate (S2O32−) may play a crucial role as a ligand in the formation of ore deposits, including Au, Ag, and Pt-group ore deposits and as an intermediate in sulfur isotope exchange between sulfate (SO42−) and dissolved sulfide in hydrothermal solutions.4,5 Moreover, much research6,7 on the hydrothermal treatment of sulfur-containing compounds like upgrading of heavy oil have found the formation of H2S and suggested that it was effective to the wall of the alloy lined reactor, especially in supercritical water.8,9 Hydrogen sulfide itself is a highly toxic waste with mass production and should be well considered.10 Recently, a novel hydrogen process11,12 was proposed with HS− as a reductant under hydrothermal conditions according to the following reaction: HS− + H 2O → S2 O32 − + SO32 − + SO4 2 − + H 2↑

(2)

(n − 1)Na 2SO3 + H 2Sn = (n − 1)Na 2S2 O3 + H 2S

(3)

Air oxidation of S2O32− is a very slow process with less than 10% change for aeration of 4 months under normal pressures and temperatures,14 but it disproportionates in acid solutions at high temperature.23 Xu et al.23 showed that, at pH 4 and 150 °C, S2O32− decomposes according to eqs 4 and 5.

(1)

In addition, sustainable hydrogen production could be available through regenerating the sulfur oxyanions into HS− in a glucose solution at 105 °C.13 Therefore, the understanding of the mechanism of reaction of sulfur species under hydrothermal conditions is of great importance. However, the chemistry of sulfur species is very complex in aqueous solution due to the existence of various types of redox reactions and disproportionated reactions which lead to the © 2013 American Chemical Society

(2n + 1)S + 2NaOH + H 2O = Na 2SO3 + 2H 2Sn

S2 O32 − = SO32 − + S

(4)

3SO32 − + 2H+ = 2SO4 2 − + S + H 2O

(5)

Received: Revised: Accepted: Published: 5616

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Sulfite (SO32−) can be easily oxidized to stable SO42− in air especially in the presence of Fe3+ and Cu2+ in acidic solution.19 However, the reaction of sulfur species under hydrothermal conditions is far from fully understood,5,24−26 especially in the presence of other substances, such as metals and mineral. Generally, metals including alloys and mineral can react with sulfur species within hot compressed water environment in the industrial and natural process. These facts make the clarity of effects of metals and mineral on the reaction of sulfur species under hydrothermal conditions highly desirable. Our previous research has revealed the promotion effect of Co, Ni, and W27 and the catalytic role of Ni3S212 on hydrogen production with HS− as a reductant under hydrothermal conditions. Besides, the reaction mechanism of HS− and Fe under hydrothermal conditions was proposed according to the following equations.28 Fe + HS− + H 2O = FeS + H 2 ↑ + OH−

(6)

FeS + OH− = FeO + HS−

(7)

3FeO + H 2O = Fe3O4 + H 2↑

(8)

mL of degassed water were added into the reactor, which was subsequently sealed and placed in an oven that had been preheated to the desired temperature. The reactor was purged with nitrogen for 5 min prior to the reaction. The heating up rate in the reactor used in this study is hard to measure due to its structure. However, in our previous report,38 the heating up rate in a steel based reactor was measured at 8 °C/min. The shortest reaction time was set at 1 h in this study to ensure the reactor reaches the desired temperature. The reaction time was defined as the time of the reactor staying in the oven, and did not include cool-down time which takes about 1 h. After a given time, the reactor was taken out of the oven and cooled to room temperature. Thus, the reaction time defined is a little shorter than the real reaction time. However, this should do not affect our conclusion. The initial concentration of sulfur species (Na2S·9H2O, S, Na2S2O3) was 200 mM based on sulfur atoms in the experiments. The initial pH of the reaction solution was adjusted to about 13. Variations of pH value before and after the reactions are shown in Table S1 (Supporting Information). Another experiment was conducted in a SUS 316 alloy tubing reactor to investigate the wall corrosion of SUS 316 alloy by HS− at 300 °C, and experimental details can be found in our previous report.31 2.3. Product Analysis. Sulfur anions (HS−, S2O32−, SO32−, and SO42−) were characterized by capillary electrophoresis (P/ ACE MDQ, Beckman) equipped with a UV detector.12,28 Typical electropherograms of liquid sampes after reactions were given in Figure 2. The percentage of sulfur anions was calculated on the basis of total input sulfur atoms. The liquid samples were diluted 50 times and tested immediately after opening the reactor within 2 h because of the oxidation of HS− in the air. The solid samples collected were analyzed by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation at an acceleration voltage of 40 kV and an emission current of 40 mA. The cross section morphology and compositions of two alloy specimens after hydrothermal treatment of HS− solution were characterized by scanning electron microscopy (SEM, Magellan 400, FEI) attached to an energy-dispersive X-ray analysis system (EDS, Oxford). Prior to cross section SEM observations, the specimens were mounted in resin to ensure the intactness of the films. The gas samples were separated by a carbon molecular sieve (TDX-01) column and detected with TCD in a GC system (HP-5890 Series II).

Thus, the aim of the present study was to investigate the effects of metals and Ni3S2 on the reactions of sulfur species under alkaline hydrothermal conditions. Fe, Co, Ni, and W were selected as the model metals because they are the main constitutions of alloy. Alloys (Hastelloy C-276 alloy and SUS 316 alloy) were also used in this study. Ni3S2, called heazlewoodite and a sulfur poor nickel sulfide mineral found in serpentinitized dunite, was selected to represent mineral.

2. MATERIALS AND METHODS 2.1. Materials. Na2S·9H2O (Sigma-Aldrich, ≥98%) was used as a source of sulfide to simplify handling. The pH of the reaction solution at room temperature and 101 KPa is between 12 and 13; thus, HS− is the primary species in solution. For example, the concentration of HS− is 193 mM in a 200 mM Na2S·9H2O solution at a pH of 12.7. Na2S2O3 (≥98.0%) and Ni3S2 (99.7% trace metals basis, 150 mesh) were purchased from Sigma-Aldrich. Sulfur (≥99.5%), Na2SO4 (≥99.0%), Fe (200 mesh, 96%), Co (200 mesh, ≥99.0%), Ni (200 mesh, ≥99.5%), and W (200 mesh, ≥99.8%) were purchased from Sinopharm Chemical Reagent Co., Ltd. SUS 316 alloy powder (40−80 mesh) was purchased from Alfa Aesar. Two specimens of Hastelloy C-276 alloy plate (1 cm × 1 cm × 0.2 cm, about 2.8 g) and SUS 316 alloy tubing (12 cm long, 3/8 in., 1 mm wall thickness) were provided by Haynes International and Swagelok, respectively. The main compositions of Hastelloy C276 alloy are 57 wt % Ni, 16 wt % Cr, and 16 wt % Mo with a small number of Fe, W, Co, and Mn. The main compositions of SUS 316 alloy are 68 wt % Fe, 11 wt % Ni, and 17 wt % Cr with a small number of Mo and Mn. 2.2. Experimental Procedure. Although our previous research has discussed the effects of zerovalent metals27,28 and Ni3S212 on hydrogen production with HS− as a reductant under hydrothermal conditions, it cannot eliminate the interference of wall material when using a Hastelloy alloy or SUS 316 alloy lined reactor. To avoid this, most experiments were conducted in a Teflon-lined stainless steel batch reactor with an inner volume of 30 mL in this study. Its highest operational temperature is 250 °C. The experimental procedure has been described in detail elsewhere.29,30 Briefly, the desired amount of starting materials (sulfur species and metals or Ni3S2) and 10

3. RESULTS AND DISCUSSION 3.1. Oxidation of HS− Solution in Air. Since HS− can be oxidized readily in air with the dominant product being elemental sulfur and a small portion of sulfur oxyanions, to determine the rate and extent of oxidation of HS− solution in the stage of experimental setup, freshly prepared HS− solution was tested continually by capillary electrophoresis (CE) under ambient environment with increasing time. Figure 1 illustrates the trend of oxidation of HS− solution in air. As shown in Figure 1, HS− underwent slow oxidation into S2O32−, SO32−, and SO42− when exposed in the air. This result is similar to the findings of Petre et al.32 However, in their study,32 S2O32− was the dominant ion in all the sulfur species after 40 min, which was not observed in this study. This disparity may be caused by the difference of the initial pH in the solution because the specific rate of oxidation of sulfide is largely dependent on the pH, which “increases significantly as pH increases through 7 to a maximum of pH 8.0 or so, then decreases to a minimum near pH 9, increases again near pH 11 and finally decreases in more 5617

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value by subtracting the corresponding value reacting from the blank HS− solution which was absent of metals and Ni3S2. 3.2. Effects of Metals and Ni3S2 on the Reactions of HS− under Hydrothermal Conditions. Experiments with HS− in the presence/absence of metals, including Hastelloy C276 and SUS 316 alloys, and Ni3S2 were conducted at 250 °C for 12 h in the Teflon lined reactor under alkaline hydrothermal conditions. In the absence of metals and Ni3S2 (referred to as blank), the percentage of remaining HS− after the reaction was 87.0% with a little formation of 4.5% S2O32−, 1.7% SO42−, and 4.6% SO32−. As pointed out above, the formation of a little amount of S2O32−, SO42−, and SO32− can be partly attributed to the setup of experiments including analysis procedure. Another reason for this was the oxidation of HS− by water via eq 1 that can also lead to formation of a sulfur oxyanion.11−13,27 One method of evaluating the extent of oxidation of HS− by water is to determine the hydrogen production via eq 1. Our previous research12 showed that considerable hydrogen amount (H2/ HS−: 1.7) can be produced in the Hastelloy C-276 lined reactor at 300 °C for 2 h and the addition of Ni3S2 would increase the hydrogen production (H2/HS−: 2.6). However, in a capillary quartz reactor, 2.1 × 10−3 (H2/HS−) hydrogen was obtained from HS− and water after the reaction at 300 °C for 2 h.12 These results revealed the enhancement effects of Ni3S2 and alloy on the oxidation of HS− by water. On the other hand, the very low yield of hydrogen production in the capillary quartz reactor suggests that the oxidation of HS− by water is limited in the absence of metals and Ni3S. That is, HS− itself is very stable under oxygen-free alkaline hydrothermal conditions. Table 1 shows the effects of different metals and Ni3S2 on the relative distributions of sulfur species after the reactions. As shown in Table 1, there was a decrease in S2O32− for all metals and Ni3S2. Furthermore, in the case of adding Co, Ni, and Ni3S2, there was no S2O32− species detected, indicating these materials would promote the transformation of S2O32− or inhibit its formation. In the case of adding W and Ni3S2, there was an increase in SO32−, which was also found by our previous research in the Hastelloy alloy lined reactor.27 Besides, two unknown peaks (peaks 5 and 6 in Figure 2, possibly attributed to S3O62−, S2O82−, or S4O62−)35 were found in the electropherograms of liquid samples after the reactions in the case of adding W and Hastelloy alloy, suggesting that HS− was oxidized into new sulfur oxyanions due to their presence. In the case of adding Fe, the remaining percentage of HS− increased obviously compared to the blank solution after both 12 and

Figure 1. Oxidation of HS− solution under atmosphere with time (initial concentration: 4 mM Na2S·9H2O, initial pH 11.9). Error bars represent standard deviations of duplicate tests.

alkaline solutions”.33 In Figure 1, it was found that the percentage of detected sulfur species (HS− + S2O32− + SO32− + SO42−) decreased after 2 h, which may be due to the accumulation of Sn2− and elemental sulfur (S8).32 The overall reaction can be summarized as the following equations based on the previous studies: nHS− + (n − 1)/2O2 = Sn 2 − + H 2O + (n − 2)OH− (9)

Sn

2−

+ 3/2O2 = S2 O3

2−

+ (n − 2)/8S8

(10)

Sn 2 − + 3/2O2 = SO32 − + (n − 1)/8S8

(11)

SO32 − + 1/2O2 = SO4 2 −

(12)

32−34

where n = 2−4. In this regard, the oxidation process of HS− solution cannot be fully eliminated in the air even in a very short time. The products of S2O32−, SO32−, and SO42− from HS− through hydrothermal reactions can be mixed with the oxidation products of HS− in the air during setup of experiments. Therefore, to minimize the interference when starting from HS− reaction solution, the data of HS−, S2O32−, SO32−, and SO42− should be discussed in the sense of relative

Table 1. Relative Distributions of Sulfur Species after the Reactions in Addition of Metals and Ni3S2 by Subtracting the Results Reacting from the Blank HS− Solution from the Actual Values additive Fe Fe Co Ni W Ni3S2 Ni3S2 Hastelloy SUS 316

reaction temperature (°C) 250 250 250 250 250 250 250 250 250

reaction time (h)

S2O32− (%)

12 48 12 12 12 12 48 12 12

−2.7 ± 1.5 (4.0 ± 1.1) −4.2 ± 1.6 (1.0 ± 1.4) N.D. N.D. −3.3 ± 0.1 (3.4 ± 2.5) N.D. N.D. −3.4 ± 0.0 (0.7 ± 0.0) −3.1 ± 0.1 (0.9 ± 0.1)

a

SO42− (%)

HS− (%)

SO32− (%)

N.D. −0.9 ± 0.3 (0.5 ± 0.2) N.D. −1.5 ± 0.3 (0.9 ± 0.5) 0.1 ± 0.7 (2.4 ± 0.1) −0.9 ± 0.7 (1.4 ± 0.1) −0.7 ± 0.3 (0.7 ± 0.1) −0.8 ± 0.0 (0.7 ± 0.0) −0.4 ± 0.2 (1.1 ± 0.2)

7.2 ± 3.3 (93.9 ± 3.9) 15.1 ± 1.2 (91.1 ± 0.8) −29.4 ± 4.4 (57.3 ± 4.9) −49.0 ± 2.1 (37.7 ± 1.5) 12.2 ± 6.1 (98.9 ± 6.6) −5.8 ± 2.3 (80.9 ± 2.8) −0.8 ± 0.8 (75.2 ± 0.4) 5.3 ± 1.2 (92.7 ± 1.2) 1.6 ± 1.9 (89.0 ± 1.9)

0.4 ± 1.2 (5.0 ± 1.2) −0.8 ± 0.5 (2.0 ± 0.5) −0.9 ± 1.9 (3.8 ± 2.0) −0.5 ± 1.7 (4.1 ± 1.7) 2.9 ± 1.1 (7.5 ± 1.1) 2.6 ± 1.3 (7.1 ± 1.3) 1.5 ± 0.1 (4.2 ± 0.1) 0.4 ± 0.3 (3.7 ± 0.3) 0.6 ± 0.5 (3.9 ± 0.5)

b

a

The numbers in parentheses represent the absolute values before subtraction of the blank. bNot detected. Initial concentration: 200 mM Na2S·9H2O, 2 mmol of Fe, 2 mmol of Co, 2 mmol of Ni, 2 mmol of W, 2 mmol of Ni3S2, 0.1 g of SUS 316 alloy powders, one Hastelloy C-276 alloy plate (about 2.8 g). Error bars represent standard deviations of duplicate or triplicate tests. 5618

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Our previous report12 showed that the existence of corrosion of HS− to Hastelloy C-276 alloy and SUS 316 alloy under alkaline hydrothermal conditions and corrosion products were identified as Ni3S2 and Fe3O4 by XRD results, respectively. However, the details of corrosion structure were not determined. In this study, the cross section morphology and compositions of these two alloys after hydrothermal treatment of HS− have been characterized by SEM/EDS. Figure 4a illustrates the backscattering electron (BSE) cross section of film grown on Hastelloy C-276 alloy in HS− solution at 250 °C for 12 h which was found to be Ni3S2 by XRD results.12 The BSE cross section image shows that the average thickness of the sulfide film was about 40 μm. The EDS line scan profile as shown in Figure 4b indicates that in addition to Ni3S2 there still existed minor phases of oxide or chromium sulfide in the film that were lower than the detection limit of XRD. As shown in Figure 4c,d, the sulfide film exhibited a layered structure dotted with small particles. Furthermore, another EDS mapping of the layered structure was conducted and given in Figure 5. As shown in Figure 5, it was composed of elements of Ni, S, and O which could be Ni3S2 and nickel oxide. The characterization of the cross section of SUS 316 alloy was shown in Figure 6. The film grown appeared to be a duplex layer with a total depth of 300−400 nm, as also has been reported in other study of steel in oxidizing high temperature water.37 Fe, Ni, O, and S were present in this dual layer (Figure 6c−g). 3.3. Effects of Fe and Ni3S2 on the Reactions of S. Sulfur is mainly disproportioned into S2O32− and HS− under alkaline hydrothermal conditions without the addition of metals and Ni3S2.21 To investigate the effects of Fe and Ni3S2, the reactions of sulfur in addition with Fe or Ni3S2 were conducted. The quantitative results of sulfur anions after the reactions were given in Figure 7. As shown in Figure 7a, in the case of adding Fe and S, S2O32− and HS− were mainly formed with traces of SO42− and SO32−, which is similar to the results of a previous report.21 The percentage of S2O32− and HS− increased in the first hour, and then S2O32− decreased slightly with a minor increase of HS−. About 40−45% of sulfur species (S2O32− + SO42− + HS− + SO32−) were detected after reactions in the liquid samples. As for characterization of solids by XRD shown in Figure 8a, with the increase in reaction time from 1 to 4 h, the patterns ascribed to FeS increased while the patterns ascribed to Fe decreased. At 12 h, patterns ascribed to FeS2 appeared with a decrease of patterns of FeS. Therefore, the rest of the portion of sulfur not detected in liquids reacted with Fe according to eqs 16 and 17.

Figure 2. Typical electropherograms of liquid samples after reactions (the initial concentration: 200 mM Na2S·9H2O, (a) without addition of metals and Ni3S2, (b) W, (c) Hastelloy C-276 alloy, 250 °C, 12 h).

48 h. It suggested that Fe would inhibit the oxidation of HS− by H2O under alkaline hydrothermal conditions. However, Fe would enhance the formation of polysulfides by reacting with HS−.28 In sum, the difference of the distribution of sulfur oxyanions (S2O32−, SO32−, and SO42−) from HS− at 250 °C between the case of adding metals and Ni3S2 and the case of blank solution is not so distinctive as the results obtained at 300−350 °C.11,12,27 The different reaction temperatures may account for the distinction of results, since hydrothermal water exhibited great changes of physical and chemical properties from 250 to 350 °C.36 Solid residues after the reactions were characterized by XRD, and the corresponding patterns were given in Figure 3. As shown in Figure 3, Co was completely sulfided to Co9S8; Ni was partly sulfided to Ni3S2; and Ni3S2 was partly sulfided into Ni7S6 and NiS. Therefore, the remaining percentage of HS− decreased compared to the blank solution, especially in the case of adding Co and Ni (Table 1). Although hydrogen cannot be quantified in this study due to the limitation of the reactor used, a quick gas sampling using a syringe from the headspace of the reactor immediately after opening the reactor is possible and hydrogen was detected in all the gas samples. These results suggest that HS− may react with metals under alkaline hydrothermal conditions according to the following reactions. 8HS− + 9Co + 8H 2O = Co9S8 + 8OH− + 8H 2

(13)

2HS− + 3Ni + 2H 2O = Ni3S2 + 2OH− + 2H 2

(14)

(16)

FeS + S = FeS2

(17)

Similarly, in the case of adding Ni3S2, S2O32− and HS− were also found and increased in the first hour (Figure 7b). However, at 2 h, the percentage of S2O32− decreased with an obvious increase of SO32− which was not detected in the case of adding Fe. S2O32− and HS− became stable with a slight decrease of SO32− after 2 h. About 32−39% of sulfur species were determined in the liquids. As shown in Figure 8b, the solid residues were all mainly NiS. At the first hour, there existed a small phase of NixS6. As the reaction time further increased to 2 h and even longer, the patterns of NixS6 nearly disappeared, indicating the sulfidation of Ni3S2 almost stopped after 1 h. Thus, the formation of NiS can be formed according to eq 18.

3Ni3S2 + 2HS− + 2H 2O = 2NiS + Ni 7S6 + 2OH− + 2H 2

Fe + S = FeS

(15)

It should be noted that Ni3S2 maintained its valence state contacting with HS− solution in the Hastelloy C-276 alloy lined reactor,12 which may be due to the protection of Hastelloy C276 alloy as a sacrificial agent. In addition, according to XRD results, Fe was oxidized into Fe3O4 rather than sulfidation (Figure 3a). The reaction mechanism of Fe with HS− was proposed in eqs 6−8.28 W did not change after the reaction (Figure 3e), suggesting it is a good element for antisulfidation under hydrothermal conditions.

Ni3S2 + S = 3NiS 5619

(18)

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Figure 3. XRD patterns of (a) Fe, (b) Ni, (c) Ni3S2, (d) Co, and (e) W after reactions (the initial concentration: 200 mM Na2S·9H2O, 100 mM Na2S2O3, 2 mmol of Fe, 2 mmol of Ni, 2 mmol of Ni3S2, 2 mmol of Co, 2 mmol of W, 250 °C).

The theoretical ratio of S2O32− to HS− based on sulfur atoms by a complete sulfur disproportion should be 1:1. However, the actual percentages of S2O32− were higher than HS− in the case of adding Fe or Ni3S2. Thus, the excess of S2O32− should be attributed to oxidation of sulfur by water (eq 19), confirmed by the detection of H2 that remained in the headspace of reactor. 2S + H 2O + 2OH− = S2 O32 − + 2H 2

tion), in the case of adding Fe and S, the pH value first dropped to 7.9 at 1 h and then slightly increased to 8.3 at 4 h and 11.1 at 12 h. While in the case of adding Ni3S2 and S, the pH value dropped to 9.9 at 1 h and then increased to a stable 11.7 at 4 h. Both drops of pH in the first hour can be explained by the nature of consuming alkali of sulfur disproportion (eqs 2 and 3) and sulfur oxidation (eq 19). In addition, when starting from HS−, there was a slight rise of pH after the reactions. When starting from S2O32−, the pH almost maintained the same. Thus, the increase of pH in the latter stages in the case of

(19)

The pH value of liquid samples before and after the reactions was examined. As shown in Table S1 (Supporting Informa5620

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Figure 4. (a) Backscattering electron cross section of Hastelloy C-276 alloy, (b) EDS lines scan profile of cross section of Hastelloy C-276 alloy, (c and d) SEM images of sulfide film of Hastelloy C-276 alloy (reaction conditions: 200 mM Na2S·9H2O, Hastelloy C-276 alloy, 250 °C, 12 h).

Figure 5. (a) Backscattering electron image and (b−f) EDS mapping of the sulfide film of Hastelloy C-276 alloy (reaction conditions: 200 mM Na2S·9H2O, Hastelloy C-276 alloy, 250 °C, 12 h).

adding S was possibly due to the presence of HS− through its producing alkali reactions such as eq 15. 3.4. Effects of Metals and Ni3S2 on the Reactions of S2O32−. For transformation of S2O32− in addition with metals and Ni3S2, the quantitative results were given in Figure 9. As

shown in Figure 9a, in the absence of metals and Ni3S2, the conversion of S2O32− was very slow. After 24 h, only 35% S2O32− was transformed with the main formation of SO32− and SO42− and a few HS−. In the case of adding Fe, similar results were observed (Figure 9b). After 8 h, about 20% S2O32− was 5621

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Figure 6. (a, b) Backscattering electron cross section of SUS 316 alloy tubing and (c−g) EDS mapping of image b (reaction conditions: 500 mM Na2S·9H2O, 300 °C, 2 h).

Figure 7. Distributions of sulfur species after reactions with different additives and reaction times (the initial concentration: 200 mM S, 2 mmol of Fe, 2 mmol of Ni3S2, 250 °C). Error bars represent standard deviations of duplicate tests. Figure 8. XRD patterns of (a) Fe and (b) Ni3S2 after the reactions at 250 °C (initial concentration: 200 mM S, 2 mmol of Fe, 2 mmol of Ni3S2).

transformed and SO32−, SO42−, and traces of HS− were formed. However, in the case of adding Ni or Ni3S2, the conversion of S2O32− was very fast (Figure 9c,d). In the initial 2 h, the 5622

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Figure 9. Distributions of sulfur species after reactions with different additives and reaction times (the initial concentration: 100 mM Na2S2O3, 2 mmol of Fe, 2 mmol of Ni, 2 mmol of Ni3S2, 250 °C). Error bars represent standard deviations of duplicate tests.

formation of SO32− and SO42− increased significantly with a sharp decrease of S2O32−. After 8 h, almost all S2O32− were consumed with about 50% (SO32− + SO42−) formed. These results indicate that Ni and Ni3S2 can significantly promote the transformation of S2O32−. It is noteworthy that the formed S2O32− as shown in Figure 7b was relatively stable after 2 h in the case of adding S and Ni3S2 maybe because Ni3S2 was already fast sulfided into NiS after 2 h in the above case (Figure 8b) and NiS cannot transform S2O32−. From Figure 3a−c, Fe maintained its zero valence after the reaction, Ni was partly sulfided to Ni3S2, and Ni3S2 was nearly all sulfided to NiS after the reactions. In the case of adding Ni and Ni3S2, about half the sulfur species were not detected in the liquid samples after the reactions. The other half of the sulfur species reacted with Ni and Ni3S2 to form Ni3S2 and NiS, respectively. According to eqs 4 and 5,23 S would be formed as a final product in the transformation of S2O32− under acidic hydrothermal conditions. Therefore, one reasonable explanation for the transformation of S2O32− under alkaline hydrothermal conditions may be that elemental sulfur was also first formed and sulfided Ni and Ni3S2 consecutively. The sulfidation process would move the equilibrium of reactions 4 and 5 forward to produce more SO32− and SO42−. However, it cannot be explained why the presumably formed elemental

sulfur could not sulfide Fe, which was possible, as shown in Figure 8a. Furthermore, if there existed sulfur, then HS− should have been formed in the case of adding Ni3S2, as shown in Figure 7b, and sulfide cannot be fully consumed under hydrothermal conditions as discussed. The truth was no peak of HS− was found in the electropherograms of liquid samples both in the case of adding Ni and Ni3S2 when starting from S2O32−. These results suggest that the reaction pathway of S2O32− with Ni or Ni3S2 was different from eqs 4 and 5. In other words, it can be considered that the whole reaction was through one step in the case of adding Ni and Ni3S2 without the formation of sulfur as an intermediate, unlike two steps in reactions 4 and 5. They were herein predicted to follow these two reactions: S2 O32 − + Ni3S2 + H 2O → SO32 − + SO4 2 − + NiS + H 2 (20)

S2 O32 − + Ni + H 2O → SO32 − + SO4 2 − + Ni3S2 + H 2 (21)

3.5. Reaction Pathways. On the basis of the results discussed above, a possible reaction pathway of sulfur species in the presence of metals and Ni3S2 under alkaline hydrothermal conditions was proposed and given in Figure 10. As illustrated 5623

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Figure 10. Proposed reaction pathways of sulfur species with water and metal or mineral under hydrothermal conditions (the solid lines represent confirmed reactions; the dotted lines represent unconfirmed reactions).

in Figure 10, HS− is very stable with respect to the oxidation at 250 °C even in the presence of metals and Ni3S2. Sulfur is reactive to disproportionate to HS− and S2O32−. In addition to disproportion, sulfur would also be oxidized by water to form S2O32− and H2 in the presence of Fe and Ni3S2. For S2O32−, in the presence of Ni and Ni3S2, S2O32− was fast consumed with formation of SO32− and SO42−, while S2O32− is relatively stable in the case of adding Fe and without addition. A thorough study of theoretical calculation39 based on these results in the following study will strengthen our understanding of sulfur chemistry under hydrothermal conditions.

(4) Xu, Y.; Schoonen, M. A. A. The stability of thiosulfate in the presence of pyrite in low-temperature aqueous-solutions. Geochim. Cosomochim. Acta 1995, 59, 4605. (5) Xu, Y.; Schoonen, M. A. A.; Nordstrom, D. K.; Cunningham, K. M.; Ball, J. W. Sulfur geochemistry of hydrothermal waters in Yellowstone National Park: I. the origin of thiosulfate in hot spring waters. Geochim. Cosomochim. Acta 1998, 62, 3729. (6) Sato, T.; Mori, S.; Watanabe, M.; Sasaki, M.; Itoh, N. Upgrading of bitumen with formic acid in supercritical water. J. Supercrit. Fluids 2010, 55, 232. (7) Sato, T.; Trung, P. H.; Tomita, T.; Itoh, N. Effect of water density and air pressure on partial oxidation of bitumen in supercritical water. Fuel 2012, 95, 347. (8) Zhang, Q.; Tang, R.; Yin, K.; Luo, X.; Zhang, L. Corrosion behavior of Hastelloy C-276 in supercritical water. Corros. Sci. 2009, 51, 2092. (9) Smanio, V.; Fregonese, M.; Kittel, J.; Cassagne, T.; Ropital, F.; Normand, B. Wet hydrogen sulfide cracking of steel monitoring by acoustic emission: discrimination of AE sources. J. Mater. Sci. 2010, 45, 5534. (10) Kim, K.; Asaoka, S.; Yamamoto, T.; Hayakawa, S.; Takeda, K.; Katayama, M.; Onoue, T. Mechanisms of Hydrogen Sulfide Removal with Steel Making Slag. Environ. Sci. Technol. 2012, 46, 10169. (11) Setiani, P.; Watanabe, N.; Kishita, A.; Tsuchiya, N. Temperature- and pH-dependent mechanism of hydrogen production from hydrothermal reactions of sulfide. Int. J. Hydrogen Energy 2012, 37, 18679. (12) Wang, Y.; Jin, F.; Zeng, X.; Ma, C.; Wang, F.; Yao, G.; Jing, Z. Catalytic activity of Ni3S2 and effects of reactor wall in hydrogen production from water with hydrogen sulphide as a reducer under hydrothermal conditions. Appl. Energy 2013, 104, 306. (13) Setiani, P.; Vilcáez, J.; Watanabe, N.; Kishita, A.; Tsuchiya, N. Enhanced hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions. Int. J. Hydrogen Energy 2011, 36, 10674. (14) Rolia, E.; Chakrabarti, C. L. Kinetics of decomposition of tetrathionate, trithionate, and thiosulfate in alkaline media. Environ. Sci. Technol. 1982, 16, 852. (15) Etchebehere, C.; Muxi, L. Thiosulfate reduction and alanine production in glucose fermentation by members of the genus Coprothermobacter. Antonie van Leeuwenhoek 2000, 77, 321. (16) O’Reilly, J. W.; Dicinoski, G. W.; Shaw, M. J.; Haddad, P. R. Chromatographic and electrophoretic separation of inorganic sulfur and sulfur-oxygen species. Anal. Chim. Acta 2001, 432, 165. (17) Druschel, G. K.; Schoonen, M. A. A.; Nordstrom, D. K.; Ball, J. W.; Xu, Y.; Cohn, C. A. Sulfur geochemistry of hydrothermal waters in Yellowstone National Park, Wyoming, USA. III. An anion-exchange resin technique for sampling and preservation of sulfoxyanions in natural waters. Geochem. Trans. 2003, 4, 12. (18) Petre, C. F.; Larachi, F. Reaction between hydrosulfide and iron/cerium (hydr)oxide: Hydrosulfide oxidation and iron dissolution kinetics. Top. Catal. 2006, 37, 97. (19) Roy, A. B.; Trudinger, P. A. The biochemistry of inorganic compunds of sulfur; Cambridge University Press: Cambridge, U.K., 1970.

4. CONCLUSION In summary, there are mainly three types of reactions between sulfur species and metals/Ni3S2 under alkaline hydrothermal conditions: (i) sulfidation of metals and Ni3S2 by sulfur species; (ii) disproportionation of sulfur species; (iii) oxidation of sulfur species by water to produce hydrogen. The presence of metals and Ni3S2 would change the reaction pathway of sulfur species in underground water and could be utilized in industry for waste treatment and energy production.

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ASSOCIATED CONTENT

S Supporting Information *

Table showing pH value variations before and after reactions at 250 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: 86-21-54742283. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Natural Science Foundation of China (Grant Nos. 21077078 and 21277091) and the National High Technology Research and Development Program (863 Program) of China (No. 2009AA063903).

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REFERENCES

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