Mechanisms of Hydrogen Sulfide Removal with Steel Making Slag

Article pubs.acs.org/est

Mechanisms of Hydrogen Sulfide Removal with Steel Making Slag Kyunghoi Kim,*,† Satoshi Asaoka,‡ Tamiji Yamamoto,† Shinjiro Hayakawa,§ Kazuhiko Takeda,† Misaki Katayama,∥ and Takasumi Onoue⊥ †

Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan Environmental Research and Management Center, Hiroshima University, Higashi-Hiroshima 739-8513, Japan § Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan ∥ Ritsumeikan University SR Center, Ristumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan ⊥ Steel Products Departments, Nisshin Steel Co., Ltd., 11-1 Showa-cho, Kure City 737-8520, Japan ‡

ABSTRACT: In the present study, we experimentally investigated the removal of hydrogen sulfide using steel-making slag (SMS) and clarified the mechanism of hydrogen sulfide removal with the SMS. The results proved that SMS is able to remove hydrogen sulfide dissolved in water, and the maximum removal amount of hydrogen sulfide per unit weight of the SMS for 8 days was estimated to be 37.5 mg S/g. The removal processes of hydrogen sulfide were not only adsorption onto the SMS, but oxidation and precipitation as sulfur. The chemical forms of sulfide adsorbed onto the SMS were estimated to be sulfur and manganese sulfide in the ratio of 81% and 19%, respectively. It is demonstrated here that the SMS is a promising material to remediate organically enriched coastal sediments in terms of removal of hydrogen sulfide. Furthermore, using SMS is expected to contribute to development of a recycling-oriented society.

1. INTRODUCTION Hydrogen sulfide is formed in organically enriched sediments through the reduction of sulfate ions by sulfate-reducing bacteria under an anoxic condition.1 Hydrogen sulfide is highly toxic and fatal to living organisms, and consumes oxygen when it is oxidized. As a result, the presence of hydrogen sulfide in sediments has a negative impact on the benthic ecosystem and sometimes leads to economic losses in aquaculture activities. It is thus important to reduce the hydrogen sulfide concentration in sediments to restore and maintain healthy aquatic ecosystems. Especially in developing countries, the rapid unidirectional development of the process industry creates large amounts of byproducts and is one factor in the deterioration of the aquatic environment. In this age of recycling and growing environmental consciousness, there are many efforts to explore and test new applications for industrial byproducts, such as the removal of phosphate using coal ash and oyster shells.2−6 Slag is a byproduct of iron and steel manufacturing. It is roughly classified into two types: blast furnace slag produced from the conversion process of iron ore into pig iron, and steelmaking slag (SMS) produced from the purification process of pig iron into steel. In 2010, a total of 37 Mt was generated in Japan with 33% as SMS.7 The slag has commonly been used for roadbed construction material, as coarse aggregate for concrete, and as raw material for cement.7 However, the remainder has been discarded, and this has become an environmental issue because reclamation of shallow coastal areas with the discarded © 2012 American Chemical Society

materials is resulting in the loss of valuable marine habitat for marine organisms. Therefore, there have been many efforts to explore new applications of slag. Previous studies have revealed that slag can effectively remove phosphate and acid volatile sulfide (AVS) in marine sediments,2,8,9 while several articles have reported that the other kind of slag can release iron, phosphorus and silicon, which can improve the growth of phytoplankton and other marine plants.10,11 It is known that granulated coal ash effectively removes hydrogen sulfide and its process is both an oxidation−reduction reaction with trace or subtrace elements and oxidation by several oxide compounds.12 The chemical composition of the SMS is similar to granulated coal ash, and accordingly, it is expected that the SMS can also remove hydrogen sulfide with similar processes. The purposes of this study are (1) to prove the hydrogen sulfide-removing efficiency of the SMS, and (2) to reveal the removal mechanisms of hydrogen sulfide of the SMS.

2. MATERIALS AND METHODS 2.1. Steel-Making Slag (SMS). The SMS used in this study was 2−5 mm in diameter and was provided by NISSHIN STEEL CO., Ltd., Hiroshima, Japan. SMS is mainly composed Received: Revised: Accepted: Published: 10169

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of sample. After making hydrogen sulfide precipitate as ZnS, the supernatant solution was filtered through a hydrophilic PVDF filter with a 0.45-μm pore size (MILLEX: Millipore). After adjusting the pH of the filtrate, the sulfate concentration was determined using an ion chromatograph (DX-120: DIONEX) attached with an anion exchange column (Ion PacAS12A: DIONEX). According to a pretest, the sulfate ion was perfectly recovered (98%) using this method. Experiments were conducted in a triplicate fashion. The SMS used for 200 mg S/L of the initial hydrogen sulfide condition (final SMS) was analyzed by the XAFS analyses described below. 2.3. X-ray Absorption Fine Structure (XAFS) Analyses and Data Processing. Sulfur K-edge measurements (range 2460−2485 eV) were made with BL11 in the Hiroshima Synchrotron Research Center, HiSOR.16 The synchrotron radiation from a bending magnet was monochromatized with a Si(111) double-crystal monochromator. The sample chamber was filled with He gas to suppress any X-ray absorption and scattering from air, and the XAFS spectra were measured both by an X-ray fluorescence yield (XFY) mode using a SDD detector (XR-100SDD; AMPTEK) and a conversion electron yield (CEY) mode. The step size for measurement was 0.25 eV. The sulfur K-edge was calibrated with the cupric sulfate (CuSO4·5H2O) peak set at 2481.6 eV.17 As references, sulfur (Katayama Chemical, analytical grade), FeS2 (Stream Chemicals, analytical grade), Al2S3 (Sigma-Aldrich, analytical grade), MnS (Sigma-Aldrich, analytical grade), CaS (Sigma-Aldrich, 99.9%), and CaSO4 (Alfa Aesar, 99%) were measured by the CEY mode. The slag samples were mounted on a double-stick tape (NW-K15; Nichiban) placed in the central hole (15 mm in diameter) of a copper plate. The angle between the incident Xray and sample surface was adjusted at 20°, and the X-ray fluorescence was detected from the direction normal to the incident beam in the plane of the electron orbit of the storage ring. The manganese and iron K-edge spectra (range 6400−6725 eV for manganese and range 7080−7250 eV for iron) were measured at BL3 in the Ritsumeikan SR Center, Japan. BL3 has a Si(220) double-crystal monochromator. The XAFS spectra were measured by X-ray fluorescence yield mode using a threeelements Ge solid state detector, SSD (GUL0110S: Canberra). The samples were sealed with polypropylene film and then positioned at 45° to the incident beam in the fluorescence mode. The step size for measurement was 0.3 eV. The X-ray energy was calibrated by defining the K-edge pre-edge peak of δ−MnO2 and hematite fixed at 6540 and 7112 eV, respectively. For the manganese standard, δ−MnO2 (Wako Pure Chemical Industries, analytical grade), Mn2O3 (Sigma-Aldrich, analytical grade), Mn3O4 (Wako Pure Chemical Industries, analytical grade), MnSO 4·5H2O (Wako Pure Chemical Industries, analytical grade), and MnS (Sigma-Aldrich, analytical grade), and for the iron standard, hematite (Stream Chemicals, analytical grade), iron hydroxide (III) (Alfa Aesar, analytical grade), FeS2 (Stream Chemicals, analytical grade), FeS (Wako Pure Chemical Industries, analytical grade), and FeSO4·7H2O (Wako Pure Chemical Industries, analytical grade), were also measured by the transmission mode using an ionization chamber filled with mixed gases: Ar 15% and N2 85% for the incident chamber (Io) and Ar 50% and N2 50% for the transmitted chamber (I). XAFS analyses were carried out using XAFS spectra processing software (REX2000 ver. 2.5: Rigaku Co. Ltd.).

of CaO, SiO2, Fe, Al2O3, MgO, and MnO, at compositions of 45.93, 15.48, 13.75, 5.97, 5.28, and 4.85%, respectively (Table 1). The chemical composition of the SMS was analyzed by an Table 1. Chemical Composition of the Steel Making Slag Used in the Present Study substance

composition ratio (%)

CaO SiO2 Fe Al2O3 MgO MnO P Cr Na C S F

45.93 15.48 13.75 5.97 5.28 4.85 0.61 0.17 0.04 0.26 0.05 0.37

X-ray fluorescence method. The compositions of C and S were measured using a carbon/sulfur elemental analyzer (EMIA820W Series; HORIBA, LTD), and Na and F contents were determined by a flame atomic absorption spectrometric method and absorption photometry. The SMS satisfied the Japanese environmental criteria for soil pollution.13 2.2. Hydrogen Sulfide Removal Experiment. A removal experiment was carried out using a 150-mL vial bottle containing 100 mL of hydrogen sulfide solution with a concentration of 0, 25, 50, 75, 100, 150, and 200 mg S/L representing the possible range in pore water of organically enriched sediments. The hydrogen sulfide solution was prepared as follows: Tris-HCl buffer (Wako Pure Chemical Industries, analytical grade) was added to 100 mL of pure water deaerated with N2 gas to a final concentration of 30 mmol/L to stabilize the pH of the solution. Thereafter, an aliquot of Na2S·9H2O (Nacalai Tesque, analytical grade) was dissolved into pure water to produce the hydrogen sulfide solution with appropriate concentrations, and the pH of the solution was adjusted to 8.2 with HCl. The hydrogen sulfide solution was slowly dispensed into the bottle and 0.2 g of the SMS was added into the solution. Hereafter, the SMS before use in the experiment is called the “initial” SMS. The amount of SMS was optimized on the basis of a preliminary experiment to minimize pH increase. The bottle was plugged with a rubber cork and sealed with an aluminum cap after the air in the head space was displaced with N2 gas. The bottle was agitated moderately at 60 rpm at 25 °C in a water bath for 8 days, and then the concentration of hydrogen sulfide, pH, and oxidation reduction potential (ORP) were measured with a detection tube (200SA or 200SB: Komyo Rikagaku Kogyo), a pH electrode (F-22: Horiba Ltd.), and an ORP electrode (RM-20P: DKK-TOA Co.), respectively. The ORP value was converted to an Eh value following the standard method.14 To evaluate the amount of hydrogen sulfide other than the SMS, control experiments were carried out without the SMS. The sulfate concentration was also determined after scavenging the remaining hydrogen sulfide with the formation of ZnS so as to prevent further oxidation of hydrogen sulfide, a partial modification of the method provided by Ito.15 Briefly, 1 mol/L of Zn(CH3COO)2·2H2O solution (0.1 mL) and 6 mol/ L NaOH solution (0.1 mL) were successively added to 10 mL 10170

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Figure 1. Experimental results for hydrogen sulfide removal using the steel-making slag. Comparison in (a) hydrogen sulfide concentration, (b) pH, (c) Eh, and (d) sulfate concentration between the initial and the final conditions. The hydrogen sulfide removal experiments were conducted in a water bath controlled at 25 °C for 8 days after the addition of 0.2 g of steel-making slag into 100 mL of hydrogen sulfide solution.

sulfide concentrations below 100 mg S/L after 8 days from the initial value of ca. −150 mV, while those over 100 mg S/L were kept below −150 mV (Figure 1c). The increase in the Eh value can be attributed to the removal of hydrogen sulfide. The concentration of sulfate increased from 1 to 12 mg S/L in accordance with the initial concentration of hydrogen sulfide (Figure 1d). This is probably due to oxidation of the hydrogen sulfide by metal oxides.18,19 The increase in sulfate at the initial hydrogen sulfide concentration of 0 mg S/L was probably due to dissolution of the sulfate contained in the SMS. The content of sulfate in the initial SMS is given in Section 3.2. The amount of hydrogen sulfide removed at the condition of the initial hydrogen sulfide concentration of 200 mg S/L was 0.328 mM and the increase of sulfate was only 0.015 mM. From these, the oxidation to sulfate from hydrogen sulfide was only 5% of the total amount of hydrogen sulfide removed. As discussed later, the white colored precipitation appeared to be sulfur compounds. From these results, it was determined that two processes, i.e., the oxidation of hydrogen sulfide and formation of sulfur compound precipitates, simultaneously occur with the addition of the SMS. Another possibility is that the removal of hydrogen sulfide causes the formation of sulfide with trace or subtrace elements contained in the SMS. 3.2. Removal Mechanism of Hydrogen Sulfide onto the SMS. In this study, to investigate the formation of sulfide with the elements contained in the initial and final SMS, the sulfur K-edge XANES spectra were measured for the SMS under the experimental condition of 200 mg S/L hydrogen sulfide and the precipitate formed under the condition of 75 mg S/L (Figure 2). The standard spectra of sulfur species and sulfide of several elements are shown in Figure 2 along with

The background absorption was approximated by a leastsquares fitting. The absorption edge (Eo) was defined as the inflection point in the spectrum. The Eh−pH diagram of manganese in the presence of sulfite and sulfate ion was illustrated by geochemical modeling software, Geochemist’s Workbench 8.0 (RockWare). The parameters used in this thermodynamic calculation were as follows: the activities were referred to liquid phase concentration and set to be 1.8 × 10−9 for Mn2+ and 6.3 × 10−3 for H2S. The temperature and pressure were set at 25 °C and 1.013 hPa, respectively. Manganese concentrations in the liquid phase were measured to illustrate the Eh−pH diagram using an ICPAES (Optima 7300DV: PerkinElmer) after filtering the water sample through a 0.45-μm hydrophilic PVDF filter (MILLEX: Millipore) and HNO3 was added in a final concentration of 2% to keep the pH of the solution low until the analysis.

3. RESULTS AND DISCUSSION 3.1. Removal of Hydrogen Sulfide from Liquid Phase. The experimental results showed that the concentration of hydrogen sulfide was significantly decreased in all experimental cases with the maximum decrease of ca. 100 mg S/L (Figure 1a). The concentration of hydrogen sulfide was below the detection limit (less than 0.1 mg S/L) at the conditions below 100 mg S/L in the initial concentration. The maximum amount of removed hydrogen sulfide per g-SMS for 8 days was estimated to be ca. 37.5 mg S/g and was proportional to the initial concentration of hydrogen sulfide. It was marked that the white colored precipitate was formed at the initial conditions of 25−100 mg S/L hydrogen sulfide concentrations. The pH values were kept ca. 8.3 in all conditions (Figure 1b). The Eh values increased to over 200 mV at the initial hydrogen 10171

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Figure 3. Iron K-edge XANES spectra of the experimented SMS and several iron standards.

identified on the final SMS. From these results on iron XANES, iron does not play an important role in the removal of hydrogen sulfide in the case of the SMS used in the present study, even though it is contained in the SMS at ca. 15%. The manganese K-edge peak of the initial SMS was observed at 6554 eV and corresponded well with Mn2O3, indicating that the major manganese species in the initial SMS was trivalent (Figure 4). The peak area at 6554 eV decreased and the peak edge at 6546.5 eV became remarkable after the adsorption of hydrogen sulfide. Fractions of several manganese species formed on the final SMS were calculated by curve fitting using the software REX2000. After adsorption of hydrogen sulfide, the content of MnS increased significantly. According to the Eh−pH diagram of manganese, Mn2O3, Mn3O4, and MnSO4 are considered to be the thermodynamically most stable manganese species under the present experimental conditions, and MnS can be formed under a highly reduced condition (Figure 5). Consequently, it can be said that the hydrogen sulfide oxidation coupled with manganese reduction on the surface of the SMS.21 The sulfur species that formed on the surface of the final SMS was composed of sulfur (81%) and MnS (19%), as shown in Figure 2. Whereas the MnS is thermodynamically unstable (Figure 5), it was possible to identify the MnS spectra by XAFS analysis. It is assumed that hydrogen sulfide reacted with the manganese of the SMS to form MnS and successively the MnS was oxidized to sulfur because the manganese redox boundary was higher than that of sulfur. In other words, hydrogen sulfide is oxidized to sulfur by reduction of manganese oxide. It was reported that the oxidation of sulfides by manganese oxide occurs in natural environment such as manganese oxide in

Figure 2. Sulfur K-edge XANES spectra of the experimented SMS and precipitate formed during the experiments along with several sulfur standards.

their curve fittings to the samples. The initial SMS has a peak at 2482 eV and represents sulfate. However, the peak intensity is very weak, and the sulfate content in the initial SMS may thus be low. A new peak at 2472 eV was observed after 8 days of experiments. The peak at 2472 eV is quite similar to that of sulfur, which can be supported by the fact that the sulfur Kedge has a peak at 2472 eV.20 Although the peak of the sulfur compounds formed on the surface of the final SMS is quite similar to the peak of the sulfur standard, the peaks of both MnS and FeS2 are also found around 2472 eV but not for that of CaS (Figure 2). However, the minor peak around 2475 eV found for Al2S3 was not observed for the sample. The precipitates showed a peak at 2472 eV, which perfectly overlapped with the peak of the sulfur standard. Therefore, it can be assumed that the major pathways of hydrogen sulfide removal from the liquid phase are the formations of iron/ manganese sulfides and precipitation of sulfur by oxidation. The iron and manganese K-edge XANES spectra were also measured in order to double-check the formation of MnS and FeS2 on the surface of the final SMS. The iron K-edge peak of the final SMS was 7128 eV, and this indicates that the major iron species in the final SMS was Fe(OH)3 (Figure 3). The iron K-edge spectra of the final SMS did not show any change between the initial and final conditions (Figure 3). Furthermore, the FeS2 adsorption edge at 7117 eV was lower than that of the final SMS, indicating that FeS2 was not 10172

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Figure 5, since MnSO4 is thermodynamically more stable compared to MnS under this experimental condition. Although further experiments and development of the methodology are needed in order for it to be utilized, SMS was demonstrated to be a promising material for remediating organically enriched coastal sediments in terms of removal of hydrogen sulfide. In addition, the use of SMS is expected to contribute to the development of a recycling-oriented society.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-82-424-7945; fax: +81-82-424-7998; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Nisshin Steel Co. Ltd., for their financial support and for providing the steel-making slag used as the experimental material. The experiments at HiSOR were carried out under the approval of the HSRC Program Advisory Committee (11-B11). XAFS analyses were carried out at the Ritsumeikan SR Center under the approval of the “Nanotechnology Network Japan Program” (S23-15).

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Figure 4. Manganese K-edge XANES spectra of the experimented SMS and several manganese standards.

Figure 5. Eh−pH diagram of manganese. The activities were set to be 1.8 × 10−9 for Mn2+ and 6.3 × 10−3 for H2S. The initial conditions were 25 °C and 1.013 hPa.

seawater and surface coastal marine sediment.22,23 In the case with the SMS, the oxidized reaction of the hydrogen sulfide is assumed as eq 1 under the reduced condition. Mn2O3 + H 2S + 4H+ → 2Mn 2 + + S0 + 3H 2O

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In this study, it was revealed that the SMS was able to remove hydrogen sulfide in the water by formation of MnS and sulfur on the surface, and the oxidation of MnS to sulfur and partially to sulfate. The oxidation of MnS can be supported by 10173

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