Environ Sci Pollut Res DOI 10.1007/s11356-016-6267-3
SHORT RESEARCH AND DISCUSSION ARTICLE
Selenium catalyzed Fe(III)-EDTA reduction by Na2SO3: a reaction-controlled phase transfer catalysis Kaisong Xiang 1 & Hui Liu 1,2 & Bentao Yang 1 & Cong Zhang 1 & Shu Yang 1 & Zhilou Liu 1 & Cao Liu 1 & Xiaofeng Xie 1 & Liyuan Chai 1,2 & Xiaobo Min 1,2
Received: 29 September 2015 / Accepted: 8 February 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract Fe(II)-EDTA, a typical chelated iron, is able to coordinate with nitric oxide (NO) which accelerates the rates and kinetics of the absorption of flue gas. However, Fe(II)-EDTA can be easily oxidized to Fe(III)-EDTA which is unable to absorb NO. Therefore, the regeneration of fresh Fe(II)EDTA, which actually is the reduction of Fe(III)-EDTA to Fe(II)-EDTA, becomes a crucial step in the denitrification process. To enhance the reduction rate of Fe(III)-EDTA, selenium was introduced into the SO32−/Fe(III)-EDTA system as catalyst for the first time. By comparison, the reduction rate was enhanced by four times after adding selenium even at room temperature (25 °C). Encouragingly, elemental Se could precipitate out when SO32− was consumed up by oxidation to achieve self-separation. A catalysis mechanism was proposed with the aid of ultraviolet–visible (UV–Vis) spectroscopy, Tyndall scattering, horizontal attenuated total reflection Fourier transform infrared (HATR-FTIR) spectroscopy, and X-ray diffraction (XRD). In the catalysis process, the
Responsible editor: Santiago V. Luis Electronic supplementary material The online version of this article (doi:10.1007/s11356-016-6267-3) contains supplementary material, which is available to authorized users. * Hui Liu [email protected] * Xiaobo Min [email protected]
1
School of Metallurgy and Environment, Central South University, Changsha 410083, China
2
Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China
interconversion between SeSO32− and nascent Se formed a catalysis circle for Fe(III)-EDTA reduction in SO 3 2− circumstance. Keywords Selenium . Reaction-controlled phase transfer . Catalysis . Fe(III)-EDTA reduction . Desulfurization and denitrification
Introduction The extensive utilization of fossil fuels has caused serious environmental problems such as acid rain, photochemical smog, and ozone depletion (Dimitriades 1972; Novakov et al. 1974; Ravishankara et al. 2009). It was widely considered that NOx and SO2 are the main pollutants abundant in fuel gases (Wang et al. 2002). For these reasons, the emissions of NOx and SO2 are strictly regulated by governments around the world. Up to date, SO2 removal technology such as wet fluegas desulfurization (WFGD) has met the requirements of SO2 control (Raju et al. 2008). As well, selective catalytic reduction and selective non-catalytic reduction technologies have been commercially applied in large scale due to its excellent denitrification performance (Muzio et al. 2002). However, these monotechnics cannot purify this kind of typical multipollutants (SO2 and NOx) simultaneously. Compared with technologies for desulfurization and denitrification separately, combined removal of SO2 and NOx is more cost effective and flexible, and it has become the research hot topic all over the world (Sumathi et al. 2010; Zhang et al. 2006; Zhang et al. 2014). Liquid-phase denitrification and desulfurization simultaneously is an important research direction in air pollution control; however, the low solubility of NO became the choke point to improve the denitrification efficiency. In the latest decades, the simultaneously removal of SO2 and NOx by
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using transition metal complex, especially Fe(II)-EDTA (Guo et al. 2014; Zhou et al. 2012; Zhu et al. 2010), has received much attention for its potential competitive advantages in markets. Fe(II)-EDTA is able to improve the absorption of NO because Fe(II)-EDTA can coordinate with NO efficiently, while Fe(II)-EDTA can be easily oxidized to Fe(III)-EDTA which is unable to absorb NO. So it is crucial to reduce Fe(III)-EDTA to Fe(II)-EDTA timely and effectively. Na2SO3, a kind of readily accessible desulfurization product, has been used as a reducing agent for Fe(III)-EDTA reduction. However, this approach is not yet in large-scale applications because of the low efficiency of Fe(II)-EDTA regeneration in simplex Na2SO3 reducing system. The rapid reduction of Fe(III)-EDTA to regenerate Fe(II)EDTA for sustained NO absorption becomes a critical step for effective removal of NOx from flue gas. Up to date, the processes which have been developed for the regeneration of Fe(II)-EDTA can be classified in three categories, such as electrochemical reduction (Guo et al. 2014; Guo et al. 2013; Zhou et al. 2012), microbial reduction (Dong et al. 2012; Wang et al. 2013) and catalytic reduction (Yang et al. 2011; Yang et al. 2013; Zhu et al. 2010). As regards the catalytic reduction, reduction of Fe(III)-EDTA by Na2SO3 catalyzed by activated carbon was the first approach to be reported (Zhu et al., 2010). This kind of catalysis process was carried out in heterogeneous condition and the catalytic efficiency could not satisfy the practical application. In contrast, homogeneous catalysis has higher catalysis activity and milder reaction conditions (Cole-Hamilton 2003; Costentin et al. 2014). Despite all these, there is a common problem in homogeneous catalytic systems that the separation and recycle of homogeneous catalyst is difficult (Cole-Hamilton 2003; Dijkstra et al. 2002). Phase transfer is an excellent strategy to meet the need to recover the homogeneous catalyst effectively and realize the high catalyst activity at the same time (Horváth and Rábai 1994; Zuwei et al. 2001). As a phase transfer catalyst (PTC), selenium (Se) has been applied in catalytic carbonylation and reduction in organic synthesis (Mei et al. 2003; Miyata et al. 1980). Elemental Se was introduced into the reaction system firstly and then combined with reducing agents such as CO to form an activated intermediate, which has the higher nucleophilic ability or stronger reducibility. Meanwhile, elemental Se will be formed again when the intermediates react with oxidizing substances and then return to new catalysis cycles. At final, the catalyst will precipitate from solution as the intermediates consumed up. Inspired by this, we proposed here a strategy of applying elemental Se as a phase transfer catalyst to enhance the regeneration of Fe(II)-EDTA in SO32− reducing system. The advantages of both homogeneous and heterogeneous catalysts are combined in this catalytic system. Moreover, the catalyst Se could precipitate easily when SO32− was consumed up, realizing the aim of self-separation (Dioumaev and Bullock 2000)
and fitting in with the characteristics of reaction-controlled phase transfer catalysis (Zuwei et al. 2001).
Materials and methods Chemicals and preparation of catalyst system Ethylenediaminetetraacetic acid ferric sodium (EDTAFeNa·3H 2 O) was purchased from Tianjin Fengchuan Chemical Co., Ltd. for Fe(III)-EDTA sources; formaldehyde and Na2SO3 were purchased from Tianjin Kermel Chemical Reagent Co., Ltd.; trigonal Se (t-Se) was purchased from Shanghai Shanpu Chemical Co., Ltd.; 1,10phenanthroline monohydrate, ammonium acetate (NH4Ac), acetic acid (HAc, 36∼38 %), hydroxylamine hydrochloride (NH 2 OH·HCl) and hydrochloric acid (HCl, ∼35 %) were all purchased from Sinopharm Chemical Reagent Co., Ltd. for the determination of ferric content. Na2SeO3 (98.0 %) was purchased from Beijing Zhonglian Chemical Reagent Co., Ltd.; H2SO4 (95∼98 %) was purchased from Zhuzhou Shiyinhuabo Co., Ltd. A total of 0.2 mol/L Na2SeSO3 solution was prepared by dissolving 1.579 g Se powders and 12.994 g Na2SO3 in distilled water for 4 h at 80 °C under the protection of N2 (99.999 %, Changsha Gaoke Gas Co., Ltd., China). In order to keep kinetic stability, a glass fiber filter (0.22 μm) was used to filter any trace of Se crystals. Then, the filtrate was transferred into a 100-mL volumetric flask and diluted to the fixed volume. Se colloid was prepared by adding proper amounts of diluted H2SO4 solution (0.1 mol/L) into the Na2SO3-Na2SeSO3Fe(III)-EDTA system or the same volume of distilled water for blank test under stirring conditions. Chemical analysis and characterization Fe(II)-EDTA determination The regenerated Fe(II)-EDTA was measured colorimetrically with the 1,10-phenanthroline method at 510 nm. Because of the existence of SeSO32− in the solution, Se may precipitate out and form a colloidal state when the NH4Ac/HAc buffer was added in. To eliminate the influence of Se colloid on ultraviolet absorption, all samples were centrifuged (10, 000×g for 5 min). Then, the supernatant was used to determinate the Fe(II)-EDTA concentration. UV–Vis spectroscopy UV–Vis spectroscopic measurements of the samples were performed on a Hitachi U4100 spectrophotometer using 1-cm quartz cells. The absorbance of the solution contained Fe(II)-
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disturbance of water for its sensitive responds to infrared energy. X-ray diffraction The fresh and recovered selenium were identified by the X-ray diffraction (XRD) method using a Rigaku TTR III diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.15406 nm, 40 kV, 250 mA). Measurements of each sample were performed in the scattering 2θ range from 10° to 80° with a step of 0.02°. Fig. 1 Fe(III)-EDTA conversion at different temperatures (25, 45, and 60 °C) after adding Na2SO3 solution (Se free), and Na2SO3/Na2SeSO3 mixture (with Se). Fe(III)-EDTA solution (50 ml, 10 mmol/L) was added with 50 ml 1 mol/L Na2SO3 solution, and 10 ml 0.2 mol/L Na2SeSO3 solution mixed with 40 ml 1 mol/L Na2SO3 solution separately
Results and discussion Catalysis test of Se
EDTA was measured at 510 nm. The characteristic absorption of colloidal selenium was measured in the range of 800∼250 nm. Tyndall scattering tests A red laser pointer was employed to identify if there was any Se colloid formed in mixtures by directing the red beam through the solution. Because the color of selenium colloid is close to that of solution from Na2SO3-Na2SeSO3-Fe(III)EDTA system, it is hard to differentiate them with the naked eyes. The Tyndall scattering test is an effective way to detect if any of elemental selenium precipitates out. HATR-FTIR spectroscopy Nicolet IS10 FT-IR spectrometer (Thermo Scientific, USA) is equipped with a universal horizontal attenuated total reflection (HATR) accessory (Thermo Scientific, USA) with a ZnSe crystal at 4 cm−1 resolution. The spectrum of water was collected as the background of all the IR tests to eliminate the Fig. 2 a UV–Vis spectrum of Se colloid (a, b) and the solution from reduction system (c). b Tyndall scattering tests of solution (a) and (c). Se colloid (a) (0.08 mmol/L) and (b) (0.20 mmol/L) were prepared via acidulation of solution from Na2SO3-Na2SeSO3-Fe(III)EDTA reduction system
Se was applied as a catalyst and SO32− as the reductant to reduce Fe(III)-EDTA. It exhibited high performance in Fe(III)-EDTA conversion. To trigger this catalysis process, the initial catalytic intermediates Na2SeSO3 was originally prepared by dissolution of t-Se in Na2SO3 at 80 °C (Chai et al. 2015). t−Se þ SO3 2− ¼ SeSO3 2−
ð1Þ
When the intermediates were introduced into the Na2SO3Fe(III)-EDTA system, the conversion rate of Fe(III)-EDTA was improved substantially (Fig. S1 in Electronic Supplementary Material). The conversion rate reached to 47.6 %, which is much higher than that of active carbon catalyzed regeneration of Fe(II)-EDTA at the same temperature (Zhu et al. 2010). In order to verify that the significant enhancement of Fe(III)-EDTA conversion is not due to the redox reaction between SeSO32− and Fe(III)-EDTA, the Na2SO3, Na2SeSO3, and the Na2SO3-Na2SeSO3 mixture were independently added into Fe(III)-EDTA to compare the conversion rates. As a result, the Na2SO3-Na2SeSO3 mixture contributed to higher Fe(III)-EDTA conversion than the case of excessive
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Na 2 SO 3 and Na 2 SeSO 3 independently did (Fig. S2 in Electronic Supplementary Material). Figure 1 shows that at different temperatures, the reduction rates of Fe(III)-EDTA all increased significantly with the addition of Na2SeSO3 as compared to initial Na2SO3/Fe(III)-EDTA system (see Table S1 in Electronic Supplementary Material for details.). From the abovementioned, it can be deduced that the intermediate, Na2SeSO3, played a specific role in the Fe(III)-EDTA reduction process.
Mechanism of catalysis Prior to catalyst precipitation, the speciation of Se in the reduction system was investigated to reveal the pathway of the enhancement effect of Na2SeSO3 on Fe(III)-EDTA reduction. The ion chromatograph (Fig. S3 in Electronic Supplementary Material) disclosed that SeSO32− could not be oxidized to SeO32− and have less possibility to form SeO42− by Fe(III)EDTA. So, elemental selenium might be the oxidation product in the oxidation-reduction process (Eq. 2). In order to detect the possible state of selenium, UV–Vis and Tyndall scattering tests were employed. As shown in Fig. 2, curve a and b show obvious strong characteristic absorptions around 300 nm−1 (Ng and Fan 2014) whereas curve c shows no absorption peak, which illustrates there is no Se precipitates in the Na2SO3-Na2SeSO3-Fe(III)-EDTA system at initial stage. Besides, the Tyndall scattering was rather weak in the solution c sampled from Na2SO3-Na2SeSO3-Fe(III)-EDTA system compared to Se colloid a. It demonstrates indirectly that no elemental selenium exist, and the elemental Se was just in a transient state in the Fe(III)-EDTA-SO32− system. As speculated, the absence of elemental Se could be ascribed to the rapid reaction between newly generated Se and SO32− which was hardly detectable. As evidenced, the introduced red Se colloid dissolved immediately by SO32− solution (see VIDEO in Electronic Supplementary Material). Thus, the reacted Se
Fig. 3 HATR-FTIR spectra of (a) Fe(III)-EDTA solution (0.1 mol/L), (b) mixed solution with equal volumes of Na2SeSO3 (0.2 mol/L) and Fe(III)EDTA (0.1 mol/L) and (c) Na2SeSO3 solution (0.2 mol/L)
Scheme 1 The basic route of Se catalyzed Fe(III)-EDTA reduction
should probably be transformed into SeSO32−which was undetectable by UV–Vis and Tyndall scattering. As we known, the quality of the catalyst remains the same before and after the catalysis reaction. It is important to make it clear that whether SeSO32− possesses this characteristic of catalyst. To verify the issue, HATR-FTIR spectrometer was applied to analyze whether Se was complexed with other elements. In Fig. 3, at 1359 cm−1, a peak was observed both in spectrum a and b, respectively. Moreover, the peak at 1620 cm−1 in spectrum b is almost the half of the overlap of peak at 1612 cm−1 from spectrum a and 1645 cm−1 from spectrum c. These information implied no new chemical bonds were formed between Fe(III) complex and SO32− or SeSO32−. Meanwhile, the characteristic peaks of SeSO32− (993 cm−1 for Se–S bond and 1117 cm−1 for S–O bond (see Fig. S4 in Electronic Supplementary Material)) were reduced almost by half after the SeSO32− solution was mixed with equal volume of Fe(III)-EDTA solution (see Table S2 in Electronic Supplementary Material). Moreover, the peak area ratio of spectrum b to c is lower than 1/2 at 930 cm−1 (see Table S2), which reflects the consumption of SO 3 2− (Siriwardane and Woodruff 1997). The results demonstrated that the total amount of SeSO32− was unchanged even after catalysis process, and no other complex was formed between Se and other elements. Depending on above results, the following reaction scheme for the lifecycle of Se in homogeneous condition is proposed. On the one hand, the generated Se was consumed rapidly by the excessive SO32− to form SeSO32− for continual circulation (Eq. 3), so the SeSO32− had a rapid regeneration approach to
Fig. 4 XRD patterns of fresh t-Se powder (a) and the red Se recycled from reduction system (b)
Environ Sci Pollut Res Table 1 Recycling of catalyst for Fe(III)-EDTA reduction
Catalyst
Catalyst:Na2SO3:Fe(III)-EDTA (molar ratio)
Conversion (%)
Recovery (%)
Fresh
4:100:1
47.6
96.01
Cycle 1 Cycle 2
3.84:100:1 3.55:100:1
46.9 47.1
92.38 89.60
Reaction conditions are as follows. Fresh catalyst:Na2SO3:Fe(III)-EDTA = 4:100:1 at 25 °C. The catalyst was recovered by filtration after acidification and used for the next catalytic reaction
compensate for the consumption caused by the reaction with Fe(III)-EDTA (Eq. 2). On the other hand, SeSO32− could not be oxidized to SeO32− let alone SeO42− because the oxidative potential of Fe(III)-EDTA is limited. Thus, the SeSO32− should be always present in the Fe(III)-EDTA-SO32− system, and the amount of SeSO32− is in balance as long as excessive SO32− exists. So, the net effect, as shown in Eq. 4, is that the Fe(III)-EDTA reduction only consumes SO32−. In other words, the selenium participated process in Fe(III)-EDTA reduction in SO32− circumstance formed a closed loop, as shown in Scheme 1, which is a typical catalysis pathway. SeSO3
2−
þ 2 FeðIIIÞ−EDTA þ H2 O
¼ Se ðnascentÞ þ 2 FeðIIÞ−EDTA þ SO4 2− þ 2Hþ ð2Þ Se ðnascentÞ þ SO3 2− ¼ SeSO3 2
−
ð3Þ
Net : 2FeðIIIÞ−EDTA þ SO3 2− þ H2 O ¼ 2FeðIIÞ−EDTA þ SO4 2− þ 2Hþ
ð4Þ
The scheme demonstrated the mechanism of Fe(III)-EDTA reduction by SO32− catalyzed by Se. First, fresh selenium (tSe) itself was stable in SO32− solution at room temperature, but with the assistance of external heat, it could be dissolved in Fig. 5 Photograph of (a) mixture with Se powder and Na2SO3 solution before heating; (b) Na2SeSO3/Na2SO3 solution; (c) Fe(III)-EDTA solution; (d) mixture after mixing b and c; (e) suspension liquid after acidification or stabilization; ( f ) separation of catalyst from solution
SO32− to form SeSO32− which acted as a trigger intermediate for the start-up of the catalysis. The regenerated SeSO32− reacted with Fe(III)-EDTA and released nascent Se and SO42 − . The activated nascent Se reacted with residual SO32− to form SeSO32− again and continued to new catalysis cycles. In the whole cycle, Se served as a porter for SO32− and catalyzed the SO32− to reduce Fe(III)-EDTA circularly. Recovery and reuse of catalyst Because the nascent Se is active enough to react with SO32−, the reaction between them is so fast that it is hard to detect the elemental Se in solution containing excessive SO32−. While, it is believed to be that, with the consumption of SO32−, Se would finally precipitate out gradually because the decrease of SO32− favorably shifts the equilibrium towards the reverse direction of Eq. 3. To verify this theory, formaldehyde was employed to stabilize SO32−. It has been reported that formaldehyde can react with SO32− and forms a stable sulfite-formaldehyde adduct (Koh and Miura 1987). When the formaldehyde was added into the catalysis system, red substance was precipitated out, which was ascribed to the reaction shifted towards to the opposite direction of Eq. 3 when SO32− was consumed. Energy-
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dispersive X-ray spectroscopy (EDS) result shows that the red precipitate is pure elementary Se (Fig. S5 in Electronic Supplementary Material). Equally valid method such as acidification was also tested here, and rapid precipitation of Se can be achieved by adding acids. External additive-induced Se precipitation provides extra alternative approaches for rapid separation of the catalyst. Through simple methods such as filtration, the red precipitate can be easily separated from the solution. Characterized by XRD, it is also observed that the fresh catalyst is transformed from initial t-Se (PDF#73-0465) to amorphous Se (Zhang et al. 1995) after catalysis process (Fig. 4). It was recognized that amorphous nanocrystals be more reactive (Kataby et al. 1997), which is also the reason why red selenium can dissolve in SO32− immediately in this reduction system. As we speculated before, in practical catalysis process, Se precipitated as the SO32− was oxidized gradually by Fe(III)-EDTA and oxygen. At final, the catalysis reaction ends up with completely self-precipitation of catalyst. So, this process could be identified as a reaction-controlled phase transfer catalysis. Then, the selenium was separated from the solution through filtration. The collected selenium could be reused again and still had a good catalytic performance. After repeating two cycles, the recovery of catalysts could achieve 96.01, 92.38, and 89.60 %, respectively (Table 1). So the catalyst features not only good catalytic performance but also ease in separation and simplicity in processing. The whole procedures of Se catalyzed reduction, self-separation, and reuse of the catalyst are illustrated in Fig. 5. In the catalysis process, the catalysis pathway is actually the circle between Se and SeSO32− coupled with Fe(II)EDTA/Fe(III)-EDTA conversion. It was observed that by controlling the amount of SO32− in the catalysis system, the conversion between Se and SeSO32− is well mediated. It should be noted here that the precipitated red Se can be also easily dissolved into the Na2SO3 solution at room temperature (see VIDEO in Electronic Supplementary Material) because red amorphous selenium is highly reactive, thereby providing a more facile approach for catalyst reactivation. So the catalytic activity can be regained by just adding fresh Na2SO3 into the original system. Predictably, the catalytic activity can be regained and sustained by continuously absorbing SO2, a waste gas itself urgently needs to be treated, to produce SO32− in situ to regenerate Fe(II)-EDTA in theWFGD process. Thus, the technique of Se catalyzed Fe(III)EDTA reduction has a great application potential in simultaneous desulfurization and denitrification and fits with the concept of Bwaste control by waste^.
Conclusion A high-efficiency reduction system for Fe(II)-EDTA regeneration catalyzed by elemental Se is successfully designed. This
catalysis strategy allows not only highly efficient homogeneous reduction of Fe(III)-EDTA but also easy catalyst recovery by simple filtration when the SO32− in solution is consumed up. The catalysis process can proceed at the temperature in desulfurization scrubber as long as the initiating reaction between Se powder (t-Se) and SO32− was completed. Further investigations to implement this new protocol to remove NOx are underway in our laboratory. We will utilize SO2 as depletion agent instead of Na2SO3 for the goal of simultaneous desulfurization and denitrification. Acknowledgments The financial support from the National Natural Science Foundation of China (51474246,51404306) and the Key Project of Science and Technology of Hunan Province, China (2013FJ1009) is gratefully acknowledged. Compliance with ethical standards Conflict of interests The authors declare that they have no conflict of interest.
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SHORT RESEARCH AND DISCUSSION ARTICLE
Selenium catalyzed Fe(III)-EDTA reduction by Na2SO3: a reaction-controlled phase transfer catalysis Kaisong Xiang 1 & Hui Liu 1,2 & Bentao Yang 1 & Cong Zhang 1 & Shu Yang 1 & Zhilou Liu 1 & Cao Liu 1 & Xiaofeng Xie 1 & Liyuan Chai 1,2 & Xiaobo Min 1,2
Received: 29 September 2015 / Accepted: 8 February 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract Fe(II)-EDTA, a typical chelated iron, is able to coordinate with nitric oxide (NO) which accelerates the rates and kinetics of the absorption of flue gas. However, Fe(II)-EDTA can be easily oxidized to Fe(III)-EDTA which is unable to absorb NO. Therefore, the regeneration of fresh Fe(II)EDTA, which actually is the reduction of Fe(III)-EDTA to Fe(II)-EDTA, becomes a crucial step in the denitrification process. To enhance the reduction rate of Fe(III)-EDTA, selenium was introduced into the SO32−/Fe(III)-EDTA system as catalyst for the first time. By comparison, the reduction rate was enhanced by four times after adding selenium even at room temperature (25 °C). Encouragingly, elemental Se could precipitate out when SO32− was consumed up by oxidation to achieve self-separation. A catalysis mechanism was proposed with the aid of ultraviolet–visible (UV–Vis) spectroscopy, Tyndall scattering, horizontal attenuated total reflection Fourier transform infrared (HATR-FTIR) spectroscopy, and X-ray diffraction (XRD). In the catalysis process, the
Responsible editor: Santiago V. Luis Electronic supplementary material The online version of this article (doi:10.1007/s11356-016-6267-3) contains supplementary material, which is available to authorized users. * Hui Liu [email protected] * Xiaobo Min [email protected]
1
School of Metallurgy and Environment, Central South University, Changsha 410083, China
2
Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China
interconversion between SeSO32− and nascent Se formed a catalysis circle for Fe(III)-EDTA reduction in SO 3 2− circumstance. Keywords Selenium . Reaction-controlled phase transfer . Catalysis . Fe(III)-EDTA reduction . Desulfurization and denitrification
Introduction The extensive utilization of fossil fuels has caused serious environmental problems such as acid rain, photochemical smog, and ozone depletion (Dimitriades 1972; Novakov et al. 1974; Ravishankara et al. 2009). It was widely considered that NOx and SO2 are the main pollutants abundant in fuel gases (Wang et al. 2002). For these reasons, the emissions of NOx and SO2 are strictly regulated by governments around the world. Up to date, SO2 removal technology such as wet fluegas desulfurization (WFGD) has met the requirements of SO2 control (Raju et al. 2008). As well, selective catalytic reduction and selective non-catalytic reduction technologies have been commercially applied in large scale due to its excellent denitrification performance (Muzio et al. 2002). However, these monotechnics cannot purify this kind of typical multipollutants (SO2 and NOx) simultaneously. Compared with technologies for desulfurization and denitrification separately, combined removal of SO2 and NOx is more cost effective and flexible, and it has become the research hot topic all over the world (Sumathi et al. 2010; Zhang et al. 2006; Zhang et al. 2014). Liquid-phase denitrification and desulfurization simultaneously is an important research direction in air pollution control; however, the low solubility of NO became the choke point to improve the denitrification efficiency. In the latest decades, the simultaneously removal of SO2 and NOx by
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using transition metal complex, especially Fe(II)-EDTA (Guo et al. 2014; Zhou et al. 2012; Zhu et al. 2010), has received much attention for its potential competitive advantages in markets. Fe(II)-EDTA is able to improve the absorption of NO because Fe(II)-EDTA can coordinate with NO efficiently, while Fe(II)-EDTA can be easily oxidized to Fe(III)-EDTA which is unable to absorb NO. So it is crucial to reduce Fe(III)-EDTA to Fe(II)-EDTA timely and effectively. Na2SO3, a kind of readily accessible desulfurization product, has been used as a reducing agent for Fe(III)-EDTA reduction. However, this approach is not yet in large-scale applications because of the low efficiency of Fe(II)-EDTA regeneration in simplex Na2SO3 reducing system. The rapid reduction of Fe(III)-EDTA to regenerate Fe(II)EDTA for sustained NO absorption becomes a critical step for effective removal of NOx from flue gas. Up to date, the processes which have been developed for the regeneration of Fe(II)-EDTA can be classified in three categories, such as electrochemical reduction (Guo et al. 2014; Guo et al. 2013; Zhou et al. 2012), microbial reduction (Dong et al. 2012; Wang et al. 2013) and catalytic reduction (Yang et al. 2011; Yang et al. 2013; Zhu et al. 2010). As regards the catalytic reduction, reduction of Fe(III)-EDTA by Na2SO3 catalyzed by activated carbon was the first approach to be reported (Zhu et al., 2010). This kind of catalysis process was carried out in heterogeneous condition and the catalytic efficiency could not satisfy the practical application. In contrast, homogeneous catalysis has higher catalysis activity and milder reaction conditions (Cole-Hamilton 2003; Costentin et al. 2014). Despite all these, there is a common problem in homogeneous catalytic systems that the separation and recycle of homogeneous catalyst is difficult (Cole-Hamilton 2003; Dijkstra et al. 2002). Phase transfer is an excellent strategy to meet the need to recover the homogeneous catalyst effectively and realize the high catalyst activity at the same time (Horváth and Rábai 1994; Zuwei et al. 2001). As a phase transfer catalyst (PTC), selenium (Se) has been applied in catalytic carbonylation and reduction in organic synthesis (Mei et al. 2003; Miyata et al. 1980). Elemental Se was introduced into the reaction system firstly and then combined with reducing agents such as CO to form an activated intermediate, which has the higher nucleophilic ability or stronger reducibility. Meanwhile, elemental Se will be formed again when the intermediates react with oxidizing substances and then return to new catalysis cycles. At final, the catalyst will precipitate from solution as the intermediates consumed up. Inspired by this, we proposed here a strategy of applying elemental Se as a phase transfer catalyst to enhance the regeneration of Fe(II)-EDTA in SO32− reducing system. The advantages of both homogeneous and heterogeneous catalysts are combined in this catalytic system. Moreover, the catalyst Se could precipitate easily when SO32− was consumed up, realizing the aim of self-separation (Dioumaev and Bullock 2000)
and fitting in with the characteristics of reaction-controlled phase transfer catalysis (Zuwei et al. 2001).
Materials and methods Chemicals and preparation of catalyst system Ethylenediaminetetraacetic acid ferric sodium (EDTAFeNa·3H 2 O) was purchased from Tianjin Fengchuan Chemical Co., Ltd. for Fe(III)-EDTA sources; formaldehyde and Na2SO3 were purchased from Tianjin Kermel Chemical Reagent Co., Ltd.; trigonal Se (t-Se) was purchased from Shanghai Shanpu Chemical Co., Ltd.; 1,10phenanthroline monohydrate, ammonium acetate (NH4Ac), acetic acid (HAc, 36∼38 %), hydroxylamine hydrochloride (NH 2 OH·HCl) and hydrochloric acid (HCl, ∼35 %) were all purchased from Sinopharm Chemical Reagent Co., Ltd. for the determination of ferric content. Na2SeO3 (98.0 %) was purchased from Beijing Zhonglian Chemical Reagent Co., Ltd.; H2SO4 (95∼98 %) was purchased from Zhuzhou Shiyinhuabo Co., Ltd. A total of 0.2 mol/L Na2SeSO3 solution was prepared by dissolving 1.579 g Se powders and 12.994 g Na2SO3 in distilled water for 4 h at 80 °C under the protection of N2 (99.999 %, Changsha Gaoke Gas Co., Ltd., China). In order to keep kinetic stability, a glass fiber filter (0.22 μm) was used to filter any trace of Se crystals. Then, the filtrate was transferred into a 100-mL volumetric flask and diluted to the fixed volume. Se colloid was prepared by adding proper amounts of diluted H2SO4 solution (0.1 mol/L) into the Na2SO3-Na2SeSO3Fe(III)-EDTA system or the same volume of distilled water for blank test under stirring conditions. Chemical analysis and characterization Fe(II)-EDTA determination The regenerated Fe(II)-EDTA was measured colorimetrically with the 1,10-phenanthroline method at 510 nm. Because of the existence of SeSO32− in the solution, Se may precipitate out and form a colloidal state when the NH4Ac/HAc buffer was added in. To eliminate the influence of Se colloid on ultraviolet absorption, all samples were centrifuged (10, 000×g for 5 min). Then, the supernatant was used to determinate the Fe(II)-EDTA concentration. UV–Vis spectroscopy UV–Vis spectroscopic measurements of the samples were performed on a Hitachi U4100 spectrophotometer using 1-cm quartz cells. The absorbance of the solution contained Fe(II)-
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disturbance of water for its sensitive responds to infrared energy. X-ray diffraction The fresh and recovered selenium were identified by the X-ray diffraction (XRD) method using a Rigaku TTR III diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.15406 nm, 40 kV, 250 mA). Measurements of each sample were performed in the scattering 2θ range from 10° to 80° with a step of 0.02°. Fig. 1 Fe(III)-EDTA conversion at different temperatures (25, 45, and 60 °C) after adding Na2SO3 solution (Se free), and Na2SO3/Na2SeSO3 mixture (with Se). Fe(III)-EDTA solution (50 ml, 10 mmol/L) was added with 50 ml 1 mol/L Na2SO3 solution, and 10 ml 0.2 mol/L Na2SeSO3 solution mixed with 40 ml 1 mol/L Na2SO3 solution separately
Results and discussion Catalysis test of Se
EDTA was measured at 510 nm. The characteristic absorption of colloidal selenium was measured in the range of 800∼250 nm. Tyndall scattering tests A red laser pointer was employed to identify if there was any Se colloid formed in mixtures by directing the red beam through the solution. Because the color of selenium colloid is close to that of solution from Na2SO3-Na2SeSO3-Fe(III)EDTA system, it is hard to differentiate them with the naked eyes. The Tyndall scattering test is an effective way to detect if any of elemental selenium precipitates out. HATR-FTIR spectroscopy Nicolet IS10 FT-IR spectrometer (Thermo Scientific, USA) is equipped with a universal horizontal attenuated total reflection (HATR) accessory (Thermo Scientific, USA) with a ZnSe crystal at 4 cm−1 resolution. The spectrum of water was collected as the background of all the IR tests to eliminate the Fig. 2 a UV–Vis spectrum of Se colloid (a, b) and the solution from reduction system (c). b Tyndall scattering tests of solution (a) and (c). Se colloid (a) (0.08 mmol/L) and (b) (0.20 mmol/L) were prepared via acidulation of solution from Na2SO3-Na2SeSO3-Fe(III)EDTA reduction system
Se was applied as a catalyst and SO32− as the reductant to reduce Fe(III)-EDTA. It exhibited high performance in Fe(III)-EDTA conversion. To trigger this catalysis process, the initial catalytic intermediates Na2SeSO3 was originally prepared by dissolution of t-Se in Na2SO3 at 80 °C (Chai et al. 2015). t−Se þ SO3 2− ¼ SeSO3 2−
ð1Þ
When the intermediates were introduced into the Na2SO3Fe(III)-EDTA system, the conversion rate of Fe(III)-EDTA was improved substantially (Fig. S1 in Electronic Supplementary Material). The conversion rate reached to 47.6 %, which is much higher than that of active carbon catalyzed regeneration of Fe(II)-EDTA at the same temperature (Zhu et al. 2010). In order to verify that the significant enhancement of Fe(III)-EDTA conversion is not due to the redox reaction between SeSO32− and Fe(III)-EDTA, the Na2SO3, Na2SeSO3, and the Na2SO3-Na2SeSO3 mixture were independently added into Fe(III)-EDTA to compare the conversion rates. As a result, the Na2SO3-Na2SeSO3 mixture contributed to higher Fe(III)-EDTA conversion than the case of excessive
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Na 2 SO 3 and Na 2 SeSO 3 independently did (Fig. S2 in Electronic Supplementary Material). Figure 1 shows that at different temperatures, the reduction rates of Fe(III)-EDTA all increased significantly with the addition of Na2SeSO3 as compared to initial Na2SO3/Fe(III)-EDTA system (see Table S1 in Electronic Supplementary Material for details.). From the abovementioned, it can be deduced that the intermediate, Na2SeSO3, played a specific role in the Fe(III)-EDTA reduction process.
Mechanism of catalysis Prior to catalyst precipitation, the speciation of Se in the reduction system was investigated to reveal the pathway of the enhancement effect of Na2SeSO3 on Fe(III)-EDTA reduction. The ion chromatograph (Fig. S3 in Electronic Supplementary Material) disclosed that SeSO32− could not be oxidized to SeO32− and have less possibility to form SeO42− by Fe(III)EDTA. So, elemental selenium might be the oxidation product in the oxidation-reduction process (Eq. 2). In order to detect the possible state of selenium, UV–Vis and Tyndall scattering tests were employed. As shown in Fig. 2, curve a and b show obvious strong characteristic absorptions around 300 nm−1 (Ng and Fan 2014) whereas curve c shows no absorption peak, which illustrates there is no Se precipitates in the Na2SO3-Na2SeSO3-Fe(III)-EDTA system at initial stage. Besides, the Tyndall scattering was rather weak in the solution c sampled from Na2SO3-Na2SeSO3-Fe(III)-EDTA system compared to Se colloid a. It demonstrates indirectly that no elemental selenium exist, and the elemental Se was just in a transient state in the Fe(III)-EDTA-SO32− system. As speculated, the absence of elemental Se could be ascribed to the rapid reaction between newly generated Se and SO32− which was hardly detectable. As evidenced, the introduced red Se colloid dissolved immediately by SO32− solution (see VIDEO in Electronic Supplementary Material). Thus, the reacted Se
Fig. 3 HATR-FTIR spectra of (a) Fe(III)-EDTA solution (0.1 mol/L), (b) mixed solution with equal volumes of Na2SeSO3 (0.2 mol/L) and Fe(III)EDTA (0.1 mol/L) and (c) Na2SeSO3 solution (0.2 mol/L)
Scheme 1 The basic route of Se catalyzed Fe(III)-EDTA reduction
should probably be transformed into SeSO32−which was undetectable by UV–Vis and Tyndall scattering. As we known, the quality of the catalyst remains the same before and after the catalysis reaction. It is important to make it clear that whether SeSO32− possesses this characteristic of catalyst. To verify the issue, HATR-FTIR spectrometer was applied to analyze whether Se was complexed with other elements. In Fig. 3, at 1359 cm−1, a peak was observed both in spectrum a and b, respectively. Moreover, the peak at 1620 cm−1 in spectrum b is almost the half of the overlap of peak at 1612 cm−1 from spectrum a and 1645 cm−1 from spectrum c. These information implied no new chemical bonds were formed between Fe(III) complex and SO32− or SeSO32−. Meanwhile, the characteristic peaks of SeSO32− (993 cm−1 for Se–S bond and 1117 cm−1 for S–O bond (see Fig. S4 in Electronic Supplementary Material)) were reduced almost by half after the SeSO32− solution was mixed with equal volume of Fe(III)-EDTA solution (see Table S2 in Electronic Supplementary Material). Moreover, the peak area ratio of spectrum b to c is lower than 1/2 at 930 cm−1 (see Table S2), which reflects the consumption of SO 3 2− (Siriwardane and Woodruff 1997). The results demonstrated that the total amount of SeSO32− was unchanged even after catalysis process, and no other complex was formed between Se and other elements. Depending on above results, the following reaction scheme for the lifecycle of Se in homogeneous condition is proposed. On the one hand, the generated Se was consumed rapidly by the excessive SO32− to form SeSO32− for continual circulation (Eq. 3), so the SeSO32− had a rapid regeneration approach to
Fig. 4 XRD patterns of fresh t-Se powder (a) and the red Se recycled from reduction system (b)
Environ Sci Pollut Res Table 1 Recycling of catalyst for Fe(III)-EDTA reduction
Catalyst
Catalyst:Na2SO3:Fe(III)-EDTA (molar ratio)
Conversion (%)
Recovery (%)
Fresh
4:100:1
47.6
96.01
Cycle 1 Cycle 2
3.84:100:1 3.55:100:1
46.9 47.1
92.38 89.60
Reaction conditions are as follows. Fresh catalyst:Na2SO3:Fe(III)-EDTA = 4:100:1 at 25 °C. The catalyst was recovered by filtration after acidification and used for the next catalytic reaction
compensate for the consumption caused by the reaction with Fe(III)-EDTA (Eq. 2). On the other hand, SeSO32− could not be oxidized to SeO32− let alone SeO42− because the oxidative potential of Fe(III)-EDTA is limited. Thus, the SeSO32− should be always present in the Fe(III)-EDTA-SO32− system, and the amount of SeSO32− is in balance as long as excessive SO32− exists. So, the net effect, as shown in Eq. 4, is that the Fe(III)-EDTA reduction only consumes SO32−. In other words, the selenium participated process in Fe(III)-EDTA reduction in SO32− circumstance formed a closed loop, as shown in Scheme 1, which is a typical catalysis pathway. SeSO3
2−
þ 2 FeðIIIÞ−EDTA þ H2 O
¼ Se ðnascentÞ þ 2 FeðIIÞ−EDTA þ SO4 2− þ 2Hþ ð2Þ Se ðnascentÞ þ SO3 2− ¼ SeSO3 2
−
ð3Þ
Net : 2FeðIIIÞ−EDTA þ SO3 2− þ H2 O ¼ 2FeðIIÞ−EDTA þ SO4 2− þ 2Hþ
ð4Þ
The scheme demonstrated the mechanism of Fe(III)-EDTA reduction by SO32− catalyzed by Se. First, fresh selenium (tSe) itself was stable in SO32− solution at room temperature, but with the assistance of external heat, it could be dissolved in Fig. 5 Photograph of (a) mixture with Se powder and Na2SO3 solution before heating; (b) Na2SeSO3/Na2SO3 solution; (c) Fe(III)-EDTA solution; (d) mixture after mixing b and c; (e) suspension liquid after acidification or stabilization; ( f ) separation of catalyst from solution
SO32− to form SeSO32− which acted as a trigger intermediate for the start-up of the catalysis. The regenerated SeSO32− reacted with Fe(III)-EDTA and released nascent Se and SO42 − . The activated nascent Se reacted with residual SO32− to form SeSO32− again and continued to new catalysis cycles. In the whole cycle, Se served as a porter for SO32− and catalyzed the SO32− to reduce Fe(III)-EDTA circularly. Recovery and reuse of catalyst Because the nascent Se is active enough to react with SO32−, the reaction between them is so fast that it is hard to detect the elemental Se in solution containing excessive SO32−. While, it is believed to be that, with the consumption of SO32−, Se would finally precipitate out gradually because the decrease of SO32− favorably shifts the equilibrium towards the reverse direction of Eq. 3. To verify this theory, formaldehyde was employed to stabilize SO32−. It has been reported that formaldehyde can react with SO32− and forms a stable sulfite-formaldehyde adduct (Koh and Miura 1987). When the formaldehyde was added into the catalysis system, red substance was precipitated out, which was ascribed to the reaction shifted towards to the opposite direction of Eq. 3 when SO32− was consumed. Energy-
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dispersive X-ray spectroscopy (EDS) result shows that the red precipitate is pure elementary Se (Fig. S5 in Electronic Supplementary Material). Equally valid method such as acidification was also tested here, and rapid precipitation of Se can be achieved by adding acids. External additive-induced Se precipitation provides extra alternative approaches for rapid separation of the catalyst. Through simple methods such as filtration, the red precipitate can be easily separated from the solution. Characterized by XRD, it is also observed that the fresh catalyst is transformed from initial t-Se (PDF#73-0465) to amorphous Se (Zhang et al. 1995) after catalysis process (Fig. 4). It was recognized that amorphous nanocrystals be more reactive (Kataby et al. 1997), which is also the reason why red selenium can dissolve in SO32− immediately in this reduction system. As we speculated before, in practical catalysis process, Se precipitated as the SO32− was oxidized gradually by Fe(III)-EDTA and oxygen. At final, the catalysis reaction ends up with completely self-precipitation of catalyst. So, this process could be identified as a reaction-controlled phase transfer catalysis. Then, the selenium was separated from the solution through filtration. The collected selenium could be reused again and still had a good catalytic performance. After repeating two cycles, the recovery of catalysts could achieve 96.01, 92.38, and 89.60 %, respectively (Table 1). So the catalyst features not only good catalytic performance but also ease in separation and simplicity in processing. The whole procedures of Se catalyzed reduction, self-separation, and reuse of the catalyst are illustrated in Fig. 5. In the catalysis process, the catalysis pathway is actually the circle between Se and SeSO32− coupled with Fe(II)EDTA/Fe(III)-EDTA conversion. It was observed that by controlling the amount of SO32− in the catalysis system, the conversion between Se and SeSO32− is well mediated. It should be noted here that the precipitated red Se can be also easily dissolved into the Na2SO3 solution at room temperature (see VIDEO in Electronic Supplementary Material) because red amorphous selenium is highly reactive, thereby providing a more facile approach for catalyst reactivation. So the catalytic activity can be regained by just adding fresh Na2SO3 into the original system. Predictably, the catalytic activity can be regained and sustained by continuously absorbing SO2, a waste gas itself urgently needs to be treated, to produce SO32− in situ to regenerate Fe(II)-EDTA in theWFGD process. Thus, the technique of Se catalyzed Fe(III)EDTA reduction has a great application potential in simultaneous desulfurization and denitrification and fits with the concept of Bwaste control by waste^.
Conclusion A high-efficiency reduction system for Fe(II)-EDTA regeneration catalyzed by elemental Se is successfully designed. This
catalysis strategy allows not only highly efficient homogeneous reduction of Fe(III)-EDTA but also easy catalyst recovery by simple filtration when the SO32− in solution is consumed up. The catalysis process can proceed at the temperature in desulfurization scrubber as long as the initiating reaction between Se powder (t-Se) and SO32− was completed. Further investigations to implement this new protocol to remove NOx are underway in our laboratory. We will utilize SO2 as depletion agent instead of Na2SO3 for the goal of simultaneous desulfurization and denitrification. Acknowledgments The financial support from the National Natural Science Foundation of China (51474246,51404306) and the Key Project of Science and Technology of Hunan Province, China (2013FJ1009) is gratefully acknowledged. Compliance with ethical standards Conflict of interests The authors declare that they have no conflict of interest.
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