Article pubs.acs.org/est
Mercury Re-Emission in Flue Gas Multipollutants Simultaneous Absorption System Yue Liu,†,‡ Qingfeng Wang,†,‡ Rongjun Mei,†,‡ Haiqiang Wang,†,‡ Xiaole Weng,†,‡ and Zhongbiao Wu*,†,‡ †
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, P. R. China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 310058, P.R. China
‡
S Supporting Information *
ABSTRACT: Recently, simultaneous removal of SO2, NOx and oxidized mercury in wet flue gas desulfurization (WFGD) scrubber has become a research focus. Mercury re-emission in traditional WFGD system has been widely reported due to the reduction of oxidized mercury by sulfite ions. However, in multipollutants simultaneous absorption system, the formation of a large quantity of nitrate and nitrite ions as NOx absorption might also affect the reduction of oxidized mercury in the aqueous absorbent. As such, this paper studied the effects of nitrate and nitrite ions on mercury re-emission and its related mechanism. Experimental results revealed that the nitrate ions had neglected effect on mercury re-emission while the nitrite ions could greatly change the mercury re-emission behaviors. The nitrite ions could initially improve the Hg0-emission through the decomposition of HgSO3NO2−, but with a further increase in the concentration, they would then inhibit the reduction of bivalent mercury owing to the formation of Hg-nitrite complex [Hg(NO2)x2‑x]. In addition, the subsequent addition of Cl− could further suppress the Hg0 emission, where the formation of a stable Hg-SO3−NO2−Cl complex was assumed to be the main reason for such strong inhibition effect. mercury by gas-phase oxidation6,7,11 or catalytic oxidation processes.12−17 According to the previous reports,5,18 the absorption efficiency of oxidized mercury (Hg2+) by WFGD device could reach as high as 80−90%. However, it was widely known that oxidized mercury could be reduced by aqueous sulfite ions in WFGD system,19−23 leading to the re-emission of elemental mercury and hence lowering the total mercury capture efficiency. Moreover, some liquid-phase ions in WFGD solution (e.g., halogen ions, sulfate ion, carbonate ions, hydroxyl ion, and metal ions, etc.) could also affect the oxidized mercury reduction process as reported by many researchers.21,22,24−29 These aforementioned ions could interact with the oxidized mercury and/or sulfite ions to affect oxidized mercury reduction behaviors. Taking Cl− and Mg2+ as examples, Cl− could react with Hg2+ and sulfite ions to form stable complex, HgSO3Cl−, HgSO3Cl22−, or HgClx−(x‑2) (x = 1− 4), hence limiting the mercury reduction by the sulfite ions;21,22,28 Mg2+ could combine with SO32− to generate neutral MgSO30 ion pair in Dual-Alkali FGD adsorbents, which could also suppress the oxidized mercury reduction by the sulfite.29
1. INTRODUCTION Mercury emissions have attracted increasing attention due to the characteristics of the mercury for its toxicity, volatility, persistence and bioaccumulation in the environment.1,2 As a multimedia pollutant, mercury can be emitted, deposited, and reemitted on both a local and global scale over the terrestrial and marine environments.3 Therefore, it could cause a wide range of pollutions. Although various anthropogenic and natural activities could be counted as the emission sources of the mercury, coal burning, especially in power plant, has been considered as the largest anthropogenic source.1−3 Hence, effectively controlling the mercury emissions from coal-fired power plants recently becomes an urgent need. The wet flue gas desulfurization (WFGD) device in coal-fired power plants could provide cobenefits of simultaneously removing SO2 and other water-soluble pollutants.4,5 As such, there is a rising research interest in simultaneous removal of SO2, NOx and mercury in current WFGD system by transforming NO and elemental mercury into more soluble forms via various oxidation technologies. For instance, NO, as the main component of NOx in flue gas, could effectively be oxidized into high valence nitrogen oxides (e.g., NO2) by means of pulsed corona discharge,6 ozone injection7 and catalytic oxidation.8−10 Elemental mercury could be also transformed into oxidized © XXXX American Chemical Society
Received: August 6, 2014 Revised: October 27, 2014 Accepted: October 31, 2014
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Figure 1. Schematic of the lab-device for Hg0 re-emission experiment.
with continuously stirring. The testing part contained a washing bottle with 100 mL 30% NaOH to retain water vapor and remove acid gas. A RA-915 M Lumex Zeeman portable mercury spectrometer was utilized to determine the emission Hg0 (Measurement uncertainty is 1 μg/m3). This test method has been reported in our previous work.36 The tail gas treatment part included two washing-bottles and one adsorbent column. The bottles contained 200 mL 4% (w/v) KMnO4 with 10% (v/v) H2SO4 solution and the adsorbent column contained 200 mL Climpregnated AC. 2.2. UV−visible Spectra Analysis. UV−visible spectra was used to identify the key reaction intermediate species in the reduction of oxidized mercury to elemental mercury under the wet flue gas desulfurization (FGD) conditions, as well as to track their changes in the concentration as the reactions proceeded.28 The measurement was conducted on a UV−visible spectrophotometer (UV−vis DRS: TU-1901, China) by scanning the prepared solution at a speed of 300 nm/min. The solution, which prepared by mixing mercuric-containing solution and other reactants-containing solution with certain molar ratio, was filled into a 1 cm square quartz cuvette with the volume of 2.5 mL. The reactants-containing solution included sulfite-containing, chloride-containing or nitrite-containing solutions, which were prepared by dissolving sodium sulfite anhydrous (Na2SO3), sodium chloride (NaCl) or sodium nitrite (NaNO2), respectively in a N2-saturated ultrapure water before experiment. 2.3. Materials. Sodium nitrite, Sodium sulfite anhydrous and perchloric acid (70%) were bought from Aladdin reagent (China) Co., Sodium chloride was supplied by Hangzhou Chemical Reagent Co., Ltd., Sulfuric acid and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd.. Sulfuric acid and perchloric acid (70%) were guarantee reagent while others were analytical grade. HgO were bought from Shanghai Shenbo Chemical Co., Ltd. and the purity was higher than 99.9%.
In the simultaneous removal system as mentioned above, the absorption of NOx could result in the accumulation of nitrite (NO2−) and nitrate ions (NO3−) in the adsorption solvents.30−33 The molar ratio of NO2− to NO3− is reported able to reach as high as 16:133 and other studies31,32 have suggested the ratio of NO2− to NO3− ranged from 1:2 to 2:1 during NO2 absorption. In literature, the effects of nitrate ions on Hg0 re-emission have been investigated by several works. Ochoa-Gonzalez et al.34 suggested that the presence of NO3− ions has little effect on the oxidized mercury reduction. However, Wo et al.21 showed that the NO3− ions in the slurry would inhibit the Hg0 re-emission due to its inhibition effect on HgSO3 generation. Such contrary result implies that the effect of NO3− ion is still needed to be further confirmed. Besides, to the best of our knowledge, there are few studies focusing on the effects of nitrite ions on Hg0-emission in the WFGD system. As a matter of fact, the presence of nitrite ions may have complicated influences on the reduction behavior of the oxidized mercury and Hg0 re-emission. First, nitrite ions could act as a reduction agent like sulfite species, which may increase the Hg0-emission. On the other hand, NO2−, as a borderline base between hard and soft, could easily combine with Hg2+ (soft acid) to reduce the Hg0-emission based on the hard and soft acid−bases theory (HSAB).4,35 As such, in order to evaluate the Hg0 re-emission behaviors in the multipollutants (SO2, NOx and oxidized mercury) control system, this paper investigated the effect of nitrate and nitrite ions on the oxidized mercury reduction and Hg0 re-emission through a set of experimental tests. UV−visible spectra analysis was used to evaluate the mechanism regarding the interactions between the oxidized mercury and the corresponding ions as concerned in this paper.
2. EXPERIMENTAL SECTION 2.1. Mercury Re-Emission Experimental Apparatus. To investigate the reactions involved in the Hg0 re-emission in wet flue gas desulfurization solution, a lab-scale device was set up as shown in Figure 1, which consists of three parts: the reactor part, testing part and tail gas treatment part. The reactor part is a 500 mL round-bottom flask fitted with three connections: a carry gas inlet, an outlet and a placement of agitator blade. As only focusing on the reaction in liquid phase, inert gas (N2) was used to pass though from above liquid level as the carry gas to take the reemission Hg0 that was released from the liquid phase. The flask was submerged in a water bath to maintain a desired temperature at 45 °C. During the test, the simulate solution with certain concentration of reactant ions was first put into flask reactor. The initial pH value of solution was adjusted to be 5.5. Thereafter, the prepared bivalent mercury was quickly injected into the solution
3. RESULTS AND DISCUSSION 3.1. Mercury Re-Emission in the Presence of Sulfite Ions (SO32−) and Nitrate Ions (NO3−) or Nitrite Ions (NO2−). In order to explore the effects of nitrate ions (NO3−) and nitrite ions (NO2−) on the mercury reduction and re-emission behaviors, a set of corresponding tests were carried out and the results are shown in Figure 2 and Figure 3, respectively. From Figure 2, it could be clearly seen that the presence of nitrate ions (NO3−) had hardly affected the mercury re-emission, which was in good accordance with the results from the literature.24 However, this was very different referring to nitrite ions (NO2−). It could be obtained from Figure 3 that the presence of NO2− at B
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Similarly, NO2− as a borderline base may also combine with oxidized mercury to form Hg-nitrite complex. Moreover, NO2− is also a reduction agent, which may deoxidize bivalent mercury ion into Hg0. To confirm this, the following experiment was conducted regarding the mercury re-emission in the presence of Hg2+ and NO2− and the results are shown in Figure 4. It could be
Figure 2. Effect of NO3− on mercury re-emission. (Experimental conditions: N2 flow rate = 1.5L/min, [Hg2+] = 9.97 μM, [SO32−] = 50 mM, solution volume = 300 mL, initial pH value = 5.5, solution temperature = 45 °C).
Figure 4. Reduction of bivalent mercury by nitrite ion (NO2−) in the liquid phase. (Experimental conditions: N2 flow rate = 1.5L/min, [Hg2+] = 9.97 μM, solution volume = 300 mL, initial pH value = 5.5, solution temperature = 45 °C).
found in Figure 4 that low concentration of NO2− could induce a significant Hg0 re-emission, which was then effectively suppressed with the rising of nitrite ion concentration. This may suggest that the NO2− could deoxidize Hg2+ into Hg0 and the Hg related complex formed in the presence of nitrite ions at higher content may be relatively stable. Figure 5 showed the UV−visible spectra of the mixed or individual solution with 0.06 mM Hg2+ and/or 0.12 mM NO2− Figure 3. Effect of NO2− on mercury re-emission. (Experimental conditions: N2 flow rate = 1.5L/min, [Hg2+] = 9.97 μM, [SO32−] = 50 mM, solution volume =300 mL, initial pH value = 5.5, solution temperature = 45 °C).
low concentration could slightly increase the mercury-emission in the solution, where with the further increasing of the concentration of NO2−, the outlet Hg0 was initially decreased and then rapidly increased. Considering such complex effects of the nitrite ions on mercury re-emission, the relative experimental tests and mechanism studies were then carried out in the following sections. 3.2. The Interactions between Nitrite Ions (NO2−) and Oxidized Mercury (Hg2+). According to HSAB theory, the complexes formed by combination of the same acid−base class would show strong bindings. Hg2+ is classified as a typically soft acid, thus easily forming complexes with soft bases or borderline bases.4,35 HSO3−/SO32− in the FGD absorbent are categorized as borderline bases. And it was widely reported that the HSO3−/ SO32− could react with Hg2+ to form HgSO3 and Hg(SO3)22−,19,20,22,28 where the HgSO3 is very unstable and its decomposition was considered as the main reason for Hg0 reemission.19 The Hg(SO3)22− is more stable than HgSO3, the formation of which could in some extent suppress the Hg0 reemission.19,20
Figure 5. UV−visible spectra of different reaction agent after putting them on scanning cuvette over 20 min. (Conditions: pH value = 4.5(HClO4), solution temperature = 15 °C.).
ions. By compared the spectra of the mixed solution to the individual ones, it could be clearly obtained that some new substance (the absorbency at 230 nm with as high as 0.5 Abs) was generated by mixing the Hg2+ and NO2− ions in the solution because neither the spectra of Hg2+ nor NO2− have the significantly absorbency around this region. According to previous work,37 the absorbency increase may be assigned to the generation of Hg-nitrite species. The insert figure indicated the absorbency changes at 230 nm (the absorbency of mixed solution subtracted by that of individual ones) with different C
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Figure 6. Effect of NO2− on the evolution of UV−visible spectra of 0.06 mM Hg (SO3)22− in the presence of 0.24 mM SO32− over 20 min.((a) [NO2− ] = 0 mM, (b) [NO2− ] = 0.06 mM, (c) [NO2− ] = 0.12 mM, (d) [NO2− ] = 0.24 mM). Conditions: pH value = 4.5(HClO4), solution temperature = 15 °C, Scanning interval = 40 s.
ratios of [NO2−]/[Hg2+]. It could be found that the absorbency change had a maximum value when [NO2−]/[Hg2+] was equal to 2. As such, we can conclude that the increased absorbency at around 230 nm may be due to the formation of Hg(NO2)2. The Hg-NO2 complex with higher coordination number might also form, which have no or lower absorbency at around 230 nm. Cram and Davies 37 have reported that the maximum coordination number of Hg-NO2 complex could reach 4.0. Both in the literature37 and our results, there are no evidence for the formation of HgNO2+. Therefore, we assumed that the HgNO2+ may be unstable, which could decompose quickly into Hg0 and NO3−. This also confirmed by the increased Hg0 at low content of NO2− as shown in Figure 4. Based on the discussion above, it could be concluded that there existed strong interactions between NO2− ions and Hg2+ ions. The reduction effect on Hg2+ ions may be due to the decomposition of Hg-nitrate intermediate species (HgNO2+) by the reaction paths listed below: Hg 2 + + NO−2 → HgNO+2 → Hg 0 + NO−3
from 2.0 to 1.0 within 20 min owing to the decomposition of Hg(SO3)22−. The Hg(SO3)22− would then transfer into unstable HgSO3 and quick decompose into release Hg0.19,20 Once the NO2− was added in the solution, the initial UV absorbency due to Hg(SO3)22− increased and its rate of decay became more rapid (see Figures 6b, c). This implied that the presence of NO2− could accelerate the reduction of oxidized mercury, which fitted well with the experimental results as shown in Figure 3. Based on the above analysis, it could be assumed that the NO2− addition may induce the formation of one kind of HgNO2−SO3 or Hg-NO2 complex, which are more unstable than HgSO3. Its quick decomposition would lead to the improvement of Hg0 re-emission. The Hg-NO2 complex could be excluded as the mercury re-emission for Hg-sulfite solution (see SI Figure S1) is higher than that of Hg-nitrite solution (Figure 4). As such, the possible reaction paths could be described as follows: Hg(SO3)22 − + NO−2 ↔ HgSO3NO−2 + SO32 − HgSO3NO−2 + H 2O → Hg 0 + SO24 − + NO−2 + 2H+
(R1)
(R4)
When the concentration of nitrite ions was much higher than Hg2+, the HgNO2+ could coordinate with other NO2− ions to form more stable Hg- nitrite complex. The main reaction pathways were given below: HgNO2+ + x NO−2 ↔ Hg(NO2 )−x +(x1− 1)(x = 1 − 3)
(R3)
However, it could be obtained in Section 3.2 that the oxidized mercury could combine with NO2− to form [Hg(NO2)x2‑x] complex, possibly inhibiting the oxidized mercury reduction. Moreover, it was found that the decay rate of the absorbency declined when the NO2− content continues to increase (Figure 6d) and the Hg0 re-emission results (see SI Figure S2) for higher NO2− content (compared to that used in Figure 3) showed that the mercury reduction was greatly inhibited when the initial NO2− content reached 240 mM. Accordingly, the reaction paths can be concluded as bellow:
(R2)
3.3. The Reaction Mechanism of the Bivalent Mercury Reduction with Coexistence of Nitrite Ions (NO2−) and Sulfite Ions (SO32−). As we can see in Figure 3, the increase in the concentration of NO2− could promote the Hg0 re-emission in the presence of SO32−. To further disclose the reaction mechanism of this interesting fact, UV−visible spectra tests were carried out to reveal the variation of the complex formed in the solution that contained Hg2+, NO2− and SO32− and the results are shown in Figure 6. For nitrite ion free solution (Figure 6a), the absorbency of the peak at around 230 nm that was due to the formation of Hg(SO3)22−,19 which was gradually decreased
HgSO3NO−2 + NO−2 ↔ Hg(NO2 )2 + SO32 −
(R5)
Hg(NO2 )2 + n NO−2 ↔ Hg(NO2 )−2 +n n (n = 1 − 2)
(R6)
The reaction pathway proposed above could also explain the uniqueness of mercury re-emission at 80 mM NO2− (see Figure 3). The initial inhibition on the mercury re-emission could be D
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attributed to the formation of Hg(NO2)x−(x‑2)(x > 1), which was then gradually transformed into HgNO2SO3− via the reverse process of the reactions R6 and R5. The HgNO2SO3− would quickly decompose to form Hg0, leading to the increase of mercury re-emission. 3.4. The effect of Cl− ions on the reduction of bivalent mercury. As a matter of fact, the WFGD absorbent is a very complex system that contains various ions. One of the important ions is Cl−, which would accumulate and had the concentration at a very high level.38,39 Therefore, the impact of Cl− on Hg0 reemission in the coexistence systems of SO32− and NO2− is also needed to be considered. Figure 7 illustrated that even very little
contrast, the inhibition effect was much weak in the solution without NO2− ions in the presence of identical Cl− concentration (see SI Figure.S3). UV−vis scan results (Figure 8) further showed that the peak absorbency decay rate was also much reduced when little amount of Cl− ions (see Figure 8b) were added into the solution (see Figure 8a). This implied that some stable Hg-related substances may be generated in the solution, which had greatly inhibited the bivalent mercury reduction. As the concentration of Cl− ions continue to increase in the solution (see Figure 8c,d), the absorbency of the solution was maintained at low level, indicating the predominant Hg contained species had been changed. As discussed in Section 3.3, the main species that led to Hg0 emission was HgSO3NO2−. The addition of Cl− ions in the solution may transfer it into very stable species. Since only the coexistence of NO2− and Cl− could result in such strong inhibition effect, this substance was assumed to be HgSO3NO2Cl2−, which is very stable and limit the decomposition of HgSO3NO2−, thereby suppressing the Hg0 emission. The reaction could be described as HgSO3NO−2 + Cl− → HgSO3NO2 Cl2 −
(R7)
In summary, the mercury re-emission in flue gas multipollutants (SO2, NOx and oxidized mercury) simultaneous absorption system could be much inhibited as compared with that in traditional WFGD system, as long as the presence of high concentration of NO2−.
Figure 7. Effect of Cl− on mercury re-emission in the presence of NO2and SO32−. (Experimental conditions: N2 flow rate =1.5L/min; [Hg2+] = 9.97 μM; [SO32−] = 50 mM; [NO2−] = 80 mM; solution volume = 300 mL; initial pH value = 5.5; solution temperature = 45 °C).
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ASSOCIATED CONTENT
S Supporting Information *
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Additional information as noted in the text is available. This material is available free of charge via the Internet at http://pubs. acs.org.
amount of Cl (2 mM) could effectively inhibit the emission of Hg0. As the concentration of Cl− reached 5 mM, the highest emission concentration of Hg0 even as low as about 7 μg/m3. By
Figure 8. Effect of Cl− ions on the evolution of the UV−visible spectra of 0.06 mM Hg (SO3)22− in the presence of 0.24 mM SO32− and 0.24 mM NO2− over 20 min. ((a) [Cl−] = 0 mM, (b) [Cl−] = 0.03 mM, (c) [Cl−] = 0.045 mM; (d) [Cl−] = 0.06 mM). Conditions: pH value = 4.5(HClO4), solution temperature = 15 °C, Scanning interval = 40 s. E
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-571-87953088; fax: +86-571-87953088; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was financially supported by the National HighTech Research and Development Program (863) of China (2011AA060801) and Changjiang Scholar Incentive Program, Ministry of Education, PR China (2009).
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Mercury Re-Emission in Flue Gas Multipollutants Simultaneous Absorption System Yue Liu,†,‡ Qingfeng Wang,†,‡ Rongjun Mei,†,‡ Haiqiang Wang,†,‡ Xiaole Weng,†,‡ and Zhongbiao Wu*,†,‡ †
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, P. R. China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 310058, P.R. China
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S Supporting Information *
ABSTRACT: Recently, simultaneous removal of SO2, NOx and oxidized mercury in wet flue gas desulfurization (WFGD) scrubber has become a research focus. Mercury re-emission in traditional WFGD system has been widely reported due to the reduction of oxidized mercury by sulfite ions. However, in multipollutants simultaneous absorption system, the formation of a large quantity of nitrate and nitrite ions as NOx absorption might also affect the reduction of oxidized mercury in the aqueous absorbent. As such, this paper studied the effects of nitrate and nitrite ions on mercury re-emission and its related mechanism. Experimental results revealed that the nitrate ions had neglected effect on mercury re-emission while the nitrite ions could greatly change the mercury re-emission behaviors. The nitrite ions could initially improve the Hg0-emission through the decomposition of HgSO3NO2−, but with a further increase in the concentration, they would then inhibit the reduction of bivalent mercury owing to the formation of Hg-nitrite complex [Hg(NO2)x2‑x]. In addition, the subsequent addition of Cl− could further suppress the Hg0 emission, where the formation of a stable Hg-SO3−NO2−Cl complex was assumed to be the main reason for such strong inhibition effect. mercury by gas-phase oxidation6,7,11 or catalytic oxidation processes.12−17 According to the previous reports,5,18 the absorption efficiency of oxidized mercury (Hg2+) by WFGD device could reach as high as 80−90%. However, it was widely known that oxidized mercury could be reduced by aqueous sulfite ions in WFGD system,19−23 leading to the re-emission of elemental mercury and hence lowering the total mercury capture efficiency. Moreover, some liquid-phase ions in WFGD solution (e.g., halogen ions, sulfate ion, carbonate ions, hydroxyl ion, and metal ions, etc.) could also affect the oxidized mercury reduction process as reported by many researchers.21,22,24−29 These aforementioned ions could interact with the oxidized mercury and/or sulfite ions to affect oxidized mercury reduction behaviors. Taking Cl− and Mg2+ as examples, Cl− could react with Hg2+ and sulfite ions to form stable complex, HgSO3Cl−, HgSO3Cl22−, or HgClx−(x‑2) (x = 1− 4), hence limiting the mercury reduction by the sulfite ions;21,22,28 Mg2+ could combine with SO32− to generate neutral MgSO30 ion pair in Dual-Alkali FGD adsorbents, which could also suppress the oxidized mercury reduction by the sulfite.29
1. INTRODUCTION Mercury emissions have attracted increasing attention due to the characteristics of the mercury for its toxicity, volatility, persistence and bioaccumulation in the environment.1,2 As a multimedia pollutant, mercury can be emitted, deposited, and reemitted on both a local and global scale over the terrestrial and marine environments.3 Therefore, it could cause a wide range of pollutions. Although various anthropogenic and natural activities could be counted as the emission sources of the mercury, coal burning, especially in power plant, has been considered as the largest anthropogenic source.1−3 Hence, effectively controlling the mercury emissions from coal-fired power plants recently becomes an urgent need. The wet flue gas desulfurization (WFGD) device in coal-fired power plants could provide cobenefits of simultaneously removing SO2 and other water-soluble pollutants.4,5 As such, there is a rising research interest in simultaneous removal of SO2, NOx and mercury in current WFGD system by transforming NO and elemental mercury into more soluble forms via various oxidation technologies. For instance, NO, as the main component of NOx in flue gas, could effectively be oxidized into high valence nitrogen oxides (e.g., NO2) by means of pulsed corona discharge,6 ozone injection7 and catalytic oxidation.8−10 Elemental mercury could be also transformed into oxidized © XXXX American Chemical Society
Received: August 6, 2014 Revised: October 27, 2014 Accepted: October 31, 2014
A
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Figure 1. Schematic of the lab-device for Hg0 re-emission experiment.
with continuously stirring. The testing part contained a washing bottle with 100 mL 30% NaOH to retain water vapor and remove acid gas. A RA-915 M Lumex Zeeman portable mercury spectrometer was utilized to determine the emission Hg0 (Measurement uncertainty is 1 μg/m3). This test method has been reported in our previous work.36 The tail gas treatment part included two washing-bottles and one adsorbent column. The bottles contained 200 mL 4% (w/v) KMnO4 with 10% (v/v) H2SO4 solution and the adsorbent column contained 200 mL Climpregnated AC. 2.2. UV−visible Spectra Analysis. UV−visible spectra was used to identify the key reaction intermediate species in the reduction of oxidized mercury to elemental mercury under the wet flue gas desulfurization (FGD) conditions, as well as to track their changes in the concentration as the reactions proceeded.28 The measurement was conducted on a UV−visible spectrophotometer (UV−vis DRS: TU-1901, China) by scanning the prepared solution at a speed of 300 nm/min. The solution, which prepared by mixing mercuric-containing solution and other reactants-containing solution with certain molar ratio, was filled into a 1 cm square quartz cuvette with the volume of 2.5 mL. The reactants-containing solution included sulfite-containing, chloride-containing or nitrite-containing solutions, which were prepared by dissolving sodium sulfite anhydrous (Na2SO3), sodium chloride (NaCl) or sodium nitrite (NaNO2), respectively in a N2-saturated ultrapure water before experiment. 2.3. Materials. Sodium nitrite, Sodium sulfite anhydrous and perchloric acid (70%) were bought from Aladdin reagent (China) Co., Sodium chloride was supplied by Hangzhou Chemical Reagent Co., Ltd., Sulfuric acid and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd.. Sulfuric acid and perchloric acid (70%) were guarantee reagent while others were analytical grade. HgO were bought from Shanghai Shenbo Chemical Co., Ltd. and the purity was higher than 99.9%.
In the simultaneous removal system as mentioned above, the absorption of NOx could result in the accumulation of nitrite (NO2−) and nitrate ions (NO3−) in the adsorption solvents.30−33 The molar ratio of NO2− to NO3− is reported able to reach as high as 16:133 and other studies31,32 have suggested the ratio of NO2− to NO3− ranged from 1:2 to 2:1 during NO2 absorption. In literature, the effects of nitrate ions on Hg0 re-emission have been investigated by several works. Ochoa-Gonzalez et al.34 suggested that the presence of NO3− ions has little effect on the oxidized mercury reduction. However, Wo et al.21 showed that the NO3− ions in the slurry would inhibit the Hg0 re-emission due to its inhibition effect on HgSO3 generation. Such contrary result implies that the effect of NO3− ion is still needed to be further confirmed. Besides, to the best of our knowledge, there are few studies focusing on the effects of nitrite ions on Hg0-emission in the WFGD system. As a matter of fact, the presence of nitrite ions may have complicated influences on the reduction behavior of the oxidized mercury and Hg0 re-emission. First, nitrite ions could act as a reduction agent like sulfite species, which may increase the Hg0-emission. On the other hand, NO2−, as a borderline base between hard and soft, could easily combine with Hg2+ (soft acid) to reduce the Hg0-emission based on the hard and soft acid−bases theory (HSAB).4,35 As such, in order to evaluate the Hg0 re-emission behaviors in the multipollutants (SO2, NOx and oxidized mercury) control system, this paper investigated the effect of nitrate and nitrite ions on the oxidized mercury reduction and Hg0 re-emission through a set of experimental tests. UV−visible spectra analysis was used to evaluate the mechanism regarding the interactions between the oxidized mercury and the corresponding ions as concerned in this paper.
2. EXPERIMENTAL SECTION 2.1. Mercury Re-Emission Experimental Apparatus. To investigate the reactions involved in the Hg0 re-emission in wet flue gas desulfurization solution, a lab-scale device was set up as shown in Figure 1, which consists of three parts: the reactor part, testing part and tail gas treatment part. The reactor part is a 500 mL round-bottom flask fitted with three connections: a carry gas inlet, an outlet and a placement of agitator blade. As only focusing on the reaction in liquid phase, inert gas (N2) was used to pass though from above liquid level as the carry gas to take the reemission Hg0 that was released from the liquid phase. The flask was submerged in a water bath to maintain a desired temperature at 45 °C. During the test, the simulate solution with certain concentration of reactant ions was first put into flask reactor. The initial pH value of solution was adjusted to be 5.5. Thereafter, the prepared bivalent mercury was quickly injected into the solution
3. RESULTS AND DISCUSSION 3.1. Mercury Re-Emission in the Presence of Sulfite Ions (SO32−) and Nitrate Ions (NO3−) or Nitrite Ions (NO2−). In order to explore the effects of nitrate ions (NO3−) and nitrite ions (NO2−) on the mercury reduction and re-emission behaviors, a set of corresponding tests were carried out and the results are shown in Figure 2 and Figure 3, respectively. From Figure 2, it could be clearly seen that the presence of nitrate ions (NO3−) had hardly affected the mercury re-emission, which was in good accordance with the results from the literature.24 However, this was very different referring to nitrite ions (NO2−). It could be obtained from Figure 3 that the presence of NO2− at B
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Similarly, NO2− as a borderline base may also combine with oxidized mercury to form Hg-nitrite complex. Moreover, NO2− is also a reduction agent, which may deoxidize bivalent mercury ion into Hg0. To confirm this, the following experiment was conducted regarding the mercury re-emission in the presence of Hg2+ and NO2− and the results are shown in Figure 4. It could be
Figure 2. Effect of NO3− on mercury re-emission. (Experimental conditions: N2 flow rate = 1.5L/min, [Hg2+] = 9.97 μM, [SO32−] = 50 mM, solution volume = 300 mL, initial pH value = 5.5, solution temperature = 45 °C).
Figure 4. Reduction of bivalent mercury by nitrite ion (NO2−) in the liquid phase. (Experimental conditions: N2 flow rate = 1.5L/min, [Hg2+] = 9.97 μM, solution volume = 300 mL, initial pH value = 5.5, solution temperature = 45 °C).
found in Figure 4 that low concentration of NO2− could induce a significant Hg0 re-emission, which was then effectively suppressed with the rising of nitrite ion concentration. This may suggest that the NO2− could deoxidize Hg2+ into Hg0 and the Hg related complex formed in the presence of nitrite ions at higher content may be relatively stable. Figure 5 showed the UV−visible spectra of the mixed or individual solution with 0.06 mM Hg2+ and/or 0.12 mM NO2− Figure 3. Effect of NO2− on mercury re-emission. (Experimental conditions: N2 flow rate = 1.5L/min, [Hg2+] = 9.97 μM, [SO32−] = 50 mM, solution volume =300 mL, initial pH value = 5.5, solution temperature = 45 °C).
low concentration could slightly increase the mercury-emission in the solution, where with the further increasing of the concentration of NO2−, the outlet Hg0 was initially decreased and then rapidly increased. Considering such complex effects of the nitrite ions on mercury re-emission, the relative experimental tests and mechanism studies were then carried out in the following sections. 3.2. The Interactions between Nitrite Ions (NO2−) and Oxidized Mercury (Hg2+). According to HSAB theory, the complexes formed by combination of the same acid−base class would show strong bindings. Hg2+ is classified as a typically soft acid, thus easily forming complexes with soft bases or borderline bases.4,35 HSO3−/SO32− in the FGD absorbent are categorized as borderline bases. And it was widely reported that the HSO3−/ SO32− could react with Hg2+ to form HgSO3 and Hg(SO3)22−,19,20,22,28 where the HgSO3 is very unstable and its decomposition was considered as the main reason for Hg0 reemission.19 The Hg(SO3)22− is more stable than HgSO3, the formation of which could in some extent suppress the Hg0 reemission.19,20
Figure 5. UV−visible spectra of different reaction agent after putting them on scanning cuvette over 20 min. (Conditions: pH value = 4.5(HClO4), solution temperature = 15 °C.).
ions. By compared the spectra of the mixed solution to the individual ones, it could be clearly obtained that some new substance (the absorbency at 230 nm with as high as 0.5 Abs) was generated by mixing the Hg2+ and NO2− ions in the solution because neither the spectra of Hg2+ nor NO2− have the significantly absorbency around this region. According to previous work,37 the absorbency increase may be assigned to the generation of Hg-nitrite species. The insert figure indicated the absorbency changes at 230 nm (the absorbency of mixed solution subtracted by that of individual ones) with different C
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Figure 6. Effect of NO2− on the evolution of UV−visible spectra of 0.06 mM Hg (SO3)22− in the presence of 0.24 mM SO32− over 20 min.((a) [NO2− ] = 0 mM, (b) [NO2− ] = 0.06 mM, (c) [NO2− ] = 0.12 mM, (d) [NO2− ] = 0.24 mM). Conditions: pH value = 4.5(HClO4), solution temperature = 15 °C, Scanning interval = 40 s.
ratios of [NO2−]/[Hg2+]. It could be found that the absorbency change had a maximum value when [NO2−]/[Hg2+] was equal to 2. As such, we can conclude that the increased absorbency at around 230 nm may be due to the formation of Hg(NO2)2. The Hg-NO2 complex with higher coordination number might also form, which have no or lower absorbency at around 230 nm. Cram and Davies 37 have reported that the maximum coordination number of Hg-NO2 complex could reach 4.0. Both in the literature37 and our results, there are no evidence for the formation of HgNO2+. Therefore, we assumed that the HgNO2+ may be unstable, which could decompose quickly into Hg0 and NO3−. This also confirmed by the increased Hg0 at low content of NO2− as shown in Figure 4. Based on the discussion above, it could be concluded that there existed strong interactions between NO2− ions and Hg2+ ions. The reduction effect on Hg2+ ions may be due to the decomposition of Hg-nitrate intermediate species (HgNO2+) by the reaction paths listed below: Hg 2 + + NO−2 → HgNO+2 → Hg 0 + NO−3
from 2.0 to 1.0 within 20 min owing to the decomposition of Hg(SO3)22−. The Hg(SO3)22− would then transfer into unstable HgSO3 and quick decompose into release Hg0.19,20 Once the NO2− was added in the solution, the initial UV absorbency due to Hg(SO3)22− increased and its rate of decay became more rapid (see Figures 6b, c). This implied that the presence of NO2− could accelerate the reduction of oxidized mercury, which fitted well with the experimental results as shown in Figure 3. Based on the above analysis, it could be assumed that the NO2− addition may induce the formation of one kind of HgNO2−SO3 or Hg-NO2 complex, which are more unstable than HgSO3. Its quick decomposition would lead to the improvement of Hg0 re-emission. The Hg-NO2 complex could be excluded as the mercury re-emission for Hg-sulfite solution (see SI Figure S1) is higher than that of Hg-nitrite solution (Figure 4). As such, the possible reaction paths could be described as follows: Hg(SO3)22 − + NO−2 ↔ HgSO3NO−2 + SO32 − HgSO3NO−2 + H 2O → Hg 0 + SO24 − + NO−2 + 2H+
(R1)
(R4)
When the concentration of nitrite ions was much higher than Hg2+, the HgNO2+ could coordinate with other NO2− ions to form more stable Hg- nitrite complex. The main reaction pathways were given below: HgNO2+ + x NO−2 ↔ Hg(NO2 )−x +(x1− 1)(x = 1 − 3)
(R3)
However, it could be obtained in Section 3.2 that the oxidized mercury could combine with NO2− to form [Hg(NO2)x2‑x] complex, possibly inhibiting the oxidized mercury reduction. Moreover, it was found that the decay rate of the absorbency declined when the NO2− content continues to increase (Figure 6d) and the Hg0 re-emission results (see SI Figure S2) for higher NO2− content (compared to that used in Figure 3) showed that the mercury reduction was greatly inhibited when the initial NO2− content reached 240 mM. Accordingly, the reaction paths can be concluded as bellow:
(R2)
3.3. The Reaction Mechanism of the Bivalent Mercury Reduction with Coexistence of Nitrite Ions (NO2−) and Sulfite Ions (SO32−). As we can see in Figure 3, the increase in the concentration of NO2− could promote the Hg0 re-emission in the presence of SO32−. To further disclose the reaction mechanism of this interesting fact, UV−visible spectra tests were carried out to reveal the variation of the complex formed in the solution that contained Hg2+, NO2− and SO32− and the results are shown in Figure 6. For nitrite ion free solution (Figure 6a), the absorbency of the peak at around 230 nm that was due to the formation of Hg(SO3)22−,19 which was gradually decreased
HgSO3NO−2 + NO−2 ↔ Hg(NO2 )2 + SO32 −
(R5)
Hg(NO2 )2 + n NO−2 ↔ Hg(NO2 )−2 +n n (n = 1 − 2)
(R6)
The reaction pathway proposed above could also explain the uniqueness of mercury re-emission at 80 mM NO2− (see Figure 3). The initial inhibition on the mercury re-emission could be D
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attributed to the formation of Hg(NO2)x−(x‑2)(x > 1), which was then gradually transformed into HgNO2SO3− via the reverse process of the reactions R6 and R5. The HgNO2SO3− would quickly decompose to form Hg0, leading to the increase of mercury re-emission. 3.4. The effect of Cl− ions on the reduction of bivalent mercury. As a matter of fact, the WFGD absorbent is a very complex system that contains various ions. One of the important ions is Cl−, which would accumulate and had the concentration at a very high level.38,39 Therefore, the impact of Cl− on Hg0 reemission in the coexistence systems of SO32− and NO2− is also needed to be considered. Figure 7 illustrated that even very little
contrast, the inhibition effect was much weak in the solution without NO2− ions in the presence of identical Cl− concentration (see SI Figure.S3). UV−vis scan results (Figure 8) further showed that the peak absorbency decay rate was also much reduced when little amount of Cl− ions (see Figure 8b) were added into the solution (see Figure 8a). This implied that some stable Hg-related substances may be generated in the solution, which had greatly inhibited the bivalent mercury reduction. As the concentration of Cl− ions continue to increase in the solution (see Figure 8c,d), the absorbency of the solution was maintained at low level, indicating the predominant Hg contained species had been changed. As discussed in Section 3.3, the main species that led to Hg0 emission was HgSO3NO2−. The addition of Cl− ions in the solution may transfer it into very stable species. Since only the coexistence of NO2− and Cl− could result in such strong inhibition effect, this substance was assumed to be HgSO3NO2Cl2−, which is very stable and limit the decomposition of HgSO3NO2−, thereby suppressing the Hg0 emission. The reaction could be described as HgSO3NO−2 + Cl− → HgSO3NO2 Cl2 −
(R7)
In summary, the mercury re-emission in flue gas multipollutants (SO2, NOx and oxidized mercury) simultaneous absorption system could be much inhibited as compared with that in traditional WFGD system, as long as the presence of high concentration of NO2−.
Figure 7. Effect of Cl− on mercury re-emission in the presence of NO2and SO32−. (Experimental conditions: N2 flow rate =1.5L/min; [Hg2+] = 9.97 μM; [SO32−] = 50 mM; [NO2−] = 80 mM; solution volume = 300 mL; initial pH value = 5.5; solution temperature = 45 °C).
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ASSOCIATED CONTENT
S Supporting Information *
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Additional information as noted in the text is available. This material is available free of charge via the Internet at http://pubs. acs.org.
amount of Cl (2 mM) could effectively inhibit the emission of Hg0. As the concentration of Cl− reached 5 mM, the highest emission concentration of Hg0 even as low as about 7 μg/m3. By
Figure 8. Effect of Cl− ions on the evolution of the UV−visible spectra of 0.06 mM Hg (SO3)22− in the presence of 0.24 mM SO32− and 0.24 mM NO2− over 20 min. ((a) [Cl−] = 0 mM, (b) [Cl−] = 0.03 mM, (c) [Cl−] = 0.045 mM; (d) [Cl−] = 0.06 mM). Conditions: pH value = 4.5(HClO4), solution temperature = 15 °C, Scanning interval = 40 s. E
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-571-87953088; fax: +86-571-87953088; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was financially supported by the National HighTech Research and Development Program (863) of China (2011AA060801) and Changjiang Scholar Incentive Program, Ministry of Education, PR China (2009).
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