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Oxidation removal of gaseous Hg0 using enhanced-Fenton system in a bubble column reactor
Fuel 246 (2019) 358–364
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Full Length Article
Oxidation removal of gaseous Hg0 using enhanced-Fenton system in a bubble column reactor
T
⁎
Yangxian Liua, , Ying Lia, Hui Xub, Jinjin Xuc a
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China c School of Energy and Power Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Elemental mercury (Hg0) removal Cu2+-enhanced Fenton Hydroxyl radical Advanced oxidation
A novel removal process of elemental mercury (Hg0) from gas using Cu2+-enhanced Fenton system in a bubble column reactor was developed. Experiments were conducted to explore the influence of several factors (concentrations of H2O2, Fe2+ and Cu2+, reagent pH, reaction temperature, concentrations of Hg0, NO and SO2) on Hg0 removal. Mechanism and routes of Hg0 removal using Cu2+-enhanced Fenton system were also proposed. The results revealed that adding Cu2+ obviously strengthened removal process of Hg0 in Fenton system. The contrast results of free radical yield found that the enhancement effect was originated from generation of more hydroxyl radicals that were produced from the synergistic role between Cu2+ and Fe2+ in Fenton system. Hg0 removal efficiency was raised by increasing concentrations of Cu2+ and Fe2+, and was reduced via increasing reaction temperature. Increasing concentration of H2O2 and reagent pH exhibited double impact on Hg0 removal. Hg0 was removed by four pathways: (1) Hg0 was oxidized by ·OH that was produced from the synergistic role between Cu2+ and Fe2+ in Fenton system; (2) Hg0 was oxidized by %OH that was produced from Fe2+/H2O2 system; (3) Hg0 was oxidized by %OH that was produced from Cu2+/H2O2 system; (4) Hg0 was oxidized by H2O2. The pathways (1) and (2) were the main routes of Hg0 removal.
1. Introduction Mercury is a toxic pollutant that has neurological health effect on human. Coal combustion is the biggest pollution source among the main anthropogenic mercury emission sources [1,2]. Typically, mercury mainly exists in three forms in coal combustion flue gas: particulate mercury (Hgp), elemental mercury (Hg0), and oxidized mercury (Hg2+) [3–5]. Both Hg2+ and Hgp can be removed by current wet desulfurization devices and dust control units, respectively. Hg0 is extremely hard to be captured owing to its very low water solubility and high volatility at room temperature [6–8]. In order to effectively control the emission of Hg0, a large number of Hg0 controlling technologies have been developed, mainly containing adsorption, catalytic, oxidation, etc [1–19]. However, due to the complexity of mercury control, there are still no economical and effective mercury control technologies that are suitable for commercial application. In various mercury control technologies, Fenton oxidation is considered as a kind of promising Hg0 removal technology because it has the potential to achieve simultaneous removal of multi-pollutants (e.g., SO2, NO, Hg0, H2S, VOCs, etc.), and has no secondary pollution [20–22]. In the area of gaseous Hg0
⁎
removal, Tan and Lu et al. [20,21] preliminarily used Fenton solution to remove Hg0 from flue gas. Liu et al. [22] further systematically studied the removal process of Hg0 from mixture flue gas using Fenton solution in a bubble column reactor. However, they also found that traditional Fenton solution has a low oxidation efficiency for Hg0 from gas because of low free radical yield (see Fig. 7a and 7b). In recent years, some results had reported that adding trace Cu2+ was able to greatly enhance the free radical yield of Fenton solution [23–25]. The Cu2+-enhanced Fenton oxidation system has shown good prospects for development as compared with original Fenton solution because of stronger oxidation capacity [23–25]. In addition, many industrial processes will discharge containing-trace Cu2+ wastewater [24]. It is also a quite valuable exploration to try to use containing-trace Cu2+ wastewater to strengthen the removal of Hg0 from flue gas using Fenton solution. However, up to now, the related studies on the removal of Hg0 from gas phase using Cu2+-enhanced Fenton oxidation system are rare. The main purpose of this article is to explore the removal of Hg0 from gas phase using Cu2+-enhanced Fenton oxidation system in a bubbling reactor. The main research contents of this article involve the following works: (1) to examine the technical feasibility of
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.fuel.2019.03.018 Received 8 December 2018; Received in revised form 8 February 2019; Accepted 4 March 2019 Available online 05 March 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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gaseous Hg0 removal using Cu2+-enhanced Fenton oxidation system; (2) to evaluate effects of several key parameters (concentrations of H2O2, Fe2+ and Cu2+, reagent pH and reaction temperature) on Hg0 removal; (3) to investigate the active species, products, and mechanism of gaseous Hg0 removal.
prepared Cu2+-enhanced Fenton reagent was added into the bubbling reactor by opening reactor lid after the pH value of the prepared reagent was adjusted based on the measure of an intelligent acidometer (sodium hydroxide and hydrochloric acid were used as buffer solutions). The temperature of Cu2+-enhanced Fenton reagent was adjusted to the required value using a constant temperature water and a thermocouple. The containing-Hg0, N2, NO, SO2 and O2 mixture gas began to enter the bubbling reactor to carry out a gas-liquid contact reaction by switching two gas valves. The outlet concentrations of Hg0 were measured by gas mercury analyzer. The mean value of instantaneous concentration in 16 min was used as the final outlet concentration of Hg0 because of the relatively better stability.
2. Experimental 2.1. Experimental installation The main components of the experimental installation in this article are as follows: (1) A Hg0-containing artificial mixture gas blending system, which contains four simulated gas cylinders (SO2, NO, N2 and O2), an elemental mercury generator, a glass gas mixing container, several flowmeters and gas valves; (2) A reaction temperature regulation system that mainly includes a thermometer and a thermostatic water bath; (3) A bubbling reactor (internal diameter of 90 mm and length of 300 mm; corrosion resistant glass), which comprises a reactor lid (rubber plug) and a flue gas distributor; (4) An analytical and tail purification system, mainly containing a tail flue gas analyzer (with three electrochemical sensors such as O2, SO2, NO), a gas mercury analyzer and an absorption container. The schematic diagram of experimental installation is described in the Fig. 1.
2.3. Analytical methods Free radicals had been detected using ESR spectrometer (i.e., electron spin resonance spectrometer) (Bruker ESP-300) combining with 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) (Sigma) that was used as the spin trap agent of free radicals. The concentrations of mercury, and the other ions in reaction solutions were analyzed using liquid fluorescence mercury analyzer and ion chromatography (IC) (ICS-1600: Dionex, USA), respectively, and the analyzed methods can be found from the literatures [26–28].
2.2. Experimental methods 2.4. Hg0 removal efficiency
Containing-Hg0, N2, NO, SO2 and O2 mixture gas (1.6 L/min) was produced by using the cylinder gases of N2, NO, SO2 and O2 (Zhenjiang Dagang Gas Co., Ltd., China), elemental mercury generator (Vici Metronics, USA) and corresponding flowmeters (Changzhou Shuanghuan Thermal Instrument Co., Ltd., China). The inlet concentrations of Hg0 and other flue gas components in mixture gas were measured using a gas mercury analyzer (QM201H, China) and flue gas analyzer (German MRU Instrument Company), respectively. 0.5 L of Cu2+-enhanced Fenton reagent (Cu2+-Fe2+/H2O2 mixed solution; solution height is about 7.9 cm) were produced using the corresponding analytical reagents (CuCl2·2H2O, FeCl2·4H2O and H2O2, Sinopharm Chemical Reagent Co., Ltd., China) and self-made deionized water. The
The concentrations of Hg0 measured by the bypass (i.e., the bubbling reactor inlet) were used as the inlet concentration of Hg0. The concentrations measured by the bubbling reactor outlet were used as the outlet concentrations of Hg0. Removal efficiency of Hg0 was calculated by the following equation (1):
Removal efficiency of Hg 0 =
Cin − Cout × 100% Cin
(1)
where Cin is inlet concentration of Hg0 in mixture gas; Cout is outlet concentration of Hg0 in mixture gas.
Fig. 1. Schematic diagram of experimental installation. 359
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Fig. 2. Effects of H2O2 concentrations on Hg0 removal efficiency. Basic conditions: Fe2+ concentrations of 0.1 mol/L and 0.06 mol/L; Reagent pH of 2.81; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
Fig. 3. Effects of Fe2+ concentrations on Hg0 removal efficiency. Basic conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Reagent pH of 2.81; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
3. Results and discussions
harmful for oxidation removal of Hg0 through consuming the produced %OH in the liquid phase [32–34].
3.1. Effects of H2O2 concentration on Hg0 removal efficiency
·OH + H2 O2 → H2 O+ HO2 ·
0
Fig. 2 shows the effects of H2O2 concentration on Hg removal efficiency in Cu2+-enhanced Fenton oxidation system. The results indicate that under the Fe2+ concentration of 0.1 mol/L, when H2O2 concentration increases from 0 to 1.5 mol/L, Hg0 removal efficiency increases from 0 to 94.8% at first, and then decreases from 94.8% to 91.6%. The optimized value of H2O2 concentrations had been found to be 0.8 mol/L. Under the Fe2+ concentration of 0.06 mol/L, when H2O2 concentration increases from 0 to 1.5 mol/L, Hg0 removal efficiency raises from 0 to 82.9% at first, and then decreases from 82.9% to 75.4%. The optimized value of H2O2 concentrations had been found to be 0.5 mol/L. A large number of results [20–22] reported that H2O2 could react with Fe2+ to produce %OH in reaction solution through the following reactions (2) and (3).
Fe2 + + H2 O2 ⇔ Fe3 + + ·OH + OH−
(2)
Fe3 + + H2 O2 ⇔ Fe2 + + HO2 ·+H+
(3)
·OH + ·OH → H2 O2
heat
k= 1. 0 × 1010 M−1 s−1
(6) (7) (8)
Therefore, adding excess H2O2 shows a negative effect on the Hg0 removal in the two studied systems using different Fe2+ concentrations. 3.2. Effects of Fe2+ concentrations on Hg0 removal efficiency Fig. 3 shows the effects of Fe2+ concentrations on Hg0 removal efficiency in Cu2+-enhanced Fenton oxidation system. The results show that when Fe2+ concentration increases from 0 to 0.18 mol/L, Hg0 removal efficiency greatly increases from 19.2% to 98.6% for the system using H2O2 concentration of 0.8 mol/L, and from 15.8% to 87.8% for the system using H2O2 concentration of 0.5 mol/L, respectively. Based on the above reactions (2)-(5), the increase of Fe2+ concentration will effectively improve the yield of ·OH in the reaction solution, and thereby can effectively promote the Hg0 removal. 3.3. Effects of reagent pH value on Hg0 removal efficiency Fig. 4 shows the effects of reagent pH value on Hg0 removal efficiency. The results show that with increasing reagent pH value (from 0.72 to 8.05), Hg0 removal efficiencies increase at first, and then decrease in the two systems using H2O2 concentration of 0.8 mol/L and 0.5 mol/L, respectively. Both of the optimized reagent pH values are 2.81. It is observed from the equation (3), H+ of high concentration (low reagent pH value) will hinder the reaction (3) that is the rate control step of the whole Fenton reaction. Hence, appropriate increase of reagent pH value will effectively improve the oxidation removal of Hg0. Nevertheless, some results [29,35,36] also had reported that ·OH apparently showed instability in strong alkaline medium because it would be consumed by the side reaction (9) (its rate constant is extremely high and almost reaches the limit of mass transfer rate of molecules in liquid phase). The oxidation capacity of the by-product O %− is far weaker than that of %OH [36]. Consequently, excessive high concentration of OH− will hinder the oxidation of Hg0.
(4)
In addition, some results [21,22,31] also had proved that H2O2 also could directly oxidize Hg0 from gas into mercury oxide in liquid phase according to the reaction (5) as below.
Hg 0 + H2 O2 → H2 O + HgO
k= 4. 2 × 109 M−1 s−1
·OH + HO2 ·→H2 O + O2
The produced %OH has strong oxidizability since its redox potential is up to 2.80 V [28–30]. The results of the previous investigators [2,3,19,26] proved that %OH could effectively oxidize Hg0 from gas into mercury hydroxide and mercury oxide in liquid phase according to the reaction (4) as below.
Hg 0 + 2·OH → Hg(OH)2 → HgO + H2 O
k= 2. 7 × 107 M−1 s−1
(5)
Based on the above equations (2)–(5), an increase of H2O2 concentration will effectively promote the oxidative removal of Hg0 in solution. However, many results of the previous researchers [18,19,22] also had indicated that in the Fenton reaction, adding excess H2O2 might result in the self-consumption of %OH through the following equations (6)–(8). The reactions (6)–(8) have very high reaction rates, which are 360
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Fig. 4. Effects of reagent pH value on Hg0 removal efficiency. Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
Fig. 6. Effects of reaction temperature on Hg0 removal efficiency. Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reagent pH of 2.81; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
·OH + OH - → H2 O + O - ·
using H2O2 concentration of 0.8 mol/L and 0.5 mol/L, respectively. Related results [23,25,31] had showed that Cu2+ could also personally catalyze H2O2 to produce %OH in solution to oxidize Hg0 from gas phase (it is named as the Fenton-like system), which can be described via the equation (11) and (12) as follows.
k= 1. 3× 1010 M−1 s−1
(9)
Moreover, a large number of results [25,31] also proved that in alkaline mediums, transition metal ions, Cu2+, and Fe2+, would react with OH− to form the precipitations of Cu(OH)2 and Fe(OH)2 in solutions, which can be described by the general equations (10) as below.
M2+ + 2OH - → M(OH)2 ↓
M =(Cu2 +, Fe 2 +)
(10)
High reagent pH value resulted in the reduction of Cu2+ and Fe2+ concentrations in solution, being not conducive for Hg0 removal. Hence, in this article, reagent pH value showed a dual influence on removal efficiency of Hg0.
Cu2 + + H2 O2 → Cu+ + ·OH + OH−
(11)
Cu+
(12)
+ H2 O2 →
Cu2 +
+ HO2·
Besides, the results [23–25] of the previous researchers proved that Cu2+ could effectively enhance the Fenton reaction through accelerating the reduction rate of Fe3+ to Fe2+. The above equation (3) (the reduction steps of Fe3+ to Fe2+) is widely recognized as the main rate controlling step of the whole Fenton reaction process. Addition of Cu2+ can effectively accelerate the circle rate between Fe2+ and Fe3+ [23–25], thereby being able to effectively enhance Hg0 removal in the Fenton oxidation system.
3.4. Effects of Cu2+ concentrations on Hg0 removal efficiency Fig. 5 shows the effects of Cu2+ concentrations on Hg0 removal efficiency. The results show that with increasing the Cu2+ concentration from 0 to 0.009 mol/L, Hg0 removal efficiency markedly increases from 54.3% to 96.5%, and from 43.8% to 86.2% in the two systems
3.5. Effects of reaction temperature on Hg0 removal efficiency Fig. 6 shows the effects of reaction temperature on Hg0 removal efficiency. It can be observed that when reaction temperature increases from 298 K to 348 K, Hg0 removal efficiencies decrease from 97.3% to 85.2%, and from 85.7% to 73.5% in the two systems using H2O2 concentration of 0.8 mol/L and 0.5 mol/L, respectively. A large number of results [33,37,38] pointed out that increasing temperature (it was equivalent to increasing solution temperature) would cause a decrease of gas solubility in liquid phase. The decrease of Hg0 solubility in solution would elevate the mass transfer resistance of Hg0 between gas & liquid two phases, which would hinder the Hg0 removal. In this study, increasing temperature caused the decrease of Hg0 solubility, and thus resulted in the decline of Hg0 removal efficiency. 3.6. Comparative study of different systems, and mechanism and routes of Hg0 removal Fig. 7(a) and (b) shows the contrast of Hg0 removal efficiency in different systems. As exhibited in Fig. 7(a) and (b), as the contrast, all of H2O (A), H2O/Fe2+ (B) and H2O/Cu2+ (C) removal systems under different H2O2 concentrations (0.8 mol/L and 0.5 mol/L) cannot remove Hg0 from gas. In the H2O2 (D) systems, Hg0 removal efficiencies are 6.5% and 5.8% using H2O2 concentrations of 0.8 mol/L and
Fig. 5. Effects of Cu2+ concentrations on Hg0 removal efficiency. Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Reagent pH of 2.81; Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm. 361
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Fig. 7. Comparative experiments of Hg0 removal efficiency in different systems: H2O2 concentrations of 0.8 mol/L (a) and 0.5 mol/L (b); ESR spectrums of ·OH free radical adducts in Cu2+-enhanced Fenton: blank sample (c) and reference value (d); Comparative experiments of hydroxyl radical yield in different systems: H2O2 concentrations of 0.8 mol/L (e) and 0.5 mol/L (f). Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reagent pH of 2.81; Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
that were obtained via the other literatures [37–39], which represent the radical adduct DMPO-OH (the reaction product of ·OH with DMPO). The result proved that ·OH had been produced in Cu2+/H2O2 (E) and Fe2+/H2O2 (F) under H2O2 concentrations of 0.8 mol/L and 0.5 mol/L. These newly increased Hg0 removed (19.2–6.5% and 15.8–5.8%) and (54.3–6.5% and 43.8–5.8%) may be oxidized by ·OH that is produced by Cu2+/H2O2 (E) and Fe2+/H2O2 (F) removal systems. When a small amount of Cu2+ was added to Fenton (Fe2+/H2O2) system, the Hg0 removal efficiencies obviously increase to 94.8% under the H2O2 concentration of 0.8 mol/L, and obviously increase to 82.9% under the H2O2 concentration of 0.5 mol/L. Compared with the Hg0 removal efficiencies of 54.3%/43.8% in Fenton (Fe2+/H2O2) system, and the Hg0 removal efficiencies of 19.2%/15.8% in Cu2+/H2O2 system, the new increases of Hg0 removal efficiencies (27.8% = [94.8%
0.5 mol/L, respectively. Besides, as shown in Fig. 7(c)-(f), there are no free radical signals in H2O (A), H2O/Fe2+ (B) and H2O/Cu2+ (C) and H2O2 (D) removal systems. The results show that Hg0 cannot be removed by absorption of Cu2+, Fe2+ or/and H2O, but can be oxidized by H2O2 (although only having low-level oxidation efficiency). As shown in Fig. 7(a) and (b), when Cu2+ and Fe2+ were added into H2O2 solution, respectively, Hg0 removal efficiencies achieve 19.2%/ 54.3% and 15.8%/43.8% in Cu2+/H2O2 (E) and Fe2+/H2O2 (F) removal systems under the two H2O2 concentrations of 0.8 mol/L and 0.5 mol/L, respectively. As shown in Fig. 7(c)-(f), clear free radical signals were detected using ESR spectrometer (free radical measurements were repeated twice, and the average was used). The key constants of hyperfine splitting such as aN = 15.0 G and aH = 14.6 G show a satisfactory agreement with the data (aN = 15.0 G and aH = 14.8 G) 362
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Table 1 Summary of ion products in reaction solutions. Types of ions →
SO42−
SO32−
NO3−
NO2−
Total mercury
Hg0
Determined ion concentration (mg/L) (H2O2, 0.8 mol/L) Determined ion concentration (mg/L) (H2O2, 0.5 mol/L))
231.23 206.15
– –
14.28 12.31
– –
3.09 (μg/L) 2.76 (μg/L)
– –
Fig. 8. Mechanism and routes of Hg0 removal using Cu2+-enhanced Fenton oxidation system.
oxidized by %OH that was produced from the synergistic role between Cu2+ and Fe2+ in Fenton system; (2) Hg0 was oxidized by %OH that was produced from Fe2+/H2O2 system; (3) Hg0 was oxidized by %OH that was produced from Cu2+/H2O2 system; (4) Hg0 was oxidized by H2O2. The pathways (1) and (2) were proved to be the main routes of Hg0 removal. The new removal process can achieve zero waste liquid and gas discharge, which can provide new option for removal of Hg0 from gas.
− (54.3% − 6.5%) − (19.2% − 6.5%) − 6.5%] under H2O2 concentration of 0.8 mol/L, and 29.1% = [82.9% − (43.8% − 5.8%) − (15.8% − 5.8%) − 5.8%] under H2O2 concentration of 0.5 mol/L) in the Cu2+-enhanced Fenton oxidation system had been found. Related studies [23–25] have confirmed that some transition metals and Fe2+ exhibited remarkable synergistic activation role for H2O2 to produce more %OH in the liquid phase. In this article, Cu2+ may effectively accelerate the reduction rate of Fe3+ to Fe2+ [23–25], which is responsible for the enhancement effect of Hg0 removal in Cu2+-enhanced Fenton oxidation system. Based on the comparison of Hg0 removal efficiencies and yields of %OH in different systems, it can be inferred that Hg0 is removed by oxidation of %OH and H2O2, and ·OH oxidation is the main pathway in Cu2+-enhanced Fenton oxidation system. ·OH is mainly produced by the Fe2+/H2O2 and Cu2+/H2O2 systems, and the synergistic activation effect between Cu2+ and Fe2+ in the Fenton system. In addition, to further verify the results, the oxidation products of Hg0 in the liquid phase were measured and the results are shown in Table 1 (the post-processing of products is a key issue for a new environmental remediation process, which has been added in the Supporting Information). The detection results of liquid oxidation products showed that Hg2+ was definitely detected in the reaction solution, which further verified the above oxidation reactions (4) and (5). The mechanism and routes of Hg0 removal using Cu2+-enhanced Fenton oxidation system can be also described by the following schematic diagram in Fig. 8.
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4. Conclusions Removal process of Hg0 from flue gas using Cu2+-enhanced Fenton system in a bubble column reactor was explored. The results reveal that adding Cu2+ obviously strengthen the removal process of Hg0 in Fenton system. The enhancement effect was originated from generation of more hydroxyl radicals that were produced from the synergistic role between Cu2+ and Fe2+ in the Fenton system. Hg0 removal efficiency was raised through increasing concentrations of Cu2+ and Fe2+, and was reduced via increasing reaction temperature. Increasing concentration of H2O2 and solution pH exhibited double impact on the Hg0 removal. Hg0 was mainly removed by four pathways: (1) Hg0 was 363
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Full Length Article
Oxidation removal of gaseous Hg0 using enhanced-Fenton system in a bubble column reactor
T
⁎
Yangxian Liua, , Ying Lia, Hui Xub, Jinjin Xuc a
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China c School of Energy and Power Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Elemental mercury (Hg0) removal Cu2+-enhanced Fenton Hydroxyl radical Advanced oxidation
A novel removal process of elemental mercury (Hg0) from gas using Cu2+-enhanced Fenton system in a bubble column reactor was developed. Experiments were conducted to explore the influence of several factors (concentrations of H2O2, Fe2+ and Cu2+, reagent pH, reaction temperature, concentrations of Hg0, NO and SO2) on Hg0 removal. Mechanism and routes of Hg0 removal using Cu2+-enhanced Fenton system were also proposed. The results revealed that adding Cu2+ obviously strengthened removal process of Hg0 in Fenton system. The contrast results of free radical yield found that the enhancement effect was originated from generation of more hydroxyl radicals that were produced from the synergistic role between Cu2+ and Fe2+ in Fenton system. Hg0 removal efficiency was raised by increasing concentrations of Cu2+ and Fe2+, and was reduced via increasing reaction temperature. Increasing concentration of H2O2 and reagent pH exhibited double impact on Hg0 removal. Hg0 was removed by four pathways: (1) Hg0 was oxidized by ·OH that was produced from the synergistic role between Cu2+ and Fe2+ in Fenton system; (2) Hg0 was oxidized by %OH that was produced from Fe2+/H2O2 system; (3) Hg0 was oxidized by %OH that was produced from Cu2+/H2O2 system; (4) Hg0 was oxidized by H2O2. The pathways (1) and (2) were the main routes of Hg0 removal.
1. Introduction Mercury is a toxic pollutant that has neurological health effect on human. Coal combustion is the biggest pollution source among the main anthropogenic mercury emission sources [1,2]. Typically, mercury mainly exists in three forms in coal combustion flue gas: particulate mercury (Hgp), elemental mercury (Hg0), and oxidized mercury (Hg2+) [3–5]. Both Hg2+ and Hgp can be removed by current wet desulfurization devices and dust control units, respectively. Hg0 is extremely hard to be captured owing to its very low water solubility and high volatility at room temperature [6–8]. In order to effectively control the emission of Hg0, a large number of Hg0 controlling technologies have been developed, mainly containing adsorption, catalytic, oxidation, etc [1–19]. However, due to the complexity of mercury control, there are still no economical and effective mercury control technologies that are suitable for commercial application. In various mercury control technologies, Fenton oxidation is considered as a kind of promising Hg0 removal technology because it has the potential to achieve simultaneous removal of multi-pollutants (e.g., SO2, NO, Hg0, H2S, VOCs, etc.), and has no secondary pollution [20–22]. In the area of gaseous Hg0
⁎
removal, Tan and Lu et al. [20,21] preliminarily used Fenton solution to remove Hg0 from flue gas. Liu et al. [22] further systematically studied the removal process of Hg0 from mixture flue gas using Fenton solution in a bubble column reactor. However, they also found that traditional Fenton solution has a low oxidation efficiency for Hg0 from gas because of low free radical yield (see Fig. 7a and 7b). In recent years, some results had reported that adding trace Cu2+ was able to greatly enhance the free radical yield of Fenton solution [23–25]. The Cu2+-enhanced Fenton oxidation system has shown good prospects for development as compared with original Fenton solution because of stronger oxidation capacity [23–25]. In addition, many industrial processes will discharge containing-trace Cu2+ wastewater [24]. It is also a quite valuable exploration to try to use containing-trace Cu2+ wastewater to strengthen the removal of Hg0 from flue gas using Fenton solution. However, up to now, the related studies on the removal of Hg0 from gas phase using Cu2+-enhanced Fenton oxidation system are rare. The main purpose of this article is to explore the removal of Hg0 from gas phase using Cu2+-enhanced Fenton oxidation system in a bubbling reactor. The main research contents of this article involve the following works: (1) to examine the technical feasibility of
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.fuel.2019.03.018 Received 8 December 2018; Received in revised form 8 February 2019; Accepted 4 March 2019 Available online 05 March 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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gaseous Hg0 removal using Cu2+-enhanced Fenton oxidation system; (2) to evaluate effects of several key parameters (concentrations of H2O2, Fe2+ and Cu2+, reagent pH and reaction temperature) on Hg0 removal; (3) to investigate the active species, products, and mechanism of gaseous Hg0 removal.
prepared Cu2+-enhanced Fenton reagent was added into the bubbling reactor by opening reactor lid after the pH value of the prepared reagent was adjusted based on the measure of an intelligent acidometer (sodium hydroxide and hydrochloric acid were used as buffer solutions). The temperature of Cu2+-enhanced Fenton reagent was adjusted to the required value using a constant temperature water and a thermocouple. The containing-Hg0, N2, NO, SO2 and O2 mixture gas began to enter the bubbling reactor to carry out a gas-liquid contact reaction by switching two gas valves. The outlet concentrations of Hg0 were measured by gas mercury analyzer. The mean value of instantaneous concentration in 16 min was used as the final outlet concentration of Hg0 because of the relatively better stability.
2. Experimental 2.1. Experimental installation The main components of the experimental installation in this article are as follows: (1) A Hg0-containing artificial mixture gas blending system, which contains four simulated gas cylinders (SO2, NO, N2 and O2), an elemental mercury generator, a glass gas mixing container, several flowmeters and gas valves; (2) A reaction temperature regulation system that mainly includes a thermometer and a thermostatic water bath; (3) A bubbling reactor (internal diameter of 90 mm and length of 300 mm; corrosion resistant glass), which comprises a reactor lid (rubber plug) and a flue gas distributor; (4) An analytical and tail purification system, mainly containing a tail flue gas analyzer (with three electrochemical sensors such as O2, SO2, NO), a gas mercury analyzer and an absorption container. The schematic diagram of experimental installation is described in the Fig. 1.
2.3. Analytical methods Free radicals had been detected using ESR spectrometer (i.e., electron spin resonance spectrometer) (Bruker ESP-300) combining with 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) (Sigma) that was used as the spin trap agent of free radicals. The concentrations of mercury, and the other ions in reaction solutions were analyzed using liquid fluorescence mercury analyzer and ion chromatography (IC) (ICS-1600: Dionex, USA), respectively, and the analyzed methods can be found from the literatures [26–28].
2.2. Experimental methods 2.4. Hg0 removal efficiency
Containing-Hg0, N2, NO, SO2 and O2 mixture gas (1.6 L/min) was produced by using the cylinder gases of N2, NO, SO2 and O2 (Zhenjiang Dagang Gas Co., Ltd., China), elemental mercury generator (Vici Metronics, USA) and corresponding flowmeters (Changzhou Shuanghuan Thermal Instrument Co., Ltd., China). The inlet concentrations of Hg0 and other flue gas components in mixture gas were measured using a gas mercury analyzer (QM201H, China) and flue gas analyzer (German MRU Instrument Company), respectively. 0.5 L of Cu2+-enhanced Fenton reagent (Cu2+-Fe2+/H2O2 mixed solution; solution height is about 7.9 cm) were produced using the corresponding analytical reagents (CuCl2·2H2O, FeCl2·4H2O and H2O2, Sinopharm Chemical Reagent Co., Ltd., China) and self-made deionized water. The
The concentrations of Hg0 measured by the bypass (i.e., the bubbling reactor inlet) were used as the inlet concentration of Hg0. The concentrations measured by the bubbling reactor outlet were used as the outlet concentrations of Hg0. Removal efficiency of Hg0 was calculated by the following equation (1):
Removal efficiency of Hg 0 =
Cin − Cout × 100% Cin
(1)
where Cin is inlet concentration of Hg0 in mixture gas; Cout is outlet concentration of Hg0 in mixture gas.
Fig. 1. Schematic diagram of experimental installation. 359
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Fig. 2. Effects of H2O2 concentrations on Hg0 removal efficiency. Basic conditions: Fe2+ concentrations of 0.1 mol/L and 0.06 mol/L; Reagent pH of 2.81; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
Fig. 3. Effects of Fe2+ concentrations on Hg0 removal efficiency. Basic conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Reagent pH of 2.81; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
3. Results and discussions
harmful for oxidation removal of Hg0 through consuming the produced %OH in the liquid phase [32–34].
3.1. Effects of H2O2 concentration on Hg0 removal efficiency
·OH + H2 O2 → H2 O+ HO2 ·
0
Fig. 2 shows the effects of H2O2 concentration on Hg removal efficiency in Cu2+-enhanced Fenton oxidation system. The results indicate that under the Fe2+ concentration of 0.1 mol/L, when H2O2 concentration increases from 0 to 1.5 mol/L, Hg0 removal efficiency increases from 0 to 94.8% at first, and then decreases from 94.8% to 91.6%. The optimized value of H2O2 concentrations had been found to be 0.8 mol/L. Under the Fe2+ concentration of 0.06 mol/L, when H2O2 concentration increases from 0 to 1.5 mol/L, Hg0 removal efficiency raises from 0 to 82.9% at first, and then decreases from 82.9% to 75.4%. The optimized value of H2O2 concentrations had been found to be 0.5 mol/L. A large number of results [20–22] reported that H2O2 could react with Fe2+ to produce %OH in reaction solution through the following reactions (2) and (3).
Fe2 + + H2 O2 ⇔ Fe3 + + ·OH + OH−
(2)
Fe3 + + H2 O2 ⇔ Fe2 + + HO2 ·+H+
(3)
·OH + ·OH → H2 O2
heat
k= 1. 0 × 1010 M−1 s−1
(6) (7) (8)
Therefore, adding excess H2O2 shows a negative effect on the Hg0 removal in the two studied systems using different Fe2+ concentrations. 3.2. Effects of Fe2+ concentrations on Hg0 removal efficiency Fig. 3 shows the effects of Fe2+ concentrations on Hg0 removal efficiency in Cu2+-enhanced Fenton oxidation system. The results show that when Fe2+ concentration increases from 0 to 0.18 mol/L, Hg0 removal efficiency greatly increases from 19.2% to 98.6% for the system using H2O2 concentration of 0.8 mol/L, and from 15.8% to 87.8% for the system using H2O2 concentration of 0.5 mol/L, respectively. Based on the above reactions (2)-(5), the increase of Fe2+ concentration will effectively improve the yield of ·OH in the reaction solution, and thereby can effectively promote the Hg0 removal. 3.3. Effects of reagent pH value on Hg0 removal efficiency Fig. 4 shows the effects of reagent pH value on Hg0 removal efficiency. The results show that with increasing reagent pH value (from 0.72 to 8.05), Hg0 removal efficiencies increase at first, and then decrease in the two systems using H2O2 concentration of 0.8 mol/L and 0.5 mol/L, respectively. Both of the optimized reagent pH values are 2.81. It is observed from the equation (3), H+ of high concentration (low reagent pH value) will hinder the reaction (3) that is the rate control step of the whole Fenton reaction. Hence, appropriate increase of reagent pH value will effectively improve the oxidation removal of Hg0. Nevertheless, some results [29,35,36] also had reported that ·OH apparently showed instability in strong alkaline medium because it would be consumed by the side reaction (9) (its rate constant is extremely high and almost reaches the limit of mass transfer rate of molecules in liquid phase). The oxidation capacity of the by-product O %− is far weaker than that of %OH [36]. Consequently, excessive high concentration of OH− will hinder the oxidation of Hg0.
(4)
In addition, some results [21,22,31] also had proved that H2O2 also could directly oxidize Hg0 from gas into mercury oxide in liquid phase according to the reaction (5) as below.
Hg 0 + H2 O2 → H2 O + HgO
k= 4. 2 × 109 M−1 s−1
·OH + HO2 ·→H2 O + O2
The produced %OH has strong oxidizability since its redox potential is up to 2.80 V [28–30]. The results of the previous investigators [2,3,19,26] proved that %OH could effectively oxidize Hg0 from gas into mercury hydroxide and mercury oxide in liquid phase according to the reaction (4) as below.
Hg 0 + 2·OH → Hg(OH)2 → HgO + H2 O
k= 2. 7 × 107 M−1 s−1
(5)
Based on the above equations (2)–(5), an increase of H2O2 concentration will effectively promote the oxidative removal of Hg0 in solution. However, many results of the previous researchers [18,19,22] also had indicated that in the Fenton reaction, adding excess H2O2 might result in the self-consumption of %OH through the following equations (6)–(8). The reactions (6)–(8) have very high reaction rates, which are 360
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Fig. 4. Effects of reagent pH value on Hg0 removal efficiency. Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
Fig. 6. Effects of reaction temperature on Hg0 removal efficiency. Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reagent pH of 2.81; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
·OH + OH - → H2 O + O - ·
using H2O2 concentration of 0.8 mol/L and 0.5 mol/L, respectively. Related results [23,25,31] had showed that Cu2+ could also personally catalyze H2O2 to produce %OH in solution to oxidize Hg0 from gas phase (it is named as the Fenton-like system), which can be described via the equation (11) and (12) as follows.
k= 1. 3× 1010 M−1 s−1
(9)
Moreover, a large number of results [25,31] also proved that in alkaline mediums, transition metal ions, Cu2+, and Fe2+, would react with OH− to form the precipitations of Cu(OH)2 and Fe(OH)2 in solutions, which can be described by the general equations (10) as below.
M2+ + 2OH - → M(OH)2 ↓
M =(Cu2 +, Fe 2 +)
(10)
High reagent pH value resulted in the reduction of Cu2+ and Fe2+ concentrations in solution, being not conducive for Hg0 removal. Hence, in this article, reagent pH value showed a dual influence on removal efficiency of Hg0.
Cu2 + + H2 O2 → Cu+ + ·OH + OH−
(11)
Cu+
(12)
+ H2 O2 →
Cu2 +
+ HO2·
Besides, the results [23–25] of the previous researchers proved that Cu2+ could effectively enhance the Fenton reaction through accelerating the reduction rate of Fe3+ to Fe2+. The above equation (3) (the reduction steps of Fe3+ to Fe2+) is widely recognized as the main rate controlling step of the whole Fenton reaction process. Addition of Cu2+ can effectively accelerate the circle rate between Fe2+ and Fe3+ [23–25], thereby being able to effectively enhance Hg0 removal in the Fenton oxidation system.
3.4. Effects of Cu2+ concentrations on Hg0 removal efficiency Fig. 5 shows the effects of Cu2+ concentrations on Hg0 removal efficiency. The results show that with increasing the Cu2+ concentration from 0 to 0.009 mol/L, Hg0 removal efficiency markedly increases from 54.3% to 96.5%, and from 43.8% to 86.2% in the two systems
3.5. Effects of reaction temperature on Hg0 removal efficiency Fig. 6 shows the effects of reaction temperature on Hg0 removal efficiency. It can be observed that when reaction temperature increases from 298 K to 348 K, Hg0 removal efficiencies decrease from 97.3% to 85.2%, and from 85.7% to 73.5% in the two systems using H2O2 concentration of 0.8 mol/L and 0.5 mol/L, respectively. A large number of results [33,37,38] pointed out that increasing temperature (it was equivalent to increasing solution temperature) would cause a decrease of gas solubility in liquid phase. The decrease of Hg0 solubility in solution would elevate the mass transfer resistance of Hg0 between gas & liquid two phases, which would hinder the Hg0 removal. In this study, increasing temperature caused the decrease of Hg0 solubility, and thus resulted in the decline of Hg0 removal efficiency. 3.6. Comparative study of different systems, and mechanism and routes of Hg0 removal Fig. 7(a) and (b) shows the contrast of Hg0 removal efficiency in different systems. As exhibited in Fig. 7(a) and (b), as the contrast, all of H2O (A), H2O/Fe2+ (B) and H2O/Cu2+ (C) removal systems under different H2O2 concentrations (0.8 mol/L and 0.5 mol/L) cannot remove Hg0 from gas. In the H2O2 (D) systems, Hg0 removal efficiencies are 6.5% and 5.8% using H2O2 concentrations of 0.8 mol/L and
Fig. 5. Effects of Cu2+ concentrations on Hg0 removal efficiency. Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Reagent pH of 2.81; Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm. 361
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Fig. 7. Comparative experiments of Hg0 removal efficiency in different systems: H2O2 concentrations of 0.8 mol/L (a) and 0.5 mol/L (b); ESR spectrums of ·OH free radical adducts in Cu2+-enhanced Fenton: blank sample (c) and reference value (d); Comparative experiments of hydroxyl radical yield in different systems: H2O2 concentrations of 0.8 mol/L (e) and 0.5 mol/L (f). Experimental conditions: H2O2 concentrations of 0.8 mol/L and 0.5 mol/L; Cu2+ concentration of 0.006 mol/L (for H2O2 concentration of 0.8 mol/L); Cu2+ concentration of 0.004 mol/L (for H2O2 concentration of 0.5 mol/L); Reagent pH of 2.81; Reaction temperature of 318 K; Hg0 concentration of 60 μg/m3; NO concentration of 300 ppm; SO2 concentration of 1000 ppm.
that were obtained via the other literatures [37–39], which represent the radical adduct DMPO-OH (the reaction product of ·OH with DMPO). The result proved that ·OH had been produced in Cu2+/H2O2 (E) and Fe2+/H2O2 (F) under H2O2 concentrations of 0.8 mol/L and 0.5 mol/L. These newly increased Hg0 removed (19.2–6.5% and 15.8–5.8%) and (54.3–6.5% and 43.8–5.8%) may be oxidized by ·OH that is produced by Cu2+/H2O2 (E) and Fe2+/H2O2 (F) removal systems. When a small amount of Cu2+ was added to Fenton (Fe2+/H2O2) system, the Hg0 removal efficiencies obviously increase to 94.8% under the H2O2 concentration of 0.8 mol/L, and obviously increase to 82.9% under the H2O2 concentration of 0.5 mol/L. Compared with the Hg0 removal efficiencies of 54.3%/43.8% in Fenton (Fe2+/H2O2) system, and the Hg0 removal efficiencies of 19.2%/15.8% in Cu2+/H2O2 system, the new increases of Hg0 removal efficiencies (27.8% = [94.8%
0.5 mol/L, respectively. Besides, as shown in Fig. 7(c)-(f), there are no free radical signals in H2O (A), H2O/Fe2+ (B) and H2O/Cu2+ (C) and H2O2 (D) removal systems. The results show that Hg0 cannot be removed by absorption of Cu2+, Fe2+ or/and H2O, but can be oxidized by H2O2 (although only having low-level oxidation efficiency). As shown in Fig. 7(a) and (b), when Cu2+ and Fe2+ were added into H2O2 solution, respectively, Hg0 removal efficiencies achieve 19.2%/ 54.3% and 15.8%/43.8% in Cu2+/H2O2 (E) and Fe2+/H2O2 (F) removal systems under the two H2O2 concentrations of 0.8 mol/L and 0.5 mol/L, respectively. As shown in Fig. 7(c)-(f), clear free radical signals were detected using ESR spectrometer (free radical measurements were repeated twice, and the average was used). The key constants of hyperfine splitting such as aN = 15.0 G and aH = 14.6 G show a satisfactory agreement with the data (aN = 15.0 G and aH = 14.8 G) 362
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Table 1 Summary of ion products in reaction solutions. Types of ions →
SO42−
SO32−
NO3−
NO2−
Total mercury
Hg0
Determined ion concentration (mg/L) (H2O2, 0.8 mol/L) Determined ion concentration (mg/L) (H2O2, 0.5 mol/L))
231.23 206.15
– –
14.28 12.31
– –
3.09 (μg/L) 2.76 (μg/L)
– –
Fig. 8. Mechanism and routes of Hg0 removal using Cu2+-enhanced Fenton oxidation system.
oxidized by %OH that was produced from the synergistic role between Cu2+ and Fe2+ in Fenton system; (2) Hg0 was oxidized by %OH that was produced from Fe2+/H2O2 system; (3) Hg0 was oxidized by %OH that was produced from Cu2+/H2O2 system; (4) Hg0 was oxidized by H2O2. The pathways (1) and (2) were proved to be the main routes of Hg0 removal. The new removal process can achieve zero waste liquid and gas discharge, which can provide new option for removal of Hg0 from gas.
− (54.3% − 6.5%) − (19.2% − 6.5%) − 6.5%] under H2O2 concentration of 0.8 mol/L, and 29.1% = [82.9% − (43.8% − 5.8%) − (15.8% − 5.8%) − 5.8%] under H2O2 concentration of 0.5 mol/L) in the Cu2+-enhanced Fenton oxidation system had been found. Related studies [23–25] have confirmed that some transition metals and Fe2+ exhibited remarkable synergistic activation role for H2O2 to produce more %OH in the liquid phase. In this article, Cu2+ may effectively accelerate the reduction rate of Fe3+ to Fe2+ [23–25], which is responsible for the enhancement effect of Hg0 removal in Cu2+-enhanced Fenton oxidation system. Based on the comparison of Hg0 removal efficiencies and yields of %OH in different systems, it can be inferred that Hg0 is removed by oxidation of %OH and H2O2, and ·OH oxidation is the main pathway in Cu2+-enhanced Fenton oxidation system. ·OH is mainly produced by the Fe2+/H2O2 and Cu2+/H2O2 systems, and the synergistic activation effect between Cu2+ and Fe2+ in the Fenton system. In addition, to further verify the results, the oxidation products of Hg0 in the liquid phase were measured and the results are shown in Table 1 (the post-processing of products is a key issue for a new environmental remediation process, which has been added in the Supporting Information). The detection results of liquid oxidation products showed that Hg2+ was definitely detected in the reaction solution, which further verified the above oxidation reactions (4) and (5). The mechanism and routes of Hg0 removal using Cu2+-enhanced Fenton oxidation system can be also described by the following schematic diagram in Fig. 8.
Acknowledgements This study was supported by National Natural Science Foundation of China (Nos. U1710108; 51576094), Jiangsu ‘‘Six Personnel Peak” Talent-Funded Projects (GDZB-014). References [1] Yang W, Arshad H, Zhang J. Removal of elemental mercury from flue gas using red mud impregnated by KBr and KI reagent. Chem Eng J 2018;341:483–94. [2] Zhao Y, Hao RL, Guo Q. A novel pre-oxidation method for elemental mercury removal utilizing a complex vaporized absorbent. J Hazard Mater 2014;15:118–26. [3] Zhao Y, Hao RL, Zhang P, Zhou SH. An integrative process for Hg0 removal using vaporized H2O2/Na2S2O8. Fuel 2014;136:113–21. [4] Xu W, Hussain A. A review on modification methods of adsorbents for elemental mercury from flue gas. Chem Eng J 2018;346:692–711. [5] Yang W, Xu W, Liu ZY. Removal of elemental mercury from simulated flue gas using sargassum chars modified by NH4Br reagent. Fuel 2018;214:196–206. [6] Zhao Y, Hao Rl, Qi M. Integrative process of preoxidation and absorption for simultaneous removal of SO2, NO and Hg0. Chem Eng J 2015;269:159–67. [7] Xu W, Pan JF, Fan BW. Removal of gaseous elemental mercury using seaweed chars impregnated by NH4Cl and NH4Br. J Clean Prod 2019;216:277–87. [8] Liu ZY, Yang W, Xu W. Removal of elemental mercury by bio-chars derived from sargassum and enteromorpha impregnated with potassium iodine. Chem Eng J 2018;339:468–78. [9] Yang W, Liu YX, Wang Q, Pan JF. Removal of elemental mercury from flue gas using wheat straw chars modified by Mn-Ce mixed oxides with ultrasonic-assisted impregnation. Chem Eng J 2017;326:169–81. [10] Xu Y, Luo GQ, Yao H. Adsorption and catalytic oxidation of elemental mercury over regenerable magnetic Fe-Ce mixed oxides modified by non-thermal plasma treatment. Chem Eng J 2019;358:1454–63. [11] Liu ZX, Adewuyi YG, Shi S. Removal of gaseous Hg0 using novel seaweed biomassbased activated carbon. Chem Eng J 2019;339:468–78. [12] Wu J, Li CE, Chen XT, Zhang J. Photocatalytic oxidation of gas-phase Hg0 by carbon spheres supported visible-light-driven CuO–TiO2. J Ind Eng Chem 2017;46:416–25. [13] Liu YX, Wang Y. Gaseous elemental mercury removal using VUV and heat
4. Conclusions Removal process of Hg0 from flue gas using Cu2+-enhanced Fenton system in a bubble column reactor was explored. The results reveal that adding Cu2+ obviously strengthen the removal process of Hg0 in Fenton system. The enhancement effect was originated from generation of more hydroxyl radicals that were produced from the synergistic role between Cu2+ and Fe2+ in the Fenton system. Hg0 removal efficiency was raised through increasing concentrations of Cu2+ and Fe2+, and was reduced via increasing reaction temperature. Increasing concentration of H2O2 and solution pH exhibited double impact on the Hg0 removal. Hg0 was mainly removed by four pathways: (1) Hg0 was 363
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