Selective Removal of Elemental Mercury from High-Concentration

National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China. Ind. Eng. Chem. Res. , 2017, 56 (15...
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Selective Removal of Elemental Mercury from High-Concentration SO2 Flue Gas by Thiourea Solution and Investigation of Mechanism Zhilou Liu,† Bing Peng,†,‡ Liyuan Chai,†,‡ Hui Liu,*,†,‡ Shu Yang,† Bentao Yang,† Kaisong Xiang,† Cao Liu,† and Dongli Wang† †

School of Metallurgy and Environment, Central South University, 932 South Lushan Road, Changsha 410083, China Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China

‡

S Supporting Information *

ABSTRACT: Nonferrous metals smelting industry is a major anthropogenic Hg emission source in China. A new method using transition metal-Tu (thiourea) solution was proposed to selectively remove Hg0 from high SO2 concentration flue gas. It is found that this solution displays significantly high Hg0 removal efficiency, up to above 88.7%, under optimal conditions, whereas the loss of SO2 is only 6.8%. High Tu concentration, low solution pH, and low temperature are helpful for Hg0 removal. The promoting effect of Fe3+ is better than that of Cu2+. The mechanism of selective Hg0 removal was studied by various methods. First, the cyclic voltammetry (CV) curves show that part of Tu could be oxidized to FDS (formamidine disulfide), which could combine with Hg0 to form Hg−Tu complex. Additionally, the UV−vis spectra indicate H2SO3 could improve the stability of FDS and facilitate Hg0 removal. In summary, the use of transition metal-Tu solution to selectively remove Hg0 from smelting flue gas displays excellent efficiency, and the proposed mechanism of Hg0 selective removal is persuasive. risk.12 Taking Boliden−Norzink technology for example, Hg0 can be removed by the absorption of HgCl2 solutions. However, HgCl2 poses a huge environmental risk on account of high toxicity. In addition, there are many literature reports that Fenton, ammonium persulfate, or diperiodatocuprate (III) solutions could efficiently oxidize and absorb Hg0 from coalfired flue gas.13−17 Although these technologies could realize the simultaneous removal of Hg0 and SO2, they are not suitable for high SO2 smelting flue gas. The concentration of SO2 in smelting flue gas is much higher than in coal-fired flue gas, about (2−6) × 104 ppm versus (0.5−2) × 103 ppm.9 Additionally, it could cause great loss of valuable sulfur resources, which is unacceptable for nonferrous metals smelting. Meanwhile, Hg0 removal efficiency will drop rapidly because of the competitive oxidation between Hg0 and SO2. Therefore, the selective removal of Hg0 in high SO2 flue gas is an urgent problem for nonferrous metals smelting. Adding a coordination agent has become an effective method for improving reaction rate and is widely used in many fields, such as gold leaching and electrode positioning.18,19 For example, inert metals are prone to oxidation in the presence of ligand even under weak oxidation conditions. The process is known as oxidation-complexation technology. The addition of

1. INTRODUCTION Mercury, due to its characteristics of high toxicity, volatility, bioaccumulation, and persistence in the global environment, has attracted worldwide attention.1−3 In 2013, more than 80 countries signed The Minamata Convention on Mercury, aiming to reduce atmospheric mercury emission. As reported by some researchers,4,5 nonferrous metals smelting industry is a major anthropogenic Hg emission source in China. There are three forms of mercury in the flue gas of nonferrous metals smelters: elemental mercury (Hg0), gaseous oxidized mercury (Hg2+), and particulate mercury (Hgp). Most Hg2+ and Hgp in flue gas can be removed efficiently with typical air pollution control devices (APCDs), such as electrostatic precipitator or wet flue gas desulfurization.6 However, Hg0 is difficult to remove from flue gas by APCDs due to its low solubility and high volatility. In addition, Hg is often accompanied by high concentrations of SO2, where the concentration of SO2 can reach up to 10 vol % in some copper or zinc smelting flue gases.7−9 The high-concentration SO2 needs to be converted to SO3 to recover sulfur in the form of sulfuric acid.10 During the production of sulfuric acid, part of Hg0 would be absorbed by concentrated sulfuric acid, leading to the deterioration of sulfuric acid product,11 and the remaining Hg0 would be released into the atmosphere. Some technologies have been reported to remove Hg0 from smelting flue gas, such as the Bolchem and Boliden−Norzink technologies.11 However, these technologies are not widely used on account of strong corrosion and high environmental © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 4, 2017 March 16, 2017 March 29, 2017 March 29, 2017 DOI: 10.1021/acs.iecr.7b00044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Functional schematic of experimental apparatus for Hg0 removal.

UV−visible spectroscopic (UV2450) techniques to characterize the intermediate species. Chemical reagents used in the experiment are given in Supporting Information. 2.3. Removal Efficiency. The concentrations of Hg0 in simulated flue gas before and after absorption by acidic Tu solution are used as inlet and outlet concentration, respectively. The removal efficiency of Hg0 is calculated by use of eq 1:

transition metal ions could improve the redox potential of solution and reaction rate while it also maintains the stability of complex agent.20 The technology using transition metal and coordination agent is a promising method to realize Hg0 oxidation under weak oxidative conditions, which also means low SO2 consumption. Hence, it is a feasible method for selective removal of Hg0 from nonferrous metals smelting of high SO2. In this paper, the method of using transition metal and thiourea solution for selective removal of Hg0 from high SO2 flue gas is put forward for the first time. Hg0 removal efficiency under different conditions was evaluated by a lab-scale simulation scrubbing reactor. Additionally, a possible mechanism of Hg0 removal was proposed by using techniques such as cyclic voltammetry (CV) and UV−visible spectroscopy.

ηHg 0 =

C Hg(inlet) − C Hg(outlet) C Hg(inlet)

× 100 (1)

where ηHg0 represents the Hg removal efficiency (percent) and CHg(inlet) and CHg(outlet) are the average Hg0 concentrations within 1 min in flue gas at the inlet and outlet of the reaction system, respectively (micrograms per cubic meter).

3. RESULTS AND DISCUSSION 3.1. Effect of Thiourea Concentration. The removal efficiency of Hg0 as a function of the concentration of thiourea (Tu) is shown in Figure 2. When there is no Tu in solution,

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus and Method. The experimental apparatus, as shown in Figure 1, consists of flue gas blending, bubbling reactor, mercury test, and tail gas treatment systems. Three cylinder gases (N2, O2, and SO2) and a Hg0 generator (VICI Metronics) were used to provide simulated smelting flue gas. The simulated flue gas (600 mL/ min) was prepared by joint use of the cylinder gases. The proportions of N2, O2, and SO2 were controlled by a mass flow meter (MFM). High-purity nitrogen gas was used as a carrier gas at a flow of 200 mL/min to take out Hg0 from the Hg0 generator. All the tubing was Teflon and heated with a temperature-controlled heating belt at 120 °C to avoid condensation of moisture and adsorption of mercury on the tube surface. A three-neck round-bottom flask of 500 mL with inlet and outlet for gas and a neck for installing electrode was used as bubbling reactor. The flask was placed in a constanttemperature water bath to control solution temperature. Solutions containing 30% (w/v) NaOH and 5% (w/v) KMnO4 were employed to absorb SO2 and Hg0 in the tail gas, respectively. 2.2. Analytical Method. Hg0 in outlet gas was continuously monitored by a Lumex Zeeman mercury spectrometer (RA-915M, detection limit = 2 ng/m3), which employed resonance absorption of mercury atoms at wavelength 253.7 nm.21 SO2 in flue gas was measured by ECOM-J2KN gas analyzer. The pH of the solution was measured by a pH meter (Mettler-S220). During the experiment, samples (5 mL) were collected at selected time intervals and then analyzed by Fourier transform infrared (FTIR) spectrometric (Nicolet IS10) and

Figure 2. Effect of Tu concentration on Hg0 removal. Experimental conditions: initial Hg0 concentration = 247 μg/m3; pH = 1; solution temperature = 40 °C; SO2 concentration = 2 vol %; O2 concentration = 8 vol %; gas flow rate = 0.6 L/min; solution volume = 300 mL.

only less than 1% Hg0 could be removed under 8 vol % O2 and 2 vol % SO2. With an increase in Tu concentration from 0 to 0.4 mol/L, Hg0 removal efficiency sharply increases from 0.54% to 65.6%. This shows that Tu plays an important role in Hg0 oxidation even with high SO2 concentrations. When the Tu concentration exceeds 0.2 mol/L, the mass transfer step of Hg0 or O2 from gas phase to liquid phase gradually begins to play a B

DOI: 10.1021/acs.iecr.7b00044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

short reaction time of actual operation. When the Cu2+ concentration is 0.01 and 0.03 mol/L, the Hg0 removal efficiency can reach 71.5% and 78.4%, respectively. On the contrary, the removal efficiency of Hg0 is only 55.4% in the absence of transition metal ions. In addition, Fe3+ plays a better role in promoting Hg0 removal than Cu2+, and the removal efficiency could increase to 82.5% and 88.7% corresponding to 0.01 and 0.03 mol/L Fe3+, respectively. When transition metal ions were added to Tu solution, the solution redox potential increased. This indicates high-valence transition metal ions may play the role of oxidants for Hg0 removal. The redox reaction between Tu and transition metal ions is quite slow due to the formation of relatively stable metal−Tu complex species.20 In this case, low consumption of Tu and high potential during the scrubbing process could be maintained. It shows that adding metal ions could significantly improve Hg0 removal efficiency. To examine the stability for Hg0 removal efficiency, a longduration test was conducted. The results show the solution of 0.1 mol/L Tu and 0.03 mol/L Fe3+ possesses stable Hg0 removal efficiency, remaining at over 85% after 8 h. This implies high stability of Fe3+−Tu solution for Hg0 removal. 3.3. Loss of SO2. The removal efficiency of SO2 during Hg0 removal was investigated and is shown in Figure 5. The results

leading role in the whole reaction process. Therefore, the growth rate of Hg0 removal efficiency gradually becomes smaller with further enhancement of Tu concentration. Tu has a strong affinity for Hg2+ to form stable complexes such as Hg(Tu)22+, Hg(Tu)32+, and Hg(Tu)42+. The stability constants of those complexes are 22.1, 24.7, and 26.8 from previous studies, respectively.22,23 The final product in this experiment is Hg(Tu)42+ due to the presence of excess Tu. The formation of stable Hg−Tu complexes could greatly promote the oxidation of Hg0 since the Eh of Hg(Tu)42+/Hg0 shown in Figure 3 decreases from 0.85 V for the Eh of Hg2+/Hg0 to 0.058

Figure 3. Eh−pH diagram of the Hg−Tu−H2O system under 1 mol/L Tu concentration at 25 °C.

V. Meanwhile, the decrease in standard redox potential of Hg0 oxidation implies that Hg0 would be preferentially oxidized relative to SO2 (the standard redox potential of SO42−/SO2 is 0.17 V). It should be the reason why Hg0 in high SO2 flue gas could be removed by Tu solution. The chemical reaction is as follows: 2Hg 0 + 8Tu + O2 + 4H+ = 2Hg(Tu)4 2 + + 2H 2O 2+

(2)

3+

3.2. Effects of Cu and Fe Concentration. The effects of Cu2+ and Fe3+ on Hg0 removal efficiency are shown in Figure 4 under the condition of 0.1 mol/L Tu in solution and 2 vol % SO2 and 8 vol % O2 in flue gas. Obviously, the addition of Cu2+ and Fe3+ could dramatically accelerate the reaction rate and Hg0 removal efficiency at the beginning. These phenomena are good for the application of gas−liquid reaction because of the

Figure 5. Loss of SO2 during scrubbing process. Experimental conditions: Tu concentration = 0.1 mol/L; pH = 1; solution temperature = 40 °C; Fe3+ concentration =0.03 mol/L; other conditions were the same as in Figure 2.

indicate that about 5.2% SO2 is oxidized to sulfuric acid during the scrubbing process in the presence of 0.1 mol/L Tu in solution and 8 vol % O2 in flue gas. This shows that only a small part of SO2 would be removed during the scrubbing process. When the 0.03 mol/L Cu2+ is added to solution, the loss of SO2 shows a small increase. With the addition of 0.03 mol/L Fe3+, the loss of SO2 improves from 5.2% to 6.8% after 120 min. Many studies reported that metal ions in solution played a catalytic role in the oxidation of SO2 by O2 in the desulfurizing process.24−26 However, the results illustrate that the addition of transition metal ions would not lead to the substantial increase of SO2 loss. The main species of transition metal ions in Tu solution is metal−Tu complex such as Fe(Tu)23+ rather than free metal ions, which leads to reduction of catalytic activity for SO2 oxidation. In addition, the standard redox potential of metal−Tu is far lower than that of individual metal solution. Taking Fe3+ for example, the standard redox potential for Fe3+/ Fe2+ decreases from 0.77 to 0.28 V [Fe(Tu)23+/Fe2+]. It may be one of the important reasons why the loss of SO2 only increases a little when transition metal ions are added to Tu solution.

Figure 4. Effects of Cu2+ and Fe3+ concentration on Hg0 removal. Experimental conditions: Tu concentration = 0.1 mol/L; pH = 1; solution temperature = 40 °C; other conditions were the same as in Figure 2. C

DOI: 10.1021/acs.iecr.7b00044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 3.4. Effect of Solution Temperature. Figure 6 indicates the effect of solution temperature on Hg0 removal. The results

Figure 7. FTIR spectra of Tu solution at different temperatures.

form stable HgS. This could explain the reason why Hg0 removal efficiency at 55 °C increases in the initial reaction period. With increasing decomposition of Tu, colloidal S0 becomes unstable and is inclined to agglomerate. Finally, after a period of time, yellow sulfur precipitate would be formed. The mercury species in yellow sulfur precipitate is identified by mercury temperature-programmed desorption technique.31 There are two peaks in Figure S1, the main one at 173 °C and the second one of low intensity at 275 °C, which correspond to black HgS and red HgS, respectively.32 Therefore, formation of HgS is the reason for the increased Hg0 removal efficiency. Under high-temperature conditions, colloidal S0 becomes unstable and easily agglomerates to give sulfur precipitation. The sulfur precipitate displays low activity with respect to colloidal S0, which leads to the sharp fall in Hg0 removal. Although Hg0 removal efficiency could be improved to a certain extent by raising temperature, excessive temperature reduces the stability of colloidal S0 and the concentration of Tu, resulting in rapid decline in Hg0 removal. Additionally, the decomposition products of Tu are complex and their effects on Hg0 removal would need to be studied in further work. 3.5. Effect of pH. Figure 8 indicates the effect of pH on Hg0 removal efficiency. When the pH increases from 1 to 9, Hg0 removal efficiency decreases from 86.5% to 63.4% at 0.1 mol/L Tu. Obviously, low pH contributes to Hg0 removal. The tautomeric form of Tu [HSC(NH)NH2] could easily be generated under acidic conditions, which can improve the

Figure 6. Effect of solution temperature on Hg0 removal. Experimental conditions: Tu concentration = 0.1 mol/L; pH = 1; Fe3+ concentration = 0.03 mol/L; other conditions were the same as in Figure 2.

show that solution temperature has a complex impact on Hg0 removal. When the temperature increased from 25 to 40 °C, the Hg0 removal efficiency increased slightly, from 83.4% to 88.7%. In general, rising temperature could promote Hg0 oxidation due to increasing reaction rate. Meanwhile, the Hg0 removal efficiency should remain stable or weaken with time. However, an interesting phenomenon was found for the first time that the Hg0 removal efficiency at 55 °C would increase to 94.5% in the first 20 min and then fall precipitously. At high temperature, the stability of Tu ligand would decrease and Tu could be easily oxidized into a series of products, leading to reduced Tu concentration in solution. When solution temperature exceeds 55 °C, the solution becomes turbid with the generation of yellow precipitates during the experiments. From previous literature,27,28 Tu could decompose to elemental sulfur (S0) and cyanamide (CN·NH2) at high temperature. Hence, it is proposed that the S0 which comes from irreversible oxidation of Tu could combine with Hg0 to generate HgS, resulting in increased Hg0 removal efficiency. The reaction path is as follows: 2SC(NH 2)2 + O2 → 2S0 + 2CNNH 2 + 2H 2O

(3)

S0 + Hg 0 = HgS

(4)

To confirm this assumption, the structure changes of Tu at different temperatures are identified by FTIR spectroscopy and the results are shown in Figure 7. Some reports are available on the characteristic peaks of Tu, which are at 1485, 1404, 1083, and 730 cm−1.29,30 The band at about 1485 cm−1 is attributed to symmetric stretching of CS (ν-CS), and the band at 1085 cm−1 is due to rocking of NH2 (ρ-NH2). With increasing solution temperature, the characteristic peaks of Tu at 1485, 1404, and 1084 cm−1 gradually disappear, which implies the decomposition of Tu. The bands at 2242 and 1668 cm−1 are attributed to the stretching of CN of CN·NH2 (ν-CN) and the rocking of NH2 of Tu(δ-NH2), respectively. The increased peak at 2242 cm−1 with temperature indicates the formation of CN· NH2. Although no characteristic peak of S0 is found, the formation of CN·NH2 could prove the generation of S0 on the basis of reaction 4. At the beginning of the experiment, S0 is in a colloidal state, in accordance with literature.20 Colloidal S0 possesses high activity and could easily combine with Hg0 to

Figure 8. Effect of solution pH on Hg0 removal. Experimental conditions: Tu concentration = 0.1 mol/L; solution temperature = 40 °C; Fe3+ concentration = 0.03 mol/L; other conditions were the same as in Figure 2. D

DOI: 10.1021/acs.iecr.7b00044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research stability of Tu due to formation of low-energy hydrosulfonyl.33 Hence, the high stability of Tu under acidic conditions is one reason for the removal of Hg0. Meanwhile, the majority of the sulfur is present as SO32− in both neutral and alkaline solutions.34 Many studies report that SO32− ions would lead to mercury re-emission due to formation of unstable HgSO3.35−37 Thus, in neutral or alkaline solutions, that HgSO3 is easier to form may be another reason. 3.6. Mechanism of Mercury Removal. Hg0 could be selectively removed from high SO2 flue gas by Fe3+−Tu or Cu2+−Tu solutions. In previous studies,3,14 Hg0 could be removed until it was oxidized to Hg2+ during the scrubbing process.3,14 However, Hg0 could not be removed in only 0.1 mol/L Tu solution without the presence of oxidant, whereas the removal efficiency is only about 8.5% in 0.03 mol/L Fe3+ (see Figure S2). These results reveal Hg0 could not be efficiently oxidized by Fe3+ and O2 oxidants. The results also show the presence of SO2 is favorable for Hg0 removal (see Figure S3). A new mechanism called Tu intermediate-oxidation mechanism is proposed. First, Tu is oxidized to an intermediate, which is stable in solution with SO2. Then it could directly react with Hg0 to form a Hg(Tu)x2+ complex. To analyze the mechanism proposed above, experiments on electrochemical oxidation of Tu were conducted in acidic solutions. The cyclic voltammetry (CV) curves for 0.1 mol/L Tu at platinum electrode are shown in Figure 9. A pair of the

Curve 1 exhibits peak potentials for A and C of about 0.59 V and −0.76 V, respectively. Curve 2 was run for 0.05 mol/L formamidine disulfide (FDS) in 0.1 mol/L H2SO4 under the same conditions as curve 1. The anodic and cathodic peaks on the CV for FDS solution are the same as the A and C peaks for Tu solution. The electrochemical reaction is considered to be oxidation of Tu to FDS and the corresponding reduction of FDS to Tu, according to reaction 5. Additionally, the anodic and cathodic peaks also indicate that the transformation between Tu and FDS is reversible. Li and Miller20 report that the first Tu oxidation by moderate oxidants such as ferric ion and oxygen is FDS and the standard redox potential of FDS/Tu is 0.42 V, which indicates FDS may act as the oxidant for Hg0. On the basis of this discussion, FDS may be the key intermediate for Hg0 removal. 2SC(NH 2) → NH 2(NH)CSSC(NH)NH 2 + 2H+ + 2e−

(5)

Mercury film electrode was used as a working electrode to study the oxidation of Hg in acidic Tu solution. Figure 10a shows only one anodic peak (A1) at about 0.56 V, which is considered as the oxidation of Tu to FDS at the first scan. Upon further scanning of Tu solution, a new peak (A2) in curve 2 at 0.02 V would appear. There are two possible oxidation reactions in this system: oxidation of Tu to FDS and oxidation of Hg to Hg(Tu)x2+. In other words, the new peak A2 ought to represent the oxidation of Hg. A phenomenon should be noted that peak A2 in curve 1 would not appear during the first cycle. To explain this phenomenon, 0.05 M FDS was added to Tu solution and the results are shown in Figure 10b. The peak of the oxidation of Hg to Hg(Tu)x2+ appears at first as long as there is FDS in solution. This demonstrates that the presence of FDS is the key to oxidation of Hg in Tu solution. The oxidation process of Hg in the presence of Tu solution could be explained as follows. In the beginning, the amount of FDS derived from Tu oxidation is very small on the mercury film electrode, and Hg could not be oxidized in this case. After several cycles, the amount of FDS gradually increases, and then Hg could react with FDS to form Hg(Tu)x2+. Hence, only after the formation of FDS could the Hg be oxidized in Tu solution. To further confirm the effect of FDS on Hg0 oxidation, the removal efficiency of Hg0 in Tu−FDS solution was investigated (see Figure S4). The results indicate that Hg0 could be removed in the presence of FDS and Tu solution in which there were no other oxidants, such as O2 or Fe3+. It also confirmed that FDS could oxidize Hg0 to Hg(Tu)x2+. The

Figure 9. CV cycles for different solutions containing 0.1 mol/L H2SO4 on glassy carbon electrode at a scan rate of 100 mV·s−1. Experimental conditions: (1) 0.1 mol/L Tu; (2) 0.02 mol/L FDS.

anodic and cathodic peaks, oxidation and reduction peaks (A and C), were observed over the potential range −1 to 1 V.

Figure 10. (a) First and fifth cycles of CV in 0.1 M Tu and 0.1 M H2SO4 solution on mercury film electrode at a scan rate of 100 mV·s−1. (b) Cycles of CV in 0.05 M FDS + 0.05 M Tu and in 0.02 M FDS solution under the same conditions. E

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Figure 11. Evolution of UV−visible spectra of Tu or FDS agents in the absence or presence of H2SO3 over 10 min. (a) [FDS] = 0.125 mM; (b) [FDS] = 0.125 mM and [H2SO3] = 20 mM. Experimental conditions: pH value = 2.0 (HClO4), solution temperature = 25 °C, scanning interval = 30 s.

above results prove that FDS is the key intermediate for Hg0 oxidation in acidic Tu solutions, and the reaction is as follows:

4. CONCLUSIONS A novel method to realize selective removal of Hg0 from high SO2 smelting flue gas was proposed and the evaluation of different parameters of Hg0 removal efficiency was carried out. The results indicate that high Tu concentration, low solution pH, and low temperature promote Hg0 removal efficiency. Fe3+ provides a better promoting effect than Cu2+. During the scrubbing process, the loss of SO2 is only about 6.8%, and the Hg0 removal efficiency could reach up to 88.7% under optimal condition. The mechanism of selective removal of Hg0 was also studied. Tu could be oxidized to FDS, and then FDS as oxidant could oxidize Hg0 to Hg(Tu)x2+ compound in Tu solution. In addition, the presence of SO2 could improve the stability of FDS and facilitate the removal of Hg0.

Hg 0 + NH 2(NH)SSC(NH)NH 2 + 2H+ + 2SC(NH 2)2 → Hg[SC(NH 2)]4 2 +

(6)

To understand the relationship between FDS and SO2, UV− vis analysis was used and the spectra of individual or mixed solutions are shown in Figure 11. The absorption peak of Tu is around 235 nm, and its intensity does not change over time in acidic solution (see Figure S5). Although the absorption peak of FDS in Figure 11a is also at 235 nm, its peak gradually increases from 1.35 to 2.11 within 10 min, owing to the decomposition of FDS to Tu. The addition of H2SO3 only results in the change of absorption spectrum between 200 and 220 nm, and it does not affect the peak at 235 nm. Compared to Figure 11a, the decay rate of FDS at 235 nm in the presence of H2SO3 significantly decreases, which illustrates that the presence of H2SO3 inhibits decomposition and improves the stability of FDS. This is the reason why the presence of SO2 facilitated the removal efficiency of Hg0. The results above show the mechanism of selective Hg0 removal from high SO2 concentration flue gas is supported. First, the CV results show that part of Tu could be oxidized to FDS, an intermediate during the process of Tu oxidation. Next, FDS can react with Hg0 to form Hg(Tu)x2+ complex. The UV− visible spectrum demonstrates that the existence of H2SO3 could improve the stability of FDS. Therefore, the formation of FDS in Tu solution is the reason Hg0 could be selectively removed. 3.7. Application of This Method. After the traditional scrubbing purification process, the temperature of smelting flue gas drops below 50 °C. Then, the transition metal−Tu solution can be used to selectively remove Hg0. Commercialized gas− liquid reaction devices, such as spray tower and bubble tower, ensure the application of this technology. After scrubbing, the solution containing Hg2+ can be treated with sodium sulfide to produce HgS precipitation, which can be recovered by filtration separation in a filter. The filtrate can be reused in mercury removal process. This can realize the circulation utilization of Tu and reduce the cost of Hg0 removal. Overall, this method has large potential application prospects for purification of smelting flue gas due to the characteristics of high selective removal of Hg0 and circulation utilization of Tu.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00044. Additional text describing chemical reagents; five figures showing Hg thermal decomposition profile, Hg0 removal efficiency, effect of SO2 and FDS on Hg0 removal, and evolution of UV−vis spectra of Tu agent with time (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hui Liu: 0000-0002-6640-9429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is financially supported by the Project of Innovation-driven Plan in Central South University (2016CXS010) and the Natural Science Foundation of China (51474246, 51404306).

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REFERENCES

(1) Schroeder, W. H.; Munthe, J. Atmospheric mercuryan overview. Atmos. Environ. 1998, 32 (5), 809−822. (2) Pirrone, N.; Cinnirella, S.; Feng, X.; Finkelman, R.; Friedli, H.; Leaner, J.; Mason, R.; Mukherjee, A.; Stracher, G.; Streets, D.; Telmer, F

DOI: 10.1021/acs.iecr.7b00044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research K. Global mercury emissions to the atmosphere from anthropogenic and natural sources. Atmos. Chem. Phys. 2010, 10 (13), 5951−5964. (3) Liu, Y.; Adewuyi, Y. G. A review on removal of elemental mercury from flue gas using advanced oxidation process: Chemistry and process. Chem. Eng. Res. Des. 2016, 112, 199−250. (4) Wu, Y.; Wang, S.; Streets, D. G.; Hao, J.; Chan, M.; Jiang, J. Trends in anthropogenic mercury emissions in China from 1995 to 2003. Environ. Sci. Technol. 2006, 40 (17), 5312−5318. (5) Streets, D. G.; Hao, J.; Wu, Y.; Jiang, J.; Chan, M.; Tian, H.; Feng, X. Anthropogenic mercury emissions in China. Atmos. Environ. 2005, 39 (40), 7789−7806. (6) Córdoba, P.; Font, O.; Izquierdo, M.; Querol, X.; Leiva, C.; López-Antón, M. A.; Díaz-Somoano, M.; Ochoa-González, R.; Rosa Martinez-Tarazona, M.; Gómez, P. The retention capacity for trace elements by the flue gas desulphurisation system under operational conditions of a co-combustion power plant. Fuel 2012, 102, 773−788. (7) Wang, Q.; Qin, W.; Chai, L.; Li, Q. Understanding the formation of colloidal mercury in acidic wastewater with high concentration of chloride ions by electrocapillary curves. Environ. Sci. Pollut. Res. 2014, 21 (5), 3866−3872. (8) Yang, B.; Chai, L.; Zhu, F.; Yan, X.; Xiang, K.; Liu, H. Kinetics and Mechanism of Se-Catalyzed Disproportionation of Bisulfite: The Critical Role of Selenosulfate. Ind. Eng. Chem. Res. 2016, 55 (16), 4435−4442. (9) Yang, B.; Chai, L.; Zhu, F.; Yan, X.; Xiang, K.; Liu, Z.; Zhang, C.; Liu, H. Selenium-Assisted Reduction of Sulfur Dioxide by Carbon Monoxide in the Liquid Phase. Ind. Eng. Chem. Res. 2017, 56 (8), 1895−1902. (10) Moeller, W.; Winkler, K. The double contact process for sulfuric acid production. J. Air Pollut. Control Assoc. 1968, 18 (5), 324−325. (11) Habashi, F. Metallurgical plants: how mercury pollution is abated. Environ. Sci. Technol. 1978, 12 (13), 1372−1376. (12) Hylander, L. D.; Herbert, R. B. Global emission and production of mercury during the pyrometallurgical extraction of nonferrous sulfide ores. Environ. Sci. Technol. 2008, 42 (16), 5971−5977. (13) Liu, Y.; Wang, Y.; Wang, Q.; Pan, J.; Zhang, Y.; Zhou, J.; Zhang, J. A study on removal of elemental mercury in flue gas using fenton solution. J. Hazard. Mater. 2015, 292, 164−172. (14) Zhao, Y.; Xue, F.; Ma, T. Experimental study on Hg 0 removal by diperiodatocuprate (III) coordination ion solution. Fuel Process. Technol. 2013, 106, 468−473. (15) Liu, Y. X.; Wang, Q. Removal of Elemental Mercury from Flue Gas by Thermally Activated Ammonium Persulfate in A Bubble Column Reactor. Environ. Sci. Technol. 2014, 48 (20), 12181−12189. (16) Zhao, Y.; Hao, R.; Qi, M. Integrative process of preoxidation and absorption for simultaneous removal of SO2, NO and Hg0. Chem. Eng. J. 2015, 269, 159−167. (17) Zhou, C.; Sun, L.; Zhang, A.; Ma, C.; Wang, B.; Yu, J.; Su, S.; Hu, S.; Xiang, J. Elemental mercury (Hg0) removal from containing SO2/NO flue gas by magnetically separable Fe2.45Ti0.55O4/H2O2 advanced oxidation processes. Chem. Eng. J. 2015, 273, 381−389. (18) Isaia, F.; Aragoni, M. C.; Arca, M.; Caltagirone, C.; Demartin, F.; Garau, A.; Lippolis, V. Gold oxidative dissolution by (thioamide)-I2 adducts. Dalton Trans 2013, 42 (2), 492−8. (19) Xiang, K.; Liu, H.; Yang, B.; Zhang, C.; Yang, S.; Liu, Z.; Liu, C.; Xie, X.; Chai, L.; Min, X. Selenium catalyzed Fe (III)-EDTA reduction by Na2SO3: a reaction-controlled phase transfer catalysis. Environ. Sci. Pollut. Res. 2016, 23, 8113−8119. (20) Li, J.; Miller, J. D. A Review Of Gold Leaching In Acid Thiourea Solutions. Miner. Process. Extr. Metall. Rev. 2006, 27 (3), 177−214. (21) López-Antón, M. A.; Díaz-Somoano, M.; Ochoa-González, R.; Martínez-Tarazona, M. R. Analytical methods for mercury analysis in coal and coal combustion by-products. Int. J. Coal Geol. 2012, 94, 44− 53. (22) Smyth, M. R.; Osteryoung, J. G. Determination of some thiourea-containing pesticides by pulse voltammetric methods of analysis. Anal. Chem. 1977, 49 (14), 2310−2314. (23) Dean, J. A., Lange’s Handbook of Chemistry, 15th ed.; McGrawHill Professional, Knoxville, TN, 1999.

(24) Zhang, Y.; Zhou, J.-t.; Wang, Y.-o. Removal of sulfur dioxide from flue gas by aqueous Fê 2̂+ catalytic oxidation. Mod. Chem. Ind. 2002, 22 (7), 22−26 ( http://caod.oriprobe.com/articles/5164863/ Removal_of_sulfur_dioxide_from_flue_gas_by_aqueous_Fe2__ catalytic_oxid.htm). (25) Brandt, C.; Van Eldik, R. Transition metal-catalyzed oxidation of sulfur (IV) oxides. Atmospheric-relevant processes and mechanisms. Chem. Rev. 1995, 95 (1), 119−190. (26) Conklin, M. H.; Hoffmann, M. R. Metal ion-sulfur (IV) chemistry. 3. Thermodynamics and kinetics of transient iron (III)sulfur (IV) complexes. Environ. Sci. Technol. 1988, 22 (8), 899−907. (27) Wang, S.; Gao, Q.; Wang, J. Thermodynamic analysis of decomposition of thiourea and thiourea oxides. J. Phys. Chem. B 2005, 109 (36), 17281−17289. (28) Shaw, W. H. R.; Walker, D. G. The decomposition of thiourea in water solutions. J. Am. Chem. Soc. 1956, 78 (22), 5769−5772. (29) Roshan S, A.; Joseph, C.; Ittyachen, M. A. Growth and characterization of a new metal-organic crystal: potassium thiourea bromide. Mater. Lett. 2001, 49 (5), 299−302. (30) Ushasree, P. M.; Jayavel, R.; Subramanian, C.; Ramasamy, P. Growth of zinc thiourea sulfate (ZTS) single crystals:: a potential semiorganic NLO material. J. Cryst. Growth 1999, 197 (1−2), 216− 220. (31) Xu, H.; Ma, Y.; Huang, W.; Mei, J.; Zhao, S.; Qu, Z.; Yan, N. Stabilization of mercury over Mn-based oxides: Speciation and reactivity by temperature programmed desorption analysis. J. Hazard. Mater. 2017, 321, 745−752. (32) Rumayor, M.; Díaz-Somoano, M.; López-Antón, M. A.; OchoaGonzález, R.; Martínez-Tarazona, M. R. Temperature programmed desorption as a tool for the identification of mercury fate in wetdesulphurization systems. Fuel 2015, 148, 98−103. (33) Rostkowska, H.; Lapinski, L.; Khvorostov, A.; Nowak, M. J. Proton-Transfer Processes in Thiourea: UV Induced Thione→ Thiol Reaction and Ground State Thiol→ Thione Tunneling. J. Phys. Chem. A 2003, 107 (33), 6373−6380. (34) Omine, N.; Romero, C. E.; Kikkawa, H.; Wu, S.; Eswaran, S. Study of elemental mercury re-emission in a simulated wet scrubber. Fuel 2012, 91 (1), 93−101. (35) Ochoa-Gonzalez, R.; Diaz-Somoano, M.; Martinez-Tarazona, M. R. Influence of limestone characteristics on mercury re-emission in WFGD systems. Environ. Sci. Technol. 2013, 47 (6), 2974−81. (36) Peng, B.; Liu, Z.; Chai, L.; Liu, H.; Yang, S.; Yang, B.; Xiang, K.; Liu, C. The effect of selenite on mercury re-emission in smelting flue gas scrubbing system. Fuel 2016, 168, 7−13. (37) Peng, B.; Liu, Z.; Chai, L.; Liu, H.; Yang, S.; Yang, B.; Xiang, K.; Liu, C. Effect of copper ions on the mercury re-emission in a simulated wet scrubber. Fuel 2017, 190, 379−385.

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DOI: 10.1021/acs.iecr.7b00044 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX