Elemental Mercury Removal by a Method of Ultraviolet-heat

Jun 26, 2019 - A novel method of ultraviolet-heat synergistically catalyzing H2O2-X (X: NaCl, NaBr, HCl, and HBr) for removal of elemental mercury (Hg...
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Article Cite This: Environ. Sci. Technol. 2019, 53, 8324−8332

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Elemental Mercury Removal by a Method of Ultraviolet-Heat Synergistically Catalysis of H2O2‑Halide Complex Runlong Hao,*,†,‡ Xinhong Dong,† Zheng Wang,† Le Fu,† Yi Han,† Bo Yuan,†,‡ Yaping Gong,† and Yi Zhao*,†,‡ †

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Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, 071003, PR China ‡ MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, PR China S Supporting Information *

ABSTRACT: A novel method of ultraviolet-heat synergistically catalyzing H2O2-X (X: NaCl, NaBr, HCl, and HBr) for removal of elemental mercury (Hg0) was developed. In terms of Hg0 removal efficiency and economy, HCl and HBr were the suitable additives. Hg0 removal efficiencies reached 93.6% for H2O2−HCl and 91.4% for H2O2−HBr, the concentrations of H2O2, HCl and HBr were 1 M, 4.2 mM and 0.5 mM. The doses of gaseous Cl and Br-oxidants were 6.27 and 0.75 ppm. The costs by using H2O2−HCl and H2O2−HBr were 1,180 USD/lb-Hg0 and 1,170 USD/lb-Hg0. The best temperature for heat catalysis was 413 K. Hg0 removal was enhanced by 500 mg/m3 SO2 and 300 mg/m3 NO due to the formation of sulfuric and NO2. Mercury distribution analyses indicated that 500 mg/m3 SO2, 300 mg/m3 NO, and 6% O2 favored KCl retaining Hg2+. When the H2O2 concentration was adjusted to 3 M, the simultaneous removal efficiencies of NO and Hg0 reached 83.7% and 99.2% for H2O2−HCl, and 82.8% and 98.8% for H2O2−HBr. Electron spin resonance demonstrated that ClOH•−/BrOH•− and Cl2•−/Br2•− played leading roles in Hg0 oxidation, besides Cl2/Br2. The mercury forms in spent KCl were HgCl2, HgBr2, and HgNO3, according to X-ray photoelectron spectroscopy. induced oxidation of Hg0 in advanced oxidation process (AOP) is another efficient method for Hg0 removal, the common radicals are mainly hydroxyl radicals (HO•) and sulfate radicals (SO4•−). Typical AOP methods include Fenton,14 UV/ Fenton,15 activation of persulfate by ultrasound or transition metal ions or heat,16,17 and activation of oxone by transition metal ions or heat.18 In contrast to the catalytic oxidation and AOP methods, gas phase oxidation is a homogeneous oxidation method, which has the advantage of lower mass transfer resistance and a rapid reaction rate. Halide compounds, such as ICl,19 S2Br220 and BrCl,21 are effective in Hg0 removal, especially if used cooperatively with fly ash. Recently, we developed a thermal catalysis oxidation method for Hg0 removal: the prepared liquid complex oxidant was thermally catalyzed to produce gaseous radicals, which were then used to homogeneously oxidize Hg0 in gas phase. The used complex oxidants included H2O2/Na2S2O8(g),22 H2O2/NaClO2(g),23 NaClO2/ NaBr(g),24 and Fenton/NaCl (or NaBr),25 the obtained removal efficiency was ranging from 80% to 94%. This method augmented the contact reaction area between reactants and radicals; but it also had the disadvantages of low utilization rate

1. INTRODUCTION Mercury emitted at high concentrations from waste incinerators and nonferrous metallurgy plants is a threat to human health and has received considerable attention. Oxidized mercury (Hg2+), particulate mercury (Hgp), and elemental mercury (Hg0) are the main existing forms of mercury in flue gas. The wet flue gas desulfurization (WFGD) and the electrostatic precipitator (ESP) can remove most of Hg2+ and Hgp. Due to the characters of high volatility and insolubility, Hg0 is hardly removed by current air pollution control device. The development of a costefficient method for Hg0 removal has been a hot topic in recent years. Oxidation is an efficient approach for Hg0 removal; it can be organized in three categories: catalytic oxidation, liquid phase oxidation, and gas phase oxidation. Catalytic oxidation accelerates the oxidation rate of Hg0 adhered to the active sites of the catalyst surface by decreasing the reaction active energy,1 and/or by generating reactive oxidation species1 such as the adsorbed state oxygen, chlorine, and hydroxyl ([O]*, [Cl]*, and [OH]*). Common active components used for catalysis oxidation of Hg0 include MnO2,1 V2O5,2 Co3O4,3 TiO2,2,4−6 CuO,2 CeO2,2−7 ZrO2,3 Si−Ag,8 and ZnS.9 Photocatalysts, such as Ag/AgI−Ag2CO3,10 AgI-BiOI/CoFe2O4,11 Ag/BiOI/ZnFe2O4,12 and Ag@AgCl/Ag2CO3,13 have been used to photocatalytically remove Hg0. The photoproduced holes and radicals are responsible for Hg0 oxidation. Radical© 2019 American Chemical Society

Received: Revised: Accepted: Published: 8324

April 8, 2019 June 22, 2019 June 26, 2019 June 26, 2019 DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

Article

Environmental Science & Technology

Figure 1. Performances (A) and cost analyses (B) of H2O2, H2O2+NaCl, H2O2+NaBr, H2O2+HCl and H2O2+HBr in Hg0 removal. Flue gas flow is 1.5 L/min, the injection rate of complex oxidant solution is 100 μL/min, the heat catalysis temperature is 393 K, the initial Hg0 concentration is 1000 μg/ m3 (0.112 ppm).

reactor. The solution 10% (v/v) H2SO4 - 4% (w/w) KMnO4 was used to totally remove the unreacted Hg0 in the tail gas after mercury detector.32 2.2. Experimental Apparatus. The experimental flowchart is shown in Supporting Information (SI) Figure S1. Five cylinders of N2 (two), SO2, NO and O2 were used to prepare the simulated flue gas, in which one N2 was used to carry Hg0 vapor. Hg0 was released from a mercury osmotic tube (1000 ng/min, VICI Metronics Co.). The gas flow ranged from 1.0 to 3.5 L/ min. The complex oxidant solution was pumped by a peristaltic pump (BT100−1F, Longerpump Co., China) into a heat catalysis reactor with 353−433 K (±5 K). The produced oxidant vapor then carried by simulated flue gas went through UV catalysis reactor, which is made by quartz and heated by a thermostat oil bath (HH-ZK4, Yuhua instrumental Co., China). UV catalysis reactor is a tandem double-cylinder, the diameter and height of inner cylinder are 50 and 120 mm respectively, the diameter and height of outer cylinder are 65 and 95 mm, respectively. Two low-pressure mercury lamps (100 mm length, 10 W, 254 nm, Philips Co., Germany) were put in the inner cylinders. The oxidation of Hg0 occurred in the UV-heat catalysis reactor. Then the produced Hg2+ was absorbed by the KCl solution at 0 °C. Hg0 concentration in the tail gas was detected by a mercury detector (RA-915M, Lumex Co. Russia), the baseline was calibrated by highly pure N2. The initial concentration of Hg0 was recorded until the Hg0 concentration was stable at least 10 min. The flue gas composition (SO2, NO, NO2 and O2) was recorded by a flue gas analyzer (ECOMJ2KN, RBR Co., Germany). The removal efficiencies of Hg0 and NO were calculated via eq 6.

of oxidant, a relatively high cost, and Na-salt scaling in the reactor. To yield more radicals, we recently created a reactor of ultraviolet-heat (UV-heat) synergistic catalysis to produce HO• from H2O2 at low concentrations,26 from which both H2O2 dose and the operational cost decreased, the removal efficiency of SO2 and NO was still satisfactory, but Hg0 removal was not. Thus, a method to enhance Hg0 removal is needed. Halide-radicals, such as Cl•, ClOH•−, and Cl2•−, have high reactivity toward Hg0.27−30 Cl•, ClOH•−, and Cl2•− can be produced through chain reactions between HO• and Cl− (eqs 1−5).31,32 Hence, UV-heat catalyzing H2O2−Cl− may be useful for Hg0 removal. The objective of this paper was to study the feasibility of Hg0 removal by the UV-heat/H2O2-X (X: NaCl, NaBr, HCl, and HBr) method. The impact of H2O2-X ingredient, the doses of Cl and Br-oxidants, the temperature of heat catalysis, the residence time of flue gas, and the coexistent gases (SO2, NO, and O2) on Hg0 removal were evaluated. The cost of Hg0 and NO removal was calculated. Mercury balance under different atmospheric conditions was determined by atom fluorescence spectrometry (AFS). Radical species were identified by electron spin resonance (ESR). The mercury forms in the products were identified by X-ray photoelectron spectroscopy (XPS). HO • + Cl− → HO− + Cl •

(1)

HO • + Cl−→ClOH •−

(2)

ClOH •− + Cl− → Cl 2 •− + OH−

(3)

ClOH •− + H+ → Cl • + H 2O

(4)

Cl • + Cl− → Cl 2 •−

(5)

η=

2. EXPERIMENTAL SECTION 2.1. Chemicals. The reagents used in the experiments were analytical reagent (Kermel Co.). H2O2 (30 wt %), NaCl (99.5 wt %), NaBr (99.0 wt %), HCl (36.0 wt %), and HBr (40.0 wt %) were used to prepare the complex oxidant solution. 1.0 mol/L KCl solution was used to absorb Hg2+ after UV-heat catalysis

Cin − Cout × 100% Cin

(6)

where η is the removal efficiency; Cin and Cout are the inlet and outlet concentrations, the unit is μg/m3 for Hg0 and mg/m3 for NO. 2.3. Analytical Method. The mercury contents in KCl and KMnO4 were determined by a cold atom fluorescence spectrometry (AFS-9230, Jitian company, China). The binding 8325

DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

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comparable to that of Br-radicals; in fact, a key role of Br2 in Hg0 oxidation has been reported before.33 In order to avoid the deposition of Na-salts in the bottom of the reactor, HCl and HBr, two volatile additives, were used to substitute NaCl and NaBr to modify 1 M H2O2. HCl exhibited better performance in Hg0 removal compared with NaCl: Hg0 removal efficiency reached 93.6% and 95.8% with HCl at 4.2 and 8.4 mM, respectively. Similarly, HBr exhibited a better performance in Hg0 removal compared with NaBr: Hg0 removal efficiency was 91.1%, 98.9%, and 99.3, with HBr at 0.5, 2.1, and 8.4 mM, respectively. Hence, HCl and HBr were better precursors of Cl-radicals and Br-radicals; 4.2 mM and 0.5 mM were the optimal concentrations for HCl and HBr, respectively. The Hg0 removal efficiency with HCl alone (4.2 mM) and HBr alone (0.5 mM) were 8.8% and 31.1% in the absence of UV, and 65.6% and 75.8% in the presence of UV. The results indicated that, without the assistance of ClOH•−, Cl•/Br• and Cl2•−/ Br2•− could also achieve high removal of Hg0, and the main radical could be Cl2•− (eq 5) and Br2•−, because the rate constant of the reaction between Cl• and Cl− is as high as 8.5 × 109 M−1 s−1. The concentrations of Cl-radicals and Br-radicals were 6.27 and 0.75 ppm, respectively; the corresponding molar ratios of Cl-radicals and Br-radicals to Hg0 were 56 and 6.7, respectively. The calculation on concentration conversion from mM to ppm is shown in SI Test S2. 3.2. Cost Analysis. Figure 1(B) and Table 1 shows the cost comparison of all groups in Hg0 removal. The cost is the money

energy of Hg 4f in dry product was analyzed by using an X-ray photoelectron spectroscopy (XPS) (ESCALAB250 spectrometer with Al Kα source (1486.6 eV)). Radical were identified by electron spin resonance (ESR), conducted with a Bruker spectrometer (E500, Bruker Instrument, Germany), and 5, 5dimethyl-1-pyrroline-Noxide (DMPO) was used as a spintrapping agent. The analyses procedures of AFS, XPS, and ESR were shown in SI Text S1.

3. RESULTS AND DISCUSSION 3.1. Optimization of H2O2-X Ingredient and Dose. Figure 1(A) shows the performances of H2O2, H2O2−NaCl, H2O2−NaBr, H2O2−HCl, and H2O2−NaBr in Hg0 removal with and without UV. The injection rate of the complex oxidant was fixed at 100 μL/min. In the absence of UV, 4 and 1 M H2O2 removed 19.0% and 5.5% Hg0, respectively; after UV addition, Hg0 removal efficiency increased to 26.7% and 12.3%. Hence, H2O2 and HO• did not efficiently remove Hg0, which is consistent with our previous work.25 To enhance Hg0 removal, NaCl and NaBr were used to modify H2O2. After adding 34.0 mM NaCl, 5.0 and 50.0 mM NaBr, and with UV, Hg0 removal efficiency increased to 74.1%, 99.6%, and 99.9%. Thus, the produced Cl- and Br-oxidants were more useful in Hg0 removal. In the absence of UV, Hg0 removal was mainly attributed to Cl2 and Br2, which were produced from the reactions between H2O2 and Br− (eq 7) and between H2O2 and Cl− (eq 8), as confirmed by the comparison of their standard electrode potentials, H2O2/ OH− (1.776 V), Cl2/Cl− (1.358 V) and Br2/Br− (1.087 V). The possible intermediates, such as HClO and HBrO, would also decompose as Cl2 and Br2 during heat catalysis. After UV addition, Cl-radicals and Br-radicals yielded from HO•-Cl− and HO•-Br− played leading roles in Hg0 removal; and the Broxidants, including Br-radicals and Br2, exhibited higher reactivity toward Hg0. From the results, it could be found that with a sufficient dose of Br2 (11.2 ppm Br2, equivalent to the addition of 5 mM NaBr), Br2 could completely oxidize Hg0. H 2O2 + 2Br − + Heat → Br2 + 2OH−

(7)

H 2O2 + 2Cl− + Heat → Cl 2 + 2OH−

(8)

Table 1. Mercury Control Cost Evaluation Comparison emission source

control method

cost (USD/lbHg0)

municipal waste activated carbon injection incineration activated carbon bed

211−870 4530−8,950

industrial boiler

activated carbon injection activated carbon bed

824−2,800 13,100−29,900

coal-fired flue gas

ultraviolet-heat synergistically catalysis of H2O2-halide (the current work)

1,170−1,180

(in USD) needed to remove 1 pound of Hg0, including the costs for H2O2 and other halide additives. The industrial chemical prices and specific calculation formulas are shown in SI Text S3. In general, adding UV increased Hg0 removal efficiency and decreased the cost of Hg0 removal, which further demonstrated the key roles of Cl-radicals and Br-radicals in Hg0 removal. In regard to the cost of Hg0 removal, without UV, HCl was better than NaCl. In the presence of UV, the costs needed to remove 1 pound Hg0 were ranging from $1,000 to $2,000 USD by using NaCl, NaBr, HCl, and HBr. When Br was used as the additive, the contribution of UV on decreasing the cost of Hg0 removal was so small, suggesting that using UV was not necessary. On the other hand, if the emission standard of Hg0 in flue gas became stricter (such as 1 μg/m3, now is 30 μg/m3 for Chinese coal-fired power plants), UV was needed to conduct the deep removal of Hg0. In term of removal economy, with UV, 8.4 mM HCl and 2.0 mM HBr were the best doses for modifying H2O2; the corresponding costs were $1,180 and $1,170 USD/lb-Hg0, and Hg0 removal efficiencies reached 95.8% and 98.9%, respectively. It was reported that the cost of activated carbon adsorption of Hg0 was $211 to $29,900 USD/lb-Hg0.34 Thus, the method proposed in this paper is less expensive and avoids the need to treat the spent activated carbon. Although the dose

The effects of Cl and Br additives, and the doses of Cl and Broxidants on Hg0 removal were investigated. Groups 6−11, 12− 17, 18−21, and 22−25 in Figure 1 show the combination of H2O2+NaCl, H2O2+NaBr, H2O2+HCl, and H2O2+HBr with different Cl and Br doses. H2O2 concentration was fixed at 1 M. Without or with UV, Hg0 removal efficiency slightly increased (or was relatively stable) as NaCl increased, which suggests that the increase of NaCl dose does not increase the yields of Cl2 and Cl-radicals, possibly because NaCl can not be volatilized well during the heat catalysis. Thus, Cl− concentration in the flue gas was low, resulting in the low yields of Cl2 and Cl-radicals in gas phase. But it could be found that the contribution of Cl-radicals to Hg0 removal was higher than that of Cl2 (Hg0 removal efficiency increased by 35%−40% with UV), thus indicating that the Cl-radicals was more efficient in Hg0 removal. NaBr showed a better performance in Hg0 removal. As the dose of NaBr increased to 7.5 mM, Hg0 removal efficiency increased to 91.1% and 96.4% without and with UV. But from the results, we found that the contribution of Br-radicals to Hg0 removal decreased as the dose of NaBr increased. The most likely reason for the decrease of the contribution of Br-radicals to Hg0 removal was that the role of Br2 in Hg0 oxidation became greater and then was 8326

DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

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Environmental Science & Technology

Figure 2. Effect of the temperature (A); effect of SO2 (B); effect of NO and O2 (C); simultaneous removal of NO and Hg0 (D). The concentration of H2O2−HCl and H2O2−HBr are 1M-4.2 mM and 1M-0.5 mM respectively, the injection rate of complex oxidant solution is 100 μL/min, flue gas flow is 1.5 L/min, the temperature is 393 K, the Hg0 concentration is 1000 μg/Nm3, (0.112 ppm). Note that the unit for NO and SO2 is mg/m3.

subsequent generation of daughter oxidants, so the decrease of H2O2 lowered the yields of HO•, Cl2, Br2, Cl-radicals, and Brradicals, and thus decreased Hg0 removal efficiency. Hence, temperatures ranging from 393 to 413 K, were more suitable for heat catalysis. Effect of Flue Gas Residence Time. The residence time of flue gas determines the oxidation extent of Hg0, thus the effect of flue gas residence time on Hg0 removal was studied. As shown in SI Figure S2, when the residence time decreased from 22.8 to 7.6 s, Hg0 removal efficiency for H2O2−HBr decreased significantly from 98.7% to 64.9% and from 93.5% to 47.2% with and without UV; and those for H2O2−HCl decreased from 95.4% to 48.6% and from 83.2% to 32.5% with and without UV. Shorter residence times were adverse for the oxidation of Hg0 and should be avoided. However, although increasing residence time could favor deep oxidation of Hg0, the reactor volume and steel consumption would increase, increasing costs. Because a removal efficiency of 90% was acceptable, 11.4 s was suitable as the optimal residence time.

of used HBr was low, HCl was more suitable as an additive for H2O2 in term of the secondary environmental impact and the universality of reagent source. 3.3. Effects of Reaction Factors on Hg0 Removal. Effect of the Temperature. The effect of the heat catalysis temperature on Hg0 removal was studied. As shown in Figure 2(A), with the use of UV, rising the temperature from 353 to 413 K significantly enhanced Hg0 removal when using H2O2−HBr or H2O2−HCl, because high temperature facilitated the vaporization of the complex oxidant solution, which then increased the yields of Br and Cl-oxidants. Another interesting phenomenon was that the contributions of Br-radicals and Cl-radicals to Hg0 removal decreased when the temperature increased from 353 to 413 K. The possible reason was that the yields of Br2 and Cl2 increased as the temperature increased to 413 K.35 When the temperature was greater than 413 K, the Hg0 removal efficiency decreased for H2O2−HBr and H2O2−HCl, with or without UV. This result was due to the decay of H2O2 to O2 (the boiling-point of H2O2 is 430 K36). H2O2 was the parental oxidant, which determined the 8327

DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

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Environmental Science & Technology

Figure 3. Hg 4f XPS spectrum of the product in KCl solution under different conditions. H2O2+HBr (A); H2O2+HBr+SO2+NO (B); H2O2+HCl (C); H2O2+HCl+SO2+NO (D); mercury balance under different atmospheric conditions (E); ESR spectra of UV/H2O2 (F), UV/H2O2−HCl (G) and UV/H2O2−HBr (H).

Effects of SO2, NO, and O2. NO, SO2, and O2 are present in real flue gas, thus the impact of SO2, NO, and O2 on Hg0 removal was investigated. The effect of SO2 on Hg0 removal is shown in Figure 2(B). When H2O2−HBr was used in the absence and

presence of UV, increasing the SO2 concentration from 0 to 500 mg/m3 improved Hg0 removal; but as SO2 increasing to 2000 mg/m3, Hg0 removal efficiency reduced by 15% and 45% with and without UV. H2SO4 and SO3 can facilitate the oxidation of 8328

DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

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Environmental Science & Technology

to 1.12 ppm, Hg0 concentration increased to 98 μg/m3, suggesting that the dose of oxidant was the key factor for Hg0 removal. Then, 500 or 1000 mg/m3 SO2 and 300 mg/m3 NO were continuously introduced into the system and Hg0 concentration decreased to 65 μg/m3. Thus, the presence of SO2 and NO improved Hg0 removal, indicating that Cl-radicals and Br-radicals had higher reactivity and selectivity for Hg0 oxidation. Simultaneous Removal of NO and Hg0. Figure 2(D) shows the results of simultaneous removal of NO and Hg0 with different complex oxidant ingredients. When 1 M H2O2 + 0.5 mM HBr was used, the removal efficiency of NO and Hg0 reached 40% and 92.2% without UV, and reached 52.3% and 96.1% with UV. At 3 M H2O2 + 0.5 mM HBr, the removal efficiency of NO and Hg0 increased to 58.1% and 98.2% without UV, and to 83.7% and 99.2% with UV. At 6 M H2O2 + 0.5 mM HBr, the removal efficiency of NO and Hg0 increased only by 3% and 0.8%, respectively. As for H2O2−HCl (4.2 mM HCl), a similar conclusion was obtained, the best H2O2 concentration was 3 M, the corresponding removal efficiencies of NO and Hg0 were 52.3% and 98.4% without UV; were 83.7% and 99.2% with UV. In terms of cost, 3 M is the optimal concentration for H2O2, and the molar ratio of H2O2 to NO is 26.8; the cost for NO removal is 21,340 USD/t. Specific calculations are shown in SI Text S4. Apparently, NO removal mainly depended on the dose of H2O2, whereas Hg0 removal mainly depended on the dose of halide-radicals. According to our results, this method could replace the complicated system of selective catalytic reduction with activated carbon injection, for integrative removal of NO and Hg0, without the problem of disposing of the waste catalysts and the spent adsorbents. 3.4. Mercury Evolution, Balance, and Removal Mechanism. Figure 3(A)−(D) shows Hg 4f XPS spectra in the spent KCl with H2O2+HBr, H2O2+HBr+SO2+NO, H2O2+HCl, and H2O2+HCl+SO2+NO as oxidants, respectively. The binding energy of Hg 4f can be resolved into four individual peaks, namely 101.5, 102.0, 102.6, and 102.8 eV. From these peaks, 101.5 eV is the overlapping peak of HgNO3 and HgSO4 (4f7/ 2),40 and 102.0 and 102.4 eV are attributed to HgCl2 (Hg 4f7/ 2).41 The peak of 102.8 eV can be assigned to HgBr2.40 The peak of HgBr2 disappeared as SO2 and NO were added; a new peak (attributed to HgNO3) appeared, which indicated that HgBr2 was converted to Hg(NO3)2 (the stability of Hg(NO3)2 was higher than that of HgBr2), accounting for the promotion of NO in retaining Hg2+. Figure 3(E) shows the mercury balance under different atmospheric conditions. Under a N 2 atmosphere, Hg 2+ proportions in KCl and KMnO4 are 46.2% and 53.8% for H2O2−HBr, and 48.1% and 51.9% for H2O2−HCl, respectively. But Hg0 removal efficiency was 91.1% for H2O2−HBr and 93.6% for H2O2−HCl, indicating that approximately 45% Hg2+ was transferred from KCl to KMnO4 for H2O2−HBr and H2O2− HCl, thus KCl was not efficient in absorbing Hg2+. The addition of SO2, NO, and O2 assisted KCl to retain Hg2+: SO2 at 500 mg/ m3 significantly increased Hg2+ proportion in KCl to 72.6% for H2O2−HBr, and to 52.6% for H2O2−HCl; NO at 300 mg/m3 increased Hg2+ proportion in KCl to 92.1% for H2O2−HBr, and to 95.6% for H2O2−HCl; O2 at 6% increased Hg2+ proportion in KCl to 59.4% for H2O2−HBr, and to 52.8% for H2O2−HCl. Comparing the results of Figure 3(E) and Figure 2(B) and (C), a correlation was observed; 500 mg/m3 SO2 and 300 mg/m3 NO not only improved the gas phase oxidation of Hg0 but also promoted the retaining of Hg2+ in KCl. The key reason for the

Hg0 to HgSO4 at high temperature,37,38 thus, in the absence of UV, the improvement on Hg0 removal owing to SO2 was due to the formation of SO3(g) and H2SO4(g), which were from the reactions of SO2+HO• and SO2+H2O2. From the results, we could found that the yields of Br2 and Br-radicals (0.75 ppm) were not significantly affected by SO2 at 500 mg/m3 (175 ppm); but as SO2 concentration increased to 1000 and 2000 mg/m3 (350 and 700 ppm), the yields of Br2 and Br-radicals decreased, because a number of H2O2/HO•, the parental oxidant, were consumed by excessive SO2.36 To confirm the role of SO3(g) and H2SO4(g) in Hg0 oxidation, the experiment of Hg0 removal with H2O2 (1M) + SO2 (500 mg/m3) was conducted. 41.6% and 57.9% Hg0 were removed without and with UV, suggesting that SO3(g) and H2SO4(g) indeed promoted the Hg0 oxidation, but SO3(g) and H2SO4(g) are less effective than halogen radicals in term of Hg0 oxidation. For the case of HCl, the addition of SO2 reduced Hg0 removal efficiency without UV, suggesting that the yield of Cl2 was affected by the presence of SO2. The standard electrode potentials of Cl2/Cl− and Br2/Br− are 1.358 and 1.087 V, that of SO2/SO42− is −0.93 V, so the redox reactivity between Cl2 and SO2 is higher than that of Br2 and SO2. At a fixed residence time, the consumption of Cl2 by SO2 could be higher than that of Br2 and SO2. Though the formed H2SO4 from the reaction of Cl2 and SO2 could also facilitate Hg0 oxidation, the inhibition caused by SO2 consuming Cl2 played a leading role, thus Hg0 removal decreased in overall. The results also indicated that the yield of Cl-radicals did not decrease as the SO2 concentration increased, as concluded from the phenomenon that Hg0 removal efficiency was relatively stable when the SO2 concentration was below 1000 mg/m3 with UV. This suggests that Cl-radicals had lower reactivity toward SO2; thus, Cl-radicals contributed more to Hg0 removal when both UV and SO2 were present. The effect of NO and O2 on Hg0 removal are shown in Figure 2(C). Increasing NO from 0 to 300 mg/m3 increased Hg0 removal efficiency to 88.2% for H2O2−HBr without UV (the Hg0 removal efficiency for H2O2−HBr was stable with UV); and to 88.6% and 93.6% for H2O2−HCl, with and without UV, respectively. The contribution of Br-radicals and Cl-radicals to Hg0 removal was lower as NO concentration increased, because NO2, as an accelerator for Hg0 removal, could assist Br2 or Cl2 to deeply remove Hg0 (eqs 9 and 10). NO2 was a strong gaseous oxidant, which was produced from the oxidation of NO by H2O2/HO•.27,37 And the yields of Br-radicals and Cl-radicals were not seriously affected by the presence of NO, thus the overall Hg0 oxidation was enhanced by the produced NO2. As for the effect of O2, its role was negligible or negative for Hg0 removal because the reaction rates between Hg0 and Br/Cloxidants19−21,27,33,39 are higher than those of O2 and Hg0.39 O2 might also quench some radicals, so that Hg0 removal does not change. M + NO2 + Hg 0 → M + NO + HgO

(9)

2HNO3 + HgO → Hg(NO3)2 + H 2O

(10)

Taking HBr as an example, a transient response experiment was conducted to confirm the roles of Br-radicals, Br2, the oxidant dose, SO2, and NO in Hg0 removal. As shown in SI Figure S3, the Hg0 concentration decreased from 1050 to 480 μg/m3 when used 0.56 ppm Br2. Then, after UV addition, Hg0 concentration decreased by 100 μg/m3. When the Br dose (including Br-radicals and Br2) increased to 1.68 ppm, Hg0 concentration decreased to 46 μg/m3. As the Br dose decreased 8329

DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

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Environmental Science & Technology increase of Hg0 proportion in KCl was that the produced H2SO4 and NO2 oxidized Hg0 into HgSO4 and HgNO3, which are more stable than HgCl2,42 thus increasing the Hg2+ concentration in KCl. The AFS analyses showed that simultaneous removal of SO2, NO, and Hg0 is feasible if the concentrations of SO2 and NO are below 500 and 300 mg/m3, respectively. ESR tests were conducted to identify the radicals, as shown in SI Figure S4. There were no signals of UV/H2O2, UV/H2O2− HBr, and UV/H2O2−HCl at 0 min in the ESR spectra; at 10 min, the typical signal of DMPO−OH adduct (1:2:2:1) appeared in UV/H 2O2 (Figure 3(F)).43 The signal of DMPO−OH adduct disappeared in UV/H2O2−HBr and UV/ H2O2−HCl, a new adduct of DMPO-Ox’ appeared instead43 (Figure 3(G) and (H)), suggesting that Br − and Cl − transformed •OH to other oxidizing species. This new oxidizing species was definitely not Cl•, because the DMPO−OH adduct could be also produced when DMPO was oxidized by Cl•.43 Therefore, the formation of DMPO-Ox’ was attributed to ClOH•− and/or Cl2•−, which were produced via Reactions 1−6. The generation rate of ClOH•− from HO• and Cl− is 4.3 × 109 M−1 s−1;31 the generation rate of Cl• from HO• and Cl− is 4.3 × 109 M−1 s−1;31 the generation rate of Cl2•− from ClOH•− and Cl− or from Cl• and Cl− is 2.5 × 105 M−1 s−1 and 8.5 × 109 M−1 s−1,31 respectively. Thus, the eqs 1, 2, and 5 could be the main generation paths for ClOH•− and Cl2•−. The main paths for Hg0 removal could be summarized as follows: (1) In the case of UV/H2O2 (1 M), H2O2 and HO• only removed a small amount of Hg0 (eqs 11 and 12), accounting for 5.5% and 6.8% of Hg0 removal, respectively; (2) in the case of heat/H2O2−HBr or heat/H2O2−HCl, Br2 and Cl2 played leading roles in Hg0 removal (eqs 13 and 14), with the contributions of 75.9% and 78.1% for Hg0 removal, respectively; (3) in the case of UV-heat/H2O2−HBr or UV-heat/H2O2− HCl, in addition to Cl2 or Br2, ClOH•− and Cl2•−, and BrOH•− and Br2•− contributed more to Hg0 removal (eqs 15−18), and with the contributions of 84.3% for Br-oxidants and 89.0% for Cl oxidants in term of Hg0 removal; (4) after the addition of SO2 and NO, H2SO4, SO3, NO2, and HNO3 also partly contributed to Hg0 removal. The mechanism of ultraviolet-heat synergistically catalytic oxidation of Hg0 is concluded as that (1) heat catalysis is used to evaporate the liquid H2O2−HCl or H2O2− HBr to generate the oxidation smog that contains highly reactive H2O2(g) and Cl2 or Br2, by which the contact area among the reactants is enlarged and the mass transfer rate will also be accelerated; (2) the UV catalysis is used to motivate the formed H2O2(g), Cl−, Br−, Cl2, or Br2 to produce the radicals ClOH•− and Cl2•−, and BrOH•−, and Br2•−, which can further enhance the oxidation of Hg0. The advantage of this synergistically catalytic oxidation method is not only enlarges the reaction contact area between the oxidant and air pollutants but also yields more highly reactive radicals to facilitate the oxidation reaction, thus the mass transfer process and the chemical reaction process are all greatly promoted by this method.

2Cl 2 •− + Hg 0 → 4Cl− + Hg 2 +

(16)

2BrOH •− + Hg → 2Br− + 2OH− + Hg 2 +

(17)

2Br2 •− + Hg 0 → 4Br − + Hg 2 +

(18)

3.5. Environmental Implications. The present work proposes a novel approach of UV-heat synergistically catalyzing H2O2-X for removal of Hg0. For 0.112 ppm of Hg0, the doses of Cl- and Br-oxidants were 6.27 and 0.75 ppm, respectively. In terms of cost and maneuverability, this method was better, or comparable, to previous reports. For instance, 52 ppm Br2 oxidized 50% Hg0 (0.2 ppm) at 15 s flue gas residence time.33 0.6 ppmv S2Br2 with 30 g/m3 fly ash removed 90% Hg0 (0.16 ppm), but only in laboratory scale.20 Ninety percent Hg0 (0.08 ppm) could be removed by 5 ppm of SCl2 or S2Cl2 combined with 40 g/m3 fly ash.44 HBr at 4 ppm removed 50% Hg0 at flue gas residence time of 1.4 s.45 As we all know HCl and HBr coexist in real flue gas, thus the UV-heat/H2O2 reactor can be inserted between electrostatic precipitator and wet flue gas desulfurization to cooperate the HCl and HBr to produce the Cl-radicals and Br-radicals for Hg0 removal in situ. Additionally, when the H2O2 concentration increased to 6,600 ppm, the simultaneous removal efficiencies of NO and Hg0 were over 80% and 90%. The costs were controlled at around $10 USD/lb-NO and $1,200 USD/lb-Hg0. The overall goal of this method is to establish an integrated system to simultaneously remove SO2, NO, and Hg0. The oxidant and absorbent are suitably selected as H2O2-X and NH4OH. The spent solution will contain NH4+, SO42−, NO3−, Hg2+, and Cl− or Br−, etc., it will be treated by TMT-15 or Na2S to precipitate Hg2+ at first, then undergo an evaporative crystallization process to produce solid mixture of (NH4)2SO4 and NH4NO3. The product can be reused as complex fertilizer for agricultural production. This technology could be used for treating the flue gas with high concentration of Hg0 in nonferrous metallurgy and refuse incinerators, or used to treat the NO and Hg0 emitted from medium and small size coalfired industrial boilers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b01741. The characterization of instruments and analysis procedure (Text S1), concentration conversion (Text S2), cost calculation for Hg0 removal by using different complex oxidant (Text S3), cost calculation for NO removal by using different complex oxidant (Text S4). Figures showing experimental flowchart (Figure S1), effect of flue gas flow on Hg0 removal (Figure S2), transient response experiment involving the roles of UV, SO2, NO and the injection rate of CO solution in Hg0 removal, (Figure S3), ESR spectra of UV/H2O2, UV/ H2O2−HCl and UV/H2O2−HBr at 0 min (Figure S4). Tables showing the performances and cost analyses of H2 O 2 , H 2 O 2 +NaCl, H 2 O 2 +NaBr, H 2 O 2 +HCl and H2O2+HBr in Hg0 removal (Table S1)(PDF)

H 2O2 + Hg 0 → HgO + H 2O

(11)

2HO • + Hg 0 → HgO + H 2O

(12)

Cl 2 + Hg 0 → HgCl2

(13)

Br2 + Hg 0 → HgBr2

(14)

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2ClOH •− + Hg → 2Cl− + 2OH− + Hg 2 +

(15)

*(R.H.) E-mail: [email protected]. *(Y.Z.) E-mail: [email protected].

AUTHOR INFORMATION

Corresponding Authors

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DOI: 10.1021/acs.est.9b01741 Environ. Sci. Technol. 2019, 53, 8324−8332

Article

Environmental Science & Technology ORCID

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Runlong Hao: 0000-0002-2793-3607 Yi Zhao: 0000-0001-9974-0348 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Weppreciate the financial support provided by the Natural Science Foundation of China (No. 51708213), the Natural Science Foundation of Hebei (No. E2018502058), and the National Key Research and Development Plan (No. 2016YFC0203705) (No. 2017YFC0210600), the Fundamental Research Funds for the Central Universities (No. 2019MS103 and 2019MS128).

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