Simultaneous Removal of SO2 and NO Using a Novel Method of

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Simultaneous Removal of SO2 and NO Using a Novel Method of Ultraviolet Irradiating Chlorite−Ammonia Complex Runlong Hao,*,†,§,‡ Xingzhou Mao,† Zhen Qian,† Yi Zhao,†,§ Lidong Wang,†,§ Bo Yuan,† Kaimin Wang,† Zihan Liu,† Meng Qi,† and John Crittenden*,‡

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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 ‡ Brook Byer Institute for Sustainable Systems and School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: A novel advanced oxidation process (AOP) using ultraviolet/sodium chlorite (UV/NaClO2) is developed for simultaneous removal of SO2 and NO. NH4OH, as an additive, was used to inhibit the generation of ClO2 and NO2. The removal efficiencies of SO2 and NO reached 98.7 and 99.1%. NO removal was enhanced by greater UV light intensity and shorter wavelengths but was insensitive to changes in pH and temperature. SO2 at 500−1000 mg/m3 improved NO removal, especially in the absence of UV. The coexistence of SO2 and O2 facilitated the removal of NO by ClO2−. HCO3−, Cl−, and Br− enhanced NO removal, but their roles were negligible when UV was added. The generation of ClO2 and ClO•/HO• was verified by an UV−vis spectrometer, electron spin resonance (ESR), and radicalquenching tests. The mechanisms responsible for the removal of SO2 and NO were attributed to the synergism between acid− base neutralization and radical-induced oxidation. The ClO2− evolution and product composition were demonstrated by UV− vis and X-ray photoelectron spectroscopy (XPS). Kinetics analyses showed that the Hatta numbers were 329−798 and 747− 1000 without and with UV. Thus, the gas−film resistance mainly controlled the mass-transfer process. micropollutants.17−20 The main radicals produced in UV/ ClO− are HO• and Cl• (eq 1). Yang et al.21 and Liu et al.22 used UV/NaClO (1500 mg/L [Cl2]) and UV/Ca(ClO)2 (0.16 mol/ L) to conduct the experiment of simultaneous removal of SO2 and NO, and the NO removal efficiencies were 95.6 and 92.4%, respectively; also, as a result of its high alkalinity, the SO2 removal efficiency also reached 100%, and the presence of SO2 accelerated the formation of HClO, which further increased the yield of HO•. Hence, ClO− is a suitable radical precursor for photocatalytic removal of SO2 and NO.

1. INTRODUCTION The simultaneous removal of sulfur dioxide (SO2) and nitric oxide (NO) from flue gas is a hot topic in the field of air pollution control.1 The key is to effectively oxidize the insoluble NO to NO3− and reduce the production of NO2. Extensively studied oxidation methods include gas−solid heterogeneous catalytic oxidation, liquid phase oxidation, and gas phase oxidation.2−13 As a result of the high solubility of SO2,14 the wet oxidation method has significant potential for use in multipollutant removal. Of the various wet oxidation ways, the advanced oxidation process (AOP) is popular due to its high removal efficiency and high absorptive capacity. The ultravioletinduced AOP (UV-AOP) receives considerable attention, because it is insensitive to the pH and has few problems on wastewater treatment. The UV-AOP method mainly includes UV/H2O2,12 UV/persulfate (PS),15 UV/oxone,16 etc.; the primary radicals used to remove SO2 and NO are the hydroxyl radical (HO•) and the sulfate radical (SO4•−). However, the quantum yields of H2O2 (0.5 mol·Einstein−1) and PS (0.7 mol· Einstein−1) at 254 nm are relatively low; thus, the oxidant dose needed to deeply remove NO is high, which limits its industrial application. Compared with H2O2 and PS, hypochlorite (ClO−) has a higher quantum yield (close to 1.0 mol·Einstein−1) and is better in the degradation of emerging contaminants and some © XXXX American Chemical Society

HOCl + hv → HO• + Cl•

(1)

Similar to hypochlorite, chlorite (ClO2−) is also an alkaline Cl oxidant with a higher oxidation capacity. ClO2− even has a higher quantum yield at 254 nm, 1.0−1.53 mol·Einstein−1,23,24 which suggests that the radical yield in UV/ClO2− may be higher than that in UV/ClO−. ClO• (eqs 2−5), as a secondary radical in UV/chlorine, has a higher reactivity and selectivity toward certain organic compounds,19,25−27 and ClO• is also one of the primary radicals (ClO•, HO•, and ClO2) in UV/ClO2−.23 Received: Revised: Accepted: Published: A

December 10, 2018 June 29, 2019 July 2, 2019 July 2, 2019 DOI: 10.1021/acs.est.8b06950 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 1. Synergistic behavior between NaClO2 and NH4OH in the removal of SO2 and NO, with or without UV. The impact of NH4OH (A); the impact of NaClO2 (B). The ratio of NaClO2 to NH4OH is plotted in the x-axis as a percentage. The flue gas flow is 2.6 L/min, the temperature is 50 °C, the SO2 concentration is 2000 mg/m3, the NO concentration is 300 mg/m3, and the UV254 power is 0.03 W/cm3. (The error bars for removal efficiencies of SO2 and NO were lower than 0.5%.)

Given the above superiorities that ClO2− has a higher quantum yield and ClO• has higher reactivity, UV/ClO2− seems better in removing NO compared with UV/ClO−. Hence, this paper will study the feasibility of using UV/ClO2− to remove simultaneously SO2 and NO and to identify the role of ClO• in NO removal. ClO2− + hv → ClO• + O•−

(2)

ClO2− + hv → (ClO2−)*

(3)

(ClO2−)* + ClO2− → ClO2 + ClO− + O•−

(4)

O•− + H 2O ↔ HO• + OH−

(5)

removal of SO2 and NO. The obtained results in this paper give new insight into the simultaneous removal of SO2 and NO by an AOP.

2. MATERIALS AND METHODS 2.1. Chemicals and Reagents. All chemicals were analytical grade. The composite solution was prepared using NaClO2 (80.0%) and NH4OH (25%) with deionized water. tert-Butanol (t-BuOH) (99.0%), benzoic acid (BA) (99%), 1,4dimethoxybenzene (DMOB) (97%), and 2,5-dimethoxybenzoate (DMBA) (98%) purchased from Sigma-Aldrich were used to scavenge HO•, Cl•, and ClO•18−20,27 to identify their roles in the NO oxidation. The other chemicals and reagents used in the experiments are listed in the Supporting Information (SI), Text S1. 2.2. Experimental Setup. The experimental apparatus consisted of simulated flue gas generation, a UV-photolysis reactor, and tail gas detection, as shown in Figure S1. The core part was a cylinder and jacketed quartz-wall UV-photolysis reactor, which was heated by a thermostat water bath; the diameter and height of the inner and outer cylinder were 60/96 mm and 140/200 mm, respectively. Three low-pressure lamps (TUVPS-S, Philips Co., Beijing) with powers of 6, 12, and 18 W (the light intensities are 1.27 × 10−4, 2.54 × 10−4, and 3.82 × 10−4 Einstein·s−1, respectively, and the calculation procedure is shown in Text S2), were placed inside the cylinder. Three UV wavelengths were studied (185, 254, and 365 nm) for the 18 W lamp. The temperature and pH of the composite solution were detected online by an inside thermocouple and pH meter. After the tail gas was dried, the components of the inlet and outlet flue gases were detected using a flue gas analyzer (ECOMJ2KN, RBR Company, Germany). The efficiencies of SO2 removal and NO conversion were calculated by eq 7.

28−30

Known from previous works, ClO2 was easily generated from the photoproduction of ClO2− (eqs 3 and 4) and/or from the acid−base neutralization reactions between ClO2− and SO2/NOx (eq 6), which were harmful for outputting ClO•. The escaped ClO2 also caused the corrosion of downstream devices and induced secondary air pollution.30,31 To deal with this problem, we selected NH4OH as an additive, to preserve ClO2− and prevent its involvement in the SO2/NOx assault. It is interesting that NH4OH had another function: it suppressed the photodecomposition of ClO2− to produce ClO2, which indicated that the combination of NH4OH and UV/NaClO2 is desirable. 5ClO2− + 4H+ → Cl− + 4ClO2 + 2H 2O

(6)

Hence, the objectives of this paper were as follows: (1) to investigate the behavior and mechanisms of HO• and ClO• in the removal of SO2 and NO; (2) to reveal the interactions among UV, NaClO2, and NH4OH during the removal of SO2 and NO; (3) to systematically examine the impact of the reaction conditions (e.g., pH, temperature, UV light intensity, O2, Br−, Cl−, HCO3−, etc.) on the simultaneous removal process; (4) to study the reactants’ evolution and the radical chemistry; (5) and to calculate the kinetic parameters relating to NO removal. The result showed that compared with some other AOP methods, UV/NaClO2 was better in simultaneous

η=

(Cin − Cout) × 100% Cin

(7)

where η is the removal efficiency after a 30 min reaction time; Cin and Cout are the inlet and outlet concentrations, respectively. B

DOI: 10.1021/acs.est.8b06950 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article


Figure 2. Impact of the different reaction factors on the simultaneous removal of SO2 and NO. Impact of UV power and wavelength (A); impact of pH of NaClO2−NH4OH (B); impact of coexisting gases (C); impact of common anions (D). The black lines stand for the efficiencies of SO2 removal and NO conversion in the absence of UV, and the red lines are those in the presence of UV, under the optimal conditions of flue gas flow of 2.6 L/min, a temperature of 45 °C, a pH of 12.6 for NaClO2−NH4OH (1.0:2.5%), an SO2 concentration of 2000 mg/m3, an NO concentration of 300 mg/m3, and an UV254 light intensity of 3.82 × 10−4 E/s.

2.3. Analysis Methods. A TU-1900 double beam UV−vis (Beijing Purkinje General instrument Company, China) was employed to determine the absorbances of ClO2− and ClO2 at 260 and 360 nm; the typical characteristic UV−vis spectra of ClO2− and ClO2 are shown in Figure S2.29,30 The reaction products were measured using X-ray photoelectron spectroscopy (ESCALAB250, Thermo Scientific, American). The binding energies of Cl 2p, S 2p, and N 1s were determined at 10 kV with a base pressure of 2 × 10−9 Mbar. The XPS characterization was performed after drying and grinding the products. The ESR experiment was conducted using a Bruker E500 spectrometer (Bruker Instrument, Germany) with or without 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spintrapping agent. The detailed ESR procedure is available in Text S3.

SO2 removal efficiency but decreases the NO removal efficiency. This was because the increased alkalinity resulting from NH4OH addition facilitated SO2 removal; OH− reacted with SO2 in liquid film to form SO32− prior to the formation of SO42−, but the introduced NH4+ (N2/NH4+ = 0.092 V) consumed part of ClO2− (ClO2−/Cl− = 1.599 V), which affected NO oxidation. Introducing UV weakened this inhibition resulting from NH4+ and even significantly increased the removal efficiencies of SO2 and NO from 88.7−94.1 and 23.5−67.8% to 97.9−99.7 and 94.7−99.6%, respectively, indicating that the possible radicals HO• and ClO• greatly promoted the oxidation processes.18−20 Figure 1(B) shows the role of NaClO2, which promoted NO removal, but had no effect on SO2 removal. Thus, SO2 removal mainly depended on NH4OH and was slightly affected by the radical-induced oxidation process. The optimum ratio of NaClO2−NH4OH was selected as 1.0:2.5. From the above results, the contribution of ClO2− and the radicals to NO removal was approximately 30 and 68%. The addition of NH 4 OH could decrease the NO 2 concentration. The possible mechanisms for this were as follows: (1) NH4OH enhanced NO2 hydrolysis (eq 8); (2)

3. RESULTS AND DISCUSSION 3.1. Roles of NaClO2, NH4OH, and UV Light in the Simultaneous Removal of SO2 and NO. The synergistic behavior between NaClO2 and NH4OH in the removal of SO2 and NO with and without UV is shown in Figure 1. As shown in Figure 1(A), with or without UV, adding NH4OH increases the C


Article

Environmental Science & Technology NH4OH decreased the acidolysis of ClO2− to produce ClO2 (eq 6), which was favorable for suppressing the gaseous oxidation of NO to NO2 (eq 9). Moreover, as shown in Figures S3 and S4, NH4OH also inhibited the photoproduction of ClO2 from UV/NaClO2: the ClO2− concentration in UV/NaClO2 was lower than that in UV/NaClO2−NH4OH, but the concentration of ClO2 in the UV/NaClO2 solution and exhaust gas was higher than that in UV/NaClO2−NH4OH. In Figure S5, the color of the NaClO2 solution quickly turns yellow as UV irradiation proceeds, whereas that of the NaClO2−NH4OH solution was relatively clean and clear. The new generated yellow substance is ClO2. Figure S6 and Text S4 revealed the interactive mechanism of NaClO2 and NH4OH in the presence of UV: OH−, rather than NH4+, derived from NH4OH inhibited the photodecomposition of ClO2− to produce ClO2, which was the key reason for the decrease in NO2 production. 3NO2 + 2OH− → NO + 2NO3− + H 2O

(8)

NO + ClO2 → NO2 + ClO•

(9)

Cl 2O2 + ClO2− + OH− → ClO3− + 2ClO− + 2H+ (12) •

H 2O + hv → HO + H H• → e− + H +

(10)

3.2. Impact of the Reaction Conditions on the Simultaneous Removal of SO2 and NO. 3.2.1. Impact of the Wavelength and Light Intensity of the UV Lamp. The impact of the intensity and wavelength of UV on the simultaneous removal process was assessed. As shown in Figure 2 A), when the light intensity was increased from 0 to 3.82 × 10−4 E/s, a slight increase in SO2 removal efficiency from 93.2 to 99.8% and a significant increase in NO removal efficiency from 43.7 to 94.1% were observed. The stronger light intensity increased the yields of HO•, ClO•, and ClO2, thereby enhancing the removal process. However, at a high concentration level, ClO• was quenched to generate Cl2O2 (eq 11) and was consumed by ClO2− (eq 12). Thus, the oxidation reaction was not always significantly enhanced when the light intensity increased. As shown in Figure 2(A), 185 nm further improved NO removal, as the shorter wavelength possessed higher energy. The 185 nm wavelength can dissociate the bond of H− OH (eqs 13 and 14) to produce HO•, H•, and e−,32 which then facilitates the oxidation of NO. The 185 nm light also increased the yield of ClO2 (as revealed in Figure S7) via eqs 3 and 4, which then enhanced NO oxidation and the generation of NO2 (eq 9). 2ClO• → Cl 2O2

(13) (14)

3.2.2. Impact of the Reaction Temperature. The impact of the reaction temperature on the simultaneous removal of SO2 and NO is shown in Figure S8. Rising temperature resulted in an initial small increase in both SO2 removal and NO removal and then reduced the removal efficiencies. The optimal temperature range was 40−45 °C. Increasing temperature was also beneficial in decreasing the NO2 concentration to as low as 3−5 mg/m3 at 45−70 °C. Overall, temperature had no obvious effect, which was possibly because the radical-induced oxidation was rapid and relatively insensitive to change in temperature. Theoretically, rising temperature favored ion diffusion,33 ClO2 generation and release,30 and the chemical reaction rates. However, according to Henry’s law and the two-film theory, high temperature would decrease gas solubility and cause an increase in mass-transfer resistance at the gas−liquid interface,30 resulting in a decrease in mass transfer of NO from gas to liquid phase. Hence, increased temperature had a dual function, and the obtained results suggested that as the temperature increased, inhibition counteracted promotion, leading to relatively stable removal of SO2 and NO. Increasing temperature also accelerated the release of NH3, reduced the utilization rate of NH4OH, and caused a secondary environmental impact; thus, a lower temperature, such as 45 °C, was recommended. 3.2.3. Impact of pH. The pH value can affect the existing forms of ClO2− and NH4OH; therefore, the impact of pH was studied. As shown in Figure 2(B), in the absence of UV, increasing pH had no clear effect on SO2 removal but significantly inhibited NO removal when the pH was greater than 8. This was because the yield of ClO2 decreased with increased pH (eq 6), which was harmful for NO oxidation, but SO2 removal was cocontrolled by acid−base neutralization and the redox reaction. Although high pH impaired the redox reaction, the acid−base neutralization of SO2 and OH− was promoted; thus, SO2 removal was relatively stable. Therefore, in the absence of UV, lower pH (5−8) was beneficial for the simultaneous removal of SO2 and NO. After UV was added, the removal efficiencies of SO2 and NO increased, and the NO removal efficiency was stable at 96% at all investigated pH values, indicating that radical-induced oxidation of NO is insensitive to change in pH, and the radicals’ yield was not affected by pH value. 3.2.4. Impact of the Coexistent Gases. Flue gas concentrations vary with changes in coal-type and boiler operational conditions; thus, the effects of the concentrations of SO2, NO, and O2 on the simultaneous removal of SO2 and NO were investigated. As shown in Figure 2(C), in the absence of UV, when the NO concentration increases from 100 to 500 mg/m3, a slight increase in SO2 removal efficiency but a marked decrease in NO removal efficiency are observed. The produced NO2 facilitated SO2 removal (eq 15), but the decrease in the molar ratio of ClO2− to NO suppressed the oxidation of NO; thus, the NO removal efficiency decreased as the NO concentration increased. Introducing UV significantly improved the simultaneous removal of SO2 and NO, and the removal efficiencies increased to 97−99 and 89−96%, respectively. These results suggested that compared with

To identify the role of UV, an experiment in which SO2 and NO were simultaneously removed using UV/H2O was conducted. The removal efficiencies of SO2 and NO were 81.5 and 3.7%, respectively. Therefore, without NaClO2, the radical-induced oxidation of NO was negligible. UV mainly irradiated NaClO2 to generate HO• and ClO• to oxidize NO. UV also decreased the inhibition on NO oxidation by NH4OH (the efficiency decreased by 5%, which was far less than the decrease of 46% without UV), as shown in Figure 1(A). Although NH4+ may consume some radicals (eq 10), the NO removal efficiency with UV still reached 98%, with less NO2 and ClO2 formation. Hence, NH4OH was a desirable cooperator for UV/NaClO2. By combining the result of NO removal and NO2 concentration in Figure 1(A), it was found that, different to ClO2, ClO•/HO• tended to convert NO/NO2/NO2− to NO3−, rather than NO2. 2NH4 + + Radicals → N2 + 4H 2O + Cl−

•

(11) D


Figure 3. Cl 2p XPS spectra of the removal products after 30 min (A) and 120 min (B) of reaction; ESR spectra of DMPO−OH, DMPO−OCl, and DMPO−OClO in UV/NaClO2 (1.0 wt %) (C) and in UV/NaClO2−NH4OH (1.0:2.5 wt %) (D) at 2 min.

Environmental Science & Technology Article

E


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Environmental Science & Technology ClO2−, ClO•, HO•, and ClO2 had higher activities in NO oxidation. However, the NO2 concentration also increased during this process, which was due to the oxidation of NO by ClO2 (eq 9).

they were reported to be effective in oxidizing NO,30 especially Br2.20,31,36

SO2 + 2NO2 + 4OH− → SO4 2 − + 2NO2− + 2H 2O (15)

HO• + HCO3− → CO3•− + H 2O

(16)

Cl• + HCO3− → CO3•− + H+ + Cl−

(17)

In the presence of UV, the oxidation reactions were dominated by the radicals. Thus, the influence of HCO3−, Cl−, and Br− was mainly ascribed to the radical-induced reactions. The results showed that the addition of HCO3− slightly enhanced the removal of SO2 and NO, indicating that CO3•− cooperated with ClO• to facilitate the oxidation reaction. With regard to the impact of Cl− and Br−, a slightly harmful role of Cl− and a slightly beneficial role of Br− on NO removal were observed, and the promotion of Br− was better at a lower concentration (776 mg/L). These results indicated that Cl•, Cl2•−, and ClOH•− produced from the chain reactions of Cl− with HO• (eqs 18−20)27 were not as effective as ClO• and HO• in NO oxidation. However, Br•−, Br2•−, BrOH•−, and ClBr•− (eqs 21−25)37 were useful in oxidizing NO.

With regard to the role of SO2, in the absence of UV, the SO2 removal efficiency increased linearly with increased SO2 concentration, but the NO removal efficiency first increased and then decreased during this process. The following two mechanisms could explain the promotion of SO2 on NOx removal: (1) the absorption of acidic SO2 caused an increase in ClO2 production (eq 6); (2) the reaction between SO2 and the produced NO2 (eq 15) further promoted NO2 removal. When the SO2 concentration exceeded 1000 mg/m3, the competition between NO and SO2 for ClO2−/ClO2 adversely impaired NO oxidation. However, following the addition of UV, the removal efficiencies of SO2 and NO increased to 99−100 and 95−97%, respectively, which suggested that ClO•/HO• preferentially reacted with NO over SO2. It was also found that the increased concentration of SO2 was favorable for decreasing the NO2 concentration; thus, this method was suitable for treating SO2 and NO simultaneously.34 When O2 was added to the system, an interesting phenomenon was observed. In the absence of UV, the removal efficiencies of SO2 and NO reached 95−97 and 95−99%, respectively, at 2 and 8% O2, which were higher than those obtained in the presence of UV. O2 significantly promoted NO removal. The reason for this was due to the synergism between SO2 and O2, which accelerated the formation of ClO2 from NaClO2. This was verified by UV−vis analysis. Compared with NaClO2 solution and SO2 + NaClO2 solution, the absorbance of ClO2 (at 360 nm) in NaClO2 + O2 + SO2 was higher, suggesting that the combination of O2 and SO2 promoted the acidolysis of ClO2−. The O2 content in real flue gas is approximately 6%, which is sufficient to cooperate with the coexisting SO2 to decompose ClO2− and produce ClO2. However, the production of ClO2 should be controlled at an appropriate level, in order to avoid secondary environmental pollution and device corrosion. 3.2.5. Impact of Common Anions. The anions HCO3−, Cl−, and Br− are frequently present in the real wet flue gas desulfurization (WFGD) slurry. It was reported that HCO3− can scavenge HO•, Cl•, and Cl2•− but not ClO•19 and that CO3•− was produced (eqs 16 and 17). Therefore, HCO3− could be used to confirm the role of ClO•. In the UV/chlorine system, adding extra halogen ions (X: Cl−/Br−) can generate reactive halogen species (RHS),35 such as X•, XO•, X2•, and XOH•−. Hence, adding Cl−/Br− to the UV/NaClO2−NH4OH system may lead to new reaction pathways. As shown in Figure 2(D), in the absence of UV, SO2 removal was not affected by these anions, with the exception of slight inhibition caused by HCO3−. However, the addition of HCO3−, Cl−, and Br− was all beneficial for NO removal. Adding HCO3− (pH = 8.3) slightly decreased the pH of ClO2− (pH = 12.6), and on the basis of the UV−vis data of NaClO2−NH4OH in the absence and presence of HCO3−, we found that adding NaHCO3 to NaClO2− NH4OH caused an increase in absorption of 0.134 at 360 nm, suggesting an increase in ClO2 production. Hence, the promotional mechanism of HCO3− on NO removal was ascribed to the increase in ClO2 production. The impact of Cl− and Br− could be attributed to the production of Cl2 and Br2, as

HO• + Cl− → HO− + Cl• •

−

HO + Cl → ClOH

(18)

•−

(19)

ClOH•− + Cl− → Cl 2•− + OH−

(20)

HO• + Br − → BrOH•−

(21)

ClO• + Br − → BrO• + Cl−

(22)

BrOH•− → HO− + Br •

(23)

Br2 + hv → 2Br •

(24)

Br • + Br − → Br2•−

(25)

3.3. Reactants Evolution and Reaction Mechanism. The N 1s XPS spectra of the products after 30 and 120 min of reaction time are shown in Figures S9 and S10. The main Nspecies were NO3−and NO3−/NH4+, of which NO3− was due to the oxidation of NO, and NH4+ was from the added NH4OH. Figures S11 and S12 show the S 2p spectra of the products after 30 and 120 min of reaction time; the S-species is assigned to SO42−.30,38 The detailed XPS analyses relating to N 1s and S 2p spectra are available in Text S5. Figure 3(A,B) show the spectra of Cl 2p after 30 and 120 min of reaction time. The two spectra can be broadly classified into the following five peaks: 198.8, 200.2, 203.3, 206.2, and 207.8 eV, and the following four peaks: 197.7, 199.2, 206.2, and 207.8 eV. The peaks from 197.7 to 200.2 eV corresponded to the different states of NaCl and NH4Cl.39 The peak at 203.3 eV is ascribed to ClO2−,38 and those at 206.2 and 207.8 eV are attributed to ClO3− and ClO4−, respectively.38 The disappearance of ClO2− was due to it being used up. When the reaction time reached 240 min, the peaks of ClO3− and ClO4− disappeared (Figure S13), suggesting they were further consumed by reactants or were photodecomposed. The total concentration of Cl-species decreased significantly, as the reaction time reached 30 and 120 min, which was due to the release of ClO2 and Cl2.22−24,38,39 According to the above UV−vis and XPS results, the evolution of ClO2− can be deduced as follows: the main products of ClO2− were ClO2, Cl−, ClO3−, Cl2, and ClO4−, of which ClO2 was mainly produced from UV photocatalysis (eqs 3 and 4)24,31,38 and ClO2− acidolysis (eq 6).29,30 The selfdecomposition of ClO2− (eq 26),24,38,39 the reaction of ClO• F


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Environmental Science & Technology with OH− (eq 27),23 and the reaction of Cl2O2 with ClO2− (eq 12),23 produced ClO3−. ClO3− then reacted with Cl− to produce ClO2 and Cl2 (eq 28). As the reaction proceeded, the acidolysis of ClO3− produced ClO4− (eqs 29 and 30), and both ClO2 and Cl2 were generated. The release of ClO2 and Cl2 into the atmosphere was the main reason for the decrease in Clspecies concentrations in the solution, as shown by the XPS results. In addition, ClO3−, ClO4−, and Cl2 were thought to contribute to NO oxidation during the reaction, but they played minor roles in NO removal, as their oxidizing potentials were lower than those of ClO•, HO•, and ClO2. 5ClO2− → 3Cl− + 2ClO3− + 2O2

penetration was also investigated, and the results and discussion are available in Figure S18 and Text S8. Overall, DMBA and DMOB absorbed 2.2 and 45.2% of UV254 light, respectively; thus, using DMOB could not accurately verify the role of ClO• in NO removal but could confirm the role of UV in NO removal. The rate constants for the reactions of ClO• with DMBA and with t-BuOH were reported to be 2.1 × 109 and 107 M−1 s−1, and the rate constant for the reaction between HO• and t-BuOH was reported to be 6 × 108 M−1 s−1.41 Hence, DMBA and t-BuOH are suitable to be used as scavengers for ClO• and HO•, respectively, though the scavenger effect of tBuOH on ClO• can not be neglected. The radical-quenching results with 100 mM t-BuOH and 20 mM DMBA demonstrated the roles of HO• and ClO• in NO removal; apparently, the contribution of ClO• was greater than that of HO•; thus, ClO• played a leading role in the NO removal. The reaction mechanisms of the removal of SO2 and NO by using UV/NaClO2−NH4OH can be concluded as a synergism between acid−base neutralization and the radical-induced oxidation. Photocatalysis of ClO2− generated the primary radicals ClO•, O•− (eq 2), and ClO2 (eqs 3 and 4);23 O•− then rapidly reacted with H2O to generate •OH (eq 5, k = 1.8 × 106 M−1·s−1).31 As a result of the addition of NH4OH, the yields of ClO2 and NO2 decreased significantly, and the yields of HO• and ClO• increased as eqs 2, 5, and 31 were accelerated.40 The main radicals responsible for NO removal were HO•, ClO•, ClO2−, and ClO2, and HO• and ClO• played leading roles. Though Cl• might also be produced from UV irradiating ClO− that was produced via eq 4, Cl• could not be a major radical in NO removal, because the Cl• yield was too low, and it could not be more useful than ClO•, as confirmed by Figure S15. During the removal process, SO2 and NO first diffused to the gas− liquid interface, and then, NH4OH absorbed SO2 to form SO32− (eq 32), along with oxidation by ClO2−/radicals to generate SO42− (eq 33). The insoluble NO in the gas−liquid interface first reacted with ClO2−/radicals to produce soluble NO3− and NO2 (eq 34), and then, the fate of NO2 had three paths: (1) the hydrolysis of NO2 produced NO (eq 8); (2) NO2 reacted with SO2 (eq 15) in the liquid film to form SO42− and NO2−; (3) NO2 reacted with NO to form NO2−. All the produced NO2− was further oxidized to NO3− (eq 35), as demonstrated by the XPS results. The main removal products were determined to be (NH4)2SO4, NH4NO3, Na2SO4, and NaNO3 based on the XPS results, and these complex products could be used as compound fertilizers. All the reaction pathways for removal of SO2 and NO (eqs 33 and 34) are summarized in Text S6.

(26)

2ClO• + OH− → HClO2− + ClO• → ClO3− + H+ + Cl− (27) +

−

4H + 2Cl +

2ClO3−

→ 2ClO2 + Cl 2 + 2H 2O

(28)

4H+ + 8ClO3− → 4ClO4 − + 3O2 + 2Cl 2 + 2H 2O

(29)

2H+ + 3ClO3− → ClO4 − + 2ClO2 + H 2O

(30)

Figure 3(C,D) shows the ESR spectra of UV/NaClO2 and UV/NaClO2−NH4OH, and Figure S14 shows the ESR spectrum of UV/NaClO as a contrast, to illuminate the difference in the radical constitution of UV/NaClO2 and UV/ NaClO. It can be found that only the DMPO−OH adduct (aN = aH = 15.0 G, aN and aH are the hyperfine coupling constants of the N and H atoms) presented in UV/NaClO. In the absence of NH4OH, the UV/NaClO2 system produced ClO2 because of the appearance of the signals of the DMPO−Ox adduct (aN = 7.4 G, aH = 5.1 G), which is consistent with Marcon’s ESR result and our UV−vis results shown in Figure S4.40 Then, adding NH4OH in UV/NaClO2 eliminated the signal of the DMPO−Ox adduct but significantly increased the signal intensity of the DMPO−OH adduct (aN = aH = 15.0 G). Hence, using NH4OH could suppress the formation of ClO2 but increase the yield of HO•. One thing that should be noted is that HO• may be also formed in UV/NaClO2, but its signal of the DMPO−OH adduct is so weak that it is possibly hidden by the signal of DMPO−Ox adduct.40 In Figure 3(D), a new adduct signal appeared, which is represented by the red circle and may be assigned to ClO•, because ClO• and HO• have a symbiotic relation.23 To confirm the key role of ClO• in NO removal, we compared the behavior of UV/NaClO2−NH4OH (0.1 M), UV/NaClO (0.1 M), and UV/H2O2 (0.5 M) in NO removal (HO• and HO•/Cl• are known as the primary radicals for UV/H2O2 and UV/HClO). The results in Figure S15 showed that UV/NaClO2−NH4OH performed a best removal of NO, which suggested that some other highly reactive radical was produced in UV/NaClO2− NH4OH besides HO•. The most possible one is ClO•, because ClO• and HO• are symbiotic in UV/NaClO2. To further confirm the roles of HO• and ClO• in NO removal, the radical-quenching tests were carried out by using tBuOH (for HO•), BA (for Cl•/HO•), DMBA (for ClO•), and DMOB (for ClO•) as scavengers. As shown in Figure S16, adding t-BuOH, DMBA, and DMOB in UV/NaClO2−NH4OH decreased the NO removal efficiency; but the addition of BA did not affect NO removal. During the reaction, we found that adding BA produced ClO2 from the acidolysis of NaClO2; therefore, BA can not be used as a scavenger in UV/NaClO2. The specific result and discussions are available in Figure S17 and Text S7. The impact of the scavengers on UV light

(ClO2 )* + OH− → ClO2− + HO•

(31)

SO2 + 2OH− → SO32 − + H 2O

(32)

ClO2− /Radicals + SO32 − → 2SO4 2 − + Products

(33)

Radicals + ClO2− + NO → NO3− + NO2 + Cl−Products

(34)

NO2 + NO + 2OH− → 2NO2− + H 2O + Radicals → 2NO3− + Cl−Products

(35)

3.4. Kinetics. NO removal is the key in the simultaneous removal process. For a gas−liquid heterogeneous reaction, the mass-transfer kinetics are significant in accelerating NO G


Article


process, no NO is residual in liquid film, as aforementioned; thus, the value of P* should be 0.

removal. On the basis of the NO removal mechanism and products analyses, the overall reaction of NO removal can be described by eq 36

NNO = KNO,g(PNO,g − P*)

a NO + bClO2− + c Radicals + dOH− → eCl− + f NO3− + g NO2 + hH 2O

1 KNO,g

(36)

where a, b, c, d, e, f, g, and h are the stoichiometric coefficients for NO, ClO2−, radicals, OH−, Cl−, NO3−, NO2, and H2O, respectively. The NO removal refers to the conversion to NO3−. Because ClO2−, HO•, ClO•, and ClO2 determine the NO oxidation, the chemical reaction rate equation of NO oxidation by the UV/ NaClO2−NH4OH system can be expressed as eq 36. It had been reported that the reaction rate constant between HO• and NO reached 1010 m−1·s−1,33,42 the NO oxidation by HO• was a rapid reaction, and the NO oxidation by HO• conformed to pseudo-first-order kinetics, as revealed by Liu et al.22 and Baveje et al.;43 thus, k1[ClO2−]0 and k2[oxi-sec]0 in eq 37 could be written as k1′ and k2′, respectively, in which, [oxi-sec] refers to the second oxidants, such as ClO2, radicals, ClO3−, Cl2, and ClO4−. rNO = −

rNO = −

(38)

(39)

where pNO,i is the interface pressure of NO, which can be gained from eq 40 based on the two-film theory. Thus, eq 39 could be transformed into eq 41. NNO = kNO,g(pNO,g − pNO,i )

(40)

ij N yz C NO,i = HNOjjjjpNO,g − NO zzzz j kNO,g z k {

(41)

where NNO represents the absorption rate of NO, (mol/m2·s); kNO,g is the gas phase mass-transfer coefficient, mol/(s·m2·Pa); PNO,g is the NO partial pressure in gas phase, Pa; and HNO is the solubility coefficient of NO, mol/(L·Pa). The NO absorption rate can be calculated via eq 42.22,44 NNO

(η =

NO

·C NO,in − MNO·

C NO2,out 46

60000·MNO·aNO·V

+

1 EHNOkNO,l

(44)

D NO,l kov1 kNO,l

(45)

Calculations of the parameters, which can be found in Text S9−S12, and the physiochemical parameters, including aNO, μNO,l (solution viscosity), HNO, DNO,g, and DNO,l, are provided in Table S1. On the basis of these data, NNO, kov1, kNO,l, kNO,g, KNO,g, and the Ha number can be calculated, as shown in Table S2. Table S2 shows that in the absence of UV, NNO is enhanced by the decrease in NH4OH and pH and the increase in NaClO2 and NO concentration. Moreover, the significant promotional roles of O2, HCO3−, Cl−, and Br− on NNO were also observed (1.74 × 10−6 increased to 3.54−3.71 × 10−6, 2.86−2.97 × 10−6, 2.01−2.27 × 10−6, and 2.89−3.35 × 10−6 (mol/m2·s)). In the presence of UV, NNO* reached 3.36−3.77 × 10−6 mol/m2·s but was insensitive to changes in the reaction factors except light intensity, wavelength, and NO concentration. For kov1, the data obtained with the UV illumination range of 2.76 to 5.23 × 103 s−1, which were higher than those without UV (0.59−2.28 × 103 s−1), suggested that the NO oxidation rate by ClO•/HO•/ ClO2 was several times higher than that conducted with ClO2− alone. With regard to the mass-transfer parameters, compared to the results of kNO,l (×10−4), kNO,g (×10−6), and KNO,g (×10−7), the total mass-transfer resistance was found to be controlled by the gas−film interface. The Ha numbers were in the range of 329− 798 and 747−1000 for without and with UV, respectively, indicating that NO removal by NaClO2−NH4OH and UV/ NaClO2−NH4OH was a fast reaction; thus, the reaction rate was much higher than the mass-transfer rate, and the masstransfer rate controlled the NO removal process. 3.5. Environmental Implication. In this study, a novel AOP was developed using UV/NaClO2 to simultaneously remove SO2 and NO, and the addition of NH4OH in NaClO2 effectively decreased the yields of NO2 and ClO2. In real applications, UV/NaClO2 was superior in terms of the following parameters: (1) cost efficiency: compared with other AOPs such as UV/H2O2, UV/NaClO, and UV/PS, UV/NaClO2 performed better in the removal of SO2 and NO, with the lowest cost. These results and calculation process are shown in Figure S19 and Text S13. The dosage of NaClO2 used in this study was 1.0 wt %, which was less than that of NaClO in UV/NaClO (0.16 mol/L),17 and NO removal using UV/ NaClO2−NH4OH was much better (Figure S20 shows the simultaneous removal of SO2 and NO vs reaction time); (2) environmental impact: the estimated residual chloride ions (1569 mg/L) in the slurry were below the chloride ion standard in the WFGD slurry (20 000 mg/L). Hence, this method can be

where CNO,i (mol/L) is the interface concentration of NO, and CNO,i could be written as eq 38 based on Henry’s law C NO,i = HNO·pNO,i

kNO,g

E = Ha =

(37)

dC NO = (k1′ + k 2′)[NO] = k OV1C NO,i dt

1

For a mass-transfer process involving chemical reactions, the mass-transfer process is enhanced by chemical reactions between the gaseous and liquid phases. In eq 45, the enhancement factor (E), which is also the Hatta number, has great meaning in actual applications.44 Ha is the ratio of the flux with chemical reaction to that without chemical reaction

dC NO = k1[NO][ClO2−]0 + k 2[NO][oxi‐sec]0 dt

= (k1′ + k 2′)[NO]

=

(43)

)·Q (42)

where ηNO represents the average removal efficiency of NO, %; CNO,in and CNO2,out represent the inlet concentration of NO and outlet concentration of NO2, respectively, in mg/m3; Q represents the total flue gas flow rate, L/min; MNO represents the molecular weight of NO, g/mol; V represents the solution volume in the reactor, L; aNO represents the specific interfacial area of NO, m2/m3, shorten as m−1. The mass-transfer process can also be written as eq 43, in which the total resistance of mass transfer (1/KG) can be modeled as the sum of the gas−film (1/kg) and liquid−film (1/ kl) resistances, as given in eq 44.45 During the oxidation H


Article

Environmental Science & Technology used to clean flue gas emitted from industrial boilers and furnaces. It was reported that32 there are approximately 800 000 small- and medium-sized coal-fired boilers, industrial furnaces, and refuse incinerators present in China; therefore, this technology could be used to treat the flue gas emitted from these industrial plants.

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removal (Table S1) and mass-transfer reaction kinetics parameters of NO oxidation removal (Table S2) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.H.) *E-mail: [email protected]. (J.C.)

ASSOCIATED CONTENT

ORCID

S Supporting Information *

Runlong Hao: 0000-0002-2793-3607 Yi Zhao: 0000-0001-9974-0348 Lidong Wang: 0000-0002-0560-3455 John Crittenden: 0000-0002-9048-7208

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b06950. Supplemental text describing chemicals and reagents (Text S1), calculation procedure of UV light emission energy density (Text S2), ESR procedure for detection of hydroxyl radicals and chlorine-containing radical (Text S3), determination of the influencing mechanism of NH4OH on ClO2− (Text S4), XPS analysis for the spectra of N 1s and S 2p (Text S5), reaction paths for the removal of SO2 and NO (Text S6), impact of BA on NaClO2 (Text S7), impact of BA, DMOB, and DMBA on the UV penetration (Text S8), solubility coefficients of gas (Text S9), diffusion coefficients of gas in solutions (Text S10), diffusion coefficients of gas in gas phase (Text S11), mass-transfer parameters (Text S12), and comparison of UV/NaClO2 with other AOP methods in the simultaneous removal of SO2 and NO (Text S13). Figures showing experimental flowchart (Figure S1), UV−vis spectra of ClO2− and ClO2 at 260 and 360 nm (Figure S2), the profiles of ClO2− concentration vs reaction time in NaClO2 and NaClO2−NH4OH under UV radiation (Figure S3), the profiles of ClO 2 concentrations in NaClO2, H2O (to absorb residual ClO2 out of NaClO2) after NaClO2, and NaClO2− NH4OH under UV radiation (Figure S4), the color change of the NaClO2 and NaClO2−NH4OH solutions as the UV irradiation proceeds (Figure S5), determination of the influencing mechanism of NH4OH on ClO2− during UV photolysis (Figure S6), impact of UV light wavelength on the yield of ClO2 (Figure S7), impact of temperature on the simultaneous removal of SO2 and NO (Figure S8), N 1s XPS spectrum of removal product within 30 min (Figure S9), N 1s XPS spectrum of removal product within 120 min (Figure S10), S 2p XPS spectrum of removal product within 30 min (Figure S11), S 2p XPS spectrum of removal product within 120 min (Figure S12), Cl 2p XPS spectrum of the removal product after 240 min (Figure S13), ESP spectrum of UV/NaClO (5 wt %) (Figure S14), the comparison of UV/H2O2, UV/NaClO, and UV/NaClO2−NH4OH in terms of NO removal (Figure S15), radicals-quenching tests for determining the roles of HO·, Cl·, and ClO· in the NO conversion process (Figure S16), impact of BA (20 mM) on the evolution of NaClO2 (1%) (Figure S17), impact of BA, DMOB, and DMBA on the absorbance of NaClO2 at 254 nm (Figure S18), comparison of UV/NaClO2 with other AOP methods in the simultaneous removal of SO2 and NO (Figure S19), and the profile of simultaneous removal of SO2 and NO vs reaction time utilizing NaClO2−NH4OH in the absence and presence of UV (Figure S20). Tables showing physicochemical parameters of NO oxidation

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support provided by the Natural Science Foundation of China (No. 51708213), the Natural Science Foundation of Hebei (No. E2018502058), the National Key Research and Development Plan (No. 2016YFC0203705) (No. 2017YFC0210600), and the Fundamental Research Funds for the Central Universities (No. 2019MS103 and 2019MS128). The authors also acknowledge the China Scholarship Council, which provided a visiting scholar grant to R.H. for his study at the Georgia Institute of Technology. The authors appreciate the support from the Brook Byers Institute for Sustainable Systems, Hightower Chair and the Georgia Research Alliance at Georgia Institute of Technology. The views and ideas expressed herein are solely those of the authors and do not represent the ideas of the funding agencies in any form.

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Simultaneous Removal of SO2 and NO Using a Novel Method of

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