Environ Sci Pollut Res https://doi.org/10.1007/s11356-017-0480-6

REVIEW ARTICLE

Purification of Hg0 from flue gas by wet oxidation method and its mechanism: a review Yi Xing 1,2 & Bojun Yan 1,2 & Pei Lu 1,2

&

Xiaoxu Cui 1 & Liuliu Li 3 & Mengsi Wang 1

Received: 23 August 2017 / Accepted: 12 October 2017 # Springer-Verlag GmbH Germany 2017

Abstract The vast majority of Hg2+ can be removed while elemental mercury (Hg0) can hardly be removed due to its characteristic of high volatility and insolubility in water. Till now, how to oxidize Hg0 to Hg2+ is the key for the purification of Hg0, especially when there are others pollutants, such as HCl, SO2, and NOx. In this review, the method and mechanism of Hg0 purification from flue gas by H2O2, KMnO4, NaClO2, and O3 are reviewed comprehensively. It is concluded that the oxidation of Hg0 mainly depends on the electronic supply efficiency from the solution. The Fenton reagent, composed of H2O2 and metal cations, is superior to O3 and the solution of KMnO4 and NaClO2. Moreover, HCl, SO2, and NOx in the flue gas can influence the oxidation and purification mechanism of Hg0. It is found that HCl in flue gas had obvious auxo-action on the oxidation of mercury, and SO2 and NOx have different effects on the oxidation of Hg0 with the change of compositions and concentration of pollutants in the flue gas. In general, SO2 and NOx can slightly promote the oxidation of Hg0 due to the synergistic effect.

Bojun Yan contributed equally to this work. Responsible editor: Bingcai Pan * Pei Lu [email protected]; [email protected]

1

School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China

2

Beijing Key Laboratory of Resource-Oriented Treatment of Industrial Pollutants, University of Science and Technology Beijing, Beijing 100083, China

3

School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China

Keywords Hg0 . Flue gas . Wet oxidation method . Purification . Mechanism

Introduction Mercury (Hg) is very stable and can be evaporated at room temperature. The vapor is highly toxic due to its bioaccumulation and can be transferred worldwide. Moreover, mercury can be turned into methylmercury (CH3Hg) under complicated conditions by the action of microorganisms (Schmeltz et al. 2011). Mercury and its compounds are highly toxic pollutants which can cause serious damage to the human nervous system (Meng et al. 2011). Mercury emissions from human activities are estimated to be 2000–6000 t/a all over the word, which accounts to about 30–55% of global atmospheric mercury emissions (Yang et al. 2007; Yan et al. 2004). Mercury pollution is mainly caused by the burning of fossil fuels (Xu et al. 2008). With the increase of global consumption of coal resources, mercury emission is also increasing (Pacyna et al. 2006, 2010). Therefore, mercury emission is becoming a global problem (Rallo et al. 2012). The efficiency of methods for removing mercury depends greatly on the species composition of mercury (Pavlish and Mann, 2003). Mercury in the flue gas usually exists in varying percentages of three basic chemical forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particle-bound mercury (Hg (p)) (Wu et al. 2008; Gale et al. 2008; Yuan et al. 2012). Hg0 is very difficult to be removed because of its high volatility and low solubility in water (Wang et al. 2014a; Driscoll et al. 2013). Therefore, the mercury pollution control technology is mainly focused on the Hg0 purification (Pavlish and Mann, 2003). In recent years, a number of elemental mercury removal technologies have been developed, which can be divided into two main methods (Liu and Wang, 2014). The first one is to convert

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Hg0 to Hg2+ by oxidizing (Presto and Granite, 2006; Li et al. 2011; Xu et al. 2015), such as catalytic oxidation, photochemical oxidation, ozonation (O3), photocatalytic oxidation, potassium permanganate (KMnO4), sodium chlorite (NaClO2), and UV/ H2O2 oxidation (Chen and Mathur, 2006; Snider and Ariya, 2010; Liu, 2011, Liu et al. 2015a; Byun et al. 2014; McLarnon et al. 2005; Nick et al. 2008); the other one is to adsorb Hg0 from the flue gas (Huang et al. 2015; Lee et al. 2008; Qu et al. 2015). Till now, the oxidation technology is becoming the most feasible way to control mercury emissions due to its high efficiency. In detail, there are dry and wet methods for Hg0 oxidation. The process of removing mercury through dry method does not produce waste water, the flue gas temperature after purification is high; no need for secondary heating. Zhao found that combination of selective catalytic reduction (SCR) and flue gas desulfurization (FGD) is considered as one of the most effective methods to remove Hg from flue gas (Zhao et al. 2015). At present, the sintering flue gas denitrification in China is still in the initial stage. For most steel companies, the SCR system needs to be rebuilt. Although the pollutant purification efficiency is high, but it needs huge energy consumption and space and configure expensive catalytic systems. Meanwhile, the wet oxidation system can be easily made replacements and technical innovations on current desulfurization system. It is very simple and feasible to purify the flue gas discharged by coal-fired power plants and steel mills, especially for developing countries. Therefore, oxidation and purifying Hg0 from flue gas by wet method are specially discussed in this study. Wet mercury removal method mainly refers to the absorption of mercury using desulfurization absorbing liquid. Wet FGD (WFGD) system can remove nearly 90% of the Hg2+ but essentially none of the Hg0 (Wang et al. 2007; Zhuang et al. 2004). Thus, according to the status of wet mercury removal, a lot of efforts have been conducted to oxidize Hg0 to Hg2+ for the purpose of increasing the mercury removal efficiency (Diaz-Somoano et al. 2005). The forms of mercury and their conversions in flue gas are illustrated in Fig. 1 (FernándezMiranda et al. 2014). This review mainly focused on the purification of mercury in flue gas by wet oxidation method, and different techniques of mercury oxidants are summarized and compared comprehensively. The impact and mechanism of flue gas components on the oxidation of mercury are also discussed. Finally, reasonable suggestions were given for the study of mercury removal in flue gas.

Methods and different oxidants Oxidation by H2O2 Under different reaction conditions, H2O2 can be decomposed and produce the following four substances: ·OH + OH, HO2· +

H·, H2O + O2, and H2O + HO2·. ·OH is an ideal oxidizer with strong oxidability and high reaction rate. The redox potential of ·OH is up to 2.80 V and the oxidation reactions with ·OH can be carried out fast and completely (Li et al. 2009; Liu and Wang, 2014). The decomposition of H2O2 and the generation of ·OH were mainly through two ways: (1) excitation under high temperature (higher than 500 °C) (Cooper et al. 2002); (2) factor addition under low temperature (room temperature to 90 °C), by adding metal ions to the H2O2 such as: Fe2+ and Cu2+ or applying ultraviolet (UV) light or ultrasound (Liu et al. 2013; Ding et al. 2014). In the situation of the rapid decomposition of H2O2 in the role of the relevant catalyst such as Fe2+ and production of ·OH (Li et al. 2013), these following reactions will take place. Fe2þ þ H2 O2 →Fe3þ þ OH− þ ∙OH Fe

3þ

−

þ H2 O2 þ OH →Fe

2þ

þ H2 O þ ∙OH

ð1Þ ð2Þ

Fe3þ þ H2 O2 →Fe2þ þ Hþ þ HO2

ð3Þ

HO2 þ H2 O2 →H2 O þ O2 ↑ þ ∙OH

ð4Þ

2þ

Hg0 þ H2 O2 →Fe

=Fe3þ

HgO þ H2 O

ð5Þ

According to reactions (1)–(5), it can be found that the main function of Fe3+ or Fe2+ is to act as a catalyst to accelerate the decomposition of H2O2. Liu has studied the oxidation of mercury by Fenton solution (Liu, 2011, Liu et al. 2014a, Liu et al.2015a) by adding metal ions and ultraviolet light. The mechanism of mercury oxidation by UV/H2O2 is shown as Fig. 2. Hg0 can be easily oxidized to HgO by ·OH through the following reaction in UV/H2O2 system. Hg0 þ ∙OH→HgðOHÞ2 →heat HgO þ H2 O

ð6Þ

When the concentration of the H2O2 is high enough, Hg0 can also be oxidized by H2O2 according to the following reaction. Hg0 þ H2 O2 →HgO þ H2 O

ð7Þ

Meanwhile, Dennis (2007) has carried out an interesting study to remove Hg0 from flue gas. The results show that 75% of the Hg0 can be oxidized by Fenton solution and the best reaction pH is 1.0~3.0 (Burbano et al. 2005). Furthermore, many researchers have studied the oxidation by H2O2; the comparison studies in different reaction systems have been carried out and the results are summarized in Table 1. Table 1 and reactions (1)–(7) illustrate that H2O2 has a strong oxidizing effect on Hg0, the removal efficiency of Hg0 is high and the oxidation process does not produce other pollutants. Moreover, the removal efficiency of Hg0 can be greatly affected by the experimental condition. In Fenton reagent, the removal efficiency of Hg0 mainly depends on the

Environ Sci Pollut Res Fig. 1 The schematic diagram of the forms of mercury and its conversions in flue gas (Fernández-Miranda et al. 2014)

decomposition rate of H2O2 and it can be accelerated by adding catalysts or applied energy to the system, which results in the significant enhancement on Hg0 removal efficiency. Due to its low price and high efficiency on purifying Hg0, H2O2 with some metal cation (Fenton solution) is desirable for purifying Hg0 from flue gas. The development of photochemical reactor can be used to effectively remove Hg0 like UV/H2O2/O2 system (Liu and Wang, 2014). Till now, purifying by H2O2 has been studied for more than 10 years, and the influence of removal conditions, such as temperature, catalyst, and UV, have been studied in detail. Though it has been found that Fenton reagent is a good oxidizer for Hg0, it is still necessary to develop more effective methods to accelerate the decomposition of H2O2 to enhance the removal efficiency of Hg0 in the future work. At the same time, lowing the cost of H2O2 applied in the removal of Hg0 is Fig. 2 Reaction mechanism of Hg0 removal by UV/H2O2 process (Liu and Wang, 2014)

another important work on Fenton oxidation technology industrialization. Oxidation by NaClO2 Till now, NaClO2 is widely used in many chemical fields as an oxidizing agent due to its strong performance on oxidation (Byun et al. 2014; Qian et al. 2007). Strong oxidizing agents of ClO and Cl can be generated by the reaction between NaClO2 and SO2 or/and NO. Meanwhile, chlorine (Cl2) will be generated at the same time. The reactions are shown as (8)–(10). OClO þ NO→NO2 þ ClO

ð8Þ

ClO þ NO→NO2 þ Cl

ð9Þ

Cl þ Cl→Cl2

ð10Þ

Environ Sci Pollut Res Table 1

Removal efficiencies of mercury in different reaction systems

Reaction systems

Experiment conditions

Removal efficiency (%)

Reference

Hg0 (μg/m3)

SO2 (ppm)

O2 (%)

NO (ppm)

H2O2 (mol/L)

UV (nm)

pH

Fe2+ (mol/L)

UV/H2O2/Fe3+

30

800

6

400

0.40

254

3.4

0.006

67.9

Liu et al., 2015a

UV/H2O2

30

1500

6

400

0.50

254

3.97

–

50.6

Liu et al., 2014a

UV/H2O2/O2

30

1500

6

400

0.50

254

3.97

–

83.5

Liu et al., 2014a

H2O2/O2

30

1500

6

400

0.50

–

3.97

–

9.8

Liu et al., 2014a

Fenton solution (Fe /H2O2)

40

1000

6

400

1.2

–

3.4

0.06

H2O2/O2 UV/H2O2 UV/H2O2/O2

30 30 30

– – –

6 6 6

– – –

0.50 0.50 0.50

– 254 254

3.97 3.97 3.97

– – –

2+

It is well known that Hg0 can be easily oxidized to Hg2+ by Cl2, which can be shown as reaction (11). Hg þ Cl2 →HgCl2

ð11Þ

According the experimental results reported by Byun et al. (2010), NaClO2 has a significant oxidation effect on Hg0 when the concentration of Hg0 is 260 μg nm−3 and the concentrations of NaClO2 are 0.1, 0.18, and 0.36 g nm−3, respectively. The experiment procedure is shown in Fig. 3 (Byun et al. 2010). The reported experimental results show that NaClO2 added into the sintering flue gas can greatly promote the oxidation of Hg0, and the oxidation rate of Hg0 increases with the concentration increasing of NaClO2 (Nick et al. 2008). When the concentration of ClO is 3 mmol/L, the removal efficiency of Hg0 is as high as 100% (Jin et al. 2011). With the reactions between NaClO2 and other reducing components in flue gas, amount oxidants will be generated, such as ClO, Cl, and Cl 2 , which will greatly accelerate the Fig. 3 Temporal profile of Hg0 concentration obtained by the injection of NaClO2(s) (Byun et al. 2010)

100 11.7 52.7 85.1

Liu et al., 2015a Liu and Wang, 2014 Liu and Wang, 2014 Liu and Wang, 2014

oxidation of Hg0 (Byun et al. 2009). On considering the difference of reactions between NaClO2 and these reducing components in flue gas, the efficiencies of purifying Hg0 and its mechanism will also be different (Nick et al. 2008). According to the reported results of Zhao et al., the acid condition of solution is beneficial to the oxidation of Hg0 (Zhao et al. 2009). Though NaClO2 can obtain significant oxidation efficiency of Hg0, this method generates a number of new pollutants (Cl− and Cl2) into absorbing solution or flue gas (Fan and Deng, 2012), which is dangerous to the environmental protection device and environment. Therefore, NaClO2 and its solution should be carefully used for removing Hg0 from flue gas. Oxidation by KMnO4 Potassium permanganate (KMnO4) solution is one kind of effective absorbents for oxidizing Hg0 from flue gas, and the

Environ Sci Pollut Res

removal efficiency is very desirable due to the strong oxidizing performance of KMnO4, especially for the low concentration of Hg0. Acidic KMnO4 is suggested to be used as an absorbing solution for purifying Hg0 in the gas phase by the US Environmental Protection Agency (EPA) as absorbent method 29 (Liu and Wang, 2014), and KMnO4 solution can also be used to collect mercury from flue gas during the purification process (Hara 1975). Ye et al. also reports that KMnO4 has a strong action for the removal of Hg0 (Ye and XH, 2007). Zhao (1999) reports that the reactions of Hg0 and KMnO4 can be influenced by the pH of the KMnO4 solution. Mn2+ can be used to produce catalytic agents to speed up reactions, especially under acidic conditions. The H+ can increase its redox potential while the produced Mn2+ can play a catalytic role. The reactions of KMnO4 under different pH values are shown as follows (Liu and Wang, 2014; Ye and XH, 2007). Under strongly acidic condition: 5Hg0 þ 2MnO4 − þ 16Hþ →5Hg2þ þ 2Mn2þ þ 8H2 O ð12Þ Under weakly acidic condition: 3Hg0 þ 2MnO4 − þ 8Hþ →3Hg2þ þ 2MnO2 þ 4H2 O ð13Þ Under neutral condition: 3Hg0 þ 2MnO4 − þ H2 O→3HgO þ 2OH− þ 2MnO2

ð15Þ

In order to study the performance of KMnO4 in Hg0 oxidation, oxidant solutions of KMnO 4, NaClO 2 , NaClO, K2S2O8, and H2O2 are comprehensively studied in WFGD system by Ping et al. (2012). In Table 2 (Ping et al. 2012), it can be found that when there is no interference of other conditions, KMnO4 solution can yield the highest oxidation and removal efficiency on Hg0. Moreover, according to the results reported by Liu and Wang, 2014), pH is a key factor in the oxidation of Hg0 by KMnO4. With the increase of pH value, the removal efficiency

where c is the concentration of H+ in KMnO4 solution. It is clear that the redox potential will increase with the decreasing of pH and the oxidizing ability of KMnO4 solution will be enhanced. Therefore, it can yield the highest removal efficiency of Hg0 under the condition of strong acid (Liu, 2011). When it is under strong alkaline condition, OH− can also be oxidized to ·OH, which is also helpful for the oxidization and removing of Hg0 (Hara 1975). The redox reactions and their various potentials are summarized and shown in Table 3. According to the results of Tables 2 and 3, the removal efficiency of Hg0 by KMnO4 solution is obviously higher than that of the others. However, the cost of the method is very high, and the reaction is difficult to be controlled. Therefore, how to control the reaction process of KMnO4 exactly and decrease its cost is still a tough task and need be further studied.

It is different from other oxidants that O3 has the unique characteristic and very high oxidation-reduction potential (ORP), only slightly below to that of Cl (Jin et al. 2011). Moreover, it can exist with a long oxidation life in the flue gas (Biswajit and Ariya, 2003). Wang et al. (2007) adds O3 into flue gases of WFGD to remove Hg0. It is found that O3 will firstly react with NO when there is NO in flue gas and when the ratio of O3 and NO is 2:1, the removal efficiency of Hg0 can reach as high as 95% without the generation of secondary pollutants (Wang et al. 2007). However, HgO will be decomposed into Hg and O 2 when the temperature is higher than 371 °C. Therefore, Wang suggests that the optimal temperature for

The effect of oxidant on Hg0 removal efficiency (Ping et al.

Oxidant solution

KMnO4

ð16Þ

Oxidation by O3

Hg0 þ 2MnO4 − þ 2OH− →HgO þ 2MnO4 2− þ H2 O

NaClO2 NaClO K2S2O8 H2O2

φ ¼ φ0 þ 0:01184lg½cðHþ Þ

ð14Þ

Under strongly alkaline condition:

Table 2 2012)

will be firstly decreased and then increased, just like a semiwaveform. The pH value has a significant effect on the oxidation kinetics and reaction pathways of KMnO4, which results from the difference of the standard electrode potential (φ0) and reaction rate. The redox potential of KMnO4 solution can be illustrated as the following equation:

Experimental conditions Qgas 1 L/min pH 5.2; Oxidant 1 mmol/L; Hg0 50 μg/m3; Treaction 55 °C

Removal efficiency (%) 96.70 69.09 60.65 51.45 5.14

Table 3 Redox reactions and their potentials of various couples of KMnO4 ions (Liu et al. 2011) Half-cell reactions

Standard electrode potential φ0 (V)

pH

Mno4‐ + 8H + 5e‐ → Mn2+ + 4H2 O

1.51 1.70 0.6 0.56

< 3.5 3.5–7 7–12 > 12

Mno4‐ + 4H+ 3e‐ → MnO2 + 2H2 O Mno4‐ + 2H2O + 3e‐ → MnO2 + 4OH‐ Mno4‐ + e‐ → MnO42‐

Environ Sci Pollut Res

Hg0 oxidation should be 245–273 °C. The oxidation property by ozone at different temperatures is shown in Fig. 4. The reactions of Hg0 oxidation by O3 are shown as the following equations. Hg þ O3 →HgO þ O2

ð17Þ

Hg þ O→HgO

ð18Þ

On the oxidation mechanism of Hg0 by O3, Wang et al. (2007) reports that Hg0 can be effectively oxidized by O3 in flue gas, and the content of NO3 is a key factor to Hg0 oxidation. The reactions can be shown as (19) and (20). O3 þ NO2 →NO3 þ O2

ð19Þ

Hg þ NO3 →NO2 þ HgO

ð20Þ

Moreover, Dai and Tao, 2014) reports that the oxidation of Hg0 can be greatly enhanced with the concentration ratio of O3/NO or/and the temperature of flue gas increasing. The reaction routes of oxidization of Hg0 in flue gases by ozone are illustrated as Fig. 5(Wang et al. 2007). Calvert (2005) has studied the mechanism of the removal Hg0 in atmospheric environment by O3 and finds that reaction (17) cannot occur in the atmosphere. Hg0 and O3 will firstly react and generate HgO3, which is a metastable compound. After that, HgO3 will be decomposed to HgO and O2. Other researches find that a combining gas-phase oxidation method, ozone and alkali absorption included, will significantly improve the oxidizing and removal of Hg 0 (Zhang et al. 2014; Glomba and Kordylewski, 2014). However, the demercuration by O3 can be affected by temperature easily. When the temperature is over 150 °C, O3 will Fig. 4 Mercury oxidation property by ozone at different temperature (Wang et al. 2007)

Fig. 5 Reaction routes of elemental mercury oxidization in flue gases by ozone (Wang et al. 2007)

decompose (Wei et al. 2007) and the oxidation rate of Hg0 begins to decrease. Meanwhile, the generation of O3 is very expensive. Therefore, demercuration by O3 is not suggested for large-scale industrial applications. Obtaining ozone with low-cost and high efficiency and enhancing its oxidation efficiency are the keys in future research and application in demercuration by O3.

Effects of experimental conditions Influence by the constituents of flue gas The composition of industrial flue gas is complex, and different compositions have significantly different impacts on the

Environ Sci Pollut Res

removal of Hg0. However, due to lack of oxidant, Hg0 can not be fast oxidized in gas phase components, though there are SO2, HCl, and NOx in the flue gas. Up to now, it has been demonstrated that the oxidation and removal of Hg0 mainly occurs during the gas-liquid mass transfer process (FuenteCuesta et al. 2012). Therefore, the effect of SO2, HCl, and NOx on the oxidization of Hg0 in flue gas with wet oxidation method are mainly discussed in the following part. Effect of SO2 Sulfur dioxide is amount discharged by coal-fired factory, such as power plants and steel mills, and it has been reported that SO2 can influence the oxidation of Hg0 under different reaction conditions (Fernández-Miranda et al. 2014; Liu et al. 2014a, b; Eswaran and Stenger, 2008; Ghorishi and Kilgroe, 1999; Xu et al. 2013). Fernández-Miranda et al. (2014) finds that SO2 can enhance the oxidation of Hg0, and the oxidation rate can only be increased 3–5% when the concentration of SO2 is 1000– 2000 ppm. The oxidation of Hg0 is shown as reaction (21). Hg0 þ SO2 þ O2 →HgO þ SO3

ð21Þ 0

Liu has found that when both SO2 and NO exist with Hg in flue gas, the oxidation rate of Hg0 will be decreased (Liu et al. 2014a, b). When there is no other oxidizing gases (such as Cl2) in the flue gas, the presence of SO2 has a slight enhancement to the oxidation of Hg0. But in the coal-fired flue gas, the concentration and oxidation rate of Hg0 in flue gas are always much lower than that of SO2 (Eswaran and Stenger, 2008). Moreover, the addition of SO2 in the flue gas will inhibit the formation of Cl atoms and Cl2, which results in the fact that SO2 will compete with Hg0 for oxidizing agent. It will reduce the rate of oxidation of Hg0 (Ghorishi and Kilgroe, 1999; Xu et al. 2013). The oxidation inhibition mechanism of Hg0 by SO2 is shown as the following reactions. Cl2 þ SO2 þ H2 O→2HCl þ SO3

ð22Þ

2Cl þ SO2 þ H2 O→2HCl þ SO3

ð23Þ

Effect of HCl In the process of coal combustion, Cl− contained in flue gas can partially exist in the status of HCl, and it can oxide Hg0 from flue gas by the following reaction. 2HCl þ Hg →HgCl2 þ H2 0

ð24Þ

Hall, 1995) studies the removal of Hg0 from simulated flue gas and finds that within a certain temperature range, Hg0 can rapidly react with HCl. When the temperature of flue gas is 900 °C, about 90% of the reactions can be completed within 0.7 s. The contents of Hg2+ will increase with the increase of

the concentration of HCl in the simulated flue gas. On the other hand, HCl can react with O2 in the simulated flue gas to generate Cl2, and it is very feasible for the oxidation of Hg0. The oxidation reaction can be shown as reaction (25). 1 2HCl þ O2 →catalyst Cl2 þ H2 O 2

ð25Þ

Gao et al. 2004has conducted an experiment on the two types of flue gas system (13% CO 2 –7% O 2 –80% N 2 – 800 ppm NO system and 13% CO 2 –7% O 2 –80% N 2 – 800 ppm NO–1200 ppm SO2 system) for exploring the impact of different concentrations of HCl on Hg0 oxidation, respectively. It is found that HCl can effectively promote the oxidation of Hg0 in flue gas. The oxidation of Hg0 is heavily dependent on Cl as a medium in the oxidation, and the concentration of Hg2+ is linear positive correlation with the concentration of HCl. Thermodynamic calculation results are consistent with that of experiments carried out by Gao et al. (2004). Effect of NOx About 90% NOx in the flue gas is NO in flue gas. Meanwhile, due to the reaction between NO and O2 in flue gas, NO can be partially oxidized to NO2. Therefore, when the influence of NOx on the oxidation of Hg0 is analyzed, both NO and NO2 must be considered. The influence on removal mechanisms of Hg0 oxidation in the flue gas by NOx is similar to that by SO2. Xue reports that when there are no other oxidizing gases in flue gas, the presence of NO has a slight influence on the catalytic oxidation of Hg0 and the enhancement effect is a little less than that of SO2. When the concentration of NO is controlled within 0–500 mg/ m3, Hg0 oxidation efficiency can increase to about 4%, which results in the generation of NO2, generated by NO oxidization. It is a stronger oxidizing agent for Hg0. When the concentration of NO continues to increase, the oxidation rate of Hg0 will be decreased because of the competition between NO and Hg0 and the change of the pH value of the absorption solution (Xue 2014). Therefore, excess of NO will inhibit the oxidation of Hg0. The main reactions between NO and Hg0 can be summarized as below. 2NO þ O2 →2NO2

ð26Þ

NO2 þ Hg0 →HgO þ NO

ð27Þ

Other researchers have found similar phenomena. The oxidation effect of NO on Hg0 primarily depends on the concentration of NO in oxidation experiments of Hg0 by Fenton reagent, and the oxidation of Hg0 can be hindered due to the reaction between NO and ·OH (Liu, 2011). However, according to the results reported by Gao, the oxidation efficiency of Hg0 is always higher in the presence of NO in flue gas than that in the absence of NO (Gao et al. 2004).

Hutson 2008

Zhou et al. 2015

Gao et al. 2004

Liu and Wang, 2014

Inhibition Inhibition Promotion Inhibition Promotion Promotion under suitable amounts Promotion Promotion 80–100 81–100

Hg0 oxidation %

As discussed above, H2O2 solution can be used for wet oxidation and removing of Hg0 from flue gas. In order to increase the discharge of ·OH and enhance the removing efficiency of Hg0, the optimization of the excitation and decomposition of H2O2 by metal cation is very important (Zhou et al. 2015; Hutson 2008; Zepp et al. 1992; Malato et al. 2007).

4–5 18 – 45 48 40–81 84.5 Decrease to 78.8 85.1 Decrease to 81.4 9.8–82.8 92 Decrease to 90 94–99 92

Influence by the constituents of H2O2

Slight promotion Slight promotion No effect Promotion

Effects on mercury oxidation

Reference

Though NO2 is a more active oxidizing agent than NO, it has no effect or a slight effect in promoting the oxidation of Hg0 because of its low concentration. The reaction of the oxidation of Hg0 by NO2 can be shown as reaction (27). After summarizing a number of literatures which mentioned the influence of different constituents of the flue gas on Hg0 oxidation process, it can be concluded that under normal circumstances, HCl can always promote the oxidation and removal of Hg0, and the related experimental results are summarized in Table 4.

Nuria Fernández-Miranda et al. 2014

Environ Sci Pollut Res

55 55 8 8 – – 0–1500 1500

200 0–500

4 4 4 4 4 7 6 6 6 – – – – – 25 – 60 60 – – – – – – – 1000 – 800 800 800 400 0–2000 400 – 100–500 –

NaClO2 wet system

Fe2.45Ti0.55O4/H2O2

UV/H2O2

SO2 NOx HCl SO2 NOx HCl SO2 NOx UV power SO2 NOx Catalyst dosage (1.0 g/L) SO2 NO

1000–2000 – – 1200 1200 1200 0–3000 1500 1500 500–1000 – –

HCl (ppm) SO2 (ppm)

NO (ppm)

O2 (%)

T °C Main experimental conditions

Constituents of the flue gas

The light source can be excited under specific wavelengths of the UV to produce light quantum. Under the impact of light quantum, which is full of different energies, some substances

Impact factor

Ultraviolet/H2O2 solution

Oxidation method

ð28Þ

Effects of different factors in different experiments on mercury oxidation

Fe2þ þ OH→Fe3þ þ OH−

Table 4

The Fenton reaction mainly describes the activation of H2O2 by ferrous ions (Fe2+) to generate ·OH (Cheves Walling 1973; Pignatello et al. 2006), and the reaction equations can be summarized as reactions (1)–(3) (Liu and Wang, 2014; Dranga et al. 2012; Granite et al. 2007). Fe3+ can react with OH− to form insoluble ferric hydroxide [Fe(OH)3] in neutral pH conditions (Neyens and Baeyens, 2003). Therefore, the pH of the solution should be maintained in an acidic environment. The relationship between the forms of iron species in aqueous and the pH of the solution could be shown as Fig. 6 (Bokare and Choi, 2014). According to the reported results (Liu and Wang, 2014; Pouran et al. 2014; Pham et al. 2010; Umar et al. 2010), the optimum working pH value of Fenton reaction is 2.8–3.2. When the concentration of H2O2 remains constant, appropriately enhancing the concentration of Fe2+ can be favorable to accelerate the decomposition rate of H2O2 and the oxidation of Hg0. However, some research (Kang et al. 2002; Ashraf et al. 2006; Zazo et al. 2005) also find that when Fe2+ is excessive in Fenton solution, the following reaction (28) will occur with a high reaction rate and it will reduce the concentrations of ·OH and Fe2+. Therefore, the removal process of Hg0 will be limited.

150 150 150 300 300 127–827 50 50 50 50 50 50

Classical Fenton solution

Environ Sci Pollut Res Fig. 6 Iron species in aqueous solution as a function of pH (Bokare and Choi, 2009)

can be excited to break down. The production of ·OH in UV/ H2O2 solution is mainly by means of the energy provided by UV which can excite the decomposition of H2O2. The homolytic reaction of H2O2 can be shown as Eq. (29) (Jeong and Jurng, 2007; Liu, 2011; Jia et al. 2010). H2 O2 þ hv→2  OH

ð29Þ

Due to the hemolytic of H2O2, UV often has a significant impact on photochemical reaction. Mercury removal rate can be improved nearly 80% for H2O2 system under the stimulation of UV (Liu and Wang, 2014; Liu et al. 2015a). The UV wavelength is the most influential factor for the production of ·OH in UV/H2O2 system (Cater et al. 2000;Muruganandham and Swaminathan, 2004). Different UV wavelengths (185, 254, and 365 nm) are used to study the effects of UV wavelength on Hg0 removal efficiency by Liu and Wang, 2014), and it is illustrated that 254 nm is the most effective UV wavelength in the photochemical reaction of removing Hg0. Some other documents also report similar conclusions (McLarnon et al. 2005; Jia et al. 2010; Granite and Pennline, 2002). The energy of photon can be calculated by Planck Eq. (30) (Xu 2005). ε ¼ hv ¼ h

c λ

shorter UVeffective propagation distance will result in smaller amount of mercury removal. At the same time, UV power should also be considered in UV/H2O2 system. Decomposition of H2O2 cannot be completed when the UV power is too low. However, when the power is beyond the scope of the required for the oxidation of Hg0, the chemical reaction may transfer to the mass transfer controlling and will cause the waste of resources (Garoma and Gurol, 2004; Song et al. 2006). The most appropriate power for per unit solution is suggested as 0.006 W/mL (Liu, 2011) in UV/H2O2 solution. Non-ferrous Fenton solution In order to completely decompose H2O2 and achieving a more desirable removal efficiency of Hg0, some researchers have studied on non-ferrous Fenton system. The redox performance of metal or metal cation can be described by the characteristic of loss or gain of electrons. The ideal Fenton catalyst should

ð30Þ

where ε is the energy of photon, J; ν is the UV frequency, 1/s; h is the Planck constant, 6.626 × 10−34 J s; c is the speed of light, 2.998 × 108 m/s; and λ is the UV wavelength, 100– 380 nm. According to Eq. (30), shorter wavelength of UV light has the larger energy of photoelectron, and it is easier to stimulate the decomposition of H2O2 and produce more ·OH. However, when the wavelength of UV light is decreased, the effective propagation distance is reduced at the same time (Xu 2005), which will inhibit the oxidation of Hg0 in reactor. Therefore,

Fig. 7 Electron transfer pathways of CeO 2 under sulfuric acidpretreating (Wang et al. 2014a)

(a) H2O2 + ))) → 2 ⋅ OH (b) Fe2+ + H2O2 + Fe3+ + ⋅ OH + OH‐ (c) Fe3+ + H2O2 + ))) → Fe2+ + ⋅ OOH (d) H2O + ))) → + ⋅ OH + OH+ (e) O2 + ))) → 2 ⋅ O (f) ⋅O + H2O + → 2 ⋅ OH (a) M + xH2O2 + Mx+ + x ⋅ OH + xOH‐ (b) Mn+ + xH2O2 + Mn + x+ + x ⋅ OH + xOH‐

Sono-Fenton (Ultrasonic + Fe2+ + H2O2)

Non-ferrous Fenton solution (Ce/Al/Mn + H2O2)

H2O2 + hv → 2 ⋅ OH

(a) Fe2+ + H2O2 → Fe3+ + ⋅ OH + OH‐ (b) Fe3+ + H2O2 + hv → Fe2+ + ⋅ OH + OH‐ (c) H2O2 + hv → 2 ⋅ OH

Fe2+ + H2O2 → Fe3+ + ⋅ OH + OH‐

UV + H2O2

Photo-Fenton (UV + Fe2++H2O2)

Classical Fenton (Fe2++H2O2)

Key reaction pathway

Description of H2O2-based AOPs

Type of H2O2 solution

Table 5

(1) Enhances decomposition of H2O2 (2) Generate less solid waste

(1) Ways to generate ·OH is increased by the reaction of water and O2 (2) Fe2+ catalyst is easily regenerated

(1) Enhances decomposition of H2O2 (2) Reducing the generation of waste iron (3) Improved the efficiency of mercury removal (1) No solid waste (2) The reaction condition is easy to control

(1) The reaction conditions require lower (2) No mass transfer limitations

Advantages

(1) For some metals, the cost is high (2) For some metals, it will produce more sludge

(1) Consume more energy (2) Experimental conditions is complex

(1) UV consume more energy (2) The effectiveness is lower than photo-Fenton solution

(1) UV consume more energy (2) Impact factors is increased, reaction is instability

(1) Progress of the reaction is difficult to control (2) Large iron-containing sludge

Disadvantages

Wen et al. 2011 Chen et al. 2012 Nidheesh et al. 2013 Watts et al. 2005 Zhao et al. 2006 Rodriguez-Perez et al. 2013

Kang et al. 2002Han et al. 2007

Liu and Wang, 2014 Liu and Wang, 2014 Bokare and Choi, 2009

Ashraf et al. 2006 Bigda 1995 Adewuyi 2005

Liu et al. 2013 Ding et al. 2014 Bigda 1995 Bigda 1996 Nesheiwat and Swanson, 2000 Mosteo et al. 2007 Venkatadri and Peters, 1993

References

Environ Sci Pollut Res

Environ Sci Pollut Res

exhibit multiple oxidation states and can be easily regenerated from an inactive form through a simple redox cycle (Bokare and Choi, 2009). Many metals have been used as catalysts, such as aluminum (Al), cerium (Ce), chromium (Cr), and manganese (Mn). Among the researched metals, Ce shows an excellent catalytic performance in non-ferrous Fenton solution (Heckert et al. 2008; Mamontov et al. 2000; Wang et al. 2014a). Cerium (Ce) is a rare earth element which can exhibit both + 3 and + 4 oxidation states in solution (Heckert et al. 2008). CeO2 is widely used in wet oxidation for the presence of oxygen vacancies which can enhance the catalytic activity (Mamontov et al. 2000). Wang et al. 2014a) reports that the pretreatment of CeO2 by sulfation can improve the catalytic activity of CeO2/H2O2 system. Electron transfer from Ce3+ is initiated under the function of the protonation of peroxide species in the presence of sulfate groups, which acts as acidic sites (Wang et al. 2014a; Ji et al. 2010). The reaction principle is shown in Fig. 7. Moreover, it will enhance the decomposition of H2O2 and the removal efficiency of Hg0 will be improved. When the amount of catalyst exceeds the optimal concentration for H2O2 decomposition, removal rate of Hg0 will not change (Hua et al. 2010; Wen et al. 2011; Chen et al. 2012; Nidheesh et al. 2013; Bautista et al. 2008; Bigda 1995, 1996; Nesheiwat and Swanson, 2000; Mosteo et al. 2007). In Table 5, some H2O2-based advanced oxidation processes (AOP), their individual characteristic, and fundamentals of solutions are all summarized in detail. From Table 5, it can be found that the key point of Hg0 removal by H2O2 system is to enhance the decomposition of H2O2 and produce ·OH with suitable concentrations, which can be greatly influenced by both amount and recycling of the catalyst used in H2O2 system (Chen et al. 2012). Though ultraviolet light can achieve a good result, the cost on energy for generating ultraviolet is too high. Moreover, the ultraviolet is harmful to the health of workers and related setups. Therefore, it is still hard work to find a suitable method to catalyze the decomposition of H2O2 and the most desirable method should be economic and technically feasible; meanwhile, there is no secondary pollution generation (Mosteo et al. 2007; Venkatadri and Peters, 1993; Adewuyi 2005; Bokare and Choi, 2009; Han et al. 2007; Watts et al. 2005; Zhao et al. 2006; Rodriguez-Perez et al. 2013; Wu et al. 2008).

Conclusions Purifying Hg0 from coal-fired flue gas has been one of the most important environmental issues worldwide in the twenty-first century. Up to now, there is no established technology which is suitable for large-scale applications for removing Hg0 from flue gas. The method of wet flue gas desulfurization is a new idea to remove Hg0, and the Fenton

solution is found to be superior to the others oxidants for removing Hg0 from flue gas in industrial applications. This technology has obvious advantages in simultaneously purifying multi-pollutants such as SO2, NOx, and Hg0, and can be widely used for its high pollutant removal efficiency and low energy consumption. Engineering applications only need to improve the equipment on the basis of the original, which can save a lot of investment costs. Meanwhile, HCl in flue gas has clear auxo-action on the oxidation and purification of Hg0 and it can effectively enhance the efficiency of mercury removal especially when the chloride ion concentration is controlled in a reasonable range in the flue gas. In the future, the further study is suggested focus on the increasing of the removal efficiency of multi-pollutants and lowing its cost. Finally, the reduction of Hg2+ should be avoided as much as possible when Hg0 is oxidized and purified from flue gas, and it is another related important research field on purifying Hg0. Funding This work was supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. U1560110), the National Key R&D Program of China (No. 2017YFC0210301), Funds of Beijing Science and Technology (D161100004516001), and Fundamental Research Funds for the Central Universities.

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