Removal of NO in flue gas simulated by the Fe2+/Cu2+-activated double oxidant system

ABSTRACT $\cdot {\rm OH}$⋅OHThe wet denitrification technology has a good development prospect due to its simple system and mild reaction conditions, and related research has become a hot topic in the field of flue gas purification. In this work, a novel simultaneous removal technology of NO from flue gas using Fe2+/Cu2+-catalytic H2O2/(NH4)2S2O8 system was developed for the first time. The feasibility of this new flue gas cleaning technology was explored through a series of experiments and performance analyses. The mechanism of oxidation products, free radicals and simultaneous removal of NO was revealed. The effects of the main process parameters on the removal of NO were investigated. Relevant results demonstrated that the removal efficiency of NO was elevated when the concentration of (NH4)2S2O8 or reacting temperature increased, while it was decreased after increasing the raising of Fe2+, Cu2+ and H2O2 concentrations. The main radicals were and·${\rm SO}_4^-$SO4−, using the electron spin resonance technique in the solution, and played a very important role in NO removal. The main products were carried out by ion chromatography and elemental N material accountancy, and the results showed that it was sulfate and nitrate in the solution, which provided theoretical guidance for the subsequent treatment and resource utilization of the absorption solution. The results of the study provided a theoretical basis for the industrial application of wet denitrification. Highlights A new green process of NO removal by a wet process with Fe2+/Cu2+ activated (NH4)2S2O8 system is proposed in this paper; Elimination mechanisms and paths of NO are elucidated; The synergistic role produced by Cu2+ and Fe2+ is beneficial to the purification of NO; The synergistic role produced by (NH4)2S2O8 and H2O2 increased the concentration of free radicals in the solution; This process jointly considers the enhanced removal of NO and recycling of transition metal ions. GRAPHICAL ABSTRACT


Introduction
With the rapid development of industries, in which the combustion of the fossil has brought about increasingly serious air pollution, due to the immoderate emission of nitrogen oxides (mainly NO and NO 2 ).Nowadays, industrial waste gas treatment, energy saving and emission reduction have become the top priority of environmental governance.NO, as the main component of nitrogen oxides (NO x ), accounts for 90% to 95% of total emissions [1].Additionally, NO has extremely low solubility in water, which is only 5.6 × 10 −5 g/ml under normal temperature and pressure [2].In conclusion, NO, as a key pollutant for NO x treatment, has become the focus of research in this field.
At present, there is the main flue gas denitrification technology of dry method and wet method in industrial applications.Selective catalytic reduction (SCR) among dry denitrification is mature, efficient, and current leading technology for industrial denitration at home and abroad.There are some shortcomings of the SCR technology, such as high cost, disposal of hazardous catalyst waste, and high catalyst activity temperature window.For wet flue gas denitrification technology, many gas-phase oxidants, such as Cl 2 , ClO 2 , O 3 [3] and liquid-phase oxidants, such as KMnO 4 , NaClO 2 , NaClO, Na 2 S 2 O 8 , HClO 3 [4][5][6], etc, have been applied in the elimination of NO.Although a variety of wet methods to remove nitrogen oxide have been developed, the ever-increasing chemical costs and subsequent production of waste solutions have limited the industrial application of wet denitrification technologies.
In recent years, the advantages of advanced oxidation process (AOPs) primarily based on persulphate (S 2 O 2− 8 ) in exhaust gas treatment have emerged, •SO − 4 which refer to activating S 2 O 2− 8 to generate •SO −  4 and •OH with the strong oxidizing ability through various activation methods such as thermal [6], ultraviolet light [7], electricity [8], sound [9], and metal [10], the catalytic activation by transition metal is considered to be the simpler way [11].Among the many transition metal ions, Fe 2+ not only has excessive catalytic performance but also has the advantages of low price, rich reserves, and non-toxic and environmental protection.Therefore, the Fe 2+ /S 2 O 2− 8 system has been widely studied.Sakyi et al. [10,12] studied the removal of NO x through high temperature and Fe 2+ -activated sodium persulphate.There are some inherent disadvantages of Fe 2+ , such as rapid accumulation of Fe 3+ , Fe 2+ exists under strict conditions (pH ≤ 4) and slow conversion of Fe 3+ to Fe 2+ , which limit the application of the Fe 2+ /S 2 O 2− 8 system in different fields.On the one hand, the (NH 4 ) 2 S 2 O 8 -water is acidic.On the other hand, the products of NO absorption by (NH 4 ) 2 S 2 O 8 -water are HNO 3 , H 2 SO 4 and small amounts of HNO 2 .The above reasons lead to the system being able to maintain acidic conditions during the reaction.The experiment had been barring any phosphate buffer or other additive to exchange the pH value of persulphate solution, which is not only convenient for absorbing solution treatment, but also can reduce the material cost.
Earlier studies had revealed that the reduction of Fe 3+ to Fe 2+ could be accelerated efficiently by adding Cu 2+ [13,14] [11].Hence, (NH 4 ) 2 S 2 O 8 was chosen in this study.Besides, related results showed [15] that adding an appropriate amount of H 2 O 2 could increase the free radical yield of S 2 O 2− 8 -based activation systems by hindering the quenching of free radicals in the solution, which was beneficial to increase NO removal efficiency.
In order to make the Fe 2+ /Cu 2+ -activated double oxidant system more practical for NO removal, the effects of different experimental parameters on NO simultaneous removal were also studied.The radical species and products of the denitrification reaction were analyzed by radical capture technique and ion chromatography, and the mechanism of the denitrification reaction was initially discussed, to provide a theoretical basis for the industrial application of wet denitrification.

Experimental installation
The flow chart of experimental installation (Figure 1) for the NO oxidative absorption consisted of a bubbling scrubber made of a silicate glass (5.0 cm i.d.50-cm length and an aeration head with a diameter of 10 mm and an aperture of μm were installed at the bottom), simulation flue gas blending unit consisting of several fluid metres (Shanxi OPEC Instrument Group Co., Ltd.), high purity gas cylinders (NO, N 2 ), a mixing tube, and analytical and post-processing unit consisting of data acquisition unit, flue gas measuring instrument (Cairn 9206) and a terminal purifier.The NO purification process was operated based on a semi-batch mode (i.e. the simulated gas continuously flowed upward through stationary scrubbing solution).The absorbed liquid volume used to be 1 L, and the simulated flue gas flow was 1 L/min [16].

Experimental procedures
Before the experiment, nitrogen was used to purge the pipeline and reactor for 2 min; Open the pressure-reducing valve and NO and N 2 enter the bypass to be detected by the flue gas analyzer (3)(4).The three gases were thoroughly mixed in a buffer tank (5).The configured purification liquid is heated to the set temperature through the constant temperature water bath heating pot (9), and the valve is switched to the main road to simulate the flue gas into the bubbling reactor (8), and the aeration head is evenly distributed to the liquid phase.The gas exiting the reactor is first passed through a gas buffer bottle (10) and then through a gas drying device (11) with a solid dryer for prior analysis to remove moisture.Data were recorded when the simulated flue gas after the reaction flowed through the flue smoke detection analyzer (6).The exhaust gas could be further processed through the exhaust gas absorber (7).

Analytical method
The smoke detection analyzer (Cairn 9206) was used to determine the inlet and outlet concentrations of NO.The ion chromatography (IC, ICS-1100) was used to determine the NO − 2 and NO − 3 in a solution.The electron spin-resonance (MS5000X) spectrometer was used to detect •SO − 4 and •OH by combining with 5,5-dimethyl-1-pyrroline Noxide (DMPO) as a spin-trapping agent to trap inorganic radicals.The UV spectrophotometer (Agilent Cary5000) was used to detect the concentration of Fe 2+ in the solution.

Analytical and data handling methods
The removal efficiency of NO was defined by the following formula (1): where C in and C out were the steady-state concentrations [in parts per million (ppm)] of NO or as recorded from the flue gas analyzer at the inlet and outlet of the Bubble Column Reactor, respectively.The C in and C out are mean concentrations in 30 min.

N element material balance analysis formula
In order to further verify the oxidation and absorption reaction path of gas-phase NO in the liquid phase, on the basis of the detection results of ion chromatography and according to the conservation law of molar mass of N element in both gas and liquid phases, the concentration of NO − 3 ion in the liquid phase can be calculated by the following formula.
C calthe concentration of NO − 3 in the liquid phase, mg/ L; η -NO removal efficiency; C ininlet concentration of NO, mg/L; Q -flue gas flow, L/min; treaction time, min; M 1the Molar mass of NO − 3 , g/mol; M 2the molar mass of NO, g/mol; V Lreactive liquid volume, L.

Experiment results and discussion
3.1.Effect of (NH 4 ) 2 S 2 O 8 concentration and temperature on NO removal efficiencies As shown in Figure 2, the effect of initial (NH 4 ) 2 S 2 O 8 concentration (0.05, 0.1, 0.2, 0.3, 0.4 mol/L) on the removal efficiency of NO at different temperatures was investigated.The results revealed that in different concentration ranges, the concentration of (NH 4 ) 2 S 2 O 8 exhibited two different effects on removing NO.When it was below 0.2 mol/L, a risein the concentration of (NH 4 ) 2 S 2 O 8 obviously promoted the NO elimination.But when it exceeded 0.2 mol/L, the NO elimination was almost unaffected.
On the one hand, (NH 4 ) 2 S 2 O 8 will be ionized in the aqueous solution to produce a peroxydisulphate (S 2 O 2− 8 ), S 2 O 2− 8 /SO 2− 4 oxidation-reduction potential up to +2.01 V, which contributed to oxidation reactions between NO in the flue gas and S 2 O 2− 8 (Equation ( 3)) [17].On the other hand, the increase of (NH 4 ) 2 S 2 O 8 concentration can result in more formation of oxidative free radicals (•SO − 4 and •OH, etc.) (Equations ( 4)-( 12)) [17].which was advantageous for improving NO removal efficiency.Excessive (NH 4 ) 2 S 2 O 8 would lead to increased mass transfer resistance and the aggravation of side reactions (Equations ( 13)-( 15)) [18], which were unfavourable for the NO removal.At the same time, it also increased the operating cost of the system.Therefore, the concentration of (NH 4 ) 2 S 2 O 8 was set at 0.2 mol/L for subsequent experiments.
The influence of reacting temperature on NO elimination was researched, and the test result is depicted in Figure 2.Under normal temperature conditions (20°C), except for S 2 O 2− 8 itself, the free radical production only relies on the activation of the metal ion.So the production rate of •SO − 4 and •OH was slow, resulting in only 28.15% for NO removal efficiency.As the temperature rose to 60°C, the removal efficiency of NO increased up to 56.84%, when the reacting temperature exceeded 60°C, a rase of reacting temperature dropped clearly the efficiency of NO elimination.
These results were conducted by the break of O-O bond in S 2 O 2− 8 at the higher temperature and produced more •SO − 4 Equation ( 4), which was confirmed by previous studies [6].The rates of mass transfer and the chemical reaction will be improved by intensifying the effective collision of molecules and radicals at a high temperature.The high mass transfer rate promotes the removal of NO.The increase in temperature will not be conducive to the dissolution of NO and thus to the removal of NO.Therefore, the solution temperature was set at 60°C for subsequent experiments because the actual reaction temperature of the limestone-based wet flue gas desulphurization (Ca-WFGD) is 55-60°C [19], and the existing equipment can be used to heat the solution.

Effect of Fe 2+ concentration on NO removal efficiencies
An experiment was carried out to investigate the influence of Fe 2+ concentrations on removing NO using a heat/Fe 2+ -catalytic (NH 4 ) 2 S 2 O 8 wet scrubbing process, and the experimental result is depicted in Figure 2. As can be seen from Figure 2, when the Fe 2+ concentration was raised from 0 to 0.01 mol/L, the NO removal efficiency increased from 30.38% to 56.24%.This phenomenon was due to the increase in the concentration of Fe 2+ in the liquid phase, which increases the contact chance between (NH 4 ) 2 S 2 O 8 and Fe 2+ , through a series of reactions (Equations ( 4), ( 6), ( 13)-( 15)) [20] to promote the concentration of the two free radicals in the liquid phase to increase the NO removal efficiencies.As the Fe 2+ concentration continued to increase, the partial Fe 2+ was not all involved in the activation of (NH 4 ) 2 S 2 O 8 , but reacted with free radicals, reducing NO removal efficiencies (Equations ( 16) and ( 17)) [17].Therefore, in the activation process, it is necessary to ensure specific Fe 2+ concentration to activate (NH 4 ) 2 S 2 O 8 , and to ensure that Fe 2+ should not be excessive to avoid reduction of the two free radical reactions.The increase in the amount of free base is also accompanied by self-recombination and mutual binding between the free radicals (Equations ( 18)-( 20)) [18].Although the removal rate at 0.02 mol/L Fe 2+ was slightly higher than that at 0.01 mol/L Fe 2+ , it was twice that of 0.01 mol/L Fe 2+ .Therefore, this experiment decided to use 0.01 mol/L Fe 2+ to avoid subsequent iron separation problems.The precipitation of Fe 3+ was not observed under the (NH 4 ) 2 S 2 O 8 /Fe 2+ system because the aqueous solutions of (NH 4 ) 2 S 2 O 8 are in themselves acidic (at pH 2.45 ± 0.1 with 0.2 mol/L (NH 4 ) 2 S 2 O 8 concentration).
Both of the monovalent (Cu + ) and divalent (Cu 2+ ) oxidation states react easily with S 2 O 2− 8 (Equations ( 22) and ( 23)) [14,21].Nevertheless, the Cu 2+ ion was more stable and the generated free radicals may be mainly consumed by contaminants instead of copper ions compared to ferrous ions [22].The (NH 4 ) 2 S 2 O 8 /Fe 2+ system exhibits two-stage reaction kinetics, fast at the beginning, and followed by a slow stage, which is consistent with previous research results [23].
The reason is that the concentration ratio of (NH 4 ) 2 S 2- O 8 and Fe 2+ in the solution is much larger than the stoichiometric ratio of the reaction, which leads to the change of (NH 4 ) 2 S 2 O 8 concentration in the reaction process that can be ignored.Under such conditions, Fe 2+ is rapidly transformed into Fe 3+ , and Fe 3+ almost does not react with (NH 4 ) 2 S 2 O 8 , so it is difficult for Fe 2+ to be reduced back, while the generation of •SO − 4 and •OH mainly depends on the concentration of Fe 2+ , so the curve of NO concentration shows such changes.
When Cu 2+ concentration increased from 0 to 0.02 mol/L, the NO purification rate increased from 45.61% to 56.24%, and the overall NO concentration curve tends to be flat, which means that the addition of Cu 2+ makes the Concentration of Fe 2+ in (NH 4 ) 2 S 2 O 8- /Fe 2+ system tends to be stable.With the increase of Cu 2+ concentration in the (NH 4 ) 2 S 2 O 8 /Fe 2+ system, more and more Fe 2+ could cooperate with Cu 2+ , which promoted the production of •SO − 4 and •OH.As shown in Figure 3, when the H 2 O 2 concentration was raised from 0 to 0.9 mol/L, the NO removal rate was raised from 56.24% to 81.25%.This phenomenon could be explained as follows: the heat from part of  24)-( 34)) [27].The addition of H 2 O 2 can result in the formation of peroxy-free radicals(•HO 2 , E θ = 1.60 eV) (Equations ( 30) and ( 32)) [27].32) and ( 33)) [15].Except that Cu + can directly induce H 2 O 2 /(NH 4 ) 2 S 2 O 8 solution to produce •SO − 4 and •OH.At the same time, relevant results revealed that Cu 2+ reacts quickly with •HO 2- (Equation ( 30)) [28,29].Cu 2+ could generate Cu + through its reaction with •HO 2 to affect the formation rate of •SO − 4 , •OH and the recovery rate of Fe 2+ , thereby affecting the NO removal efficiency of the entire reaction system.
In the combined H 2 O 2 /(NH 4 ) 2 S 2 O 8 system, the concentration of H 2 O 2 exceeded 0.9 mol/L which led to generating more radicals and increased the whole concentration of the free radicals in the aqueous solution.However, more radicals would also be consumed and the reaction system was in a strong acid condition (Equations ( 35)-( 38)) [30][31][32].The decrease in the pH value of the solution can accelerate the decomposition of (NH 4 ) 2 S 2 O 8 but does not increase the amount of reactive free radicals [6].These reasons will ensure the NO removal efficiency does not increase with the increase of H 2 O 2 concentration but slightly decreases.
4. Probe into the reaction mechanism (H)) have been illustrated in Figure 4.
In the present work, Cu 2+ accelerated the conversion of Fe 3+ to Fe 2+ , thus facilitating the elimination of NO during the wet scrubbing of the Fe 2+ /Cu 2+ -catalyzed H 2 O 2 /(NH 4 ) 2 S 2 O 8 system.In order to speculate on the specific role of Cu 2+ in the Fe 2+ /Cu 2+ -catalytic H 2 O 2 / (NH 4 ) 2 S 2 O 8 system.It is necessary to detect the concentration of Fe 2+ in each system.
As shown in Figure 5, due to the high reaction rate between Fe 2+ and free radicals, Fe 2+ is consumed in a short time and then kept at a relatively low concentration in unit (A).With the addition of Cu 2+ in unit (B), Fe 2+ content increased from 0.39 to 28.66 mg/L.Partly, only part of Fe 2+ can be reduced back after Fe 2+ was oxidized to Fe 3+ (Equation ( 15)); Partly, Cu 2+ reacts with peroxymonosulphate (HSO − 5 ) to form Cu + , which increased the concentration of Fe 2+ in the solution (Equation ( 21)).As a result, the removal rate of unit (B) increased by 10.63% compared with that of unit (A).In unit (C), with the addition of Cu 2+ , the concentration of Fe 2+ began to rise, and the removal effect of NO increased from 21.59% to 36.51%.This phenomenon could be explained as follows: Cu 2+ readily reacts with •HO 2 (Equation ( 30)) and converts to a more reductant Cu + , which then reacts with Fe 3+ or two oxidants to produce free radicals (Equations ( 21), (22), and ( 28     radicals in the solution besides •SO − 4 and •OH were the predominant radicals responsible for the NO removal.

Product and reaction mechanism
Table 1 shows the mass balance of nitrogen in NO was calculated and the theoretical value was in good agreement with the calculated value (the calculation approach is described in more detail in the reference [34]).The results further showed that NO was mainly oxidized to NO − 3 .At the same time, S 2 O 2− 8 eventually converted to SO •HO 2 ) and other oxidants such as O 2 within the solution.Different oxides can oxidize NO at the same time, and the final products of the system are (NH 4 ) 2 SO 4 and (NH 4 ) 2 NO 3 .The removal mechanism of NO can also be described more intuitively by Figure 6.

Potential application of this technology
The advanced oxidation technology based on persulphate has been shown to achieve simultaneous removal of multiple components (NO, SO 2 , Hg 0 , etc.) from flue gas [4,35,36].The inlet temperature of the flue gas in the tower is about 110-180°C and it is also possible to heat the solution using the old Ca-WFGD system [37,38], which ensures that the solution can be in the optimal temperature range.The pH is adjusted by adding ammonia to remove metal ions from the solution.According to the results of the above analysis, the final products are (NH 4 ) 2 SO 4 and (NH 4 )NO 3 , which can be reused as resources through evaporation equipment.The present research results provide new purification techniques for small-and medium-sized boiler flue gas purification.The next step requires further research on a large number of key technical and engineering issues (e.g.selection of absorption equipment, long-term operation of the equipment, kinetic modelling, and effects of other flue gas components such as fly ash particles), which will effectively facilitate the scale-up and application of this technology.

Conclusion
A novel Fe 2+ /Cu 2+ -catalytic H 2 O 2 /(NH 4 ) 2 S 2 O 8 system was also developed for the removal of NO from flue gas.
The results found that the removal efficiency of NO was elevated when the concentration of (NH 4 ) 2 S 2 O 8 or reacting temperature increased, while it was decreased after increasing by rasing Fe 2+ , Cu 2+ and H 2 O 2 concentrations.The coexistence of Fe 2+ /Cu 2+ not only has a good co-activation effect but also accelerates the transfer rate of Fe 3+ to Fe 2+ , thus enhancing the NO elimination.The addition of H 2 O 2 increased the number of radicals and enriched the oxide species in the solution, which eventually led to an increase in the NO purification rate to 81.25%.
The results of the study provide theoretical support for the research, optimization and industrial application of NO removal technology and they can be applied for the direct renovation of old equipment, which is expected to show high-efficiency, green process and industrial scale-up of wet flue gas denitrification technology.
H 2 O 2 decomposition promotes (NH 4 ) 2 S 2 O 8 activation to produce more •SO − 4 and •OH free radicals contributing to the formation of a reaction system with greater oxidizability, another part of unreactive H 2 O 2 takes part in the consummation and the regeneration of H 2 O 2 , •OH •SO − 4 , •HO 2 and O 2 simultaneously by (Equations (

H
)). Unit (F) and unit (G) show that both Fe2+ and Cu 2+ have the ability to activate H 2 O 2 /(NH 4 ) 2 S 2 O 8 .The addition of H 2 O 2 increased the number of radicals and enriched the oxide species in the solution, which eventually led to an increase in the NO purification rate to 81.25%.The above studies indicate that Cu + plays an important role in the oxidation of NO in the Fe 2+ /Cu 2+ -catalytic H 2 O 2 /(NH 4 ) 2 S 2 O 8 system.On the one hand, it has a strong catalytic ability to decompose H 2 O 2 and (NH 4 ) 2 S 2- O 8 and directly promote the formation of •SO − 4 and •OH in the system.On the other hand, it indirectly promotes the formation of •SO − 4 and •OH in the system by reducing Fe 3+ , thus promoting the removal of NO.Adding an appropriate amount of H 2 O 2 could increase the free radical yield of S 2 O 2− 8 based activation systems, thus forming a stronger oxidation system, which was beneficial to increase NO removal efficiency.

α
H = 0.78 G) are similar to the data in the literature[14,33].The seven-line peaks in the figure are the typically mixed spectrum shapes of •SO − 4 and •OH measured by an ESR spectrometer, among which four circles represent •OH and three rectangles are •SO − 4 .Due to the overlap of peaks, the peak pattern of •OH free radicals approximately conforms to the characteristics of •OH(1:2:2:1), which also results in the characteristic peak of •SO − 4 that is not obvious.The results show that •SO − 4 and •OH are formed in the solution.The results showed that the presence of Fe 2+ /Cu 2+ -catalytic H 2 O 2 / (NH 4 ) 2 S 2 O 8 system did not change the types of free
. The S 2 O 2− 8 mainly include Na 2 S 2 O 8 , K 2 S 2 O 8 and peroxymonosulfate(PMS). (NH 4 ) 2 S 2 O 8 has a lower price than Na 2 S 2 O 8 , K 2 S 2 O 8 and PMS.Meanwhile, another advantage of (NH 4 ) 2 S 2 O 8 was that its denitrification products including (NH 4 ) 2 SO 4 and NH 4 NO 3 could be used as agricultural fertilizers The reaction rate of the hydrogen extraction reaction of •SO − 4 free radical with H 2 O to produce •OH radical was very slow (<3 × 10 3 M −1 s −1 ), so it was hard to quickly form •OH. Previous studies have shown that H 2 O 2 can be activated by Fe 2+ or Cu 2+ to produce •OH [13,25,26].In addition, H 2 O 2 and (NH 4 ) 2 S 2 O 8 may have several synergistic attributes rather than H 2 O 2 -activated (NH 4 ) 2 S 2 O 8 in the process of NO removal.

Table 1 .
Material balance of N elements.
2− 4 .Based on the above experimental results and discussions, the mechanism of wet NO removal by Fe 2+ /Cu 2 + -activated H 2 O 2 /(NH 4 ) 2 S 2 O 8 system can be summarized as follows: The Fe 2+ activated (NH 4 ) 2 S 2 O 8 to produce •SO − 4 and •OH with strong oxidizing ability in an acidic environment consisting of (NH 4 ) 2 S 2 O 8 hydrolysis.The addition of Cu 2+ accelerated the transfer rate of Fe 3+ to Fe 2+ .The addition of H 2 O 2 allowed the formation of more free radical species (•OH, •SO − 4 ,