Investigation on the removal of NO from marine exhaust gas using the Na₂S₂O₈-urea redox system in seawater carrier

Abstract

The sodium persulfate (Na₂S₂O₈)-urea system has been proven to be an excellent scrubbing solution for the wet removal of NO. Commonly, seawater is used as a wet carrier in marine applications. To further explore the feasibility of marine denitrification using Na₂S₂O₈-urea system, this study proposed the Na₂S₂O₈-urea-seawater composite redox system for NO removal from the marine exhaust gas. The effects of seawater carrier, reaction temperature, Na₂S₂O₈ concentration, urea concentration, pH value, and NO concentration on NO removal were investigated. Additionally, the NO₃⁻ concentration in the solution was measured. Results showed that the lowest normalized NO concentration was 0.099, with the corresponding mass of NO absorbed per unit volume of solution reaching 0.108 g·L⁻¹. The addition of seawater carrier and incremental reaction temperature, Na₂S₂O₈, and urea concentration promoted the NO removal performance. When the pH value increased within the range of 4–7, the NO removal performance decreased. The NO removal performance increased as the pH value further increased to 8, but decreased again when the pH value increased to 11. An increase in NO concentration was detrimental to NO removal. The Cl⁻, HCO₃⁻, and CO₃²⁻ in seawater could augment the total concentration of active free radicals to improve denitrification performance.

Graphical abstract

Keywords

marine diesel exhaust gas / denitrification / wet scrubbing / redox / seawater carrier

1 Introduction

Marine transportation is widely regarded as a crucial mode of international trade transportation due to its large cargo capacity, high traffic efficiency, and low cost [1]. Nevertheless, marine diesel engines utilizing heavy fuel oil as the primary fuel emit significant amounts of nitrogen oxides (NO_x), seriously endangering human health and the ecological environment. In response, the International Maritime Organization (IMO) has formulated a series of related regulations for strictly limiting NO_x emissions from marine exhaust [2]. At present, numerous studies have explored NO_x emission control technologies for ships, and several of them have already been successfully applied.

Wet scrubbing technology is deemed one of the most promising technologies for controlling marine pollutant emissions due to its economic efficiency and simple operation [3]. The abundance and accessibility of seawater make it the commonly employed carrier of wet scrubbing solutions in practical marine applications. However, the main obstacle is that more than 90% of NO_x in marine exhaust comprises water-insoluble NO, resulting in low water solubility of NO_x and limiting the effectiveness of the wet scrubbing technology for NO_x removal [4]. Relevant studies have shown that seawater could convert less than 10% of NO in the flue gas [5,6]. Although hydrodynamic cavitation could effectively promote denitrification under seawater conditions, it still cannot meet the NO emission standards from marine exhaust gas [7,8]. Consequently, existing commercial marine wet scrubbers using seawater could effectively remove SO₂ but not NO. Additionally, converting NO into water-soluble NO_x is the key to achieving effective denitrification through wet scrubbing technology. Therefore, when seawater is used for denitrification, a strong oxidizing agent should be added to seawater.

In recent years, various potent oxidants, such as H₂O₂ [9,10], KMnO₄ [11,12], O₃ [13,14], ClO₂ [15,16], NaClO [17,18], NaClO₂ [19,20], K₂S₂O₈ [21,22], and their mixtures [23−25] have been used in wet scrubbing systems for NO removal from flue gas, and the results demonstrated their effectiveness in NO removal. The presence of Cl⁻, CO₃²⁻, and HCO₃⁻ ions in seawater can impact the NO removal efficiency to some extent [26]. Seawater also has a natural acid-base buffering capacity, which can effectively stabilize pH levels during the scrubbing to prevent sharp declines due to acid generation [27]. Han et al. [28] and Lu et al. [29] investigated the NO removal using the NaClO₂ seawater solution and found that seawater with NaClO₂ had a significantly enhanced breakthrough time than freshwater with NaClO₂. The buffering ability of seawater could effectively suppress the acidic decomposition of NaClO₂ into ClO₂ and improve the utilization of NaClO₂ in the solution. The Cl⁻, HCO₃⁻ and CO₃⁻ ions in seawater could react with the oxidants to form multi-active inorganic free radicals (e.g., ·Cl, ·CO₃⁻, and ·Cl₂⁻), which might also play an important role in denitrification [30,31]. Some oxidants with high costs, high toxicity, and poor stability were unsuitable for NO removal from the marine exhaust due to the special environmental conditions and space constraints aboard ships. Sodium persulfate (Na₂S₂O₈), as an environmentally friendly potent oxidizing agent has the advantages of good stability, low cost, strong oxidation, non-toxicity, etc. Na₂S₂O₈ has been widely applied for soil remediation and the treatment of toxic organic gases [32]. For denitrification, Xi et al. [33] investigated the wet oxidative removal of NO using Na₂S₂O₈ in a bubble column reactor. The results showed that the 0.15 mol·L⁻¹ Na₂S₂O₈ solution at 80 °C completely absorbed the NO in simulated exhaust gas from marine diesel engine. The final products were SO₄²⁻ and NO₃⁻. The gas-phase resistance and liquid-phase resistance controlled the mass transfer process of NO absorption using the Na₂S₂O₈ solution. The increase in the gas-phase volume fraction could reduce the gas-phase resistance, and the increase in the concentration of oxidant, solution temperature, and pH value could reduce the liquid-phase resistance, thus improving the mass transfer performance [6]. It was inferred that Na₂S₂O₈ was a viable oxidant for removing NO from marine diesel exhaust using wet scrubbing, and the seawater carrier could promote the NO absorption in the Na₂S₂O₈-seawater solution.

Nevertheless, the oxidation process in wet scrubbing produces large amounts of NO₃⁻ as the final product. The excessive release of NO₃⁻ into the environment disrupts the nitrogen cycle, causing eutrophication, which can severely affect aquatic ecosystems [34]. IMO stipulates that the NO₃⁻ concentration in the washing waste liquid must be less than 60 mg·L⁻¹ or the amount of NO₃⁻ corresponding to the removal of 12% NO_x from the exhaust gas. Urea is a cost-effective and strong alkaline reducing agent, which is frequently used for removing SO₂ and NO from marine exhaust [35]. However, the studies on wet denitrification using urea are still in the exploratory stage, and the addition of oxidants could improve the denitrification efficiency of urea [36]. KMnO₄ [37,38], H₂O₂ [39,40], NaClO [37] and NaClO₂ [41] are the commonly used oxidants, with KMnO₄ having the best denitrification performance. The denitrification efficiency could reach 96.3% under the conditions of KMnO₄ concentration of 5 mmol·L⁻¹, reaction temperature of 70 °C, and urea concentration of 9% [37]. Meanwhile, NO_x was found to convert primarily to N₂, significantly reducing the concentration of NO₃⁻ and NO₂⁻ in the solution [42]. It was inferred that urea as a reducing agent could reduce high-valence nitrogen to N₂, effectively inhibiting NO₃⁻ formation [43]. Xi et al. [44] studied NO removal from marine exhaust using Na₂S₂O₈ as the oxidant in the Na₂S₂O₈-urea solution. The scrubbing solution with 0.1 mol·L⁻¹ Na₂S₂O₈ and 4.0 mol·L⁻¹ urea had a NO₃⁻ concentration of 14.93 mg·L⁻¹ at 80 °C, while the scrubbing solution without urea had a NO₃⁻ concentration of 194.54 mg·L⁻¹. When synergizing SO₂ and NO, the NO₃⁻ concentration was 8.56 mg·L⁻¹ at 70 °C with 0.2 mol·L⁻¹ Na₂S₂O₈ and 2 mol·L⁻¹ urea [45]. The addition of urea effectively reduced the NO₃⁻ emission in the wet scrubbing system. The Na₂S₂O₈-urea solution system showed excellent application prospects for NO removal from marine exhaust. However, the application and popularization of Na₂S₂O₈-urea system on ships are hindered since the denitrification performance of the Na₂S₂O₈-urea system in seawater remains underexplored, and the specific denitrification mechanisms are not yet fully understood. Therefore, studying the denitrification process of Na₂S₂O₈-urea redox system in seawater is imperative.

In this study, the Na₂S₂O₈-urea-seawater composite redox system was utilized to remove NO from marine diesel engine exhaust in a bubbling reactor. Thereinto, seawater, Na₂S₂O₈, and urea served as the solution carrier, oxidant, and reducing agent, respectively. The effects of various parameters, such as seawater carrier, reaction temperature, Na₂S₂O₈ concentration, urea concentration, initial NO concentration, and solution pH, on NO removal performance were systematically investigated. Additionally, for the Na₂S₂O₈-urea-seawater composite redox system, the corresponding NO₃⁻ concentration after the reaction was measured, and the denitrification mechanism was elucidated. Consequently, this study could provide further theoretical and technical support for the potential engineering application of Na₂S₂O₈-urea-seawater composite redox system.

2 Experimental

2.1 Experimental materials

The standard gases were purchased from Shandong Deyang Special Gas Co., Ltd., China, which included 99.999% N₂ and 1% NO (with N₂ as the balance gas). Na₂S₂O₈ was supplied by China Aladdin Industrial Corporation. Urea and magnesium chloride hexahydrate (MgCl₂·6H₂O) were provided by Shanghai Macklin Biochemical CO., Ltd., China, KCl was obtained from Tianjin Kermel Chemical Reagent Co., Ltd., China, CaCl₂, NaHCO₃, NaCl, and Na₂SO₄ were purchased from Sinopharm Chemical Reagent Co., Ltd., HCl and NaOH used to adjust the pH of the solution were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The primary components of the simulated seawater prepared according to the American ASTM1141 standard are shown in Tab.1 [46].

2.2 Experimental system

The experimental system for NO removal using the Na₂S₂O₈-urea-seawater composite redox system is depicted in Fig.1. The experimental system mainly included three parts: the simulated gas distribution system, the bubbling reactor absorption system, and the flue gas analysis and tail gas treatment system. The simulated gas distribution system included gas cylinders, mass flow meters, pressure-reducing valves, and gas mixers. The simulated mixed gas consisted of N₂ and NO, which were supplied by gas cylinders. The total flow rate was maintained at 1.0 L·min⁻¹, and the gas flow was accurately controlled using mass flow meters. Gas components were mixed in the gas mixer according to a predetermined ratio. The bubbling reactor absorption system primarily encompassed the plexiglass bubbling reactor and the constant temperature water bath. The inner diameter and height of the bubbling reactor were 60 and 500 mm, respectively. The gas distributor of the bubbling reactor facilitated the uniform distribution of mixed gas flow, and its diameter, height, and diameter of the inner hole were 20 mm, 30 mm, and 10 μm, respectively. The upper end of the bubbling reactor was equipped with a feeding hole, a mixed gas inlet, and a mixed gas outlet. The solids and simulated seawater could be introduced into the reactor through the feeding hole, from which the bubbling solution could be taken out. The temperature of bubbling solution and reactor is regulated by the constant temperature water bath. The flue gas analysis and tail gas treatment system included a gas dryer, gas analyzer (ECOM-EN2, ECOM GmbH, Germany), and tail gas treatment device. The gas analyzer was used to measure the NO concentration of the reaction system. To prevent corrosion or damage to the gas analyzer, granular anhydrous CaCl₂ was utilized to dry the mixed gas before entering the gas analyzer. The exhaust gas was emptied after being treated with KMnO₄/NaOH solution and activated carbon. The NO₃⁻ concentration in the liquid phase was determined by ion chromatography (ICS900, Dionex Corporation, USA). The pH value of the solution was measured by the pH meter (PHS-3E, Shanghai Yoke Instrument Co., Ltd., China).

2.3 Experimental method

For the experiments, 1.0 L of simulated seawater was precisely measured and poured into the bubbling reactor. Subsequently, the bubbling reactor was placed in the constant temperature water bath to preheat the mixed solution to the designated temperature. The gas flow was switched to the bypass before each experiment to obtain the initial NO concentration. When the NO concentration reached the target value and remained stable, the initial NO concentration was measured using the gas analyzer. The required amounts of Na₂S₂O₈ and urea solids were weighed, then injected into the bubbling reactor through the feeding hole, and dissolved completely in 1.0 L of simulated seawater. Subsequently, the pH value of the solution was adjusted to the required value. The mixed gas was then switched to the main route and entered the bubbling reactor by turning the valve. The reaction time was immediately recorded. After passing through the gas dryer, the mixed gas was analyzed using the gas analyzer. The gas analyzer readings were recorded every 5 min to monitor the outlet NO concentration. Each experiment lasted 90 min, and the total gas flow rate was maintained at 1.0 L·min⁻¹. The same experiment was repeated three times, and the obtained data was averaged before further analysis and calculation.

The normalized NO concentration, defined as the ratio of the outlet NO concentration to the inlet NO concentration, is expressed as shown in Eq. (1):

(1)\mathrm{N}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}\mathrm{N}\mathrm{O}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{C_{\mathrm{o}\mathrm{u}\mathrm{t}}}{C_{\mathrm{i}\mathrm{n}}},

where C_in and C_out represent the inlet and outlet concentration (ppm) of NO, respectively.

The mass of NO absorbed per unit volume of scrubbing solution could be calculated by Eq. (2):

(2)\mathrm{q}={\int }_0^{t}30\times Q\times \left(-C_{\text{out}}\right)/\left(22.4\times V\right)\text{d}t,

where Q represents the total gas flow rate (mL·min⁻¹); V represents the volume (L) of scrubbing liquid, t represents the reaction time (min).

3 Results and discussion

3.1 Effect of seawater carrier

Since seawater was generally used as a wet carrier in marine applications, the influence of seawater carrier on NO removal from marine exhaust gas using the Na₂S₂O₈-urea composite redox system was investigated and the results were illustrated in Fig.2. The NO concentration initially decreased and eventually stabilized as the experiment progressed. The normalized NO concentration of Na₂S₂O₈-urea-seawater composite redox system was 0.154, which was lower than that of Na₂S₂O₈-urea-deionized water solution (0.197). This indicated that the seawater carrier significantly enhanced the NO removal performance of the Na₂S₂O₈-urea composite system.

Na₂S₂O₈ could generate the sulfate radical (·SO₄⁻), and some of the ·SO₄⁻ were converted to the hydroxyl radical (·OH) in water through reactions (3) and (4). The Cl⁻, HCO₃⁻, and CO₃²⁻ ions in seawater increased the content of active free radicals in the Na₂S₂O₈-urea-seawater composite redox system, strengthening the NO removal [30]. Moreover, the alkaline buffer capacity of seawater could promote the treatment of the acidic gas NO_x. Therefore, the seawater carrier could increase the active free radical concentration in the N₂S₂O₈-urea-seawater composite redox system and provide an alkaline buffer system to promote NO removal.

(3){\text{S}}_2{\text{O}}_8^{2-}\to 2\cdot {\text{SO}}_4^{-}

(4)\cdot {\text{SO}}_4^{-}+{\text{H}}_2\text{O}\to {\text{SO}}_4^{2-}+\cdot \text{OH}+{\text{H}}^{+}

3.2 Effect of reaction temperature

The variation of NO concentration with time under different temperatures and the mass of NO absorbed per unit volume of Na₂S₂O₈-urea-seawater composite redox system were illustrated in Fig.3. The reaction temperature significantly impacted NO removal using the Na₂S₂O₈-urea-seawater composite redox system. The normalized NO concentration decreased from 0.867 to 0.143 as the reaction temperature rose from 20 to 70 °C. The equilibrium NO concentration gradually decreased with the increase in the reaction temperature. Correspondingly, as shown in Fig.3(b), the mass of NO absorbed per unit volume of solution increased from 0.012 to 0.095 g·L⁻¹ when the reaction temperature increased from 20 to 70 °C. This indicated that higher reaction temperature favored the NO removal using the Na₂S₂O₈-urea-seawater composite redox system.

The activation degree of Na₂S₂O₈ was limited when the reaction temperature was 20 °C, and S₂O₈²⁻ was the primary oxidizing substance. The oxidizing ability of S₂O₈²⁻ was weak, and the NO removal was primarily through reactions (5–7). However, reaction (7) could release NO again, leading to poor NO removal performance. As the temperature increased, the reaction rate of reaction (3) accelerated and the activation degree of Na₂S₂O₈ was gradually enhanced [6]. It led to a gradual increase in the concentration of active free radicals in the solution. As a result, the oxidation ability of the scrubbing solution increased, which strengthened the oxidation processes and achieved higher NO removal performance. The thermal activation of Na₂S₂O₈ usually occurred at the temperature greater than 60 °C [33,44,45]. Consequently, the NO removal performance was significantly enhanced when the reaction temperature was increased to 60 °C, which was consistent with Fig.3. Additionally, urea was hydrolyzed in an aqueous solution via reaction (8), which produced ammonium carbamate. The reaction rate of reaction (8) could increase with the increase in the reaction temperature. Compared to urea, ammonium carbamate was more likely to react with NO₂⁻ through reaction (9) to promote NO removal [45]. Hence, the NO removal performance of Na₂S₂O₈-urea-seawater composite redox system from marine exhaust gas was improved as the reaction temperature increased.

(5){\text{S}}_2{\text{O}}_8^{2-}+\mathrm{N}\mathrm{O}+{\text{H}}_2\mathrm{O}\to 2{\text{SO}}_4^{2-}+2{\text{H}}^{+}+{\text{NO}}_2

(6)2{\text{NO}}_2+{\text{H}}_2\text{O}\to {\text{HNO}}_2+{\text{HNO}}_3

(7)3{\text{HNO}}_2\to 2\mathrm{N}\mathrm{O}+{\text{HNO}}_3+{\text{H}}_2\text{O}

(8){\left({\text{NH}}_2\right)}_2\mathrm{C}\mathrm{O}+{\text{H}}_2\text{O}\to {\text{NH}}_2{\text{COONH}}_4

(9){\text{NH}}_2{\text{COONH}}_4+2{\text{NO}}_2^{-}+2{\text{H}}^{+}\to 2{\text{N}}_2+{\text{CO}}_2+4{\text{H}}_2\text{O}

3.3 Effect of Na₂S₂O₈ concentration

The Na₂S₂O₈ concentration was an important influencing factor for NO removal using Na₂S₂O₈-urea-seawater composite redox system. The variation of NO concentration with time during NO removal under different Na₂S₂O₈ concentrations was shown in Fig.4(a). When the Na₂S₂O₈ concentration increased from 0 to 0.1 mol·L⁻¹, the normalized NO concentration decreased from 0.974 to 0.154, indicating the continuous decrease in the NO equilibrium concentration. However, when Na₂S₂O₈ concentration continued to increase to 0.25 mol·L⁻¹, further increases in Na₂S₂O₈ concentration had slight effects, with the normalized NO concentration slightly decreasing to 0.142. Meanwhile, the mass of NO absorbed per unit volume of Na₂S₂O₈-urea-seawater composite redox system under different Na₂S₂O₈ concentrations was shown in Fig.4(b). The mass of NO absorbed per unit volume of solution increased from 0.007 to 0.095 g·L⁻¹ as the Na₂S₂O₈ concentration increased from 0 to 0.1 mol·L⁻¹. Subsequently, the mass of NO absorbed increased marginally to 0.101 g·L⁻¹ as the Na₂S₂O₈ concentration reached 0.25 mol·L⁻¹. It indicated that the NO removal performance initially increased and then remained stable with the increase in the Na₂S₂O₈ concentration. The optimal Na₂S₂O₈ concentration was determined to be 0.1 mol·L⁻¹.

The higher Na₂S₂O₈ concentration accelerated the reaction rate of reaction (3), increasing the active free radical concentration in the solution [33]. Thus, the oxidation ability of Na₂S₂O₈-urea-seawater composite redox system was strengthened. At Na₂S₂O₈ concentration below 0.1 mol·L⁻¹, the limited Na₂S₂O₈ addition restrained the generation of active free radicals. The interaction between the NO molecules and active radicals was weak, with a limited number of effective collisions between them. Therefore, an increase in the Na₂S₂O₈ concentration enhanced the interaction between NO molecules and active radicals and strengthened the NO removal performance. However, when the Na₂S₂O₈ concentration exceeded 0.1 mol·L⁻¹, increasing the Na₂S₂O₈ concentration did not significantly improve NO removal due to the constraints in the NO and urea concentrations. Therefore, the increased Na₂S₂O₈ concentration could increase the concentration of the effective oxidizing substances in the solution, which was beneficial for the NO removal using Na₂S₂O₈-urea-seawater composite redox system to a certain extent.

3.4 Effect of urea concentration

The influence of different urea concentrations on NO removal using Na₂S₂O₈-urea-seawater composite redox system was depicted in Fig.5. An increase in the urea concentration in the range of 0–0.5 mol·L⁻¹ decreased the normalized NO concentration from 0.455 to 0.154, and increased the corresponding mass of NO absorbed per unit volume solution from 0.057 to 0.095 g·L⁻¹. Consequently, this indicated that the addition of urea significantly promoted NO removal. However, the normalized NO concentration and the corresponding mass of NO absorbed per unit volume solution remained stable for a further increase in the urea concentration to 0.9 mol·L⁻¹. Therefore, the optimal urea concentration was identified as 0.5 mol·L⁻¹.

In the absence of urea, the NO₂⁻ generated during NO oxidation consumed the active free radicals via reactions (10) and (11), diminishing the oxidation ability of the composite solution [44]. In addition, NO₂⁻ would regenerate NO again through reaction (7), resulting in an increase of the outlet NO concentration. Therefore, the NO removal performance was weak without urea. When urea was added to the solution, urea consumed the intermediate products NO₂ and NO₂⁻ via reactions (12) and (13), thereby reducing the additional consumption of active free radicals and inhibiting the NO regeneration [47]. Meanwhile, urea could directly react with NO to reduce NO emission through reaction (14). Therefore, the addition of urea reduced the consumption of active free radicals and enhanced the direct removal of NO to strengthen NO removal. Furthermore, excess urea was more inclined to react with NO₂⁻, NO, and NO₂ rather than with Na₂S₂O₈, and thus the reaction between urea and Na₂S₂O₈ could be to some extent negligible, resulting in an increase in NO removal performance with increasing urea concentration [44,45]. When the urea concentration exceeded 0.5 mol·L⁻¹, the NO removal performance did not continue to increase due to the limitation of Na₂S₂O₈ and NO concentrations.

(10)\cdot {\text{SO}}_4^{-}+{\text{NO}}_2^{-}\to {\text{SO}}_4^{2-}+{\text{NO}}_2

(11)\cdot \text{OH}+{\text{NO}}_2^{-}\to {\text{NO}}_2+{\text{OH}}^{-}

(12){\text{6NO}}_2+4{\left({\text{NH}}_2\right)}_2\text{CO}\to {\text{7N}}_2+{\text{4CO}}_2+{\text{8H}}_2\text{O}

(13){\left({\text{NH}}_2\right)}_2\text{CO}+{\text{2NO}}_2^{-}+{\text{2H}}^{+}\to {\text{2N}}_2+{\text{CO}}_2+{\text{3H}}_{\text{2}}\text{O}

(14)6\mathrm{N}\mathrm{O}+2{\left({\text{NH}}_2\right)}_2\text{CO}\to {\text{5N}}_2+{\text{2CO}}_2+{\text{4H}}_{\text{2}}\text{O}

3.5 Effect of pH value

The effect of pH value on NO removal using Na₂S₂O₈-urea-seawater composite redox system was presented in Fig.6. As the initial pH value of the Na₂S₂O₈-urea-seawater composite redox system increased from 4 to 7, the normalized NO concentration increased slightly from 0.102 to 0.154, and the corresponding mass of NO absorbed per unit volume of solution decreased from 0.108 to 0.095 g·L⁻¹. The effect of the initial pH value of Na₂S₂O₈-urea-seawater composite redox system was not obvious. However, a further increase in the initial pH value to 8 decreased the normalized NO concentration, and the corresponding mass of NO absorbed reached 0.108 g·L⁻¹. With a subsequent increase in the initial pH value to 11, the normalized NO concentration increased to 0.273, while the corresponding mass of NO absorbed declined to 0.079 g·L⁻¹. Therefore, this suggested that the optimal NO removal performance was obtained when the initial pH value was set at 8.

Under acidic conditions, reactions (15)–(17) occurred. At the initial pH of 4, the acidic atmosphere facilitated the activation of S₂O₈²⁻, and reactions (15) and (16) were the primary reactions. At this point, reaction (18) enhanced the NO removal process [48]. As the initial pH increased for pH values less than 7, the activity of reactions (15) and (16) was weakened and that of reaction (17) was enhanced, which increased the consumption of Na₂S₂O₈ and reduced the oxidizing capacity of Na₂S₂O₈ with constant concentration. Urea could alter the acidity of solution through reaction (19), which weakened reactions (16) and (18). Consequently, the composite solution had a reduced oxidizing capacity. When the pH was increased in the range of 7–8, the reaction (20) occurred in a weakly alkaline environment, with a higher reaction rate than reaction (4). ·OH have superior oxidizing ability compared to ·SO₄⁻ [49]. Consequently, the rapid generation rate of ·OH increased the active free radical concentration and enhanced NO removal. As the pH continued to increase to 11, reaction (20) facilitated the NO removal, but the inert oxygen radicals (·O⁻) generated through reaction (21) in a strongly alkaline environment exhibited weaker oxidizing activity than ·OH [50]. Therefore, the NO removal performance was weakened when the pH increased from 8 to 11.

(15){\text{S}}_2{\text{O}}_8^{2-}+{\text{H}}^{+}\to {\text{SO}}_4+{\text{HSO}}_4^{-}

(16){\text{SO}}_4+{\text{H}}_2\text{O}\to {\text{H}}_2{\text{SO}}_5\to {\text{H}}^{+}+{\text{HSO}}_5^{-}

(17){\text{S}}_2{\text{O}}_8^{2-}+{\text{H}}_2\text{O}\stackrel{{\text{H}}^{+}}{\to }{\text{2H}}^{+}+{\text{2SO}}_4^{2-}+\frac{\text{1}}{\text{2}}{\text{O}}_{\text{2}}

(18)\text{2NO}+{\text{3HSO}}_5^{-}+{\text{H}}_2\text{O}\to {\text{2NO}}_2^{-}+{\text{3SO}}_4^{2-}+{\text{5H}}^{+}

(19){\left({\text{NH}}_2\right)}_2\text{CO}+{\text{H}}_2\text{O}+{\text{2HSO}}_4^{-}\to {\text{2NH}}_4^{+}+{\text{2SO}}_4^{2-}+{\text{CO}}_{\text{2}}

(20)\cdot {\text{SO}}_4^{-}+{\text{OH}}^{-}\to \cdot \text{OH}+{\text{SO}}_4^{2-}

(21)\cdot \text{OH}+{\text{OH}}^{-}\to \cdot {\text{O}}^{-}+{\text{H}}_2\text{O}

3.6 Effect of initial NO concentration

Fig.7 illustrated the variation of NO concentration with time under different NO concentrations during NO removal using the Na₂S₂O₈-urea-seawater composite redox system. The normalized NO concentration increased from 0.144 to 0.163 as the NO concentration rose from 400 to 1500 ppm. Although the increase in the NO concentration decreased the NO removal performance, the reduction was insignificant. It suggested that the Na₂S₂O₈-urea-seawater composite redox system could effectively handle a wide range of NO concentrations. The increase in the initial NO concentration enhanced the gas phase partial pressure of NO, which strengthened the gas-liquid mass transfer process and increased the number of gas molecules passing through the reactor per unit of time [51]. However, the oxidizing active groups in the solution were limited when Na₂S₂O₈ concentration, urea concentration, and reaction temperature were constant. The increase in the number of NO gas molecules decreased the relative molar ratio of oxidizing species to NO. Thus, as the NO concentration increased, the normalized NO concentration gradually increased and the NO removal performance decreased.

3.7 The NO₃⁻ concentration in Na₂S₂O₈-urea-seawater composite redox system

As illustrated in Fig.8, the NO₃⁻ concentration after the reaction was measured to investigate the effect of reaction conditions on the NO₃⁻ concentration in the Na₂S₂O₈-urea-seawater composite redox system. After the reaction, the NO₃⁻ concentrations in Na₂S₂O₈-urea-seawater composite redox system increased as the reaction temperature, Na₂S₂O₈ concentration, and NO concentration increased. Additionally, the NO₃⁻ concentrations in Na₂S₂O₈-urea-seawater composite redox system after the reaction decreased within the initial pH range of 4–7. Conversely, the NO₃⁻ concentrations increased as the initial pH value increased to 8, and the NO₃⁻ concentration further decreased as the initial pH value increased to 11. Notably, the Na₂S₂O₈-urea-seawater composite redox system had a higher NO₃⁻ concentration after reaction than the Na₂S₂O₈-urea-deionized water composite redox system. The changing trend of the NO₃⁻ concentration closely mirrored the NO removal performance of the reaction system. This was primarily because the oxidation capacity of Na₂S₂O₈-urea-seawater composite redox system was influenced by reaction temperature, Na₂S₂O₈ concentration, pH value, NO concentration, and solution carrier at a constant urea concentration. Under the condition of certain urea concentration, the reducing ability of urea was limited. The solution had a higher concentration of NO₃⁻ generated by NO oxidation when the denitrification ability of Na₂S₂O₈-urea-seawater composite redox system was stronger.

The urea concentration constrained the reduction process of the solution. Specifically, higher urea concentrations reduced NO₃⁻ concentrations in the Na₂S₂O₈-urea-seawater composite redox system, which could be attributed to two factors. On the one hand, the increased urea concentration promoted the consumption of NO₃⁻ through reaction (22). On the other hand, the reduction of NO₂⁻, NO₂, and NO reduced the production of NO₃⁻ via reactions (12–14) [45]. Therefore, urea could effectively inhibit NO₃⁻ formation, ensuring that the NO₃⁻ concentration in Na₂S₂O₈-urea-seawater composite redox system met the relevant emission standards (< 60 mg·L⁻¹).

(22)5{\left({\text{NH}}_2\right)}_2\text{CO}+{\text{6NO}}_3^{-}+{\text{6H}}^{+}\to {\text{8N}}_2+{\text{5CO}}_2+{\text{13H}}_2\text{O}

3.8 Denitrification mechanism of Na₂S₂O₈-urea-seawater composite redox system

The schematic diagram of the denitrification mechanism using Na₂S₂O₈-urea composite redox system is shown in Fig.9.

The ionization of Na₂S₂O₈ in water could produce S₂O₈²⁻, which could decompose into ·SO₄⁻ via reaction (3). Furthermore, some ·SO₄⁻ ions generated ·OH through reaction (4). The radicals exhibited higher oxidation potential than S₂O₈²⁻ [33,52]. The generated ·SO₄⁻ and ·OH reacted efficiently with NO to form NO₂⁻ via reactions (23) and (24). Subsequently, ·SO₄⁻ and ·OH oxidized the NO₂⁻ to form NO₂ and NO₃⁻, thus contributing to the NO removal from marine exhaust gas via reactions (10), (11), (25), and (26). Simultaneously, S₂O₈²⁻ could directly oxidize NO to remove NO through reaction (5).

(23)\cdot {\text{SO}}_4^{-}+{\text{H}}_2\mathrm{O}+\mathrm{N}\mathrm{O}\to {\text{SO}}_4^{2-}+{\text{NO}}_2^{-}+{\text{2H}}^{+}

(24)\cdot \text{OH}+\text{NO}\to {\text{NO}}_2^{-}+{\text{H}}^{+}

(25)\cdot {\text{SO}}_4^{-}+{\text{NO}}_2+{\text{H}}_2\text{O}\to {\text{SO}}_4^{2-}+{\text{NO}}_3^{-}+{\text{2H}}^{+}

(26)\cdot \text{OH}+{\text{NO}}_2\to {\text{NO}}_3^{-}+{\text{H}}^{+}

It should be noted that Cl⁻, HCO₃⁻, and CO₃²⁻ in seawater could react with ·SO₄⁻ and ·OH, which affect the distribution pattern and selectivity of active free radicals [30,53]. Cl^– could produce active ·Cl and ClO₃⁻ via reactions (27–29), augmenting the overall concentration of active free radicals in the solution. Subsequently, ·Cl and ClO₃⁻ could efficiently react with NO through reactions (30–33) [30]. It was inferred that ·Cl and ClO₃⁻ had a good oxidation effect on NO, which improved NO removal performance. Furthermore, the CO₃²⁻ and HCO₃⁻ could be complexed with the ·OH and ·SO₄⁻ radicals to form ·CO₃⁻ with sufficient oxidation potential via reactions (34–37).

(27)\cdot {\text{SO}}_4^{-}+{\text{Cl}}^{-}\to {\text{SO}}_4^{2-}+\cdot \text{Cl}

(28)6\cdot {\text{SO}}_4^{-}+{\text{Cl}}^{-}+{\text{3H}}_2\text{O}\to {\text{ClO}}_3^{-}+6{\text{H}}^{+}+6{\text{SO}}_4^{2-}

(29)\cdot \text{OH}+{\text{Cl}}^{-}\to {\text{OH}}^{-}+\cdot \text{Cl}

(30)\text{NO}+2\cdot \mathrm{C}\mathrm{l}+{\text{H}}_2\text{O}\to {\text{NO}}_2+{\text{2Cl}}^{-}+{\text{2H}}^{+}

(31)3\mathrm{N}\mathrm{O}+{\text{ClO}}_3^{-}\to {\text{3NO}}_2+{\text{Cl}}^{-}

(32){\text{NO}}_2+\cdot \mathrm{C}\mathrm{l}+{\text{H}}_2\text{O}\to {\text{NO}}_3^{-}+{\text{Cl}}^{-}+{\text{2H}}^{+}

(33){\text{6NO}}_2+{\text{ClO}}_3^{-}+{\text{3H}}_2\text{O}\to {\text{6NO}}_3^{-}+{\text{Cl}}^{-}+{\text{6H}}^{+}

(34)\cdot {\text{SO}}_4^{-}+{\text{CO}}_3^{2-}\to \cdot {\text{CO}}_3^{-}+{\text{SO}}_4^{2-}

(35)\cdot {\text{SO}}_4^{-}+{\text{HCO}}_3^{-}\to \cdot {\text{CO}}_3^{-}+{\text{SO}}_4^{2-}+{\text{H}}^{+}

(36)\cdot \text{OH}+{\text{CO}}_3^{2-}\to \cdot {\text{CO}}_3^{-}+{\text{OH}}^{-}

(37)\cdot \text{OH}+{\text{HCO}}_3^{-}\to \cdot {\text{CO}}_3^{-}+{\text{H}}_2\mathrm{O}

The presence of urea in Na₂S₂O₈-urea-seawater composite redox system altered the conversion path of element N during the denitrification of Na₂S₂O₈. On the one hand, urea reduced the NO₃⁻ to N₂ through reaction (22), decreasing the NO₃⁻ concentration in the solution. On the other hand, the urea could react with the intermediates (NO₂⁻ and NO₂) of the NO oxidation process through reactions (12) and (13), which reduced NO₃⁻ production [44]. Additionally, urea in the composite solution could be hydrolyzed to produce ammonium carbamate through reaction (8), which could further react with NO₂⁻ through reaction (9). The urea could also directly react with NO via reaction (14), avoiding the formation of NO₃⁻. Therefore, from the perspective of mechanism analysis, Na₂S₂O₈-urea-seawater composite redox system can effectively reduce the emission of NO and NO₃⁻ on ships.

4 Conclusions

The Na₂S₂O₈-urea-seawater composite redox system was proposed as an effective scrubbing solution for NO removal from marine exhaust gas. The NO removal performance was systematically studied under different solution carriers, reaction temperatures, Na₂S₂O₈ concentrations, urea concentrations, pH values of the solution, and NO concentrations. The experimental results revealed that the seawater carrier was advantageous for NO oxidation. Additionally, the seawater carrier increased the active free radical content and provided an alkaline buffer system to promote NO removal. The increase in reaction temperature and Na₂S₂O₈ concentration could augment the active free radical concentration in Na₂S₂O₈-urea-seawater composite redox system to strengthen the NO removal performance. The addition of urea reduced the consumption of active free radicals and enhanced the direct NO removal, with the overall NO removal performance increasing in proportion to urea concentration. Furthermore, the NO removal performance decreased as the pH values increased within the pH ranges of 4–7 and 8–11. However, the pH values in the range of 7–8 could promote NO removal. The limitation of oxidizing active groups weakened the NO removal performance as the NO concentration increased. The addition of urea decreased significantly the NO₃⁻ concentration, allowing it to meet emission standards over an extended period. It was clear that Na₂S₂O₈-urea-seawater composite redox system had an outstanding NO removal performance and ability to reduce NO₃⁻ emission. The Na₂S₂O₈-urea-seawater composite redox system was a promising scrubbing solution for NO removal from marine exhaust gases.

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