Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems, part II: NO₂⁻/NO₃⁻ generation and a brief critical review

Highlights

• •Effects of gas saturation/sparging were studied for sonochemical NO₂⁻/NO₃⁻ generation.
• •Twelve gas conditions of Ar, O₂, N₂, and binary mixtures were tested.
• •N₂:Ar (25:75) resulted in the highest NO₂⁻ and NO₃⁻ concentrations under all conditions.
• •H₂O₂/NO₂⁻/NO₃⁻ related OH radical activity was suggested.
• •Gas sparging enhanced the generation of NO₂⁻ and NO₃⁻ for O₂:N₂, N₂, and N₂:Ar.

Abstract

The sonochemical generation of NO₂⁻ and NO₃⁻ is considered to be one of the reasons for the low sonochemical oxidation activity in the presence of N₂ in the liquid phase. In this study, the generation characteristics of NO₂⁻ and NO₃⁻ were investigated using the same 28 kHz sonoreactor and the 12 gas conditions used in Part I of this study. Three gas modes, saturation/closed, saturation/open, and sparging/closed, were applied. N₂:Ar (25:75), N₂:Ar (50:50), and O₂:N₂ (25:75) in the saturation/closed mode generated the three highest values of NO₂⁻ and NO₃⁻. Ar and O₂ were vital for generating relatively large concentrations of NO₂⁻ and NO₃⁻. The absorption of N₂ from the air resulted in high generation of NO₂⁻ and NO₃⁻ for Ar 100 % and Ar/O₂ mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO₂⁻ and NO₃⁻ for N₂:Ar (25:75), O₂:N₂ (25:75), and N₂ significantly because of the change in the sonochemically active zone and the increase in the mixing intensity in the liquid phase, as discussed in Part I. The ratio of NO₃⁻ to NO₂⁻ was calculated using their final concentrations, and a ratio higher than 1 was obtained for the condition of Ar 100 %, Ar/O₂ mixtures, and O₂ 100 %, wherein a relatively high oxidation activity was detected. From a summary of the results and findings of previous studies, it was revealed that the observations of NO₂⁻ + NO₃⁻ could be more appropriate for investigating the NO₂⁻ and NO₃⁻ generation characteristics. In addition, H₂O₂/NO₂⁻/NO₃⁻ related activity rather than H₂O₂ activity was suggested to quantify the OH radical activity more appropriately in the presence of N₂.

1. Introduction

During acoustic cavitation events, various nitrogen species, including ∙NO, N₂O, HNO₂, HNO₃, and NH₃, can be detected in the liquid phase in the presence of dissolved nitrogen (N₂) [1], [2], [3], [4], [5]. Generally, NO₂⁻ and NO₃⁻ are detected as sonochemical oxidation products because they are relatively more stable than other generated nitrogen species and accumulate linearly to a certain extent [1], [2], [6], [7], [8], [9]. ∙NO is reported to be the main precursor for NO₂⁻ and NO₃⁻, generated from the reactions of ∙O∙, ∙OH, and N₂ under ultrasonic irradiation, as shown by the following reactions, which occur in the gas phase (inside cavitational bubbles) [1], [5], [10]:

$$H_{2}O\leftrightarrow ·OH+·H$$ (1)

$$O_2\leftrightarrow ·O·+·O·$$ (2)

$$\mathit{·}\mathit{OH}+\mathit{·}OH\leftrightarrow H_{2}O+\mathit{·}O\mathit{·}$$ (3)

$$N_2+\mathit{·}O\mathit{·}\leftrightarrow \mathit{·}NO+\mathit{·}N$$ (4)

$$\mathit{·}N+\mathit{·}O\mathit{·}\leftrightarrow \mathit{·}NO$$ (5)

$$\mathit{·}N+O_2\leftrightarrow \mathit{·}NO+\mathit{·}O\mathit{·}$$ (6)

$$\mathit{·}N+\mathit{·}OH\leftrightarrow \mathit{·}NO+\mathit{·}H$$ (7)

$$2\mathit{·}NO+O_2\leftrightarrow 2\mathit{·}N{O}_2$$ (8)

$$\mathit{·}\mathit{OH}+\mathit{·}NO\leftrightarrow HN{O}_2$$ (9)

$$\mathit{·}\mathit{OH}+\mathit{·}N{O}_2\leftrightarrow HN{O}_3$$ (10)

Homolysis of O₂ can occur inside the bubbles at a temperature of several thousand Kelvins owing to relatively low bond dissociation energy (498 kJ/mol). However, homolysis of N₂ rarely occurs inside the bubbles because its high bond dissociation energy (945 kJ/mol) implies that it only occurs at temperatures above 10,000 K [5]. The Zeldovich mechanism, which describes the thermal oxidation of gaseous N₂ and the generation of ∙NO, also suggests the formation of ∙N by the reaction of N₂ and ∙O [1], [11]. In addition, the extended Zeldovich mechanism suggests that Eq. (7) can be obtained at room temperature [11]. Sonochemically generated HNO₂ and HNO₃ have low pK_a values and are present in the form of NO₂⁻ and NO₃⁻, respectively, under neutral conditions. As a result of this, the pH of the liquid phase decreases significantly [2], [10]. In addition, NO₂⁻ can be further oxidized to NO₃⁻ using ∙OH, O₂, and H₂O₂ [1], [2].

Previous research has for decades investigated the sonochemical generation of NO₂⁻ and NO₃⁻ under various frequencies and gas conditions [1], [2], [3], [5], [6], [7], [8], [9], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Significant amounts of NO₂⁻ and NO₃⁻ could be generated in the presence of N₂, considering the generation of H₂O₂ and the degradation of pollutants [2], [3], [6], [7], [8], [9], which resulted in a remarkable decrease in the availability of oxidizing species such as OH radicals for target oxidation reactions. This is considered one of the main reasons why sonochemical oxidation activity is very low for gas conditions including N₂, with the low ratio of specific heat capacity, low solubility, and high thermal conductivity compared to Ar and O₂, as discussed in Part I of this study. Therefore, it is necessary to understand the generation characteristics of NO₂⁻ and NO₃⁻ to enhance the sonochemical oxidation activity. In addition, the comparison between the H₂O₂ generation and the NO₂⁻ and NO₃⁻ generation under various mixture conditions of Ar, O₂, and N₂, which are commonly used in Sonochemistry, has been rarely reported.

In this study, the effects of gas saturation and sparging on the generation of NO₂⁻ and NO₃⁻ were investigated systematically under three gas modes: saturation/closed, saturation/open, and sparging/closed. The gas conditions of Ar, O₂, N₂, and binary mixtures were applied, and the generation of NO₂⁻ and NO₃⁻ was quantitatively analyzed in a 28 kHz sonoreactor equipped with a gas supply system. To compare the generation characteristics of NO₂⁻ and NO₃⁻, a summary of the initial linear generation rate and final generated concentrations of NO₂⁻ and NO₃⁻ under various conditions, using the results and findings of previous studies, was provided.

2. Experimental methods

2.1. Chemicals

Nitrite ion standard solution (NaNO₂) and nitrate ion standard solution (NaNO₃) were acquired from Sigma–Aldrich Co. (USA). Sodium nitrite, sodium nitrate, and tert-butanol were purchased from Samchun Pure Chemical Co. ltd. (KOR). Potassium biphthalate (C₈H₅KO4) was acquired from Daejung Chemical &Metals Co. ltd. (KOR). Potassium iodide (KI) and ammonium molybdate [(NH₄)₂MoO₄] were purchased from Junsei Chemical Co. ltd. (JPN). All chemicals were used as received.

2.2. Sonoreactor and gas supply

An acrylic cylindrical sonoreactor was used in this study, equipped with a 28 kHz transducer module (Mirae Ultrasonic Tech., Bucheon, KOR) placed at the bottom. The liquid height and volume were 4.0 λ (53.6 mm) and 3.65 L, respectively. The temperature of the liquid body was maintained at 20 ℃ using a cooling system. The working electrical power was 100 W and the ultrasonic power, also called calorimetric power, was 55 W. The details of the sonoreactor are described in Part I of this study.

Three modes of gas saturation/sparging were tested using N₂, O₂, and Ar: 1) Saturation/Closed mode for twelve gas conditions including Ar, Ar:O₂ (75:25), Ar:O₂ (50:50), Ar:O₂ (25:75), O₂, O₂:N₂ (75:25), O₂:N₂ (50:50), O₂:N₂ (25:75), N₂, N₂:Ar (75:25), N₂:Ar (50:50), and N₂:Ar (25:75). 2) Saturation/Open mode for seven conditions including Ar, Ar:O₂ (75:25), Ar:O₂ (25:75), O₂, O₂:N₂ (75:25), N₂, and N₂:Ar (25:75). 3) Sparging/Open mode for six conditions including Ar, Ar:O₂ (75:25), O₂, O₂:N₂ (25:75), N₂, and N₂:Ar (25:75). Gas was delivered into the liquid body using a microporous glass sparger (pore size: 20–30 μm) equipped with a glass pipe, the sparger was placed 1 cm above the reactor bottom. The dissolved oxygen (DO) concentration was measured before and after each test to estimate the degree of gas saturation in the liquid body using a DO meter (ProODO; YSI Inc., USA). The details of the gas supply are described in Part I.

2.3. Quantification of sonochemical reactions

The concentrations of nitrite (NO₂⁻) and nitrate (NO₃⁻) ions were measured using an ion chromatography (IC) system (ICS-2100; Dionex, USA) equipped with an autosampler (AS-DV; Dionex, USA) [2]. The concentration of sonochemically generated H₂O₂ was spectrophotometrically analyzed using solution A (0.10 M potassium biphthalate), solution B (0.4 M KI, 0.06 M sodium hydroxide, and 10^-4 M ammonium molybdate), and a UV–vis spectrophotometer (Vibra S60, Biochrom ltd., UK) [22], [23].

3. Results and discussion

3.1. Gas saturation effect on NO₂⁻ and NO₃⁻ generation

The generation of H₂O₂, NO₂⁻, and NO₃⁻ under the three gas modes, saturation/closed, saturation/open, and sparging/closed, was investigated using the same sonoreactor system and the same gas conditions (N₂, O₂, Ar, and their binary mixtures) used in Part I. For the saturation/open and sparging/closed modes, selected gas conditions were applied. The concentration of H₂O₂ represents the cavitation-induced oxidizing radical activity, especially when using OH radicals, which are available for target oxidation reactions, while the concentrations of NO₂⁻ and NO₃⁻ represent undesirable consumption of the oxidizing radicals in the presence of dissolved N₂ in the liquid phase. The concentrations of H₂O₂, NO₂⁻, and NO₃⁻ increased linearly in this study for all the applied modes, and the final concentration of the sonochemical products for the saturation/closed mode is shown in Fig. 1. The irradiation duration was 90 min. The variations in the DO concentration before and after ultrasonic irradiation for the three gas modes are summarized in Table 1. The sonochemical generation characteristics of NO₂⁻ and NO₃⁻ under various gas conditions in previous studies are presented in Table 2.

Fig. 1.

Sonochemically generated concentrations of NO₂⁻, NO₃⁻. NO₂⁻ + NO₃⁻ and H₂O₂ under the saturation/closed mode using Ar, O₂, and N₂ for 28 kHz. The irradiation duration was 90 min.

Table 1.

Variation of DO concentrations before and after ultrasonic irradiation for various gas saturation and sparging conditions with a DO saturation concentration of 9.1 mg/L at 20 °C.

Gas modes		Ar	Ar:O2 (75:25)	Ar:O2 (50:50)	Ar:O2 (25:75)	O2	O2:N2 (75:25)	O2:N2 (50:50)	O2:N2 (25:75)	N2	N2:Ar (75:25)	N2:Ar (50:50)	N2:Ar (25:75)
Saturation/Closed	DO0* (mg/L)	0.5	12.1	22.8	32.9	43.8	32.8	22.5	11.0	0.3	0.4	0.4	0.4
	DO90* (mg/L)	1.0	10.6	19.9	28.9	38.4	29.1	20.6	10.2	0.7	0.9	0.9	0.6
	ΔDO**	0.5	-1.4	-2.9	-4.0	-5.4	-3.8	-1.9	-0.7	0.4	0.5	0.5	0.3
Saturation/Open	DO0 (mg/L)	0.4	11.1	-	31.1	40.3	31.1	-	-	0.4	-	-	0.4
	DO90 (mg/L)	3.0	9.6	-	18.8	23.6	18.7	-	-	5.0	-	-	4.6
	ΔDO	2.6	-1.6	-	-12.3	-16.7	-12.4	-	-	4.6	-	-	4.2
Sparging/Closed	DO0 (mg/L)	0.4	11.3	-	-	41.0	-	-	11.0	0.4	-	-	0.3
	DO90 (mg/L)	0.7	11.0	-	-	38.2	-	-	10.7	0.7	-	-	0.6
	ΔDO	0.3	-0.3	-	-	-2.8	-	-	-0.3	0.4	-	-	0.3

* DO₀ and DO₉₀ represent DO concentrations before and after 90 min of irradiation, respectively.

** ΔDO = DO₉₀-DO₀.

Table 2.

Sonochemical generation characteristics of NO₂⁻ and NO₃⁻ under various gas conditions. Some numerical data including concentrations and their corresponding irradiation durations were extracted using WebPlotDigitizer (https://automeris.io/WebPlotDigitizer/).

Experimental conditions	Sonochemical generation of NO2- and NO3-		Description/Comments	Ref.
	Initial linear generation rate	Final concentration
300 kHz		- Ar:N2O (96:4)	- NO2- and NO3- were not detected for Ar 100%.	[15]
I=3.5 W/cm2		NO2-: 940 μM@10 min	- The irradiation time for O2/N2O was not provided.
VL = 20 mL		NO3-: 680 μM@10 min	- The peak of NO2- + NO3- was observed for O2/N2O (90:10).
vessel submerged in US bath		NH3: 70 μM@10 min
gas saturated (Ar:N2O (96:4), N2O/O2 mixtures)		NO3-/NO2- = 0.72
		NO2-+NO3- : 162 μM/min
		- O2/N2O mixture: NO2-+NO3-
		(98:2): 460 μM
		(90:10): 1,200 μM
		(50:50): 60 μM
300 kHz		Overall generation rate of NO2-+NO3-	- N2 (the mixture of 14,14N2 and 15,15N2 (1:1)) was used and sonochemical formation of 14N15N was observed.	[3]
VL = 37.5 mL		- Ar/N2 mixture	- The highest generation rate for NO2-+NO3- and NH3 was observed for Ar:N2 (60:40) and Ar:N2 (40:60).
Ar/N2 saturated		Ar 100%: not detected	- As the ratio of N2 in the mixture increased, the generation rates of H2, H2O2, and O2 decreased significantly.
		(80:20): 3.9 μM/min
		(60:40), (40:60): 5.9 μM/min
		(20:80): 3.9 μM/min
		N2 100%: 1.6 μM/min
50 kHz	- air saturated	- air saturated	- •NO, the precursor of NO2- and NO3-, generation was quantified using electron paramagnetic resonance (EPR) techniques.	[5]
VL = 0.8 mL	•NO: 0.9 μM/2 min	NO2-: 5.1 μM@5 min	- The cause for no NO3- being detected, was considered as short US irradiation time.
vessel submerged in US bath		NO3-: not detected	- The EPR method only quantified •NO generated in bulk phase and 45% of •NO was detected considering the generation of NO2-. The other portion of •NO might be generated in the interior or interface of the cavitation bubbles.
air saturated, gas sparging		•NO: 3.4 μM@ 10 min	- The highest generation of •NO was observed at N2:O2 (90-78:10-22) and N2:Ar (29-57:71:43).
		- •NO for N2/O2 mixture
		N2 100%: 1.1 μM@5 min
		(90:10): 2.1 μM@5 min
		(85:15): 2.1 μM@5 min
		O2 100%: not detected
		- •NO for N2/Ar mixture
		N2 100%: 0.75 μM@5 min
		(43:57): 5.6 μM@5 min
		Ar 100%: not detected
35 kHz	- air	- air	- NO2- and NO3- were not detected for O2 100% and Ar 100%.	[14]
VL = 650 mL	NO2-: 7.9 μM/30 min	NO2-: 15 μM@60 min	- NO3- was detected after 10 min for N2.
vessel submerged in US bath	NO3-: 5.8 μM/60 min	NO3-: 5.8 μM@60 min	- As ionic strength increased in terms of NaCl concentration from 0 to 15%, the generation of NO2- and NO3- increased and H2O2 generation decreased. For higher ionic strength the generation of NO2- and NO3- decreased.
gas sparging (air, O2, N2, Ar)	NO3-/NO2- = 0.37	NO3-/NO2- = 0.39
	0	NO2-+NO3- : 0.35 μM/min
	NO2-: 1.2 μM/20 min	0
	NO3-: -	NO2-: 2.7 μM@60 min
		NO3-: 0.71 μM@60 min
		NO3-/NO2- = 0.26
		NO2-+NO3- : 0.06 μM/min
900 kHz		- air (5℃)	- For air, the generated amounts of NO2- and NO3- decreased significantly as the temperature increased from 5 to 40℃.	[13]
Pcal = 27 W		NO2-: 68 μM@20 min	- For N2/O2 mixtures, the peak value of NO2- and NO3- was observed at N2:O2 (58:42). High or low ratios of O2 resulted in low generation of NO2- and NO3-.
VL = 100 mL (5℃)		NO3-: 35 μM@20 min	- NO2- and NO3- were not detected for O2 100% and N2 100%.
gas sparging (N2/O2 mixtures)		NO3-/NO2- = 0.51	- pH@3 h = around 3.0 (air, 5℃)
		NO2-+NO3- : 5.2 μM/min
		- air (40℃)
		NO2-: 18 μM@20 min
		NO3-: 15 μM@20 min
		NO3-/NO2- = 0.83
		NO2-+NO3- : 1.7 μM/min
		#NULL!
		NO2-: 1.9 μM@20 min
		NO3-: 1.3 μM@20 min
		NO3-/NO2- = 0.68
		NO2-+NO3- : 0.16 μM/min
		#NULL!
		NO2-: 61 μM@20 min
		NO3-: 95 μM@20 min
		NO3-/NO2- = 1.56
		NO2-+NO3- : 7.8 μM/min
		#NULL!
		NO2-: 1.6 μM@20 min
		NO3-: 1.2 μM@20 min
		NO3-/NO2- = 0.75
		NO2-+NO3- : 0.14 μM/min
500 kHz	NO2-: 140 μM/50 min	NO2-: 30 μM@275 min	- NO2- concentration decreased after the peak at 100 min and the generation rate for NO3- increased after 70 min.	[7]
Pcal = 30 W	NO3-: 60 μM/50 min	NO3-: 1,400 μM@275 min	- NO2- + NO3- concentration increased linearly during the entire US irradiation duration (275 min).
VL = 200 mL	NO3-/NO2- = 0.43	NO3-/NO2- = 46.7
air saturated		NO2-+NO3- : 5.2 μM/min
500 kHz	from 40 min to 120 min	NO2-: 14.4 μM@240 min	- During the degradation of chlorobenzene, NO2- and NO3- generation was monitored.	[19]
Pcal = 25 W	NO2-: 48.4 μM/80 min	NO3-: 223.9 μM@240 min	- After the removal efficiency of chlorobenzene reached 70% at 40 min, NO2- and NO3- generation started, because the availability of oxidizing radicals for NO2- and NO3- generation was limited due to the high volatility of chlorobenzene.
VL = 250 mL	NO3-: 60.8 μM/80 min	NO3-/NO2- = 15.5	- The peak for NO2- was observed at 120 min and then NO2- concentration decreased and NO3- generation rate increased.
air saturated	NO3-/NO2- = 1.26	NO2-+NO3- : 0.99 μM/min
chlorobenzene degradation
32.6 kHz	NO2-: 0.0004 μM/min	NO2-: 0.73 μM@30 h	- A single bubble cavitation system was investigated.	[18]
VL = 60 mL	NO3-: 0.0004 μM/min	NO3-: 0.76 μM@30 h	- NO2- and NO3- concentrations increased linearly during 30 h of US irradiation.
single bubble system	NO3-/NO2- = 1	NO3-/NO2- = 1.04
air (DO : 3-6 mg/L)		NO2-+NO3- : 0.00083 μM/min
118, 224, 404, 651 kHz	-	- 118 kHz	- No significant concentration of NO2- was observed, possibly due to long irradiation duration (10 h) and much larger generation of NO3- compared to that of NO2-.	[9]
Pcal = 29 W		NO2-: not detected	- NO3- concentration was linearly generated from 2 h for 118 kHz and from 0.5 h for 224, 404, and 651 kHz.
VL = 250 mL		NO3-: 47.2 mg/L@10 h	- The final concentration of NO3- for 11.4, 29.0, and 41.5 W was 746, 2,222, and 4,127 μM@10 h, respectively.
air saturated		(762 μM@10 h)	- pH@10 h = 2.5 – 3.5 for all cases
		NO2-+NO3- : 1.3 μM/min
		- 224, 404, 651 kHz
		NO2-: not detected
		NO3-: 137.8 mg/L@10 h
		(2,222 μM@10 h)
		NO2-+NO3- : 3.7 μM/min
20 kHz (Probe)	NO2-: 50 μM/10 min	NO2-: 31 μM@60 min	- The mixture of NO (1,040 ppmv) and SO2 (2,520 ppmv) was used.	[17]
I = 86.8 W/cm2	NO3-: 11 μM/60 min	NO3-: 11 μM@60 min	- The peak of NO2- (54 μM) was observed at 20 min at which point NO concentration decreased.
VL = 1,200 mL	NO3-/NO2- = 0.04	NO3-/NO2- = 0.35	- NO3- increased linearly during the whole irradiation duration.
gas sparging (NO 1,040 ppmv)		NO2-+NO3- : 0.7 μM/min
300 kHz	NO2-: 180 μM/45 min	NO2-: 240 μM@180 min	- NO2- concentration decreased after the peak at 90 min and NO3- was generated linearly during the whole irradiation duration (180 min).	[6]
Pelec = 80 W	NO3-: 180 μM/45 min	NO3-: 760 μM@180 min	- pH: 6.6 → 3.0 (after 180 min)
VL = 300 mL	NO3-/NO2- = 1.0	NO3-/NO2- = 3.17
air sparging		NO2-+NO3- : 5.6 μM/min
404 kHz	-	- NO3- (only water)	- No significant concentration of NO2- was observed.	[8]
3.5, 9.0, 12.9 kW/m2		3.5 kW/m2: 700 μM@10 h	- NO3- generation was delayed in the presence of BPA compared to in the absence of BPA.
VL = 250 mL		NO2-+NO3- : 1.2 μM/min	- pH@10 h = around 3.0 for all cases
air saturated		9.0 kW/m2: 2,000 μM@10 h
		NO2-+NO3- : 3.3 μM/min
		12.9 kW/m2: 4,100 μM@10 h
		NO2-+NO3- : 6.8 μM/min
		- NO3- (with BPA degradation)
		3.5 kW/m2: 200 μM@10 h
		NO2-+NO3- : 0.3 μM/min
		9.0 kW/m2: 1,100 μM@10 h
		NO2-+NO3- : 1.8 μM/min
		12.9 kW/m2: 2,800 μM@10 h
		NO2-+NO3- : 4.7 μM/min
Reactor 1		- Reactor 1 (Pcal = 19.1 W)	- The US irradiation time ranged from 15 to 150 min depending on experimental conditions.	[16]
365 kHz		NO2-: 1.15 μM/min	- The larger volume of the second reactor resulted in approximately 10 times lower generation of NO2- and NO3- under the similar power condition (19.1 W and 21.6 W).
VL = 0.4 L (D = 65 mm)		NO3-: 0.2 μM/min	- As the input power increased, the generation of NO2- and NO3- increased.
Pcal = 19.1 W		NO3-/NO2- = 0.17
		NO2-+NO3- : 1.35 μM/min
Reactor 2		- Reactor 2 (Pcal = 8.4 W)
367 kHz		NO2-: 0.032 μM/min
VL = 2.0 L (D = 102 mm)		NO3-: 0.005 μM/min
Pcal = 8.4, 13.6, 21.6, 27.8 W		NO3-/NO2- = 0.16
		NO2-+NO3- : 0.04 μM/min
		- Reactor 2 (Pcal = 13.6 W)
		NO2-: 0.07 μM/min
		NO3-: 0.007 μM/min
		NO3-/NO2- = 0.10
		NO2-+NO3- : 0.08 μM/min
		- Reactor 2 (Pcal = 21.6 W)
		NO2-: 0.11 μM/min
		NO3-: 0.022 μM/min
		NO3-/NO2- = 0.20
		NO2-+NO3- : 0.13 μM/min
		- Reactor 2 (Pcal = 27.8 W)
		NO2-: 0.18 μM/min
		NO3-: 0.047 μM/min
		NO3-/NO2- = 0.26
		NO2-+NO3- : 0.23 μM/min
800 kHz	- US	- US	- NO2- and NO3- concentrations increased linearly in the US process.	[21]
Pcal = 25 W	NO2-: 41.9 μM/40 min	NO2-: 41.9 μM@40 min	- In the US/UV process, the generation rate for NO2- decreased gradually and significantly, and the generation rate for NO3- increased after 10 min.
VL = 250 mL	NO3-: 45.6 μM/40 min	NO3-: 45.6 μM@40 min	- NO2- + NO3- increased linearly during the whole irradiation period and similar final concentrations of NO2- + NO3- were obtained for both US and US/UV processes.
US and US/UV system	NO3-/NO2- = 1.09	NO3-/NO2- = 1.09
	- US/UV	NO2-+NO3- : 2.2 μM/min
	NO2-: 8.0 μM/10min	- US/UV
	NO3-: 14.8 μM/10 min	NO2-: 17.8 μM@40 min
	NO3-/NO2- = 1.85	NO3-: 75.3 μM@40 min
		NO3-/NO2- = 4.23
		NO2-+NO3- : 2.3 μM/min
200, 400, 600, 800 kHz	- 200 kHz	- 200 kHz	- For 200, 400, and 600 kHz, NO2- and NO3- concentrations increased linearly during the first 20 min and then the rates of NO2- and NO3- generation changed gradually and significantly.	[2]
I = 0.69 W/cm2, 0.19 W/mL	NO2-: 70 μM/20 min	NO2-: 170 μM@60 min	- NO2- and NO3- concentrations increased relatively linearly during the whole irradiation duration (60 min) for 800 kHz, possibly due to lower concentrations of NO2- and NO3-.
VL = 200 mL	NO3-: 44 μM/20 min	NO3-: 200 μM@60 min	- For N2:O2 (75:25), N2:O2 (50:50), and N2:O2 (25:75), NO2- decreased and the generation rate of NO3- increased gradually after the peak at 40-60 min.
air sparging	NO3-/NO2- = 0.63	NO3-/NO2- = 1.18	- NO2- and NO3- generation: N2:O2 (75:25) > N2:O2 (50:50) > N2:O2 (25:75) >> N2 100%, O2 100% ≈ 0
	- 400 kHz	NO2-+NO3- : 6.2 μM/min
	NO2-: 40 μM/20 min	- 400 kHz
	NO3-: 20 μM/20 min	NO2-: 120 μM@60 min
	NO3-/NO2- = 0.50	NO3-: 110 μM@60 min
	- 600 kHz	NO3-/NO2- = 0.92
	NO2-: 110 μM/20 min	NO2-+NO3- : 3.8 μM/min
	NO3-: 24 μM/20 min	- 600 kHz
	NO3-/NO2- = 0.22	NO2-: 170 μM@60 min
	- 800 kHz	NO3-: 280 μM@60 min
	NO2-: 28 μM/20 min	NO3-/NO2- = 1.65
	NO3-: 7 μM/20 min	NO2-+NO3- : 7.5 μM/min
	NO3-/NO2- = 0.25	- 800 kHz
		NO2-: 73 μM@60 min
		NO3-: 27 μM@60 min
		NO3-/NO2- = 0.37
		NO2-+NO3- : 1.7 μM/min
28 kHz	air	- air	- For the air saturated condition, the linear generation rate for NO2- and NO3- decreased significantly after 30 min.	[20]
Pelec = 1,200 W (US bath)	NO2-: 40 μM/30 min	NO2-: 72.3 μM@120 min	- A very low concentration of NO3- was observed for N2 100%, O2 100% and Ar 100%.
VL = 200 mL	NO3-: 42 μM/30 min	NO3-: 52.8 μM@120 min	- pH: 6.3 → 5.4 (after 120 min)
vessel submerged in US bath	NO3-/NO2- = 1.05	NO3-/NO2- = 0.73
gas saturated (air, O2, N2, Ar)		NO2-+NO3- : 1.0 μM/min
		0
		NO3-: 4 μM@120 min
		0
		NO3-: 2 μM@120 min
		- Ar
		NO3-: 0 μM@120 min
200 kHz	NO2-: 100 μM/30 min	NO2-: 160 μM@60 min	- NO2- decreased (119 μM@180 min) and NO3- increased (157 μM@180 min) after US irradiation stopped at 60 min. NO2-+NO3- remained constant after US irradiation stopped.	[1]
Pcal = 25 W	NO3-: 40 μM/30 min	NO3-: 110 μM@60 min	- The highest NO2-+NO3- was obtained at 14 W among 2.7, 5.5, 14, 19, and 25 W
VL = 65 mL	NO3-/NO2- = 0.40	NO3-/NO2- = 0.69
vessel submerged in US bath		NO2-+NO3- : 4.5 μM/min
air saturated
28 kHz		- gas saturated/closed	- The generation of H2O2 under various gas conditions was investigated in Part I of this work.	in this study
Pcal = 55 W		(12 gas conditions)	- Three gas supply modes including saturated/closed, saturated/open, and sparging/closed were investigated.
VL = 3.65 L		NO2-: 0.08–3.58 μM@90 min	- NO2- and NO3- increased linearly during whole US irradiation duration (90 min).
gas saturated/gas sparging		NO3-: 0.11–1.86 μM@90 min	- The highest concentration of NO2- and NO3- was obtained for N2:Ar (25:75) for the three gas modes.
(Ar, O2, N2, and binary mixtures)		NO3-/NO2-: 0.34–1.71	- H2O2/NO2-/NO3- related OH radical activity was suggested to consider more appropriate sonochemical oxidation activity.
		NO2-+NO3-: 0.002–0.06 μM/min
		- gas saturated/open
		(7 gas conditions)
		NO2-: 0.74–6.56 μM@90 min
		NO3-: 0.61–3.20 μM@90 min
		NO3-/NO2-: 0.41–0.82
		NO2-+NO3-: 0.015–0.10 μM/min
		- gas sparging/closed
		(6 gas conditions)
		NO2-: 0–8.23 μM@90 min
		NO3-: 0.10–4.60 μM@90 min
		NO3-/NO2-: 0.56–2.85
		NO2-+NO3-: 0.001–0.14 μM/min

For the saturation/closed mode, lower concentrations of NO₂⁻ and NO₃⁻ were obtained for the gas conditions where higher concentrations of H₂O₂ were obtained, and higher amounts of NO₂⁻ and NO₃⁻ were generated for the conditions where lower amounts of H₂O₂ were detected. The generation of NO₂⁻ and NO₃⁻ was principally attributed to the presence of N₂, an essential ingredient for the generation of nitrogen oxides, in the following order: N₂:Ar (25:75) > N₂:Ar (50:50) > O₂:N₂ (25:75) > O₂:N₂ (50:50) > N₂:Ar (75:25) > O₂:N₂ (75:25). The large generation of NO₂⁻ and NO₃⁻ may have been attributed to two factors: 1) a higher ratio of Ar leading to more severe and violent cavitation events that result in increased generation of NO, the precursor of NO₂⁻ and NO₃⁻, and highly reactive oxidizing radicals [5], [24], [25], [26]; and 2) the presence of O₂ effectively promoting the generation of oxidizing radicals such as OH radicals [24], [27], [28]. From the above-mentioned order, Ar was found to be the most important for the generation of NO₂⁻ and NO₃⁻, followed by O₂ in the presence of N₂. In addition, a higher ratio of Ar and a relatively lower ratio of O₂ were favorable for NO₂⁻ and NO₃⁻ generation in this study. Therefore, it is conceivable that very small amounts of NO₂⁻ and NO₃⁻ were produced in N₂ 100 % as shown in this study. As shown in Table 2, previous researchers have also reported similar findings under various gas mixture conditions: Hart et al. reported that the highest generation rate of NO₂⁻ + NO₃⁻ was obtained at Ar:N₂ (40–60:60–40) and less than 30 % of the highest rate was obtained at N₂ 100 % for Ar/N₂ mixtures under 300 kHz [3]; Mišík and Riesz found the highest level of NO, the precursor of NO₂⁻ and NO₃⁻, at N₂:O₂ (90–78:10–22) for N_2/O₂ mixtures and at N₂:Ar (29–57:71–43) for N₂/Ar mixtures under 50 kHz. No NO was generated at O₂ 100 % and Ar 100 % in their study [5]; in Supeno and Kruus’s study, the highest concentrations of NO₂⁻ and NO₃⁻ were detected at N₂:O₂ (58:42), and no NO₂⁻ and NO₃⁻ were generated at O₂ 100 % and N₂ 100 % under 900 kHz [13]; Yao et al.’s highest concentrations of NO₂⁻ and NO₃⁻ were obtained at N₂:O₂ (75:25), and no NO₂⁻ and NO₃⁻ were generated at O₂ 100 % and N₂ 100 % under 600 kHz [2].

In principle, NO₂⁻ and NO₃⁻ could not be generated for the conditions from Ar 100 % to O₂ 100 % [Fig. 1(a)] because of the absence of N₂. However, low concentrations of NO₂⁻ and NO₃⁻ were detected due to the residual trace of N₂, which was not removed by the 20-min sparging for gas saturation that occurred before ultrasonic irradiation [12]. Only very low concentrations of NO₂⁻ and NO₃⁻ were generated for the gas sparging conditions of Ar 100 %, Ar:O₂ (75:25), and O₂ 100 %, as shown in Fig. 3(b), because the residual N₂ was continuously and more completely removed by the gas sparging during ultrasonic irradiation.

Fig. 3.

Sonochemically generated concentration of NO₂⁻, NO₃⁻. NO₂⁻ + NO₃⁻ and H₂O₂ under the saturation/open and the sparging/closed mode using Ar, O₂, and N₂ for 28 kHz. The irradiation duration was 90 min. The yellow bar in Fig. 3(a) represents the value obtained for NO₂⁻ and NO₃⁻ under the saturation/closed mode as shown in Fig. 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The ratio of NO₃⁻ to NO₂⁻ may also provide information on the sonochemical oxidizing activity. As shown in Fig. 1S, the ratio using the concentration of NO₂⁻ and NO₃⁻ in the first linear generation period ranged from 0.34 [O₂:N₂ (25:75)] to 1.71 [Ar:O₂ (25:75)] for the saturation/closed conditions. Despite low concentrations of NO₂⁻ and NO₃⁻, the ratio was higher than 1 for the conditions from Ar:O₂ (75:25) to O₂ 100 %, where higher oxidizing activity was obtained. This indicates that sonochemically generated NO₂⁻ can be easily oxidized to NO₃⁻ in the presence of many oxidizing agents, such as ∙OH, O₂, and H₂O₂ [1], [2]. This might be also due to the low concentration of NO₂⁻ compared to the generated oxidizing radicals, considering the concentration of generated H₂O₂ in the conditions from Ar:O₂ (75:25) to O₂ 100 %. In previous studies, various ratios have been reported depending on the experimental conditions and irradiation times. It was found that the NO₂⁻ and NO₃⁻ concentrations increased linearly during the initial stage. After the NO₂⁻ concentration peaked, it decreased significantly, and correspondingly, NO₃⁻ increased with higher generation rates. The decrease in NO₂⁻ concentration is mainly attributed to the oxidation of NO₂⁻ to NO₃⁻ using sonochemically generated oxidizing species [1], [2].

As shown in Table 2, the ratio using NO₂⁻ and NO₃⁻ concentrations (NO₃⁻/NO₂⁻) in the linear generation period was less than 1.1 before 60 min had passed: 0.37 (30 min, NO₂⁻+NO₃⁻: 0.36 μM/min, air, 35 kHz) [14]; 0.43 (50 min, NO₂⁻+NO₃⁻: 40 μM/min, air, 500 kHz) [7]; 1.0 (45 min, NO₂⁻+NO₃⁻: 8 μM/min, air, 300 kHz) [6]; 1.09 (40 min, NO₂⁻+NO₃⁻: 2.19 μM/min, air, 800 kHz); 0.63 (20 min, NO₂⁻+NO₃⁻: 5.7 μM/min, air, 200 kHz), 0.50 (20 min, NO₂⁻+NO₃⁻: 3.0 μM/min, air, 400 kHz), 0.22 (20 min, NO₂⁻+NO₃⁻: 6.7 μM/min, air, 600 kHz), and 0.25 (20 min, NO₂⁻+NO₃⁻: 1.75 μM/min, air, 800 kHz) [2]. The gaseous state of air leads to relatively low oxidizing activity, causing the ratio to be lower than 1.1. Jiang et al. reported a slightly higher ratio of 1.26 (80 min, NO₂⁻+NO₃⁻:1.37 μM/min, air, 500 kHz), which was obtained during the degradation of a volatile compound (chlorobenzene) which significantly inhibited the oxidizing radical activity of chemical species existing outside the cavitation bubbles. In their research, NO₂⁻ and NO₃⁻ were not detected during the first 40 min, and it was highly likely that a substantial amount of NO, the precursor of NO₂⁻ and NO₃⁻, was not generated during this period [19]. In this study, the ratio for O₂:N₂ (25:75), a composition similar to air, was 0.34. Ratios higher than 1.1 were obtained for Ar:O₂ (75:25), Ar:O₂ (50:50), Ar:O₂ (25:75), and O₂ 100 %.

Final concentrations of NO₂⁻ and NO₃⁻ were also reported in previous research, with ratios of NO₃⁻ to NO₂⁻ ranging from 0.26 to 46.7 as shown in Table 2. A ratio much higher than 1 indicates that the concentrations of the ratio were collected over a longer period of time after the first linear generation period, allowing a large amount of NO₂⁻ to be further oxidized to NO₃⁻. Inoue et al. also reported no significant concentration of NO₂⁻ and a high concentration of NO₃⁻ after 10 h [8], [9]. Thus, the sum of NO₂⁻ and NO₃⁻ is more appropriate for understanding the sonochemical generation characteristics of NO₂⁻ and NO₃⁻. Pétrier and Casadonte [7] and Zaviska et al. [21] reported that the NO₂⁻ + NO₃⁻ concentration increased linearly over the entire duration of irradiation. Zaviska et al. also found that the final concentrations of NO₂⁻ + NO₃⁻ were highly similar for the US and US/UV processes [21]. Okitsu and Itano found that the NO₂⁻ + NO₃⁻ concentration remained constant after US irradiation stopped, even though NO₂⁻ decreased and NO₃⁻ increased [1].

As shown in Table 2, various values of NO₂⁻ + NO₃⁻ generation rate ranging from 0.04 μM/min to 162 μM/min were obtained using the final concentrations of NO₂⁻ and NO₃⁻ depending on the applied frequency, the input power, the gas mixture, and the liquid volume: 162 μM/min [300 kHz, 20 mL, Ar:N₂O (96:4)] [15]; 1.6–5.9 μM/min (300 kHz, 37.5 mL, Ar/N₂ mixtures) [3]; 0.35 μM/min for air, 0.06 μM/min for N₂ (35 kHz, 650 mL) [14]; 1.7–5.2 μM/min for air, 0.14–7.8 μM/min for N₂/O₂ mixtures (900 kHz, 100 mL) [13]; 5.2 μM/min (500 kHz, 200 mL, air) [7]; 0.99 μM/min (500 kHz, 250 mL, air, chlorobenzene degradation) [19]; 1.3–3.7 μM/min for 118, 224, 404, 651 kHz (250 mL, air) [9]; 5.6 μM/min (300 kHz, 300 mL, air) [6]; 1.2–6.8 μM/min for only water, 0.3–4.7 μM/min for BPA degradation (404 kHz, 250 mL, air) [8]; 1.35 μM/min (365 kHz, 0.4 L), 0.04–0.23 μM/min (367 kHz, 2.0 L) [16]; 2.2 μM/min for US, 2.3 μM/min for US/UV (800 kHz, 250 mL) [21]; 1.7–7.5 μM/min (200–800 kHz, 200 mL, air) [2]; 1.0 μM/min (28 kHz, 200 mL, air) [20]; 4.5 μM/min (200 kHz, 65 mL of air) [1]. From the results of previous research, it seemed that a higher frequency and lower liquid volume resulted in a higher concentration of NO₂⁻ and NO₃⁻. However, further research is needed because the results in Table 2 were obtained under very different experimental conditions. As shown in Fig. 1, relatively low concentrations of NO₂⁻ + NO₃⁻ ranging from 0.002 μM/min [O₂ 100 %] to 0.06 μM/min [N₂:Ar (25:75)] were observed in this study. This may be due to the low frequency (28 kHz) and higher liquid volume (3.65 L) used in this study.

3.2. Effect of OH radical scavenger

As shown in Fig. 2, it is clearly understood that the generation of H₂O₂, NO₂⁻, and NO₃⁻ is greatly related to the OH radical activity. In the presence of tert-butyl alcohol (TBA) (40 mM), which is one of the most common OH radical scavengers [29], [30], the concentrations of H₂O₂, NO₂⁻, and NO₃⁻ decreased significantly for Ar:O₂ (75:25), where the highest H₂O₂ generation was observed, and N₂:Ar (25:75), where the highest NO₂⁻ and NO₃⁻ generation was detected in this study. The presence of TBA more severely affected the sonochemical oxidation reactions for N₂:Ar (25:75) and it might be due to the lower availability of oxidizing radicals for N₂:Ar (25:75) than for Ar:O₂ (75:25).

Fig. 2.

Effect of the radical scavenger (tert-butyl alcohol, 40 mM) on the generation of H₂O₂, NO₂⁻, NO₃⁻ for Ar:O₂ (75:25) and N₂:Ar (25:75), with an irradiation duration of 90 min.

Considering the generation of NO₂⁻ and NO₃⁻ and their relationship with OH radicals, the sonochemical oxidation activity can be better understood. Under the simple assumption that two, one, and two OH radicals are consumed for the generation of H₂O₂, NO₂⁻, and NO₃⁻, respectively, a more realistic OH radical activity can be calculated and compared for various gas conditions, as shown in Fig. 2S. No significant difference (only 1.7–7.0 %) was observed between two activities, including H₂O₂/NO₂⁻/NO₃⁻ related activity (H₂O₂ concentration × 2 + NO₂⁻ concentration × 1 + NO₃⁻ concentration × 2) and H₂O₂ related activity (H₂O₂ concentration × 2) for the gas conditions from Ar 100 % to O₂ 100 %. However, a relatively large difference (30.5–49.7 %) was obtained from O₂:N₂ (50:50) to N₂:Ar (25:75) indicating that actual OH radical activity could be further underestimated by a third to a half for the gas conditions. It should be noted that this assumption is debatable because the radical reactions for the sonochemical generation of H₂O₂, NO₂⁻, and NO₃⁻ cannot be precisely identified under various ultrasonic conditions. In addition, the reactivity of OH radicals and the generation locations during cavitation events can be significantly different for H₂O₂, NO₂⁻, and NO₃⁻.

3.3. Saturation/open and sparging/closed mode

The generation of H₂O₂, NO₂⁻, and NO₃⁻ under saturation/open and sparging/closed modes was investigated, as shown in Fig. 3. As discussed in Part I of this study, no significant difference was observed between the sparging/closed and sparging/open modes. For the saturation/open mode, the concentrations of NO₂⁻ and NO₃⁻ increased significantly for all applied gas conditions compared to those obtained for the saturation/closed mode because of the gas exchange between the liquid and air phases. The order for the NO₂⁻ + NO₃⁻ concentration was as follows: N₂:Ar (25:75) ≈ Ar:O₂ (75:25) > Ar 100 % > Ar:O₂ (25:75) ≈ O₂:N₂ (75:25) ≈ N₂ 100 % > O₂ 100 %. The large increase in the NO₂⁻ + NO₃⁻ concentration for Ar 100 % (556 %), Ar/O₂ mixtures (722 %, 975 %) and O₂ 100 % (685 %) was attributed to the entry of N₂ into the liquid phase, the essential source of NO₂⁻ and NO₃⁻, considering the change in the DO concentration, as shown in Table 1. Relatively lower increases for N₂ 100 % (312 %) and N₂:Ar (25:75) (170 %) were observed, which might be due to relatively lower oxidation activity. However, it was again confirmed that the presence of O₂ significantly enhanced the generation of NO₂⁻ and NO₃⁻. In addition, the ratio of NO₃⁻ to NO₂⁻ was less than 1 for all applied gas conditions. The ratio of Ar:O₂ (75:25), Ar:O₂ (25:75), and O₂ 100 % was higher than 1 under the saturation/closed mode, as shown in Fig. 1S due to relatively high oxidation activity considering the H₂O₂ concentration for the saturation/open and the saturation/closed modes. This indicated that the sonochemical oxidation activity decreased owing to the entry of N₂, which resulted in the reduction of the Ar and O₂ contents in the liquid phase.

In previous studies, the gas sparging method was frequently used to maintain the gas content to investigate the effect of dissolved gases on sonochemical reactions [2], [5], [6], [13], [14], [17]. However, comparisons between gas saturation and gas sparging have rarely been reported. In the sparging/closed mode, large enhancement in the generation of NO₂⁻ and NO₃⁻ was observed for O₂:N₂ (25:75) (315 %), N₂ 100 % (290 %), and N₂:Ar (25:75) (236 %) compared to that obtained in the saturation/closed mode. This enhancement was due to the sparging effect, which induced a change in the sonochemical active zone in terms of the location, shape, and intensity, as well as violent mixing in the liquid phase, as discussed in Part I of this work. The effect of the residual trace gases could be negligible because no significant concentrations of NO₂⁻ and NO₃⁻ were detected for Ar 100 %, Ar:O₂ (75:25), and O₂ 100 %. It was found that the main source of oxygen atoms for the generation of NO₂⁻ and NO₃⁻ was the water molecules for N₂ 100 % and N₂:Ar (25:75) [5]. In addition, gas sparging was more effective for the generation of H₂O₂. The enhancement by the gas sparging for the applied six gas conditions ranged from 248 to 729 % and 18–315 % for the generation of H₂O₂ and NO₂⁻ + NO₃⁻, respectively. The highest H₂O₂/NO₂⁻/NO₃⁻ related activity was obtained for Ar:O₂ (75:25), with activity-three times higher than that of the saturation/closed mode.

4. Conclusion

The effects of gas saturation and sparging on the sonochemical generation of NO₂⁻ and NO₃⁻ were investigated using a 28 kHz sonoreactor equipped with a gas supply system as Part II of the study. Twelve gas conditions including Ar 100 %, O₂ 100 %, N₂ 100 %, and binary gas mixtures, were applied under three gas modes: saturation/closed, saturation/open, and sparging/closed. The conclusions of this study are as follows:

• 1.In the saturation/closed mode, the highest NO₂⁻ and NO₃⁻ concentrations were obtained for N₂:Ar (25:75), N₂:Ar(50:50), and O₂:N₂ (25:75), which was attributed to the presence of Ar and O₂. The generation of NO₂⁻ and NO₃⁻ for the Ar 100 %, Ar/O₂ mixtures and O₂ 100 % was due to the residual N₂.
• 2.Gas exchange between the liquid and air occurred in the saturation/open mode. Relatively higher concentrations of NO₂⁻ and NO₃⁻ were observed for the seven gas conditions due to the entry of N₂ and O₂ compared to that for the saturation/closed mode.
• 3.Gas sparging enhanced the generation of NO₂⁻ and NO₃⁻ for O₂:N₂ (25:75) and N₂:Ar (25:75) significantly. However, no significant concentrations of NO₂⁻ and NO₃⁻ were obtained for Ar 100 %, Ar:O₂ (75:25), and O₂ 100 % due to the continuous removal of residual N₂.
• 4.Results and findings regarding sonochemical generation of NO₂⁻ and NO₃⁻ were summarized and compared with the results in this study.

CRediT authorship contribution statement

Younggyu Son: Conceptualization, Methodology, Investigation, Validation, Writing – original draft, Writing – review & editing, Visualization, Supervision, Funding acquisition. Jongbok Choi: Methodology, Investigation, Validation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Data availability

No data was used for the research described in the article.