Azo dye-containing wastewater poses serious risks of environmental pollution because it is generally biologically toxic and resistant to conventional wastewater treatment methods. A novel degradation system integrating ozone, microchannel, and ultrasound was designed to effectively degrade azo dye-contaminated wastewater. The effects of discharge voltage of dielectric barrier discharge (DBD) reactor, liquid flow rate, microchannel width, ultrasonic power, initial pH, and reaction temperature on methylene blue (MB) decolorization were studied. A maximum MB decolorization efficiency of 92.7% was obtained in the ozone/microchannel/ultrasound (O3/MC/US) system with 14 min of treatment. In addition, the 14-min decolorization efficiency and TOC removal efficiency obtained in O3/MC/US system were increased by 12.6 and 6.5%, respectively, compared to those obtained in the pure O3 system. Based on the results of scavenging experiments, the combined effects of microchannel and ultrasound were proved to improve the contribution rate of hydroxyl radicals, thus improving the decolorization efficiency. The present work clearly illustrates that ozonation degradation can be effectively enhanced by microchannel and ultrasound, and also provides a feasible method for the treatment of organic wastewater.

  • Combined effects of microchannel and ultrasound on ozonation degradation were clearly observed.

  • The 14-min decolorization efficiency of methylene blue reached 92.7%, which was increased by 12.6% due to the combination of microchannel and ultrasound.

  • Ozonation degradation was enhanced by the combination effects of microchannel and ultrasound through the promotion of indirect ozonation process.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Azo dyes are characterized by containing at least one azo group (–N = N–) bound to a chromophore or aromatic system (Sun et al. 2022). Due to the advantages of firm coloring and low production cost, azo dyes bring great convenience to our daily life. However, azo substances are generally toxic and non-biodegradable, and may cause serious water pollution even at low concentrations (Srivastava et al. 2022). Therefore, efficient degradation methods of azo-contaminated effluents are urgently needed to be investigated.

Unfortunately, conventional physical (Sun et al. 2006), chemical (Wu & Wang 2001) and biological (Prasac & Aikat 2014) treatment techniques are mineralized incomplete, time-consuming and uneconomical (Rayaroth et al. 2021). In recent years, advanced oxidation processes (AOPs) have been widely used for efficient and non-selective degradation of azo dyes (Bilińska & Gmurek 2021; Liu et al. 2021; Ismail & Sakai 2022). The principle of AOPs is to utilize the strong reactivity of hydroxyl radicals (HO•) to indiscriminately attack organic molecules, thus mineralizing the target pollutants (Dong et al. 2022). AOPs can be classified either as heterogeneous or homogeneous progress (Poyatos et al. 2010). Heterogeneous AOPs, referring to the processes using catalysts, have the advantages of environmental friendliness, high efficiency, and energy saving (Hou et al. 2022). Nevertheless, the difficulty of catalyst recovery limits the applications of heterogeneous AOPs in large-scale wastewater treatment. Homogeneous AOPs refer to the processes that use Fenton reagents (hydrogen peroxide with ferrous ions) and ozone (O3) as the sources of HO•, with or without energy input. Although the Fenton process is widely used for its simplicity and efficiency, its major drawback is the production of iron sludge waste, which inevitably causes secondary pollution (Poyatos et al. 2010). Compared with other approaches, homogeneous AOPs based on ozonation have been considered a popular way due to their high oxidation potential and no generation of chemical byproducts (Zheng et al. 2022).

O3 is unstable in the aqueous medium and will spontaneously decompose through a complex mechanism involving HO• generation. Consequently, the ozonation degradation of azo dyes follows two pathways (Mehrjouei et al. 2015), including the direct pathway caused by molecular O3 (E0(O3/H2O) = +2.07 V), and the indirect pathway caused by HO• (E0(HO•/H2O) = +2.8 V). In the direct pathway, azo dyes are degraded by O3 through oxidation–reduction reaction, cycloaddition reaction, electrophilic substitution reaction, and nucleophilic reaction (Issaka et al. 2021). In the indirect pathway, since HO• has a higher oxidation potential than O3, and reacts with organic compounds 106–1012 times faster than O3 (Munter 2001), azo dyes can be completely mineralized into carbon dioxide and water. The specific process of mineralization generally includes C–N bond rupturing, desulfurization reaction, and denitrification reaction (Shamsabadi & Behpour 2021). However, the use of ozonation treatment without any enhancement method is limited by low O3 utilization (Miruka et al. 2021; Ghanbari et al. 2020) and poor HO• yield (Poyatos et al. 2010; Guo et al. 2015), because O3 is difficult to dissolve and decompose spontaneously in aqueous solution (Wu et al. 2020). Therefore, the focus of this study is to investigate the possibility of the ozonation process enhanced by microchannel and ultrasound, as well as the resultant combination effects on MB degradation.

Microchannel can be used to improve O3 utilization due to its large surface-to-volume ratio and short transport path. The ozonation degradation of azo dye Acid Red 14 enhanced by microchannel was reported to follow a pseudo-first-order kinetic model (Gao et al. 2012). The decolorization efficiency was observed to increase with the decrease of liquid volume flow rate in the microchannel. Additionally, the combination effect of microchannel on ozonation was investigated by MB degradation experiments with a microchannel reactor (Patinglag et al. 2019). It was found that the degradation efficiency was improved by a decreasing microchannel width in the range of 50–100 μm.

As another enhancement method, ultrasound can be used to improve the HO• yield and the mixing intensity of solutes. According to the hot-spot theory (Giray et al. 2018), the decomposition of O3 into HO• will be promoted by the local excessive temperature released during the rupture of cavitation bubbles. The synergistic effect of ultrasound was investigated by combining a 520 kHz ultrasound with an ozonation process to degrade textile dyes (Tezcanli-Guyer & Ince 2004). The improvement of degradation efficiency was attributed to the enhancement of O3 diffusion, electrophilic reaction, and radical reaction by ultrasound.

To the best of our knowledge, the combined applications of microchannel and ultrasound in the ozonation process are limited. In this study, we designed a novel degradation system that combines O3, microchannel and ultrasound for the treatment of azo dye wastewater. The effects of the discharge voltage of DBD reactor, liquid flow rate, microchannel width, ultrasonic power, initial pH and reaction temperature on decolorization efficiency were studied. The combination effects of microchannel and ultrasound on ozonation degradation were clearly evaluated. The contribution of HO• to decolorization was also investigated by scavenging experiments. We, therefore, anticipated that this work would provide some theoretical and technical references for the treatment of azo dyes-containing effluents.

Reagents

The target contaminant MB (C16H18N3ClS) was purchased from the Tianjin Beilian Fine Chemicals Development Co., Ltd. Tertiary butanol (t-BuOH, C₄H10O), sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were obtained from Sinopharm Chemical Reagent Co. Ltd. All reagents used in this study were of analytical grade and used without further purification. Deionized water was used for the preparation of the required solutions. The solution pH was adjusted using 0.1 mM H2SO4 or NaOH.

Experimental apparatus and procedures

Figure 1 shows the structural diagram of the O3/MC/US degradation system. O3 was generated by a DBD reactor and released into the MB solution for the primary ozonation degradation. Subsequently, a part of the MB solution and O3 gas were pumped into an ultrasonic microchannel reactor for enhanced ozonation degradation.
Figure 1

Structural diagram of the O3/MC/US degradation system: (1) DBD reactor, (2) DBD power supply, (3) oscilloscope, (4) ultrasonic power supply, (5) ultrasonic microchannel reactor, (6) peristaltic pump, and (7) wide-mouth flask with constant temperature water bath.

Figure 1

Structural diagram of the O3/MC/US degradation system: (1) DBD reactor, (2) DBD power supply, (3) oscilloscope, (4) ultrasonic power supply, (5) ultrasonic microchannel reactor, (6) peristaltic pump, and (7) wide-mouth flask with constant temperature water bath.

Close modal

O3 generation

A DBD reactor (Jiangnan University, China) with a coaxial cylinder structure was used to continuously convert air into O3 under room temperature and pressure. Two quartz glass tubes with a thickness of 2 mm and a length of 250 mm were used as the dielectric barriers, and their outer diameters were 40 and 48 mm, respectively. An air pump (Carmel Fluid Technology, China) was used to continuously feed air into the DBD reactor. A glass rotameter (Changzhou Shuanghuan, China) was used to control the inlet gas flow rate at 100 mL/min. A high-voltage power supply (CTP-2000K, Nanjing Suman Electronics, China) and a matching voltage regulator were used to apply an alternating voltage with a constant frequency of 6.8 kHz on the electrodes. The discharge voltage was measured by a voltage probe (P6015A, Tektronix, USA) and monitored by a digital oscilloscope (TBS2104B, Tektronix, USA). During the experiments, the DBD reactor was continuously air-cooled, and the residual O3 was decomposed in water at a temperature of about 70 °C.

Primary ozonation degradation

MB solutions with an initial concentration of 0.04 mM and varying pH values were configured using a precision electronic balance (Ningbo Yinzhou Huafeng, China) and a pH meter (Mettler Toledo Instrument, China). The primary ozonation degradation was carried out in a wide-mouth flask immersed in a constant temperature water bath. The O3 gas was released into the MB solution through a silicone hose, whose outlet was fixed at 10 mm below the liquid level. A peristaltic pump (Carmel Fluid Technology, China) was used to pump O3 gas and MB solution together into a microchannel through another silicone hose, whose inlet was fixed at 5 mm below the liquid level of the MB solution.

Enhanced ozonation degradation

As shown in Figure 2, an ultrasonic microchannel reactor composed of an ultrasonic bath, cover plate, capillary, and ultrasonic oscillator was designed for the enhanced ozonation degradation. Three capillaries with inner diameters of 0.8, 1.0, and 1.2 mm were used as microchannels. The ultrasonic bath with a length, width, and depth of 200 mm × 125 mm × 6 mm was installed by bolt connection with the cover plate with a length, width, and thickness of 190 mm × 100 mm × 5 mm. Six parallel grooves were carved on the underside of the cover plate for inlay of capillaries. In addition, a groove with a length, width, and depth of 190 mm × 40 mm × 2.5 mm was carved on the underside of the cover plate. The constant temperature water could flow through the groove and directly contact with the capillaries to control the reaction temperature. An ultrasonic oscillator (Wuxi Hesen Technology, China) with a resonance frequency of 28 kHz was installed at the bottom of the ultrasonic bath. An ultrasonic power supply (Shenzhen Kemeida Ultrasonic Equipment, China) was used to drive the ultrasonic oscillator with a maximum power of 100 W.
Figure 2

Diagram of the ultrasonic microchannel reactor.

Figure 2

Diagram of the ultrasonic microchannel reactor.

Close modal

Analytical methods

Percentage of color removal was used to analyze the progress of MB degradation. According to previous studies (Thangavadivel et al. 2014; Kodavatiganti et al. 2021), the decolorization efficiency, η, was defined as Equation (1).
formula
(1)
where Abs0 and Abst represent the MB concentrations obtained by spectrophotometry before and after the decolorization, in mM. The absorbance of the MB solution was measured by an ultraviolet–visible spectrophotometer (Shimadzu, Japan). The absorbance calibration plot was initially obtained according to the spectrophotometric results of MB solutions with concentrations ranging from 0.005 to 0.05 mM. 3 mL of the decolorized MB solution was sampled for each measurement and tested at a maximum absorption wavelength of 664 nm.
The total organic carbon (TOC) was determined by an organic carbon analyzer (Shimadzu, Japan). The TOC removal efficiency, E, was calculated as Equation (2) (Can et al. 2019).
formula
(2)
where TOC0 is the initial TOC concentration of MB solution, in mg/L. TOCt represents the TOC concentration of the degraded MB solution, in mg/L.
The percentage contribution of HO• to decolorization, C, was illustrated by Equation (3).
formula
(3)
where and refer to the MB decolorization efficiency obtained without and with HO• quencher, respectively. With sufficient quencher addition, the MB decolorization will be dominated only by O3 molecules, rather than by both O3 and HO•. Consequently, the difference between and represents the decolorization efficiency contributed only by HO•.

Statistical analysis

To reduce the systematic error during the measurement, each test sample was measured by taking the average of two measurements. Origin software was employed for the statistical analysis of the experimental results. The goodness-of-fit of the absorbance calibration plot was evaluated based on the R2 value (determination coefficient), where values less than 0.99 were considered statistically significant. Analysis of variance (ANOVA) was used to test the significance of the experimental results, and p < 0.05 was considered to be statistically significant.

Effect of discharge voltage of the DBD reactor

The discharge voltage of the DBD reactor affects the concentration of generated O3. The O3 generation can be improved by the increase of discharge voltage, while an excessive discharge voltage will cause a temperature rise, which may induce the decomposition of O3 into oxygen. Therefore, it is significant to study the effect of discharge voltage on MB degradation. The initial pH was adjusted to 7, and the reaction temperature was controlled at 23 °C. According to Paschen's law (Eichhorn et al. 1993; Samaranayake et al. 2000), only when the discharge voltage is higher than 7.2 kV, the air between two electrodes will be broken down and converted into O3. In the present work, the discharge voltage was adjusted by changing the regulating voltage. When the regulating voltages of 50, 60, and 70 V were applied, the corresponding discharge voltages measured by the oscilloscope were 7.8, 8.6, and 9.5 kV, respectively.

As shown in Figure 3, the MB decolorization efficiency increased with the increasing discharge voltage of the DBD reactor. When the discharge voltages were 7.8, 8.6, and 9.5 kV, the 14-min decolorization efficiency reached 71.0, 77.3, and 79.3%, respectively. The decolorization process was observed to follow first-order kinetics within 10 min, and the rate constants were 0.063, 0.065, and 0.081 s−1 at discharge voltages of 67.8, 8.6, and 9.5 kV, respectively. The increase in discharge voltage can reduce the population density of low-energy electrons, which decompose the generated O3, thereby improving the O3 yield (Kitayama & Kuzumoto 1997). Statistical results have shown that the decolorization efficiency obtained at discharge voltages of 7.8 and 8.6 kV was statistically different within 14 min. When the discharge voltage was 9.5 kV, the decolorization efficiency obtained within 10 min was statistically different. However, under the continuous treatment at 9.5 kV discharge voltage for more than 10 min, the decolorization efficiency hardly increased with duration, and no statistical difference was observed. A reasonable reason is that the DBD reactor was overheated after operating for several minutes under the discharge voltage of 9.5 kV (Miruka et al. 2021). The overheating was caused by the energy consumption during discharge. It can be inferred that the high temperature destroyed the stability of O3 gas, and also affected the discharge characteristics of the DBD reactor. In order to reduce the influence of O3 decomposition caused by excessive temperature rise, the subsequent experiments were carried out at the discharge voltage of 8.6 kV. Furthermore, for the process of large-scale wastewater treatment, overheating of the reactors are needed to be avoided via adequate cooling or appropriate reduction of the discharge voltage.
Figure 3

Effect of the discharge voltage of the DBD reactor on the MB decolorization efficiency in the O3 system.

Figure 3

Effect of the discharge voltage of the DBD reactor on the MB decolorization efficiency in the O3 system.

Close modal

Effect of liquid flow rate

The mixture of MB liquid flow and O3 airflow was pumped into the microchannel reactor by a peristaltic pump. Since the MB liquid plugs were separated by O3 gas pockets, a plunger flow was formed in the microchannel. The liquid flow rate of the MB solution can affect the length of the liquid plug, thus affecting the decolorization efficiency. As the liquid flow rate increases, the decolorization efficiency will therefore be improved as more MB solutions are involved in the degradation enhanced by microchannel. However, the increased liquid flow rate also led to a shorter residence time of the plunger flow in the microchannel, which could reduce the decolorization efficiency. Due to these two opposite effects on the decolorization efficiency, the effect of liquid flow rate was necessary to be studied. Experiments were conducted in O3/MC system with the liquid flow rates ranging from 5 to 20 mL/min, the discharge voltage was 8.6 kV, the microchannel width was 1.2 mm, the initial pH was 7, and the reaction temperature was 23 °C. Reynolds (Re) numbers were used to analyse the dominant hydrodynamic conditions of microfluidics at varying liquid flow rates. The computed Re number increased from 5.6 to 22.4 when the liquid flow rate increased from 5 to 20 mL/min. These low Re numbers indicate that the prevailing flow was laminar, hence the mass transfer process was diffusion controlled. In addition, the actual Re numbers should be slightly higher than the calculated values, because the significant mixing effect of ultrasound changed the inertia and viscous force of the microfluidics in the microchannel.

As shown in Figure 4, the decolorization efficiency increased when the liquid flow rate increased from 5 to 15 mL/min. A maximum decolorization efficiency of 86.2% was obtained within 14 min at a liquid flow rate of 15 mL/min. Nevertheless, if the liquid flow rate was higher than 15 mL/min, there was a negative effect on the MB decolorization. Correspondingly, kinetic studies revealed that the first-order rate constants were 0.046, 0.054, 0.058, and 0.054 s−1, when the liquid flow rates were 5, 10, 15, and 20 mL/min, respectively. The liquid flow rate affected both the specific surface area of the plunger flow and the residence time of the liquid slug in the microchannel. As the liquid flow rate increased from 5 to 15 mL/min, more MB solution entered the microchannel to involve in the enhanced ozonation degradation. Furthermore, the liquid convective effect in the microchannel was also intensified (Chaurasiya & Singh 2022), so the decolorization efficiency was increased. When the liquid flow rate increased from 15 to 20 mL/min, the liquid slugs of the plunger flow were excessively lengthened, thus reducing the gas–liquid mass transfer efficiency (Zhang et al. 2022). Additionally, the increasing liquid flow rate shortened the residence time of MB in microchannels, which also decreased the decolorization efficiency.
Figure 4

Effect of the liquid flow rate on the MB decolorization efficiency in the O3/MC system.

Figure 4

Effect of the liquid flow rate on the MB decolorization efficiency in the O3/MC system.

Close modal

Effect of microchannel width

For a microchannel reactor, microchannel width is one of the important factors that affect the gas–liquid mass transfer efficiency, thus affecting the decolorization performance. The effects of microchannel widths ranging from 0.4 to 1.2 mm on MB decolorization were investigated. Experiments were carried out in O3 and O3/MC systems at initial pH of 7, reaction temperature of 23 °C, discharge voltage of 8.6 kV, and liquid flow rate of 15 mL/min. The Re numbers of the microfluids in the microchannels were 50.4, 33.6, 25.2, 20.2, and 16.8, when the inner diameters of the microchannels were 0.4, 0.6, 0.8, 1.0, and 1.2 mm, respectively.

As shown in Figure 5, by comparing the results obtained in O3 and O3/MC systems, a combined effect of microchannel on ozonation degradation was clearly observed. The decolorization efficiency increased first and then decreased with the increasing microchannel width. When the microchannel width was 0.4, 0.6, 0.8, 1.0, and 1.2 mm, the 14-min decolorization efficiency reached 79.7, 82.8, 86.2, 84.7, and 80.4%, respectively. Additionally, the corresponding rate constants determined by kinetic studies were 0.066, 0.072, 0.076, 0.072, and 0.058 s−1, respectively. When the microchannel width increased from 0.4 to 0.8 mm, the 14-min decolorization efficiency increased from 79.7 to 86.2%. The improvement of decolorization efficiency can be attributed to the fact that the residence time of MB solution in the microchannel increased with the increase of microchannel width. Since the sufficient residence time ensured the contact time of MB solution and O3 gas, the decolorization efficiency was improved. When the microchannel width increased from 0.8 to 1.2 mm, the 14-min decolorization efficiency decreased from 86.2 to 80.4%. Since the prevailing flow in the microchannel was laminar, the gas–liquid mass transfer process was largely realized by molecular diffusion. The reduction of the microchannel width meant a faster molecular diffusion rate (Bingham & Dunham 1997). Therefore, despite the decreasing microchannel width reducing the residence time of the MB solution in the microchannel, the MB decolorization efficiency was still improved due to the significant enhancement of the gas–liquid mass transfer.
Figure 5

Effect of the microchannel width on the MB decolorization efficiency in O3 and O3/MC systems.

Figure 5

Effect of the microchannel width on the MB decolorization efficiency in O3 and O3/MC systems.

Close modal

Effect of ultrasonic power

Reasonable ultrasonic power helps in optimizing the operating cost for a given physicochemical transformation. Therefore, experiments were carried out in the O3/MC/US system to study the effect of ultrasonic power on MB decolorization. The discharge voltage was 8.6 kV, the initial pH was 7, the reaction temperature was 23 °C, the liquid flow rate was 15 mL/min, the microchannel width was 0.8 mm, and the Re number of the liquid plug in the microchannel was 25.2.

As shown in Figure 6, the MB decolorization efficiency increased with the increase of ultrasonic power. The 14-min decolorization efficiency reached 86.6, 89.1, and 90.4%, with corresponding kinetic rate constants as 0.99, 1.01, and 1.06 s−1, when the ultrasonic powers were 33, 66, and 100 W, respectively. The phenomenon that the application of ultrasound can promote decolorization is ascribed to the micromixing effect and thermal activation effect caused by acoustic streaming and acoustic cavitation, respectively (Ileri & Dogu 2022). With the increase of ultrasonic power, the reactions of O3 decomposition into reactive species were promoted. The energized radicals initiated a series of chain reactions with O3 to form extra free radicals (Chen et al. 2009). In the current experimental setup, the minimum power required for ultrasonic cavitation was about 33 W. Nonetheless, at high power levels (exceeded 66 W), the improvement of decolorization efficiency with the increase of ultrasonic power was not obvious. Excessive ultrasonic power resulted in bubble coalescence, bubble clustering, and acoustic impedance, thus reducing the cavitation effect. Thangavadivel et al. (2014) studied ultrasound-assisted decolorization of methyl orange in a microreactor. The results show that when the ultrasonic power was increased from 160 to 200 W, the decolorization efficiency was increased by less than 0.3%. The above observation by Thangavadivel et al. was consistent with Figure 6, which was also due to the obstruction of the cavitation process by excessive cavitation bubbles.
Figure 6

Effect of the ultrasonic power on the MB decolorization efficiency in the O3/MC/US system.

Figure 6

Effect of the ultrasonic power on the MB decolorization efficiency in the O3/MC/US system.

Close modal

In addition, the ultrasonic oscillator was overheated after a long-time operation, which is common in the applications of ultrasonic devices. In this study, the temperature rise generated by ultrasound was controlled by a constant temperature water circulation. The temperature difference between the inlet and the outlet of the microchannel remained within 3–5 °C, which was low enough to avoid the interference of temperature rise on MB decolorization.

Effect of reaction temperature

Since the macroscopic rate constant of O3 decomposition is a temperature dependent parameter (Miruka et al. 2021), the reaction temperature has a great influence on ozonation degradation. Therefore, the effect of reaction temperature was examined in the range of 23–60 °C. Experiments were carried out in O3/MC/US system with a discharge voltage of 8.6 kV, initial pH of 7, a liquid flow rate of 15 mL/min, microchannel width of 0.8 mm, the Re number of 25.2, and ultrasonic power of 100 W. The temperature was maintained with a thermostatic bath that supplied circulating water to the wide-mouth flask and ultrasonic microchannel reactor.

As shown in Figure 7, the decolorization efficiency of MB decreased with an increasing reaction temperature. At the room temperature of 23 °C, the 14-min decolorization efficiency reached 90.4%, which was 2.2, 10.7, and 30% higher than that of 30, 40, and 60 °C, respectively. When the reaction temperature was 23, 30, 40, and 60 °C, the first-order rate constants of the decolorization were 1.06, 0.72, 0.44, and 0.11 s−1, respectively. During the decolorization processes, the O3 gas was released into the MB solution and stayed in the wide-mouth flask for several seconds. Since the spontaneous decomposition of O3 into oxygen was promoted by the temperature rise, partial O3 molecules could not complete the decolorization reactions before their lifetime achievements. In addition, the excessive temperature in the microchannel has been reported to hinder the mass transfer of gas molecules into the solutions (Lian et al. 2021), thus further reducing the decolorization efficiency.
Figure 7

Effect of the reaction temperature on the MB decolorization efficiency in the O3/MC/US system.

Figure 7

Effect of the reaction temperature on the MB decolorization efficiency in the O3/MC/US system.

Close modal

Effect of initial pH

The pH of an aqueous solution can affect the number of oxides, the stability of reactive species, and the rate of chemical reactions during the ozonation process (Orhon et al. 2017). Therefore, the initial pH needs to be studied as a significant factor for ozonation degradation. It has been reported that when the pH value exceeds 11, excessive alkalinity has a scavenging effect on HO•. On the contrary, when the pH value is less than 5, the hydrolysis of O3 is negligible (Kalmaz & Trieff 1986), and no longer dependent on pH. Based on the aforementioned studies, the effect of initial pH on MB decolorization was examined in the pH range of 3–11. The experiments were performed in the O3/MC/US system with a reaction temperature of 23 °C, a discharge voltage of 8.6 kV, a liquid flow rate of 15 mL/min, a microchannel width of 0.8 mm, an Re number of 25.2, and an ultrasonic power of 100 W.

As shown in Figure 8, the decolorization was observed to conform to a first-order kinetic model, and the rate constants were 0.55, 0.78, 1.06, and 1.20 s−1, when the initial pH values were 3, 5, 7, 9, and 11, respectively. Since the HO• yield was effectively increased by the increase of HO dosage, the 14-min decolorization efficiency increased from 84.6 to 93.2% when the initial pH increased from 3 to 11. O3 interacts selectively with organics in a molecular state under acidic or circumneutral pH. While under the alkaline conditions (Munter 2001), O3 is easily hydrolyzed to produce highly reactive free radicals, mainly HO•. An obvious conversion of O3 to HO• (k = 2.2 × 106 M−1s−1) was confirmed at a pH of 9 (Gunten 2003; Fajardo et al. 2013). HO has been proven to be an initiator and promoter of O3 hydrolysis, as described by Equations (4)–(9) (Hoigne & Bader 1983).
formula
(4)
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)
Figure 8

Effect of the initial pH on the MB decolorization efficiency in the O3/MC/US system.

Figure 8

Effect of the initial pH on the MB decolorization efficiency in the O3/MC/US system.

Close modal

In addition, when the pH increased from 9 to 11, the 14-min decolorization efficiency increased from 92.7 to 93.2%. The slight improvement of the decolorization efficiency can be attributed to the gradual decrease in decolorization efficiency with the decrease of MB concentration (Zhang et al. 2013). Since a satisfactory efficiency can be obtained with a pH of both 9 and 11, an initial pH of 9 was chosen as a condition for subsequent experiments in consideration of saving reagents.

Combination effects of microchannel and ultrasound

Ultrasonic microchannel reactors have been successfully applied in liquid–liquid extraction (Thangavadivel et al. 2014) and Fenton degradation (John et al. 2016). The combination effects of microchannel and ultrasound in ozonation degradation were demonstrated, by comparing the MB decolorization efficiency obtained in O3, O3/MC, and O3/MC/US systems. Experiments were carried out at a discharge voltage of 8.6 kV, microchannel width of 0.8 mm, a liquid flow rate of 15 mL/min, the Re number of 25.2, ultrasonic power of 100 W, reaction temperature of 23 °C, and an initial pH of 9.

As shown in Figure 9(a), kinetics studies showed that the first-order rate constants were 0.56, 0.11, and 0.12 s−1 for the decolorization processed in O3, O3/MC, and O3/MC/US systems, respectively. The 14-min decolorization efficiency obtained in the O3/MC/US system was 92.7%, which was 12.6 and 2.4% higher than that in O3 and O3/MC systems. The decolorization performance observed in the O3/MC/US system was better than that in both O3 and O3/MC systems. Consequently, the combination effects of microchannel and ultrasound on ozonation process have been clearly confirmed. In addition, the combination effect provided by microchannel was found to be more obvious than that provided by ultrasound.
Figure 9

(a) Combination effects of microchannel and ultrasound on ozonation degradation. (b) TOC removal efficiency of O3, O3/MC, and O3/MC/US systems.

Figure 9

(a) Combination effects of microchannel and ultrasound on ozonation degradation. (b) TOC removal efficiency of O3, O3/MC, and O3/MC/US systems.

Close modal

The TOC removal was quantified to study the process of mineralization. As shown in Figure 9(b), the TOC removal was related to the decolorization process. For O3, O3/MC, and O3/MC/US systems, the 7-min TOC removal efficiency was 49.5, 54.8, and 56.1%, respectively, as well as the 14-min TOC removal efficiency was 57.5, 62.1, and 64%, respectively. Since part of the MB was degraded into organic intermediate compounds during ozonation, the TOC removal efficiency of each treatment method was lower than its decolorization efficiency. Anyhow, the TOC removal efficiency of both O3/MC and O3/MC/US treatments was higher than that of pure O3 treatment, indicating that both microchannel and ultrasound can enhance the ozonation process.

Contribution rates of HO• in ozonation degradation

The roles of various reactive species involved in degradation processes can be elucidated by scavenging experiments. In this study, scavenging experiments were conducted to investigate the contribution rates of HO• in O3, O3/MC and O3/MC/US systems. HO• was rapidly scavenged by t-BuOH (3.8 × 108–7.6 × 108 M−1s−1) (Miruka et al. 2021). The reaction temperature was 23 °C, initial pH was 9, discharge voltage was 8.6 kV, liquid flow rate was 15 mL/min, microchannel width was 0.8 mm, the corresponding Re number was 25.2, and ultrasonic power was 100 W.

As shown in Figure 10(a), in O3/MC/US system, the addition of t-BuOH showed an obvious inhibition in MB decolorization. The 14-min decolorization efficiency was 86.9, 79.9, and 78.3% with the t-BuOH concentrations of 30, 50, and 100%, respectively. If the t-BuOH concentration was higher than 50%, the decolorization efficiency was almost no longer related to the t-BuOH dosage. A reasonable explanation is the indirect ozonation caused by HO• was largely inhibited, and only the direct ozonation caused by O3 dominated the decolorization. The decolorization efficiency obtained with the inhibition of the radical scavenger was higher than that of some previous studies. This phenomenon can be attributed to the enhanced effect of microchannel and ultrasound on gas–liquid mass transfer. In O3/MC/US system, the nucleophilic attack of O3 was enhanced, so the decolorization efficiency can be maintained at a high level even after the HO• was quenched.
Figure 10

(a) Effect of the t-BuOH concentration on MB decolorization in the O3/MC/US system. (b) Decolorization efficiency of MB with and without t-BuOH addition in O3, O3/MC, and O3/MC/US systems. (c) Contribution of HO• to decolorization obtained by scavenging experiments in O3, O3/MC, and O3/MC/US systems.

Figure 10

(a) Effect of the t-BuOH concentration on MB decolorization in the O3/MC/US system. (b) Decolorization efficiency of MB with and without t-BuOH addition in O3, O3/MC, and O3/MC/US systems. (c) Contribution of HO• to decolorization obtained by scavenging experiments in O3, O3/MC, and O3/MC/US systems.

Close modal

Figure 10(b) shows the time-courses of MB decolorization with and without t-BuOH addition in O3, O3/MC and O3/MC/US systems. The complete scavenging of HO• reduced the 14-min decolorization efficiency by 9.8, 13.5, and 14.4% in O3, O3/MC, and O3/MC/US systems, respectively. Based on these results, the contribution rates of HO• were calculated and shown in Figure 10(c). Qi et al. (2015) found that the 5-min and 15-min contribution rates of HO• reached about 18 and 11% in the catalytic ozonation of phenacetin. Figure 10(c) shows that in O3 system, the 5-min and 14-min contribution rates of HO• reached about 26 and 12%, which were similar to the experimental results obtained by Qi et al. (2015). In addition, as the decolorization progressed from 2 to 14 min, the contribution rates of HO• decreased from about 50% to less than 20% in either system. As the decolorization proceeded, various small acidic molecules were produced as the intermediate products. The acidified solution hindered the HO• generation, thereby reducing the MB decolorization efficiency, which has been explained by Figure 8. Moreover, the contribution rates of HO• in O3/MC/US system were higher than those in O3/MC and O3 system under the same experimental conditions. The 14-min contribution rates of HO• were about 15.5, 14.8, and 12.2% in O3/MC/US, O3/MC, and O3 systems, respectively. As shown in Figure 10(b) and 10(c), the improvement rate on MB decolorization under the effect of ultrasound was not remarkably high. However, this is an observable trend, and could prove that ultrasound can enhance the ozonation process. During the experiments, the comprehensive effect of microchannel and ultrasound on the MB decolorization was existed. The MB decolorization efficiency was increased by 2.4% within 14 min due to the enhancement effect of ultrasound.

In addition, considering the complex chain reactions and interactions between reactive species, it was difficult to quantify all the contributors and precisely calculate their contributions. Therefore, the contribution rates obtained in this study were estimated values. Anyhow, the highest contribution rate of HO• was clearly observed in O3/MC/US system. The combination effects of microchannel and ultrasound were confirmed to promote the indirect ozonation caused by HO•, thus increasing the MB decolorization efficiency.

MB was experimentally degraded in the newly designed degradation systems. The effects of discharge voltage, liquid flow rate, microchannel width, ultrasonic power, initial pH and reaction temperature on decolorization performance were investigated. The results show that the MB decolorization efficiency could be increased by an increasing discharge voltage, ultrasonic power, and initial pH, as well as a decreasing reaction temperature. The decolorization efficiency would be increased by an appropriate liquid flow rate, but would be decreased by both insufficient and excessive liquid flow rates. An increase of microchannel width in the proper range would be conducive to the color removal, otherwise, the excessive microchannel width would even cause a decrease in decolorization efficiency. For 100 mL MB solution with an initial concentration of 0.04 mM, a 14-min decolorization efficiency of 92.7% was obtained in the O3/MC/US system, which was 12.6% higher than that in the pure O3 system. In O3/MC/US system, the 2-min and 14-min contribution rates of HO• reached 54.6 and 15.5%. In contrast, in pure O3 system, the HO• contribution rates at 2 and 14 min were only 48.9 and 12.2%, respectively.

The authors would like to thank Wuxi Hesen Technology Co., Ltd for providing ultrasonic oscillators and technical supports.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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