Continuous ultrasonic ozone coupling technology-assisted control of ceramic membrane fouling coupled enhanced multiphase mixing to treat dye wastewater and CFD flow field simulation

Highlights

• •A novel ultrasonic-ozone-coupled dye effluent treatment system was developed.
• •A detailed analysis of the flow state of the experimental device was conducted using CFD simulation method.
• •The addition of ultrasound increased the RhB reaction rate constant from 0.0134 min-1 to 0.0372 min-1.
• •Ultrasound can effectively control membrane fouling.

Abstract

In this study, ozone catalysts (hydrogenation-modified red mud, HM-RM) successfully prepared by hydrogenation-modification of industrial hazardous solid waste red mud (RM) as a raw material in accordance with the viewpoint of treating waste with waste and using waste. Meanwhile, as for the common phenomenon of membrane fouling, uneven distribution of multiphase solid catalysts and ozone in liquids, the addition of ultrasound can not only disperse materials, but also play a role in online cleaning of ceramic membranes and catalysts. The optimum treatment conditions for Rhodamine B (RhB) solution with volume of 2 L and concentration of 40 mg/L were catalyst concentration of 0.4 mg/L, reaction temperature of 45 °C, ultrasonic time of 1 h, ultrasonic intensity of 600 W, removal rate of RhB was up to 90 %. In addition, the computational fluid dynamics (CFD) simulation method was used to investigate the fluid flow between the two gas-liquid phases and the effect of the negative pressure of the membrane pump on the fluid by the analysis of flow, pressure and ozone flux of the ceramic membrane(CM) reaction apparatus. The CFD simulation results showed that at the inlet gas-liquid flow rate of 3 m/s and the negative pressure of 20,000 Pa, the maximum flow rates of CM-1 were 3 m/s, 0.752 m/s for CM-2, and 0.228 m/s for CM-3, respectively. Vortices, which are beneficial to solid-liquid mixing and gas-liquid mass transfer, formed between the suction port CM-1 of CM-1 and the inlets of CM-2 and CM-3. This discovery is consistent with relevant experimental research results. Significantly higher concentrations of both •OH and dissolved ozone were observed in the US/HM-RM/O₃ system compared to other systems, indicating the significant improvement in ozone utilization rate through the application of ultrasound. The superiority of the US/HM-RM/O₃ device was demonstrated. The real dye effluent was tested under optimum operating conditions and the results showed that COD and TOC were reduced by 81.34 % and 60.23 % respectively after 180 min of treatment. The above research can provide technical support for the treatment of dye wastewater using Ultrasound-enhanced ozone oxidation ceramic membranes.

1. Introduction

The textile industry consumes vast amounts of water and energy resources and is closely intertwined with national livelihoods. In the textile production process, the dyeing and finishing step consumes the greatest amount of water resources and can account for up to 20 % of global wastewater discharge [1]. This result in a substantial discharge of wastewater containing dyestuff into the aquatic environment. It should be noted that the chemical structure of different types of dye-effluent varies considerably. Many dyes have a complex aromatic hydrocarbon structure and exhibit high resilience to sunlight, oxidiser, and microorganisms, rendering them resistant to deterioration during conventional wastewater treatment [2]. This poses a non-negligible threat to both the ecosystems and human health, as their presence even at very low concentrations can exhibit strong color and COD pollution [3]. Other industries, including tanning, food processing, beverages, pharmaceuticals, cosmetics, and paper manufacturing, also discharge significant amounts of these pollutants in their wastewater [4]. In addition, it is evident that the dye wastewater also comprises numerous chemicals besides dyes. With the rise of stringent regulations, the efficient elimination of dyes from industrial wastewater has emerged as a serious issue for various sectors [5].

According to China's annual ecological environment statistics, the amount of textile wastewater in China reached approximately 184 million tonnes in 2021, equating to 1.41 % of all ten industries [6]. Therefore, there is an urgent need to develop a technology to eliminate dye particles permanently from textile wastewater would significantly benefit the environment and humanity. Currently, numerous physical, chemical, and biological methods are claimed to successfully degrade dyes in many research papers. These methods include coagulation and flocculation, membrane filtration, and oxidation [7]. Membrane filtration (e.g. reverse osmosis [8], nanofiltration, etc.) and biological treatment (e.g. anaerobic-aerobic [9], microalgae, etc.) are widely used, but membrane filtration can only concentrate the dyes, not degrade them, and biological treatment is high demanding on the influent water [2]. Advanced oxidation processes (AOPs) have been more extensively researched and adopted by the industry owing to their high oxidative capacity, high mineralization rate of organic pollutants, and lack of secondary pollution [10].

Nowadays, although a variety of applicable technologies for the oxidative degradation of dyes have been reported, the effect of using a single dye oxidation degradation technology is often unacceptable due to the inherent advantages and disadvantages of each oxidation technology [11]. To achieve discharge water quality that meets the requirements of the relevant regulations, single oxidation would be costly. According to Su et al. [12] A review had showed two or more treatment methods can be combined well without great technical difficulties. A combining process involves sequentially performing two or more treatment technologies, while a hybrid process integrates two or more treatments into individual steps. When compared to separate treatment methods and combination processes, the adoption of hybrid processes provides superior results in colour removal, COD reduction, ease of operation, and shorter degradation times. Bilińska et al. [13] used electro-flocculation and ozone coupling for the degradation of Reactive Black 5 at a concentration of 500 mg/L, which resulted in a degradation rate of up to 98 % in 10 min.

The optimum method for efficiently removing high levels of dyes from wastewater is to remove them rapidly without introducing further contamination [14]. Ozone oxidation (O₃), which has a reduction potential of 2.07 V, is a favored advanced oxidation technique [15]. One advantage is that O₃ reacts directly with organic compounds under acidic or neutral conditions and/or indirectly generates groups such as hydroxyl radicals (•OH), superoxide radicals (•O₂ ⁻ ), and other groups that cause decomposition reactions at high alkaline pH [16], [17]. Wang et al. [18] showed that ozonation has a good effect on the management of coloration (91.90 % removal rate, 64 min). Non-homogeneous catalysts have received increasing attention in recent years due to their ability to significantly improve ozone oxidation. However, heterogeneous catalysts will settle at the bottom of the liquid and be washed away with the flow of water. Ceramic membrane filtration offers a solution to the issue of the easy loss of heterogeneous catalysts and enables direct filtration of pollutants. However, during practical application, concentration polarization and membrane fouling can occur, leading to the deposition of particles and heterogeneous catalysts on the surface of the ceramic membrane. This deposition hampers the subsequent separation process [19].

The device utilized in this scientific investigation involves the combination of ultrasound(US) technology with a non-uniform catalytic ozone oxidation process for the purpose of effectively treating organic pollutants in water [20]. During the process of ultrasound catalysis, the occurrence of transient cavitation serves to enhance the turbulence within the solution, thereby facilitating the efficient mass transfer of both reactants and by-products between the solution and the catalyst surface [21] Simultaneously, the rupture of ultrasonic cavitation bubbles generates shock waves, which exhibits a cleansing effect on the catalyst and ceramic membranes surface and yield a substantial quantity of advanced oxidation radicals. On an alternative note, hazardous solid waste known as RM is typically generated as a by-product during the extraction of alumina from bauxite mines [22]. Owing to its strong alkaline properties and multifarious composition, it is difficult to find a suitable disposal or reuse solution for RM [23]. However, RM can be treated by hydrogenation modification to serve as a catalyst for catalytic ozone oxidation to reduce catalyst costs. The addition of a ceramic membrane can effectively retain the catalyst and prevent the catalyst run off while allowing the treated water to pass through the ceramic membrane, realizing catalyst retention for reuse and normal continuous operation of the treatment system. The study delved into the effects of several key variables on the degradation efficiency within the reaction device. These variables included the content of HM-RM, temperature, ultrasonic intensity, and ultrasonic time. Through systematic investigation, insights were gained into the optimal conditions for achieving efficient degradation. The mechanism of the reaction process was studied by UV–visible spectrophotometer and fluorescence photometer, these methods shed light on the effects occurring during catalytic ozone oxidation.

This study employs industrial waste red mud as the iron source for the catalyst. By undergoing simple hydrogenation modification, the formation of Fe₃O₄ is promoted, resulting in a significant improvement in the catalytic activity of RM [24]. Due to its high alkalinity and catalytic performance, HM-RM is considered is an excellent ozone catalyst for degrading dye wastewater as it eliminates the steps of dealkalization treatment. The objective of this study was to design a novel ultrasonic ozone coupling technology and incorporate HM-RM as an ozone catalyst in the device. The effectiveness of this technology was evaluated in terms of the degradation of RhB solution, actual dye wastewater, and the control of ceramic membrane fouling. Various parameters during the reaction process (catalyst content, temperature, ultrasound intensity and ultrasound time) on the degradation of RhB solution were discussed in detail. CFD simulation software was employed to simulate and analyze the flow, pressure, and ozone flow in the reaction device, investigating the fluid flow between the gas-liquid two-phase and the impact of the negative pressure of the diaphragm pump on the fluid. In addition, the recovery performance of 5 consecutive reuse cycles was studied, and the toxicity of HM-RM reaction was detected using inductively coupled plasma mass spectrometry (ICP-MS). The results obtained from this study offer an effective approach for the resource-oriented disposal of heavy metal waste, pollution control of ceramic membranes and the design and application of actual industrial wastewater facilities.

2. Experiments

2.1. Material

Red mud (RM, Chongqing Minfeng Chemical Co., LTD), ground the raw-red mud, then sieve it through a 50-mesh screen, and store it in a 60-degree oven for later use. Potassium iodide (KI, 99 %, Chengdu Kelong Chemical Reagent Factory), Rhodamine B (RhB, C₁₆H₈N₂Na₂O₈S₂, Chengdu Kelong Chemical Reagent Factory). All reagents used are analytical grade and can be used directly without further purification.

2.2. Preparation of HM-RM

The RM was ball milled at a rotational speed of 25 rpm for 40 min, and then crushed with an onyx mortar and sieved through a 50-mesh sieve to obtain small particles of RM, and labeled as raw RM. After ball milling, the RM was crushed with an agate mortar and then sieved through a 50-mesh sieve to obtain small particles of RM. Weighing 1 g of RM in a high-pressure reactor, the air in the reactor was withdrawn, and nitrogen and then hydrogen, respectively, repeated three times, the air in the reactor was exhausted, and finally the hydrogen was locked into a closed high-pressure reactor heating stirrer, the set conditions for the temperature of 200℃, the time was 1 h, and the pressure was 1 MPa to obtain the catalyst, and to be cooled to room temperature to take out the catalyst to obtain the HM-RM (Hydrogenation-modified RM)(Fig. 1).

Fig. 1.

The process of hydrogenation-modified of RM.

2.3. Experimental set-up

The reactor is mainly composed of an ozone generator (Taixing Green Ring Environmental Protection Machinery Manufacturing Co.,Ltd.), an embedded UV-2100 desktop ozone gas analyzer(Shandong Zhipu Measurement and Control Technology Co.,Ltd.), a diaphragm pump(SFDP2-050-055-42), a peristaltic pump (ChinaShanghai Zhixin Instrument Co.), and a KQ-600KED high-power CNC ultrasonic cleaner (Kunshan City, Ultrasonic Instrument Co.). The flat-plate ceramic membrane used in the setup is obtained from China Huahuai New Material Company and is known as the PCFM-A-100-3-M model. This ceramic membrane is mainly made of α-Al₂O₃ and has a surface pore size of 0.34 μm. The membrane has a porosity of 50 %. The membrane reactor and liquid storage tank are transparent reaction devices made of acrylic plates, allowing for clear visibility of the internal reaction process. The membrane reactor measures 16 cm × 13 cm × 8 cm (L × W × H) and features a specifically designed slot to securely hold the ceramic membrane in place. The liquid storage tank measures 14 cm × 14 cm × 11 cm (L × W × H). The detailed device structure diagram is shown in Fig. 2.

Fig. 2.

Schematic diagram of experimental set-up.

2.4. Device operation and analysis methods

Connected the required concentration of ozone (O₃) source, submerged the sand core aeration head in a beaker filled with pure water and then waited the airflow to stabilize, then place the membrane reactor with the ceramic membrane properly installed into the ultrasonic cleaning device and distributed the HM-RM into the membrane reactor device. Connecting the O₃ tail gas treatment device to the membrane reactor cover. Added 2 L of 40 mg/L RhB solution in reservoir 1^#, and then flow into reservoir 2^#, through the peristaltic pump would be in reservoir 2^# in the RhB solution pumped to the membrane reactor, the membrane reactor inside the liquid through the diaphragm pump with ceramic membrane filtration to remove the catalyst after the supply of the reservoir 1^#, the role of the reservoir 2^# was to avoided the diaphragm pump inflow of the peristaltic pump 1^# in the air bubbles generated by the effect of peristaltic pump. After the cycle was stabilized, 5 mL of water samples were recorded as C₀. Introduced the ozone into the membrane reactor after it had been stable for 30 min, and activated the ultrasonic device. 5 mL of sample was filtered through a polyethersulfone membrane (0.22 μm) every 5 min., recorded as C_t, and measure the absorbance at the wavelength λ = 554 nm using an ultraviolet spectrophotometer and recorded the results.

The concentration of RhB was measured using a TU-1901 dual-beam ultraviolet visible spectrophotometer. The indigo disulfonic acid sodium method is used to determine the dissolved ozone in water [25]. The fluorescence method is used to determine the concentration of dissolved •OH in water [26]. The detection methods for COD, Chemical oxygen demand (COD), Total organic carbon (TOC), Nitrogen, Phosphor, Ammonia, pH, Transparency (visual method), Chroma (Dilution times method), and Dissolved oxygen (DO) indicators in real dye wastewater refer to the《CJ/T 51–2018 Examination methods for municipal sewage》. Use the total organic carbon analyzer (TOC-L CPH) from Shimadzu, Japan to measure TOC. A three-dimensional excitation-emission matrix (3D-EEM) was obtained using a Fluorescence Spectrometer (Thermo Fisher, LF-1404014).Inductively coupled plasma mass spectrometry(ICP-MS,Thermo Fisher).

2.5. HM-RM characterization methods

An XRD-6000 X-ray diffractometer (XRD) manufactured by Shimadzu Co.Ltd. of Japan was employed to characterize the crystal morphology of the substances in the HM-RM. Cu Kα (λ = 0.154 nm) as the radiation source, accelerating voltage of 40 kV, current of 50 mA, step size of 0.02°, and scanning angle of 10°-80°. The physical structure of the catalyst was analyzed and studied mainly by scanning electron microscope (SEM) made by Zeiss (Zeiss Sigma 300), Germany, to observe the micro-morphological structure. X-ray fluorescence spectroscopy (XRF) model EDX4500H was used in this experiment to characterize the content of metal elements and metal oxides inside the catalyst. Fourier Transform Infrared Spectroscopy (FI-TR) of TENSOR27, BRUKER, Germany type was used in this experiment to detect the functional groups as well as the chemical structure of the samples. Thermo Fisher Scientific K-Alpha X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental species and valence distribution of the catalysts.

2.6. CFD simulation work information

CFD simulation work information is presented in the supporting information S1.

3. Results and discussion

3.1. HM-RM characterization

Fig. 3 depicts the XRD patterns, FT-IR spectra of samples of HM-RM and RM. The XRD analysis, as depicted in Fig. 3(a), showed that the RM samples mainly contained hematite, rutile, calcite, anatase, kaolinite, potassium magnesite, calcium olivine phases, and the main characteristic diffraction peaks corresponded to the 2θ values located at 21.5°, 24.3°, 27.54°, 33.3°, 54.2°, 64.18°and 72.06°, and the peak intensities did not change much [27]. Among them, the change in substance content is caused by the reduction of various oxides. It contains Fe₃O₄, TiO₂ and Al₂O₃, all of which are capable of catalyzing the formation of powerful oxidizing radicals from ozone [28]. XRF was used to characterize the content of metal elements and metal oxides inside the catalyst. The results are shown in Table 1, where the main substances are slightly changed.

Fig. 3.

(a) XRD patterns of RM and HM-RM; (b) FTIR patterns of RM and HM-RM.

Table 1.

XRF data sheet before and after hydrogenation modification.

Sample	Cr	Na2O	Al2O3	SiO2	K2O	CaO	TiO2	Fe2O3	Others
RM	0.14	1.45	26.38	4.97	0.44	5.36	6.74	42.81	11.71
HM-RM	0.12	0.85	25.99	5.18	0.35	7.21	4.38	44.52	11.40

The chemical state and the composition of the elements on the surface of the HM-RM were shown in the FT-IR spectra (Fig. 3(b)). The 3500–3000 cm⁻¹ region and 1632 cm⁻¹ attributed to O-H stretching vibrations (The presence of hydroxyl groups in ligand water molecules) and O-H bending vibrations (presence of physically adsorbed water on the RM), respectively [29]. The frequency band in the range of 919–999 cm⁻¹ is attributed to the antisymmetric SiO₄ stretching in the Ca₂SiO₄ tetrahedron vibration [30]. In addition, the band in the 460–500 cm⁻¹ region is the result of stretching vibrations of Fe-O bonds(hematite) [31].

The surface morphology, structure and surface elemental composition of RM and HM-RM were analyzed by SEM as shown in Fig. 4. The SEM images showed that a comparison of the images of RM and HM-RM indicated that both RM and HM-RM appeared to have uneven and irregular surfaces. Under magnification, large hexagonal particles can be observed on the surface of the sample, which should be hematite. This result is consistent with the XRD results and further confirms that the main component of RM is hematite. In addition, compared with RM, the surface particles of the RM after hydrogenation modification show agglomeration and the surface elements mainly contain O, Fe, Al and Na.

Fig. 4.

SEM images of RM (a)–(b),SEM/EDS images of HM-RM(c) -(d) and SEM/mapping images of RM(e).

XPS characterization is presented in supporting information S2.

3.2. HM-RM catalyzed ozone oxidation coupled with ceramic membrane combined with ultrasonic enhancement for dye wastewater treatment performance study

3.2.1. The degradation performance of the device

The rate of degradation of RhB under a variety of reaction conditions was shown in Fig. 6. Reactive ceramic membranes were used in combination with ultrasonic enhanced degradation of RhB by catalytic ozone oxidation was carried out on HM-RM with concentrations of 0 mg/L, 0.1 mg/L, 0.2 mg/L, 0.3 mg/L, 0.4 mg/L, and 0.5 mg/L, respectively. The reaction conditions were O₃ concentration of 1.5 g/m³, 40 mg/L 2 L RhB solution, an ultrasonic intensity of 600 W, and an ultrasonic time of 1 h. It can be seen from Fig. 6(a and b) that the concentration of RhB solution did not change significantly with the increase in HM-RM content. The degradation efficiency improved with increasing HM-RM content from 0 mg/L to 0.4 mg/L but slightly decreased when 0.5 mg/L was added. Based on the observed phenomenon, it is evident that increasing the concentration of the catalyst within a specific range leads to an increase in the number of active sites and the generation of more hydroxyl radicals. However, surpassing the concentration threshold of the catalyst results in an excess of hydroxyl radicals that compete with each other, ultimately reducing the overall number of hydroxyl radicals [32]. Therefore, the optimal HM-RM content was determined to be 0.4 mg/L.

Fig. 6.

Different Ultrasonic time Degradation effect and first-order kinetic diagram of RhB under different conditions of (a)HM-RM doses, (b) temperature, (c)intensity, (d)sonication time; first-order reaction constant k (b), (d), (f), (h),3D cylindrical graph representing the value of C_t/C₀ at the end of the reaction.

The energy during the ultrasonic process can be absorbed by the medium and then converted into thermal energy, thereby increasing the temperature of the medium, which has a significant impact on the degradation performance of RhB [33]. Set the reaction conditions to an O₃ concentration of 1.5 g/m³, HM-RM content of 0.4 mg/L, a 40 mg/L 2 L RhB solution, an ultrasound intensity of 600 W, and an ultrasound time of 1 h. By varying the reaction temperature to 15℃, 25℃, 35℃, and 45 °C, the impact of temperature on the degradation efficiency of RhB was investigated. Fig. 6(c and d) presents the obtained results, revealing a notable rise in the degradation rate constant from its initial value of 0.0138 min⁻¹ to 0.0372 min⁻¹ as the reaction temperature increases and the degradation rate can reach over 95 % after 60 min of reaction. This phenomenon occurs due to the cavitation effect, where an increase in temperature can form a sufficient number of gas nuclei, significantly enhancing the cavitation effect. [34]. On the one hand, a further increase in temperature will increase the solvent vapor pressure, leading to the collapse of the cavity buffer and reducing the dissipation of cavitation energy [35]. On the other hand, it can also intensify the interactions between substances in the reaction system, resulting in a significant increase in the reaction rate [33], [36].

The study also examined the impact of ultrasonic intensity on the rate of RhB degradation. The investigation was carried out under specific conditions of an initial O₃ concentration of 2 g/m³, an HM-RM concentration of 0.4 mg/L, a reaction temperature of 45℃, and a reaction time of 1 h. The results are shown in Fig. 6(e and f) with the ultrasonic intensity from 0 W to 600 W, the degradation rate constant rose significantly from 0.0134 min⁻¹ to 0.0372 min⁻¹ and the reaction rate also grew. The reaction rate under the condition of ultrasonic power of 600 W is 2.5 times greater than that of the unamplified ultrasonic system. This indicates that the stronger the cavitation of ultrasound, the more it can enhance the degradation of RhB. Meanwhile, because the maximum ultrasonic power of this ultrasonic reaction device is 600 W, the optimal ultrasonic power of 600 W was selected as the best treatment effect. The above trend is mainly due to the fact that higher ultrasonic power levels can intensify ultrasonic cavitation, which enhances the transfer of mass between water quality, HM-RM and O₃, whilst simultaneously stimulating the production of potent oxidizing free radicals, such as •OH. This, in turn, accelerates the reaction rate [37].

Under the given conditions of an initial ozone concentration of 2 g/m³, HM-RM concentration of 0.4 mg/L, reaction temperature of 45 ℃, and ultrasonic intensity of 600 W, the study aimed to investigate the effect of ultrasonic time on the degradation of RhB. As illustrated in Fig. 6(g) and (h), the reaction rate increased consistently with the prolongation of ultrasonic time. The rate constant for degradation gradually increased from its initial value of 0.0134 min⁻¹ to 0.0372 min⁻¹. Additionally, when ultrasonic systems were integrated throughout the process, the degradation rate of the reaction was 3.7 times higher compared to the reaction without ultrasonic systems. Due to the heavy mass of the HM-RM catalyst, when it directly added to a reactive ceramic membrane device that catalyzes ozone oxidation, once the ultrasound is stopped, the HM-RM will sink to the bottom of the device and cannot fully contact O₃ in the water body, thus inhibiting the generation of strong oxidizing free radicals and the cavitation effect will also be stopped. O₃ can also be better distributed on the membrane surface under the action of ultrasound, reducing the accumulation of organic matter [38]. The cleaning effect can be attributed to the fact that ultrasonic waves provide sufficient mechanical vibration energy, which prevents some particles in the solution from settling on the membrane surface and avoids the deposition of fine particles [39]. From two aspects, the change in ultrasound time will also change the degradation efficiency of RhB [40]. Continuous ultrasound during the reaction process can simultaneously disperse catalysts and remove pollutants on the membrane surface, ensuring the flux of the membrane and stabilizing the degradation effect. Additionally, the ceramic membrane's strong characteristics allow the cavitation bubbles generated by ultrasonic waves to effectively remove the filter cake layer formed on the membrane surface without causing any damage to the ceramic membrane [41].

3.2.2. Performance of membrane flux and degradation during long-term reaction processes

Under the given conditions of an initial ozone concentration of 2 g/m³, HM-RM concentration of 0.4 mg/L, reaction temperature of 45 ℃, and ultrasonic intensity of 600 W, the study aimed to investigate the changes of membrane flux and degradation during the reaction process. Fig. 5 demonstrates the fluctuation of the membrane flux and the degradation of RhB, initially decreasing and then increasing, as the RhB solution undergoes gradual degradation. Eventually, the membrane flux stabilizes at approximately 0.21 m³/(m²•h). This phenomenon can be attributed to the initial adsorption of a significant amount of catalyst by the ceramic membrane during the reaction. Additionally, RhB solution was rejected on the surface of the membrane to form an organic filter cake. However, as the reaction progresses, the catalyst disperses gradually from the membrane surface and also effectively controlling the growth of the organic filter cake. Consequently, the flux is restored and stabilized to a certain extent (Fig. 11).

Fig. 5.

Performance of membrane flux and degradation during long-term reaction processes.

Fig. 11.

(a)-(g) The SEM(a) and EDS(b) analysis of US 5 min-CM and US + O₃ 60 min-CM;appearance of ceramic film(h)CM,(i)US 5 min-CM and (j)US + O₃ 60 min-CM;(K)Schematic diagram of ultrasonic cavitation bubbles cleaning the membrane surface.

3.3. Flow field simulation

The above study reveals that various parameters have a direct effect on the effectiveness of the catalyst in treating wastewater, while the flow conditions inside the reactor have an effect on the distribution pattern between the polymorphic substances, which in turn affects the overall reaction process. For this reason, this part of the work will adopt the computational fluid dynamics approach to analyze the phase distribution, pressure distribution, velocity distribution, etc. in the above system from the perspective of microscopic flow field evolution, to obtaining the fluid flow migration characteristics to provide guidance for the structural and control parameters of the catalytic oxidation system. The related flow field simulation control equations, boundary conditions and other work are shown in supporting information.

3.3.1. Ceramic membrane surface flow field simulation

The effect of a fluid with an inlet velocity of 3 m/s and different pressures on the surface of the ceramic membrane is shown in Fig. 7. The ceramic membrane experiences a significant negative effect on the inlet water flow. When the membrane is closer to the WI, the velocity of the influent water generates greater velocity intensity. This, in turn, reduces the force impact on the surface of the ceramic membrane. Additionally, the velocity diffusion width decreases. This phenomenon occurs only at a flow inlet rate of 3 m/s for CM-1 and a maximum flow rate of 0 at the upper end for CM-2 is 752 m/s, CM-3 at the quickest flow rate is reduced to 0.228 m/s, which corresponds with the experimental process of the reaction apparatus. The speed disparity among the types causes a variation in the flow rate across different regions of the surfaces of CM-2 and CM-3. Because AI-1 and AI-2 are at the low end of the position, according to the turbulence pattern in Fig. 7(a), it can be seen that most of the gas flow in AI-1 flows directly into the outlet, which is caused by the impact effect of the WI flow velocity on the gas, which fails to form the gas velocity on the surface of CM-1. While the flow velocity on the other parts of the two ceramic membranes CM-2 and CM-3 is caused by the incoming gas flow of AI-2, AI-2 is subjected to the flow velocity of the water body which has been blocked by CM-1. At this time, the highest flow velocity observed from CM-2 is 0.782 m/s. This significantly reduces the impact on the gas flow, allowing for a more stable flow velocity in the other areas of CM-2 and CM-3. This phenomenon suggests that the catalysts may be more evenly mixed on CM-2 and CM-3.

Fig. 7.

(a)-(d) Effect of different pressures and (e)-(h) different flow rates on the surface of ceramic membranes.

Static pressure refers to the pressure exerted on the surface of an object when it is at rest or in uniform linear motion. Fig. 7(e–h) represents the effect of water flow on the pressure generated on the surface of the ceramic membrane. The device accommodates two phases of gas and liquid, and generates two types of static pressures: hydrostatic pressure due to water flow and pressure caused by the impact of ozone gas flow. From the figure, it can be seen that the further away from the WI among the three ceramic membranes, the smaller the pressure is, which is due to the ceramic membrane blockage the water flow, so that the static pressure of the impact point on the membrane becomes smaller. From the position of the air inlet, the pressure ranges of CM-2 and CM-3 have a significantly higher pressure range than CM-1, possibly due to the static pressure generated by the impact of the ozone gas stream. As the flow velocity decreases from bottom to top, the impact force also decreases, causing the pressure to decrease sequentially from bottom to top. From Fig. 7(c–e), the water flow enters from WI to penetrate CM-1 and then is blocked, while a small portion of the fluid can impact CM-2.AI-1 and AI-2 enter from the lower end, and the buoyancy force decreases with the height, which makes the force act on the ceramic membrane decrease, which further confirms the influence of the pressure of the gas-liquid two-phase on the surface of the ceramic membrane. It can be seen that this phenomenon results in a uniform upward flow of fluid, which promotes gas-liquid mass transfer and better degradation.

3.3.2. Simulation of eddy currents in the reaction unit

Fig. 8(a – e) shows the effect of different inlet and outlet turbulence patterns and countercurrent lines. From the Fig. 8(a and b), it can be seen that the gas-liquid phase can fill the entire interior of the reaction device. Fig. 8(c) shows the flow pattern of the water flow, which enters from the inlet and first touches CM-1 thus causing the fluid flow in both upward and downward directions, and due to the existence of the gap between the ceramic membrane and the reaction device, the water flow can flow to other parts of the reaction device. Fig. 8(d and e) shows the two ozone inlets, clearly showing that the left inlet can completely overflow the entire unit, while the right inlet, due to the combined effect of the inlet and outlet, causing the ozone stream to flow quickly with the water into the outlet and therefore cannot completely overflow the entire unit.

Fig. 8.

Effects of different inlet and outlet turbulence patterns and streamline reversal, (a)Turbulence pattern, (b)Gas-liquid two-phase streamline reversal, (c)WI streamline direction, (d)AI-1 streamline direction, (e)AI-2 streamline direction), (f)-(i)Vortex diagram inside the reaction unit.

Fig. 8(f–i) shows a vortex diagram inside the reactor unit, which shows the interaction between the gas-liquid phases at different locations to form certain vortices, which are mainly generated by the effects of jetting, impingement development and diffusion. As shown in Fig. 8(f), the vortex mainly exists in the water inlet and the ozone inlet and outlet position. After the water enters, it rapidly hits the surface of the ceramic membrane to form a vortex at the position of the water inlet, the water spreads out in different directions, and the boundary layer on the surface of the jet is unstable, forming a vortex. The right ozone inlet is affected by both the ceramic membrane and the in flowing water, forming a vortex near the inlet. The left ozone inlet, on the other hand, forms a small vortex near the inlet due to the influence of the gravity of the water itself and the buoyancy of the airflow in the water. The outlet due to the pressure difference is too large for the water and air to flow here to form a vortex. The addition of the gas phase disrupts the free flow of the liquid-phase fluid, forming a chaotic region of transitional turbulence. Under the dual action of fluid gravity and bubble buoyancy, these vortices formed make the gas-liquid two-phase contact sufficiently, so that RhB is degraded to a large extent.

3.3.3. Safety and reusability assessment of HM-RM

In order to assess the reusability and safety of HM-RM,the catalyst used was washed with deionized water and dried in a 40 °C oven, followed conduct five consecutive rounds of ultrasonic-ozone catalytic oxidation degradation experiments on it. Since red mud is a type of heavy metal waste that may pose safety risks in water treatment, metal ions in the solution after the reaction were detected using inductively coupled plasma mass spectrometry(ICP-MS)(Table. 2).

Table 2.

ICP-MS analysis of solution.

Element	Mg	Ti	V	Cr	Mn	Fe
Content(μg/L)	7.41	2.46	4.10	19.47	3.85	68.40

The catalytic performance of HM-RM did not significantly decrease after three cycle tests, as showen in Fig. 9. Degradation efficiency remained at 64.78 % in the fifth experiment, indicating that the catalyst maintained good stability during repeated use. Table 2 demonstrates that the heavy metal contents also meet the requirements of the 'Surface Water Environmental Quality Standard GB 3838–2002′, confirming the safety of HM-RM.

Fig. 9.

Recyclability of the HM-RM.

3.3.4. Degradation and clean mechanism studies

In the experiments on the mechanism of RhB degradation in O₃, US/O₃, HM-RM/O₃ and US/HM-RM/O₃ systems, the amount of dissolved ozone in the water and the concentration of •OH were determined by using the sodium indocyanine disulfate method and the fluorescence method, respectively. Based on the findings depicted in Fig. 10(a and b), the levels of dissolved ozone and •OH in the water exhibited a constant augmentation throughout the course of the reaction. The concentration of dissolved ozone in both the O₃ and US/O₃ systems demonstrated a gradual escalation, whereas the HM-RM/O₃ and US/HM-RM/O₃ systems showcased a more accelerated rise in dissolved ozone within the initial 30 min. However, after the second 30 min, there was a noticeable deceleration at the rate of increase. At the end of the reaction, the ozone concentrations in the four systems were 1.51 mg/L, 1.73 mg/L, 2.45 mg/L, and 2.99 mg/L, respectively. The US/HM-RM/O₃ system had an almost two-fold increase in ozone dissolution rate compared to the O₃ system. This increase not only improved the utilization rate of ozone but also enhanced the treatment efficiency.

Fig. 10.

(a) Concentration of dissolved O₃ in water and (b) concentration of generated •OH in water, (c) Mechanism diagram of HM-RM catalytic ozone degradation of RhB.

For the hydroxyl radical concentration in water, the trend of the •OH concentration in water in the four systems was almost steadily increasing in 60 min, but the •OH concentration in water in the O₃/HM-RM/US system was obviously faster than the other three systems, with the •OH concentrations of the four systems being 102.76 μmol/L, 128.43 μmol/L, 135.96 μmol/L and 171.20 μmol/L at 60mins respectively. The concentrations of •OH in the four systems were 102.76 μmol/L, 128.43 μmol/L, 135.96 μmol/L and 171.20 μmol/L. The concentration in the O₃/HM-RM/US system was about 1.7 times higher than that in the Ozone-alone condition, which greatly promoted the degradation of RhB.

The cleaning effect of ultrasound on the membrane is mainly manifested in the spearing of the catalyst on the membrane surface, as showen in Fig. 11(h–j). After only 5 min of ultrasonic reaction,a large amount of catalyst is adsorbed on the membrane surface, resulting in a red. After 60 min of US/O₃, a large amount of catalyst has been removed from the membrane surface, resulting in a light brown. The microstructure and elemental composition of the ceramic film surface was determined by SEM/EDS after only 5 min of ultrasound reaction(US 5 min CM) and 60 min of US/O₃ reaction(US + O₃ 60 min CM), as shown in Fig. 11(a–g). The SEM results indicated that the US 5 min CM surface was covered with many particulates, while EDS showed an iron content of 59.65 %, while the US + O₃ 60 min CM surface has much less particulates. EDS shows an iron content of 10.58 % which is consistent with the appearance of Fig. 11(i and j). It is worth noting that EDS also showed an increase in carbon content from 0 to 7.03 % from CM to US 5 min CM. However, the carbon content of from US 5 min CM to US + O₃ 60 min CM only increased by 2.05 %, which also proves that the device designed in this experiment is able to effectively clean the membrane under conditions that ensure the degradation of pollutants.

Ultrasonic waves in the experimental setup can induce cavitation phenomena, leading to the nucleation, growth, and collapse of cavitation bubbles [42]. During the cavitation process, rapid bubble growth creates a vacuum, causing the liquid to instantly occupy the space and eventually implode the vacuum space(cavity) [43]. The rupture of the cavity is accompanied by the release of a large amount of energy in a short period of time, leading to the generation of supercritical conditions, such as sonoluminescence, local high temperatures of up to 5000 K, pressures of 1000 atm, and cooling rates of 109 K/s [44]. These extreme conditions generate•OH radicals in the solution, which oxidize contaminants on the ceramic membrane's surface and control the formation and growth of the filter cake layer. Additionally, acoustic flow, microflow, microjet, and shock waves are generated [45]. These intense fluid flows remove dirt from the surface of the ceramic membrane and prevent the deposition of contaminants. In the experiment's device (Fig. 11(k)), the ultrasonic cavitation phenomenon causes bubbles to directly impact the ceramic membrane's surface, removing the HM-RM and RhB adsorbed by the membrane. Simultaneously, the ultrasonic waves disperse O₃ evenly around the ceramic membrane, significantly enhancing the removal of dirt from its surface.The results indicate that ultrasonic ozone effectively alleviates catalyst fouling and organic filter cake by cleaning the surface of the ceramic membranes and controlling the increasing concentration of organic contaminants on the membrane surface, Thereby ensuring relative stability of the flux.

3.3.5. Treatment of real textile dye effluent

The real textile dye effluent used in this study was obtained from a textile factory located in Nansiba Industrial Park, Mianyang City, Sichuan Province,which is grayish green in color and accompanied by turbidity. The wastewater was generated from the bleaching and printing process. The main characteristics of the wastewater before and after degradation are shown in Table 3.

Table 3.

Characterization of the textile dye wastewater.

Parameter	Units	0 min	180 min
Chemical oxygen demand (COD)	mg/L	2610	487
Total organic carbon(TOC)	mg/L	509.4	202.6
Nitrogen	mg/L	79.5	45.4
Phosphor	mg/L	99.3	85.2
Ammonia	mg/L	28.6	7.4
pH	\	12.56	12.10
Transparency	cm	2.7	21.6
Chroma	\	16	4
DO	mg/L	2.02	0.89

To account for the complex composition of the actual dye wastewater used in the experiment, a treatment time of 180 min was set. Samples were taken every 60 min during this period to measure COD and TOC. Additionally, Fluorescence extraction-emission matrix (EEM) spectroscopy technology was employed to characterize the dye wastewater before and after degradation (Fig. 12(b and c)).

Fig. 12.

(a) COD and TOC level as a function of time for textile wastewater, under optimal operation conditions for the O₃ device, (b) and (c) 3-D EEM spectra of the textile dye effluent before and after degradation, (d) AOS and COS values before and after treatment,(e) Color change of dye wastewater under different degradation times (0 min, 60 min, 120 min, 180 min).

Eqs. (1), (2) were used to find the COD and TOC removal rates, respectively:

Removal_COD(%)=(COD₀-COD_t)/COD₀ (1)

Removal_TOC(%)=(TOC₀-TOC_t)/TOC₀ (2)

Following a 180-minute reaction period, the removal rates of COD and TOC were 81.34 % and 60.23 %, correspondingly. Consistently throughout the degradation process, the rate of COD degradation surpassed that of TOC. This occurrence can potentially be ascribed to the oxidation procedure of colored wastewater, causing the generation of intermediate products and steady organic substances, which are then represented as TOC [46].

The EEM spectrum is divided into five areas, with the depth of color representing the fluorescence intensity. In Fig. 12(b and c), the V and IV areas show significant fluorescence intensity, which corresponds to soluble microbial byproducts (Zone IV) [47] and humic acid substances (V zone) [48]. During the HM-RM catalytic ozone oxidation process, the fluorescence intensity of the V region decreased while the fluorescence intensity of the IV region increased. This indicates that humic acids were degraded to soluble microbial products (SMP) and the biochemistry of the effluent was improved.

To further assess the enhancement in biodegradability of dye effluent, the measurement of the mean oxidation state (MOS) and carbon oxidation state (COS) is employed. The determination of AOS and COS was performed using the Eqs. (3), (4) [49]:

AOS = 4-(1.5 COD_t/TOC_t) (3)

COS = 4-(1.5 COD_t/TOC₀) (4)

where, after catalytic O₃ degradation for t min, the chemical oxygen demand (mg/L) and total organic carbon (mg/L) of the HM-RM sample were expressed as COD_t and TOC_t, respectively. The AOS or COS value falls within the range of −4 to +4. A value of −4 signifies CH₄, which represents the most reduced form of carbon (C), whereas a value of +4 signifies CO₂, indicating the most oxidized form of carbon (C) [45].

Fig. 12(d) presents the values of AOS and COS increased dramatically from −0.83 to +0.39 and +2.57, respectively. These results not only confirmed that the degree of mineralization was within reasonable limits, but also proved that more oxidizing substances were produced during the reaction process and that the biodegradability of the dye wastewater was significantly increased. Fig. 12(e) shows that as the degradation time increased, the color of the dye effluent changed from turbid grey-green to more transparent. This proves that the experimental device in this article has an excellent decolorization effect.

4. Conclusions

In conclusion, the study investigated the process parameters of the ultrasound-enhanced HM-RM-catalyzed ozone oxidation coupled with ceramic membranes treatment system, and prove the effects. In the experiment, the optimal conditions of HM-RM concentration of 0.4 mg/L, temperature of 45℃, O₃ concentration of 2 g/m³, ultrasonic intensity of 600 W, and ultrasonic time of 1 h were used to degrade 40 mg/L of 2 L RhB solution, and the degradation rate could reach 95.81 %. CFD numerical calculations were used to simulate the gas-liquid two-phase flow field mixing in this reaction system. The larger the flow rate is, the larger the velocity intensity on the surface of the ceramic membrane is. The passage of the gas phase, the gas-liquid two-phase in the ozone-ceramic membrane reactor collision to form a vortex, and the two phases’distribution are fully in contact with each other and are conducive to the mass transfer. The results indicate that ultrasound facilitates the dispersion of HM-RM catalysts and enhances vortex formation, thereby favoring gas-liquid mass transfer and enhancing the catalytic ozone oxidation efficiency of powder catalysts.

Compared to the RhB solution, the experimental device exhibited lower efficiency in reducing real dye wastewater. This can be attributed to the presence of complex and recalcitrant compounds in the actual dye wastewater. During the operation of the device, Under the action of ultrasound ozone, there is no significant change in the flux of ceramic membranes at the end of the reaction. The 3D-EEM spectrum demonstrates that the experimental device successfully reduces the pollutant concentration and enhances the biodegradability of the wastewater. ICP-MS analysis further confirms the safety of HM-RM in the treatment of dye-effluent. In addition, HM-RM showed good stability in the reuse experiment. The research results indicate that the device has a good degradation effect on dye wastewater and has the potential for practical industrial treatment of dye wastewater.

CRediT authorship contribution statement

Jinshan Tang: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Zhiliang Cheng: Writing – review & editing, Validation, Supervision, Investigation, Funding acquisition, Formal analysis, Data curation. Xuan Zhang: Writing – review & editing, Supervision, Project administration, Formal analysis, Data curation. Jinyu Sun: Supervision, Conceptualization. Zhaoqiang Liu: Validation, Methodology. Hao Zhang: Investigation, Data curation. Shengmei Tan: Software, Investigation. Facheng Qiu: Writing – review & editing, Supervision, Methodology.

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: