Sonophotocatalytic degradation of malachite green in aqueous solution using six competitive metal oxides as a benchmark

A comparison study examines six different metal oxides (CuO, ZnO, Fe3O4, Co3O4, NiO, and α-MnO2) for the degradation of malachite green dye using four distinct processes. These processes are as follows: sonocatalysis (US/metal oxide), sonocatalysis under ultra-violet irradiation (US/metal oxide/UV), sonocatalysis in the presence of hydrogen peroxide (US/metal oxide/H2O2), and a combination of all these processes (US/metal oxide/UV/H2O2). The effective operating parameters, such as the dosage of metal oxide nanoparticles (MONPs), the type of the process, and the metal oxides’ efficiency order, were studied. At the same reaction conditions, the sonophotocatalytic is the best process for all six MOsNPs, CuO was the better metal oxide than other MOsNPs, and at the sonocatalysis process, ZnO was the best metal oxide in other processes. It was found that the metal oxide order for sonocatalytic process is CuO > α-MnO2 ≥ ZnO > NiO ≥ Fe3O4 ≥ Co3O4 within 15–45 min. The order of (US/metal oxide/UV) process is ZnO ≥ NiO ≥ α-MnO2 > Fe3O4 ≥ CuO ≥ Co3O4 within 5–40 min. The order of (US/ MOsNPs/ H2O2) process is ZnO ≥ CuO ≥ α-MnO2 ≥ NiO > Co3O4 > Fe3O4 within 5–20 min. The maximum removal efficiency order of the sonophotocatalytic process is ZnO ≥ CuO > α-MnO2 > NiO > Fe3O4 ≥ Co3O4 within 2–8 min. The four processes degradation efficiency was in the order US/MOsNPs ˂ US/MOsNPs/UV ˂ US/MOsNPs/H2O2 ˂ (UV/Ultrasonic/MOsNPs/H2O2). Complete degradation of MG was obtained at 0.05 g/L MONPs and 1 mM of H2O2 using 296 W/L ultrasonic power and 15 W ultra-violet lamp (UV-C) within a reaction time of 8 min according to the MOsNPs type at the same sonophotocatalytic/H2O2 reaction conditions. The US/metal oxide/UV/H2O2 process is inexpensive, highly reusable, and efficient for degrading dyes in colored wastewater.


Introduction
Human health and the environment are at risk due to increased organic pollutants in wastewater. Malachite green (MG) has long been used as a representative dye for dyeing wool, silk, paper, textiles, and leather [20]. Its task of removing industrial effluents is critical [55,56].
Due to secondary pollution, chemical treatment of wastewater containing dyes is inappropriate. To more effectively remove hazardous dyes from wastewater, additional novel research is required [22,56]. Coagulation-flocculation, ozonation, membrane filtration, electrolysis, oxidation, active sludge biochemical processes, bio-decolorization, and other physical and chemical techniques have been widely used to remove dyes from wastewater [9,24,27,28,43,45]. These well-established technologies are frequently incapable of lowering the dye concentration to the desired effective level.
Among the advanced oxidation processes (AOPs), photocatalysis by semiconducting materials has become a leading technology in wastewater purification because of these materials' high stability and activity [6,10,26,47,54]. The heterogeneous photocatalytic process is a legitimate method for oxidizing organic pollutants to carbon dioxide and water in an aqueous system. On the other hand, the UV/H 2 O 2based advanced oxidation processes are important because of the formation of hydroxyl radicals (OH • ). These radicals act as a non-selective oxidizing agent with a high oxidation capacity to degrade or oxidize most organic contaminants in water streams [35]. The H 2 O 2 can react with the surfaces of metal oxides via catalytic decomposition and redox reactions, where the . OH radicals are stabilized by forming bonds between their unpaired electrons and the oxide surface [32].
Ultrasound (US) energy creates positive holes that can mineralize various organic pollutants. This technique has several advantages, including simplicity, safety, and environmental friendliness. It is based on the acoustic cavitation phenomenon, which involves the growth, nucleation, and violent collapse of microbubbles, resulting in sonoluminescence (SL) emission and the generation of hot spots at extremely high temperatures and pressures [34]. It produces OH radicals due to water dissociation, which oxidize organic pollutants into CO 2 and H 2 O [21,41].

3
The combination of the ultrasonic field with ultra-violet radiation and a semiconducting material was thus developed as sonophotocatalysis (SPC). The SPC has attracted attention to removing hazardous organic pollutants from wastewater. Under an ultrasonic field, the growth and collapse of gas bubbles improve mass transfer and surface cleaning due to its synergistic effect in the sonophotocatalytic process [3,4,48]. Both processes act together where the sonolysis can degrade the hydrophobic materials while the photocatalysis can degrade the hydrophilic ones. In the practical application of dye-wastewater treatment with ultrasonic, photocatalytic, and sonophotocatalytic, there is a need to determine the optimal conditions of experimental parameters [44]. Little information in the literature was found on the sonocatalysis, sonophotocatalysis, and sonophotocatalysis/H 2 O 2 processes with different metal oxides. Recent studies focused on the most preferred conventional semiconductor (TiO 2 ), widely investigated as sonocatalyst due to its low-cost, good chemical and thermal stability, excellent photo-thermal-catalytic properties, and environmental integrity [5,8,23,38,51,54].
This paper reports a benchmark survey of the first six transition metal oxides of 3d elements for the first time. These metal oxides were evaluated to remove MG dye as a model of an organic pollutant in combination with the two ultrairradiation systems, ultrasonic and ultra-violet, with and without H 2 O 2 . The metal oxide(s)/H 2 O 2 system act as a onepot powerful and facile process for the degradation of MG. Suitable low concentrations of H 2 O 2 and small metal oxide dose were used to achieve the highest efficiency with the ultra-MONPs (Ultrasonic/UV/Metal Oxide/H 2 o 2 ). Different six types of metal oxides have been synthesized. These metal oxides are ZnO, CuO, α-MnO 2 , NiO, Fe 3 O 4, and Co 3 O 4 . They have been used under optimum reaction conditions to reach the desirable degradation efficiency of MG.

Materials
Malachite green dye (MG) is purchased from Aldrich and used as received, Scheme (S1

Instruments
A Shimadzu 2100-S UV/Vis double-beam recording spectrophotometer (Japan). Ultrasonic cleaner set (Volts 230, Watts 296, distilled water was used all over the entire work) and constant frequency 60 Hz (WUC-A03H, Korea), and ultra-violet lamp (PHILIPS UV-C 15 W) were used. Fourier Transform Infrared (FT-IR) spectra were recorded on (JASCO FT-IR-4100) spectrophotometer using KBr pellets with a spectrum of wavenumber ranges from 4000 to 200 cm −1 with an accuracy of 2 cm −1 . The crystal structure of the synthesized oxides nanoparticles was analyzed by X-ray powder diffraction (XRD; Regaku D/max-250) with Cu K a radiation (λ = 1.540598 Å) and 2θ range from 10° to 90° at a step size of 0.020 (2θ) and scanning step 0.2/2θ. The operational voltage and current were kept at 40 kV and 300 mA, respectively. The band gap was measured by spectrometer/data system-Jasco Corp., V-570, rev. 1.00. The size and morphology of the nanoparticles were characterized using the scanning electron microscope (SEM; JEOL JSM-7000F) and transmission electron microscope (TEM; The JEM-2100 operated at 200 kV).

ZnO synthesis
500 mL of 0.2 M zinc acetate anhydrous (S1) were stirred for 15 min in a 4000 mL round flask held at 60 ºC. After 15 min, 500 mL of 0.5 M NaOH solution heated separately at 60 ºC were added slowly to the zinc acetate solution. The reaction time was 60 min, and the reaction mixture was allowed to cool to room temperature without stirring. The suspension was further ultrasonicated and then centrifuged. It was washed with water, followed by centrifugation. After the four cycles of washing and centrifuging, it was dried at 80 °C and grinded [40]. Scheme S2 illustrates the chemical reactions for the synthesis of the six metal oxides' catalysts.

CuO synthesis
400 mL of Cu(NO 3 ) 2 .3H 2 O (0.08 M) were slowly added to 400 mL of NaOH (0.5 M) solution in the round flask at 78 °C ± 2 with a rate of 20 mL/15 min. 40 mL of distilled water were added with a ratio of 20 mL/15 min. The resultant product was aged together at the same temperature for 16 h. The CuO nanocrystals were washed with water repeatedly, separated by centrifugation at a speed of 6000 rpm for 10 min, and finally dried overnight [12].

α-MnO 2 synthesis
α-MnO 2 was prepared by dissolving 4.74 g of KMnO 4 in 300 mL of water and stirring. 150 mL of ethanol (99%) were added, and the solution was heated at 50 °C for 30 min. The ethanol acts as a reducing agent for the KMnO 4 into Mn +4 . The brown precipitate formed was separated by centrifugation, washed several times with water, and dried at 90 °C. α-MnO 2 was obtained after calcination at 500 °C for 5 h [15].

NiO synthesis
300 ml of 0.1 M NiCl 2 solution was added dropwise to 300 ml of 0.8 M NaOH solution in a round flask at 80 °C. This mixture was refluxed with simultaneous stirring at 80 ± 5 °C for 6 h. The green precipitate formed was thoroughly washed with water, centrifuged, and dried overnight. The dried green Ni(OH) 2 precursor was crushed into powder and calcined at 600 °C for 5 h [37].

Fe 3 O 4 synthesis
The magnetite (Fe 3 O 4 ) was synthesized by the sonochemical method. 9.24 g of FeSO 4 0.7H 2 O were dissolved in 360 ml of water, stirred, and sonicated for 75 min. 36 ml of 3 M NaOH were added 15 min from the start of ultrasonication. The precipitate was collected with an external magnet, washed several times with water, and dried overnight at 80 ºC. The magnetite nanoparticles were then calcined at 400 ºC for 2 h [1].

Co 3 O 4 synthesis
Cobalt hydroxide was synthesized using urea as the precipitating agent by homogeneous precipitation of Co 2+ ions. An aqueous solution of CoCl 2 .6H 2 O (6 g in 500 mL water) was added to a conical flask containing 50 g of urea. The contents were refluxed at 80 ºC for 6 h under constant stirring. The precipitate was filtered, washed with water, and dried at 80 ºC overnight. The dried precipitate was heated at 350 ºC for 3 h to obtain the Co 3 O 4 nanoparticles [29].

Kinetics measurements
MG stock solutions were prepared in distilled H 2 O and were diluted to the desired concentrations. Equation (1) was used to find the degradation efficiency where C 0 and C t represent the initial and residual MG concentrations, respectively. The stock solution of MG was prepared in distilled water that was almost unaffected by the ultrasonic and the ultra-violet Lamp-C irradiation. At room temperature (24 ± 1 °C), 20 mL of MG stock solution were sonicated in a series of beakers containing 20 mL of water (100 mL). The beakers were immersed in the sonicator by maintaining a higher water level in the sonicator than in the reaction level, which was precisely 15 cm below the UV-C lamp. The water in the sonicator was replaced to avoid temperature-related effects. The mixture was sonicated (US/MOsNPs) after adding a suitable amount of metal oxide nanoparticles (MONPs). The experiment was conducted in the presence of H 2 O 2 (US/MOsNPs/H 2 O 2 ). It was again repeated under the UV irradiation (US/MOsNPs/UV), (US/ MOsNPs/H 2 O 2 /UV), Scheme (1). At definite time intervals, 10 mL samples were withdrawn from the reaction mixture and immediately centrifuged at 7000 rpm for 5 min to separate the MONPs and obtain a clear solution for measuring the absorbance of the unreacted dye.

Crystal size and structure
The X-ray diffractograms of the six metal oxide samples are depicted in Fig. (1). The diffraction peaks of these metal oxides are fully indexed to their pure phase in this study. The XRD peaks of ZnO indicate the formation of the pure phase of hexagonal cell structure for ZnO (JCPDS card No. 36-1451, hexagonal phase, space group P63mc, and unit cell a = 3.2490 Å, c = 5.206 Å) with high crystallinity. The crystallite size was obtained from Scherer (S1). The crystallite size based on the peak at 31° corresponding to the (100) plane is 19 nm. The typical XRD pattern for CuO nanoparticles is pure tenorite CuO with a space group C2/c and unit cell lattice parameters a = 4.65, b = 3.41, and c = 5.11 Å. It corresponds to the characteristic diffractions of the monoclinic phase CuO (JCPDS card no. 41-0254), verifying that the CuO obtained is phase-pure. In particular, the low Miller-indexed (002) and (200) reflections are among the strongest, which indicates that they are preferential crystal planes of the nanorods with an average crystallite size of 11 nm. XRD pattern of α-MnO 2 corresponds to the pure tetragonal cryptomelane phase (card number 29-1020, space group I4/m, with unit cell parameters a = b = 9.815 Å and c = 2.847 Å and average crystallite size 18 ± 3 nm. The XRD peaks of NiO appeared at 2θ = 37.64, 43.65, 63.24, 75.67, and 79.73 with cubic crystal system, space group Fm-3 m (225), and unit cell a = 4.1700 Å is, corresponding to the standard JCPDS card no. 04-0835 indicates that the NiO particles are crystalline with a face center cubic structure. The average crystallite size calculated by Scherer from the NiO XRD pattern showed that the particles' average diameter is 20 ± 2 nm.
The sharp and strong peaks obtained from the XRD of

FT-IR study
FT-IR spectra of the six metal oxides are shown in Fig. (S1). The spectrum of pure ZnO nanoparticles was recorded in 300-4000 cm −1 , Fig. (S1-a). Predictably, the characteristic band of wurtzite ZnO in the range of 400-500 cm -1 is the strongest in the case of ZnO pure phase. Other marked absorption bands corresponding to the O-H bending and stretching [39]. Figure (S1-b), shows the FT-IR spectrum of the CuO nanocrystals. The peaks around 413, 509, and 620 cm −1 are due to the stretching of the Cu-O bond along the [101] direction. The peak at ca. 1623 cm −1 is due to the bending vibrations of physically adsorbed water, whereas the OH in-plane bending is at 1367 cm −1 . The strong presence of the O-H stretching band at 3403 cm −1 is mainly due to water residues [14]. In

Morphology
The SEM and TEM images of ZnO are shown in Fig. 2 (A-B) and Fig. 3 (A-B). The most narrowly sized particles dominated spherical morphology with step-edged surfaces along the side with an average particle size of 20 ± 2 nm. The CuO morphology shows aggregated nanorods Fig. 2 (C-D) and Fig. 3(C-D). The average nanorods width is 11 nm, and the average length is 64 nm. The α-MnO 2 morphology is depicted in Fig. 2(E-F) and Fig. 3(E-F). α-MnO 2 shows relatively long and thin nanorods. The nanorods have an average length of 82 nm. The selected area electron diffraction (SAED) of nanorods, insets of panels of Fig. 3(E), showed that the oxide is highly crystalline. The fast Fourier transform (FFT) of nanorods shows lattice fringes with an interplanar distance of 4 nm Fig. 3(F). The NiO morphology exhibits nanocrystals of short nanorods morphology Fig. 2(G-H) and Fig. 4(G-H). The SEM and TEM images of NiO show uniform nanorods with a small particle size 1 3 of 8 ± 2 nm. The SAED shows that the NiO particles are crystalline.
The fast Fourier transform (FFT) shows that the interplanar distance between the lattice fringes of NiO is 2 nm. The SEM and TEM observed for Fe 3 O 4 in Fig. 2 (K-L) and Fig. 4(K-L) reveal hexagonal-plate like particles with little agglomeration. The electron diffraction pattern is shown in the same TEM image as the inset, where the hexagonal-plate shape with high crystallinity is revealed. This is consistent with the sharp and stronger peaks obtained from the magnetite XRD data, which confirm the cubic inverse spinel structure of the magnetite.
The SEM images of Co 3 O 4 nanoparticles given in Fig. 2(M-N) show a petal-like morphology. Typical TEM images of the Co 3 O 4 nanoparticles with SAED inset are shown in Fig. 4(M-N). They indicate the presence of porous petal-like nanoparticles with inter-connected nanoparticles and high crystallinity.

UV-Vis spectroscopy
The UV-Vis diffuse reflectance spectra show the optical comparison of the six prepared metal oxide nanoparticles, Figs. (5, 6). ZnO nanoparticles show higher UV-light absorption ability than others, with a band gap of 3.

Comparison of the efficiency of alternative processes
The catalytic degradation of MG was investigated by four different processes, sonocatalysis (US/MOsNPs), sonocatalysis under ultra-violet irradiation (US/MOsNPs/UV), sonocatalysis in the presence of hydrogen peroxide (US/ MOsNPs/H 2 O 2 ), and a combination of all processes (US/ MOsNPs/UV/H 2 O 2 ), Scheme (S1). The kinetics of the six metal oxides through the four degradation processes was studied under the same dose of catalyst (0.05 g/L) and the results are depicted in Fig. (7). The same study was also performed at different doses of metal oxides (0. 15  The US/MOsNPs process leads to more energy generation and increases the pulsation and collapse of bubbles. Thus, hydroxyl radicals are generated by breaking the bond between the hydrogen and hydroxyl ions of water. They increase with the increase in the number of the transient collapse of the cavitation bubble [41,42]. The MG degradation enhancement was also observed by applying the US/MOsNPs/UV and US/MOsNPs/H 2 O 2 systems, where an excess of OH • radicals is generated. The complete degradation of MG was achieved by the synergetic effect between the ultrasonic and the heterogeneous photocatalytic oxidation process (US/UV/MOsNPs/H 2 O 2 ). MG's high activity of this sonophotocatalytic oxidation may be due to the acoustic microstreaming that cleans, sweeps the catalyst surface, and generates more active sites on the catalyst surface. In solution, the mass transport of reactants and products increases by the facilitated transport via shockwave propagation and by ultrasonic fragmentation. In the presence of H 2 O 2 , H 2 O, and this catalyst, the concentration of OH • radicals increases, and subsequently, their reaction with the target pollutants increases [16,38,44].

Comparison between MOsNPs' efficiencies
To evaluate the efficiency of the metal oxides under investigation for degradation of MG, all metal oxides were tested under each process at the same conditions. Figure (8) displays a comparative study between the six metal oxides at each specific process at constant dose (0.05 g/L). Similar study was performed at different doses of metal oxides, 0.15  Fig. 8(a), with dose 0.05 gm/L, while for the doses 0.15, and 0.3 g/L, the results are shown in Fig. S4(a) and Fig. S5(a), respectively. Generally, the degradation rate increased with increasing the metal oxide dose, as revealed from Fig. S4(a) and Fig. S5(a) In the US/MOsNPs/UV process, MG degradation rate is given in Fig. 8(b) with a metal oxide dose 0.05 g/L. The results of 0.15 and 0.3 g/L doses are shown in Fig. S4 (b) and Fig. S5 (b), respectively. The removal percentage of MG was 80% within 5-40 min according to the metal oxide type which follows the order ZnO ≥ NiO ≥ α-MnO 2 > Fe 3 O 4 ≥ C uO ≥ Co 3 O 4 . This order is matching with band-gap values   Fig. 8(d), the metal oxide dose was 0.05 g/L, but 0.15 g/L and 0.3 g/L of metal oxide are shown in Fig. S4 (d) and Fig. S5 (d), respectively. The maximum removal efficiency reached 90% within 2-8 min, depending on the activity of each metal oxide that follows the order ZnO ≥ CuO > α-MnO 2 > NiO > Fe 3 O 4 ≥ Co 3 O 4 . Figure (9) shows the full comparison of six metal oxides, and at the same time, the four alternative processes. It was observed that the sonophotocatalytic process has the highest degradation efficiency of other processes to be considered the best in all MOsNPs types. The US/MOsNPs/H 2 O 2 process is the most competitive to the sonophotocatalytic process. The two processes efficiencies were nearly equal for ZnO, CuO, and α-MnO 2 . The four processes efficiencies are similar and equal for α-MnO 2 and Co 3 O 4 . However, a large gap was noticed between the sonophotocatalytic process and the other processes with the lowest efficiency for Fe 3 O 4 and all MOsNPs processes. The sonocatalytic process has high removal efficiency for α-MnO 2 than NiO. The US/MOsNPs/ UV process efficiency was nearly equal for ZnO, NiO, and α-MnO 2 .

Catalyst dose effect
Several experiments were performed to find the dosage effect of all six catalysts. (MOsNPs) ranging from 0.05 to 0.3 g/L on the degradation of MG from an aqueous solution. The critical parameter in the Ultra-MOsNPs like oxidation processes was hydrogen peroxide concentration, as a dominant source of OH • . However, an excessive H 2 O 2 dosage has ) have lower oxidation potential than OH • and compete with the MG molecules to occupy the adsorption sites on the surface of MOsNPs, which reduces the degradation efficiency of MG in this heterogeneous catalytic process; consequently; it raises the treatment cost [49]. Herein, we avoided the formation of hydroperoxy radicals (HO 2 • ) using the lowest hydrogen peroxide concentration (1 mM) to achieve high efficiency with the sonophotocatalytic process.
The catalyst dose in catalytic degradation reactions is a critical factor that should be optimized to reduce wastewater treatment costs. This effect was investigated and the results are shown in Figs. (S6-S11) for ZnO, CuO, NiO, Fe 3 O 4 , Co 3 O 4 , and α-MnO 2 , respectively. All figures show an enhancement in the degradation efficiency with increasing the metal oxides dose from 0.05 to 0.3 g/L. The highest removal percentage was observed at the catalyst dose of 0.3 g/L. There was also no significant difference in removal efficiency on going from 0.05 to 0.15 g/L for ZnO, CuO, α-MnO 2 , and NiO. Increasing the dose of ZnO did not affect the degradation efficiency which was relatively high at each of the four reaction processes. Increasing the dose of CuO, α-MnO 2 , and NiO increased the degradation efficiency along with the four reaction processes. This may be attributed to the less effect of the ultrasonic dispersion turbidity, which facilitates the UV-light penetration in the solution to produce more hydroxyl radicals. This finding can also be explained by the fact that MOsNPs act as a peroxidase-like catalyst. Therefore, an increase in the catalyst dose provides more active sites, accelerating the decomposition of H 2 O 2 to generate more reactive OH • radicals [52]. The low degradation efficiency of MG is principally derived from the insufficient production of hydroxyl radicals in solution with increasing the metal oxides amount more than 0.5 g/L, which causes an increase in solution turbidity by ultrasonic dispersion that reduces the penetration of the UV radiation into the Fe 3 O 4 and Co 3 O 4 suspension. Table (S1) includes different catalysts in the field of color removal.

Effect of zinc oxide
To investigate the best oxidation system for degradation of MG, the role of US/ZnO, US/ZnO/UV, US/ZnO/H 2 O 2 , and US/ZnO/H 2 O 2 /UV processes were studied, respectively, under the same conditions. ZnO is considered the more stable photocatalyst due to its high electron mobility (~ 100 cm 2 V −1 s −1 ). The effect of the sonophotocatalysis increased the surface area of zinc oxide for photocatalytic activity and increased the cavitation activity by providing additional nucleation growth. The combined effects of US and UV increased the catalyst particles' fragmentation; thus, their surface area and activity increased [18,30]. Figure 6(a) shows the higher ability of ZnO in UV-light absorption with a band gap of 3.31 eV. The hydroxyl radical population increased using the US/UV/ZnO system and the suitable particle size and morphology of ZnO led to a higher degradation of MG. Results of MG degradation in the presence of ZnO are shown in Fig. (8), where the improvement of the reaction time of the four systems was observed. The US/

Effect of copper oxide
Copper oxide can be considered the most effective oxidation catalyst for ultrasonic cavitation events. During the sonication, tiny amounts of copper oxide particles are further fragmented into smaller particles and act as nuclei sources to create an additional source to increase the cavitation activity [30]. Thus, CuO nanorods are compatible with the spherical ZnO nanoparticles for MG degradation in all processes US/ CuO, US/CuO/H 2 O 2, and US/CuO/H 2 O 2 /UV except the US/ CuO /UV process Fig. (8). The synergetic effect of the US/ UV system in CuO nanorods was not observed, Fig. 8(b). The lower MG degradation by US/CuO/UV may be due to the low ability to UV-light absorption with band gap 2.81 eV, Fig. 6(b), and may also be due to the scavenging  [18]. The active sites of the CuO may be blocked by adsorption of the dye, decreasing the photocatalytic efficiency. Overall, it can be considered that CuO is a suitable catalyst for the US compared to the US/UV process [19,33].  [31]. The MnO 2 systems were competitors in the degradation of MG by increasing the α-MnO 2 dose and due to the higher ability of α-MnO 2 in UV-irradiation with band gap 3.27 eV, Fig. 6(c). α-MnO 2 accelerates the decomposition of H 2 O 2 to produce an excess of reactive OH • radicals which attack the MG molecules to form degradation products, Table ( Fig. 6(d). The higher surface area and increasing number of cavities and radicals at higher concentrations led to a higher removal rate. Improving the mass transfer from the bulk to the catalyst surface by shock waves' sonication is also a reason for the high production of OH • radicals from the pyrolysis and sonication of H 2 O and H 2 O 2 which participate in MG degradation [53]. According to our knowledge, no information was found by a critical search in the literature on NiO/four processes under investigation for MG degradation. All the investigated processes US/NiO, US/NiO/H 2 O 2 , US/NiO/UV, and US/NiO/H 2 O 2 / UV for the degradation of MG are used for the first time.

Effect of magnetite (Fe 3 O 4 )
The solution turbidity was observed using the ultrasonic dispersion technique, which reduces the ultra-violet light penetration at the higher dose of magnetite 0.15-0.3 g/l in the four processes of US/Fe 3 Fig. (8). The best results were obtained using the US/Fe 3 O 4 /UV and US/Fe 3 O 4 /H 2 O 2 /UV (Sono-Fentonlike) processes at a lower dosage 0.05 g/l. This dose facilitates the ultra-violet light to pass into the solution to help the MG degradation within 25 and 8 min, respectively. The higher activity of the magnetite of band gap 3.27 eV comes from the absorption of the UV-light, Fig. 6(e). Although it is highly effective than sonicator dispersion which makes the solution turbid, it can easily be collected and separated by a simple magnet. Therefore, it is clear that the sonophotocatalysis processes with magnetite of high activity can be used as an efficient method for degrading recalcitrant contaminants.  Figure (9) shows the full comparison of six metal oxides and, at the same time, the four alternative processes. It was observed that the sonophotocatalytic process has the highest degradation efficiency of other processes to be considered the best in all MOsNPs types. The US/MOsNPs/H 2 O 2 process is the most competitive to the sonophotocatalytic process. The two processes' efficiency were nearly equal for ZnO, CuO, and α-MnO 2 . The four processes' efficiency is similar and equal for α-MnO 2 and Co 3 O 4 . However, a large gap was noticed between the sonophotocatalytic process and the other processes with the lowest efficiency for Fe 3 O 4 and all MOsNPs' processes. The sonocatalytic process has high removal efficiency for α-MnO 2 than NiO. The US/MOsNPs/