Application of photocatalytic proxone process for petrochemical wastewater treatment

Industrial wastewaters are different from sanitary wastewaters, and treatment complications due to their unique characteristics, so biological processes are typically disrupted. High chemical oxygen demand, dye, heavy metals, toxic organic and non-biodegradable compounds present in petroleum industry wastewater. This study intends to optimize the photocatalytic proxone process, utilizing a synthesized ZnO–Fe3O4 nanocatalyst, for petroleum wastewater treatment. The synthesis of ZnO–Fe3O4 was done by air oxidation and layer-by-layer self-assembly method and XRD, SEM, EDAX, FT-IR, BET, DRS, and VSM techniques were used to characterize the catalyst. Central composite design (CCD) method applied to investigated the effect of pH (4–8), reaction time (30–60 min), ozone gas concentration (1–2 mg/L-min), hydrogen peroxide concentration (2–3 mL/L) and the amount of catalyst (1–0.5 g/L) on the process. In the optimal conditions, biological oxygen demand (BOD5) and total petroleum hydrocarbon (TPH) removal, reaction kinetic, and synergistic effect mechanisms on the process were studied. Based on the ANOVA, a quadratic model with R2 = 0.99, P-Value = 0.0001, and F-Value = 906.87 was proposed to model the process. Based on the model pH = 5.7, ozone concentration = 1.8 mg/L-min, hydrogen peroxide concentration = 2.5 mL/L, reaction time = 56 min, and the catalyst dose = 0.7 g/L were proposed as the optimum condition. According to the model prediction, an efficiency of 85.3% was predicted for the removal of COD. To evaluate the accuracy of the prediction, an experiment was carried out in optimal conditions, and experimentally, a 52% removal efficiency was obtained. Also, at the optimum condition, BOD5 and TPH removal were 91.1% and 89.7% respectively. The reaction kinetic follows the pseudo-first-order kinetic model (R2 = 0.98). Also, the results showed that there is a synergistic effect in this process. As an advanced hybrid oxidation process, the photocatalytic proxone process has the capacity to treat petroleum wastewater to an acceptable standard.

Fe 3 O 4 synthesis.The Fe 3 O 4 nanoparticles were synthesized via the air oxidation method 23 .In this manner, 0.27 g FeSO 4 •7H 2 O was dissolved in 100 mL deionized water to obtain a concentration of 0.001 mol/L of divalent iron.Then, it was stirred vigorously at room temperature and sodium hydroxide (6 M) was gradually added to the solution until its pH reached 11 and then was stirred for 1 h at 25 °C in the presence of airflow.After filtration and washing the solution, the resulting Fe 3 O 4 was dried at 70 °C for 24 h.
ZnO-Fe 3 O 4 synthesis.ZnO-Fe 3 O 4 nanoparticle was synthesized by the layer-by-layer (LBL) self-assembly method 24 .Briefly, 2 g of ZnO powder was dissolved in 20 mL SDS (8 mol/L).After stirring at room temperature for 1 h, the resulting solution was filtered and washed.Then, the solids were added to 100 mL solution containing Fe 3 O 4 nanoparticles and stirred at room temperature for 5 h.The solids were then filtered and washed with ethanol and finally dried in a vacuum oven at 70 °C for 6 h.
Characterization of synthesis samples.To characterize ZnO, Fe 3 O 4 and ZnO-Fe 3 O 4 , FT-IR spectrophotometer (Spotlight 200i FT-IR Microscopy Systems; 4000-400 cm −1 ) was used to identify relevant created bonds.X-ray diffraction (XRD) patterns were recorded on XRD diffractometer Rigaku-ZSX Primus 400; radiations source: Cu Kα [(λ = 1.54056Å) monochromatic incident beam between 10° to 80° with the step interval of 0.02°, and a rate of 0.05°/s)] to assess the crystal structure of the samples.Also, the average crystallite size (D) of the prepared nanocomposite has been calculated from the Debye-Scherrer equation (Eq. 1) 25 : UV-visible spectrum (UV-Vis DRS) was recorded by Agilent Cary 60 spectrophotometer to study the structural features and optic properties.The surface morphology of the samples was assessed using field-emission Scanning Electron Microscopy (FE-SEM) (UN41219SEM) under vacuum condition of ≥ 1.3 × 10 -4 mbar.Energy dispersive spectrum (EDS) was used to analyzes purity and elemental mapping of samples.Transmission electron microscopes (TEM) were recorded on JEOL, JEM1200EX at 200 kV the samples were dispersed in a 1:1 methanol and water solution and deposited on a 3 mm copper grid and dried at ambient temperature after removing the excess solution using filter paper.The special surface area, volume and distribution pore size of catalysts were determined by nitrogen adsorption at 77 K with a Quantachrome Autosorb analyzer (BET).Samples were previously degassed in situ at 200 °C under vacuum for 12 h.Surface areas were calculated using the Brunauer-Emmet-Teller (BET) method over a p/p 0 range where a linear relationship was maintained.The Vibrating-sample magnetometry character of ZnO-Fe 3 O 4 was studied via VSM (LBKFB).
Reactor.The process was performed in a bench scale photo-reactor.The photo-chemical reactor was constructed with an approximate volume of 500 mL consisted of an ultraviolet lamp (UV-C 16W) and a semi-batch ozonation system.Ozone gas in different concentrations was injected into the chamber by the ozone generator.The light source was placed horizontally inside the quartz sheath in the middle of the reactor and the contents of the reactor were mixed by a magnetic stirrer.The entire system was operated within a pilot which has been shown in Fig. 1.

Procedure.
In order to study the process, prepared wastewater sample was transmitted to the laboratory under the transportation and storage conditions specified by the water and wastewater standards.Then, 500 mL of the sample was taken and poured into the reactor vessel and adjusted to the desired pH by 0.1 N sulfuric acid and sodium hydroxide.Afterwards, an amount of the catalyst and hydrogen peroxide was added into the media and the lamp was switched on.The ozone gas, generated via the ozone generator.An ozone-containing oxygen gas was bubbled continuously into the reactor via a ceramic diffuser by passing pure oxygen gas feed through the ozone generator.The concentration of ozone gas in the solution reaction was adjusted by changing the flow rate of the feed oxygen.After that, the process was started and sampling was performed at regular time.In this study, COD, BOD 5 , and TPH parameters were selected as treatment criteria.The COD was measured based on the close reflux method (No. 5220 D, colorimetric method) by UV-visible spectrophotometer (DR6000) that presented in standard methods of water and wastewater examination.Also, TPH (EPA-821/B-94-004b) 26 and BOD 5 (No. 5210 D, respirometric Method) are measured based on standard methods for the examination of (1) D = 0.9 β cos θ .water and wastewater (23rd edition) 21 .The decreasing percentage of COD, BOD 5 , and TPH in the process was calculated as (Eq.2): where, C 0 and C t are the concentrations of initial and residual of COD, BOD 5 , and TPH (mg/L), respectively.
Design of the experiment.Due to the complexity of wastewater treatment processes and the effect of various parameters, recently researchers in this field use statistical modeling methods to investigate and optimize these processes by performing the minimum experiments.One of the valuable methods in this field is the design of experiment (DOE) that for the first time developed by Fisher and Yates in 1920 27 .One of the most popular experimental design methods is the central composite design (CCD) of the response surface method 28,29 .The CCD method consists of three points: cubic, axial, and center points and the total number of experiments can be calculated as follows (Eq.3): where k is the number of factors, 2 k is the terms of cubic points, 2k is the axial points, and N 0 is the center points.
In this work the parameters of pH (4-8), the ozone concentration (1-3 mg/L-min), the hydrogen peroxide concentration (2-3 mL/L), ZnO-Fe 3 O 4 dose (0.5-1 g/L), and the reaction time (30-60 min) were selected as effective parameters.According to the related literature results, the level of effective parameters was determined.Each variable in this design was examined at five different levels, which are presented in Table 2.It should be mentioned that the UV light (16 W and λ = 254 nm) was used consistently in all runs.

Results and discussion
Characterization of the synthesized catalyst.FESEM, TEM, and EDS.In Fig. 2, FESEM, TEM and EDS images of Fe 3 O 4 -ZnO catalyst are presented.As illustrated in the FESEM image (Fig. 2A), the synthesized particles of Fe 3 O 4 -ZnO have aggregated.Core-shell structure in TEM images shows that ZnO particles are coated on iron oxide.The formation of spherical structures is observed in the size range of 20-30 nm (Fig. 2B).The results of the present study are consistent with the study of Fernández et al. 30 .Also, the result of EDS analysis indicates iron, oxygen, zinc and carbon elements in the catalyst structure (Fig. 2C).The results of the study are consistent with Ezzatzadeh et al. 31 .
XRD and BET.The synthesized samples were structurally characterized by X-ray diffraction measurements (Fig. 3a).Magnetite (Fe 3 O 4 , JCPDS PDF-2 card number 19-0629) and zinc oxide (ZnO, JCPDS PDF-2 card   32 .Other peaks correspond well to the planes of the face-centered cubic structure of Fe 3 O 4 (JCPDS 65-3107) 33 .Significantly, the intensity of ZnO diffraction peaks has decreased due to the core-shell structure.
Based on the Scherrer equation (Eq.1), the size of the ZnO-Fe 3 O 4 nanocomposite was calculated from the (101) plane as ∼ 14 nm.The results are consistent with the study of Zou et al. 34 .In Fig. 3B, the results of the  adsorption and desorption of nitrogen gas (BET analysis) related to the Fe 3 O 4 -ZnO catalyst is presented.As it is clear, the nitrogen gas adsorption and desorption pattern of the catalyst follows the H 3 model.Also, the peak diameter of pores is below 10 nm, which indicates the formation of microspores in the catalyst.The highest active surface area of the catalyst is 142 m 2 /g with a pore volume of 0.26 m 3 /g.The results of the current study are consistent with the study of Rakati et al. 35 .
UV-Vis spectra and FTIR.In Fig. 4a, the result of the UV-Vis spectrum is presented.According to the results, the catalyst has the property of absorbing light even in visible wavelengths, which indicates its photocatalytic activity.The absorption spectrum of ZnO shows a steep edge at a wavelength of about 345 nm, which indicates the main absorption in the UV light region.While the primary catalyst shows broad absorption in the wavelength range of 380-700 nm.The results are consistent with the study of Liu et al. 24 .
Figure 4b presents the FT-IR analysis of Fe 3 O 4 -ZnO catalyst.As it is clear from the Fig. 4b, the peak at 585 cm −1 is due to the stretching vibration between oxygen and iron 36 .The bands at 448, 510, and 610 cm −1 in the spectrum corresponding to ZnO are because of the different stretching modes of the Zn-O bond 37 .Due to the high concentration of ZnO in Fe 3 O 4 -ZnO, a broad band between 450 and 575 cm −1 is observed, which can be attributed to Zn-O vibrations.The broad peaks at 3447 and 1665 cm −1 are because of the stretching and bending vibrations of the O-H bond of the water molecule adsorbed on the nanoparticle surface 38 .The results of the current study are consistent with the results of Menon et al. 39 .
VSM.In Fig. 5, the results of the VSM analysis are presented.As it is clear, pure Fe 3 O 4 has more magnetic properties than the Fe 3 O 4 -ZnO catalyst, but it still has a significant magnetic property that can be collected by a magnetic field.The saturation magnetization of the catalyst was determined to be 27.7 emu/g.The results of the study are consistent with the study of Abbasi et al. 40 .
Optimizing the process with CCD.Based on CCD method for 5 effective parameters of pH, the ozone concentration, the hydrogen peroxide concentration, ZnO-Fe 3 O 4 dosage, and the reaction time, 50 experimental test were designed by the software which presented in the Table 3.The designed experiments were performed and COD removal efficiency as response was measured for all experiments.Regression analysis of the data was assessed and a second order polynomial equation (Eq. 3) was suggested by software as an appropriate model to model and predict of the process.The analysis of variance (ANOVA) was used to assess the significance of the model and terms; such that p-values less than 0.05 and greater than 0.10 identify that the model terms are significant and not significant, respectively.ANOVA analysis was presented in Table 4.As seen, the model F-value is 906.87 and the P-value < 0.0001, that implies the model is significant.Also, the F-value of 2.06 and p-value 0.1659 for the lack-of-fit implies the Lack of Fit is not significant relative to the pure error.Totally, R 2 = 0.99, predicted R-Squared of 0.97, the Adj R-Squared of 0.98, and the adequate precision of 108.1 show that the model is significant and has well accuracy to predicted the process.The effect of parameters.For assessment of different parameters effect on COD removal efficiency and interaction between the parameters, 3D graphs were used.The related graphs for pH, ozone concentration, hydrogen peroxide concentration, catalyst dose, and reaction time versus COD removal (%) are presented in Fig. 6. Figure 6 a, illustrated the COD removal variation by pH vs. reaction time.By increasing the solution pH from the acidic condition to the neutral range, the efficiency of the process increases, and the highest efficiency is achieved at pH = 6, and further, as the pH increases from this amount to the alkaline range, the efficiency decreases again.In AOPs, such as processes based on ozonation, ozone molecules are used in order to carry out direct oxidation by the ozone molecule itself and produce highly active hydroxyl radicals, and subsequently carry out oxidation with these active radicals (indirect oxidation).pH has a decisive role in the rate of reaction and the formation of active species 41 .In the conventional ozonation process, via increasing the pH to the alkaline range, the ozone molecule reacts with the hydroxyl ion (OH − ) in the aqueous media and produces the HO 2 − (Eq.5).Then HO 2 − reacts with the ozone molecule and produces hydroxyl radical (Eq.6) 42 .The production of HO 2 -from the reaction between the hydroxyl ion and the ozone molecule (Eq.7) is considered as a side reaction that reduces the concentration of dissolved ozone in the reaction medium and the reaction rate with H 2 O 2 and HO 2 − decreases and as a result less hydroxyl radical is produced.On the other hand, due to the decrease in dissolved ozone and the increase in the amount of HO 2 − compared to ozone in the reaction media, HO 2 − acts as a radical scavenger and competes with the pollutant in consuming hydroxyl radicals 43 .In a relatively acidic pH, there is a proportional relationship between hydroxyl ions and ozone molecules, and it causes the production of amounts of hydroxyl radicals, and this increases the efficiency of the process.
The results of this study are consistent with the results of Stanisova Popil and coworkers that investigated the efficiency of the conventional ozonation and the peroxon process in the decomposition of dibutyl sulfide.In this study, the best efficiency of the process was obtained at weak acidic pH 44 .
The reaction time parameter has an increasing effect on the efficiency of the process.In such a way that the highest efficiency of the process was obtained at high values of this parameter.As it is obvious, by increasing the reaction time, the presence of ozone and UV and subsequently hydroxyl radicals produced in the process and ( 5)  www.nature.com/scientificreports/hydrogen peroxide as a source of hydroxyl radical increases that increase the decomposition of pollutants.Over time, the process efficiency was proven due to the complete decomposition of the pollutant occurred.Shahamat et al. studied the UV/O 3 process performance in azo red-60 dye degradation in textile wastewater.In this research, reaction time was considered in the range of 10-60 min.The optimum reaction time 60 min reported 45 .Figure 6b, shows pH and ozone concentration effect on process performance.By increasing ozone concentration, the efficiency of the process is initially increased and then remains relatively constant.The efficiency of the process has a direct relationship with the concentration of ozone gas entering the reaction chamber.According to the theory of mass transfer, increasing the concentration of ozone entering the reaction media increases the concentration of dissolved ozone in the environment, and since ozone has a dual role as a producer of hydroxyl oxidizing radicals from the reaction with hydrogen peroxide and direct oxidation by itself.So increasing the concentration of ozone increases the efficiency of the process 46 .A study conducted by Wang and her colleagues on the decolorization of acid orange 2 indicated that increasing the ozone flow rate from 35 to 118 mg/L increased the removal efficiency from 80 to 98 percent 47 .Also, the results of Asgari et al. 's study on the removal of Butoben and Phenylmethyl ester from aqueous by the processes based on AOPs process, indicated that increasing the concentration of ozone in the UV/ZnO/O 3 process, increased the removal efficiency 48 .
Figure 6c, shows pH and hydrogen peroxide concentration effect on process performance.The results of the effect of the hydrogen peroxide concentration parameter on the efficiency of the process are incremental.This increase starts with a steep slope and then continues with a slow slope and is relatively constant at higher values.As it is clear, by changing the concentration of hydrogen peroxide in the reaction media, the efficiency of the process is also changed.In general, the ozone molecule reacts with the deproteinized form of hydrogen peroxide (Eqs.8-12) 49 .By increasing the optimal ratio of hydrogen peroxide to the ozone gas in the reaction media, secondary series reactions or side reactions started (Eqs.13-15) 49 .Finally, it causes the consumption of hydroxyl radicals and the production of compounds that are low activity or are completely inactive compounds 50 .Feng et al. investigated the pretreatment of mother liquor of gas field wastewater.The results of the ozone/hydrogen peroxide process indicated that the efficiency of the process increased by increasing the concentration of hydrogen peroxide and 6.2 mL was the optimum concentration of H 2 O 2 51 .
Figure 6d, shows the variation of COD removal efficiency versus catalyst dosage and pH changes.As seen, with increasing catalyst dosage, COD removal efficiency increases and with further increase in catalyst dosage, ( 12) www.nature.com/scientificreports/ it decreases.To overcome the disadvantages of homogeneous catalysts, solid catalysts with high stability and efficiency have been widely studied in ozonation systems 52 .The process of heterogeneous catalytic ozonation is complex and involves many reactions and several stages that are influenced by various factors.The catalyst may play various roles in this process, such as creating reaction sites for adsorption and catalysis 53 .
The organic substances adsorption depends on the polarity of the compounds and the surface characteristics of the catalyst, such as the material charge, which strongly depends on the pH of the solution.Polar compounds may be easily adsorbed on the surface, while nonpolar organic substances are hardly adsorbed on the surface, unless some hydrophobic sites are present.
In addition, the catalyst has a high adsorption capacity for organic ions to some extent, which depends on the surface charge of the catalyst and the dissociation constant of the compounds.Another key factor in determining the catalytic activity of a catalyst is the adsorption and decomposition of ozone, which is believed to occur on the catalyst surface.In addition, ozone adsorption and subsequent decomposition usually lead to surfacebound oxidizing radicals and hydroxyl on catalyst surfaces, which enhance the removal of organic matter 53 .In the study of Shikhmohammadi and coworkers studied Butyl p-hydroxybenzoate removal from liquid phase by UVC/ZnO/O 3 /HP process.The results showed that 1 g/L of ZnO had a perfect effect on pollutant degradation 54 .
Also, ultraviolet rays in the catalyst's presence can lead to the photocatalytic process and improve the efficiency of the process.An excessive increase in the amount of catalyst in a photochemical reactor leads to light scattering and prevents to reach light to other oxidants such as ozone and hydrogen peroxide and will lead to a decrease in efficiency 55 .
Finally, the predicted optimal conditions for the photocatalytic proxone process were provided by the software.In these conditions (pH = 5.7, reaction time = 56 min., ozone gas concentration = 1.8 mg/min-L, hydrogen peroxide concentration = 2.5 mL/L and catalyst dosage = 0.7 g/L), a COD removal efficiency of 85.3% was predicted.To assess the accuracy of the model, an experiment was performed at optimum condition and a COD removal efficiency of 82 was experimentally obtained.
Further studies.Removal of BOD 5 and TPH.In the photocatalytic proxone process, the BOD 5 and TPH parameters were simultaneously assessed with the COD removal efficiency in optimal conditions.Based on the results, a BOD 5 removal efficiency of 91.1% and a TPH removal efficiency of 89.7% were obtained at the optimum condition.
Reaction kinetic.Another discussion that has a great help in the design and implementation of processes is the study of reaction kinetics.It will help to model and implement the process better in the practical scale.Reactive kinetic models are used to identify the factors affecting the contaminant removal rate and process efficiency.In oxidation processes, pseudo-first-order kinetics is common.So, the reaction kinetic was evaluated based on first-order kinetics under optimal conditions (Eq.16) 49 .
where, C 0 and C t : are initial and final (at time) concentration of COD (mg/L), t is the reaction time (min), K 1 is the kinetic constant of the first-order reaction.
In studies, reaction kinetics are measured based on various factors.The initial concentration of the pollutant, the amount of the catalyst, the amount of ozone gas, the amount of the oxidizing agent such as hydrogen peroxide, etc., are among the factors that are investigated 56 .The rate of chemical reaction affects various aspects of the process.The main influencing factor is the EEO, which will be mentioned.
To this aim, an experiment was performed at optimum condition and the obtained data fitting assessment by pseudo-first-order kinetic was performed (Fig. 7).Based on the results, the kinetics of the photocatalytic proxone process follow the first-order kinetics with R 2 = 0.95 and rate constant of 0.247 (L/min).In first-order kinetic, the reaction rate has a direct and linear relationship with the amount of reactants, which in this study is the initial concentration of the pollutant.In most of the processes studied in this area, the kinetics of the process follow the first-order kinetic model.According to the study of Kermani et al., the kinetics of peroxon process follow the first-order kinetics 12 .
The synergistic effect.In the photocatalytic proxone process, the mechanisms are divided into single, binary, triple, and, finally, the fundamental process.The mechanisms of UV, ozone gas, hydrogen peroxide, and catalyst as single mechanisms.UV/O  8).Based on the results, it was found that the efficiency of the fundamental process is more than other processes and mechanisms that show the synergistic effect of the mechanisms together.Yang et al. studied the synergistic effects of ozone/peroxymonosulfate for isothiazolinone biocides degradation in aqueous solution.The resulted indicated the synergistic effect of parameters on performance of process 57 .

Analysis of electrical energy efficiency.
The electrical energy required for the photocatalytic proxone process typically has the greatest monetary impact on operating costs.Thus, it is necessary to evaluate the energy efficiency.
Utilizing the figure-of-merit electrical energy per order (EEO) is an appropriate method for computing the electrical energy efficiency.This is a powerful scale-up parameter and evaluation of the rate of purification in a fixed amount of contaminated water according to the energy consumed.The EEO value was used to compare www.nature.com/scientificreports/ the energy efficiencies of systems.For low pollutant concentrations, the EEO (kW/h-m -3 order -1 ) value can be derived using Eq. ( 17) 58 .
where P is the power (kW) of the light source and ozone generator; V is the volume (L) of solution in the reactor; and k is the pseudo-first-order rate constant (min -1 ) for mineralization.A higher EEO value corresponds to a lower energy efficiency of the system.The results show that the EEO value in the photocatalytic proxone process is more than the other two mechanisms (photolysis and ozonation), but this higher EEO leads to an increase in the process's efficiency in wastewater treatment.In Lester et al. 59 , and Joardar 60 and coworkers studies EEO value calculated.In these studies, different EEO value have been reported, which indicate the different EEO value according to the type of pollutant and the efficiency of the process in wastewater treatment.

Conclusion
The aim of this work was a study of performing photocatalytic proxone process for treatment of petroleum wastewater using ZnO-Fe 3 O 4 nanocatalyst.The ZnO-Fe 3 O 4 catalyst was synthesized and its characteristics were determined by FESEM, EDS, TEM, BET, VSM, XRD, and FTIR analysis.Also, the process was statistically modeled and optimized by CCD method.Based on the model pH = 5.7, ozone concentration = 1.8 mg/L-min, hydrogen peroxide concentration = 2.5 mL/L, reaction time = 56 min, and the catalyst dose = 0.7 g/L were proposed as optimum condition and at these conditions, an efficiency of 82% was experimentally obtained for the removal of COD.Also, a BOD 5 removal efficiency of 91.1% and a TPH removal efficiency of 89.7% were obtained at the optimum condition.Based on the results, the kinetics of the process follow the first-order kinetics and it concluded that there is a synergistic effect in the photocatalytic proxone process.The EEO value of photolysis, ozonation and photocatalytic proxone process were calculated 13.29, 34.41 and 113.8 kW/h-m -3 -order -1 , respectively.Based on the results, the photocatalytic proxone process can treat pollutants in water media, especially industrial wastewaters, because of proper efficiency.The strength of this process is its simplicity in setting up and managing.In this process, hard-to-decompose pollutants, such as slowly decomposable organic compounds, are usually completely and effectively removed and stabilized.Table 6 includes a comparison between the current study and other recent studies regarding petroleum wastewater treatment.Table 5. EEO value (kW/h-m -3 -order -1 ) of various processes under optimum condition (pH = 5.7, RT = 80 min, O 3 = 1.8 mg/min-L, HP = 2.5 mL/L and dose catalyst = 0.7 g/L).

Processes First author and references
Electrofenton process using a low cost porous graphite air-diffusion cathode Optimal conditions for COD removal were determined to be a current density = 6.66 mA/cm 2 , Fe 2+ concentration = 0.80 mM, and an electrolysis duration = 60 min.These conditions achieved a 94% RE% with a specific energy consumption (SEC) of 3.75 kWh/kg COD Marwa 61 Photocatalytic nanohybrid PSfm/Co-TiO 2 @SiO 2 Color = 88.5%,turbidity = 81.45%,TOC = 94.36%,TDS = 79.09%,COD = 73.86,and N-ammonia-nitrogen = 71.96removed in optimum condition Dalanta 62 Adsorption via silica and calcium carbonate nanoparticles Optimized processing conditions for COD reduction using SiO 2 nanoparticles are pH 4.0, dosage 0. Photocatalytic proxone process pH = 5.7, ozone concentration = 1.8 mg/L-min, hydrogen peroxide concentration = 25 mL/L, reaction time = 56 min, and the catalyst dose = 0.7 g/L were proposed as optimum condition.82% of COD, 91.1% of BOD 5 and 89.7% of TOC removed.The kinetics of the process follow the first-order kinetics.The EEO value of photolysis, ozonation and photocatalytic proxone process were calculated 13.29, 34.41 and 113.8 kW/hm -3 -order -1 respectively Current study

Figure 1 .
Figure 1.Schematic of the reactor used in the process.

Figure 3 .
Figure 3. (a) XRD analysis of samples, (b) adsorption and desorption isotherm of nitrogen gas and pore diameter distribution of Fe 3 O 4 -ZnO catalyst.

Table 1 .
Physicochemical characteristics of the wastewater.

Table 2 .
The range and levels of variables.

Table 3 .
The designed experiments by the CCD method.

Table 4 .
Analysis of variance (ANOVA) results of a quadratic model of photocatalytic proxone process.

Table 5
summarizes the EEO value of various mechanisms and processes in optimum condition.

Table 6 .
Some recent studies regarding petroleum wastewater treatment.
5 g, stirring speed 125 rpm, and 90 min stirring time.For CaCO 3 nanoparticles, the corresponding values are pH 8.0, dosage 0.4 g, stirring speed 100 rpm, and 90 min stirring time Al Rasbi 63 Sono Fenton Process Removal of 85.81% was attained under the optimal conditions of 21 min and 0.289 mM of iron concentration Jiad 64 Solar-light-driven photocatalytic/hierarchically-structured copper sulfide (CuS) hollow nanocatalysts 66% of COD in PRW was removed in 3 h degradation under conditions of 1.0 g/L catalyst and 7.6 pH and in the presence of hydrogen peroxide, 98% of COD in PRW was removed in 2 h under conditions of 1.0 g/L catalyst, 3.0 g/L H 2 O 2 and 7.6 pH Wang 65 Simultaneous adsorption-photocatalytic process/AC/TiO 2 /CeO 2 Optimized formulation was accepted to have 36.85%TDS removal, 49.23% COD removal, treated PRW with pH = 7.22 and electrical conductivity (EC) = 2937.11µS/cm, 53.76% phenol removal, and 52.86% NH 3 -N removal, by applying 53.43%-wt of AC, 21.96%-wt of TiO 2 , and 24.61%-wt of CeO 2