Modified Graphite with Tin Oxide as a Promising Electrode for Reduction of Organic Pollutants from Wastewater by Sonoelectrochemical Oxidation

Most of the studies on tin oxide coatings as electrode materials were conducted on titanium; in this study, the aim was to create pure tin oxide (SnO 2 ) films on graphite substrate, which is more prevalent than titanium. There is a lack in investigation the effect of SnCl 2 and HNO 3 concentrations on the prepared SnO 2 electrode; therefore, the aim of this work was to study these effects precisely. Also, no previous study investigated the removal of phenol sonoelectrochemically by a SnO 2 electrode, which would be accomplished in the present work. The tin dioxide electrode was produced by cathodic electrodeposition using a SnCl 2 ·2H 2 O solution in the presence of HNO 3 and NaNO 3 on a graphite plate substrate. The impact of various operating parameters (current density – CD, HNO 3 concentration, and SnCl 2 ·2H 2 O concentration) on the morphology and structure of the SnO 2 deposit layer was thoroughly investigated. The physical structures of the SnO 2 film were determined by X‑ray diffraction (XRD), surface morphology was characterized using field‑emission scanning electron microscopy (SEM), and chemical composition was analyzed using energy‑dispersive X‑ray spectroscopy (EDX). In a batch reactor, the sonoelec - trochemical oxidation of phenol was tested to determine the performance of the best SnO 2 electrodes for phenol degradation and any organic byproducts. It was discovered that 10 mA/cm 2 , 50 mM of SnCL 2 ·2H 2 O, and 250 mM of HNO 3 were the optimum conditions to prepare SnO 2 electrodes, which produced the smallest crystal size, with no appeared cracks, and gave the best phenol removal. The best prepared electrode was tested in the sonoelec-trochemical oxidation of phenol with two different electrolytes and different CD, and the results showed that the phenol removal was 76.87% and 64.68% when using NaCl and Na 2 SO 4 , respectively, as well as was 63.39, 76.87, and 100% for CD at 10, 25, and 40 mA/cm 2 , respectively.


INTRODUCTION
There is a serious issue with increased demand and a shortage of clean water supplies on a global scale because of faster industrial expansion, population growth, and extended droughts.Large-scale environmental pollution from industrial wastewater discharges necessitates the creation and implementation of effective treatment methods that can eliminate the dangerous contaminants present in these industrial streams (Al-Alawy et al., 2017).Water is used in vast amounts throughout the refining process in the petroleum industry (Dhamin et al., 2022) According to a survey, the amount of wastewater produced by the methods used to refine oil is 1.6 times greater than the total amount of crude oil treated, and between 80 and 90% of the water used in the process (Majeed, 2017).The compounds contained in contaminated water are diverse and complicated, including chemical oxygen demand (COD), varied amounts of emulsified oil, heavy metals, organic compounds, aromatic hydrocarbons, oils and greases, phenol, and occasionally radioactive elements.Phenol and its derivatives are the most prevalent organic contaminants found in Modified Graphite with Tin Oxide as a Promising Electrode for Reduction of Organic Pollutants from Wastewater by Sonoelectrochemical Oxidation

ECOLOGICAL ENGINEERING & ENVIRONMENTAL TECHNOLOGY
oil refining industry wastewaters.Because of their high toxicity, limited biodegradability, and ecological impact, these substances are particularly dangerous (Naser et al., 2021).Phenolic compounds are very dangerous for the environment because of their severe toxicity, even at extremely low concentrations (Al-Jandeel, 2013), stability, bioaccumulation, and low biodegradability.Phenolic compounds hurt human health both immediately and over time (Al-Obaidy, 2013;Mezaal, 2019).
Wastewater is treated using several methods, including coagulation, adsorption, oxidation, biological processing, and electrochemical processes (Thwaini et al., 2023).Conventional treatment methods only transfer pollutants from one medium to another while producing secondary waste and typically consume a lot of energy (Kassob et al., 2022;Fajri et al., 2023).Due to the difficulties in using conventional wastewater treatment techniques, combining ultrasound and electrochemistry, which is called sonelectrochemistry, may be a viable choice (Ang et al., 2022).There are several advantages of sonoelectrochemistry, like chemical and physical influence of ultrasound that generate oxidizing species like OH • radicals and H 2 O 2 , disturbance of the diffusion layer, enhanced ion mass transfer over the double layer, as well as periodic cleaning and reactivation of the electrode surfaces.All of these impacts work together to increase yield and enhance electrical efficiency, so sonoelectrochemistry is promising for the removal of organic pollutants (Thokchom et al., 2015).
The efficiency of sonoelectrochemistry is significantly influenced by the electrode materials.The electrodes must undergo a high oxygen evolution reaction (OER) to generate a significant amount of OH • radicals, which is advantageous for the oxidation of organic pollutants on the electrode surface.Boron-doped diamond (BDD), Ti/ PbO 2 , and Ti/SnO 2 electrodes all have quite high OER potential (Zhao et al., 2015).PbO 2 electrodes are thought to have adequate electrocatalytic capability for oxidizing organic compounds.However, the concerns about the potential toxicity of Pb leaking from the anode would preclude its use (Abbas et al., 2022).Although the BDD electrode has great chemical stability, good electrocatalytic oxidation capacity, and a significant overpotential for oxygen evolution, its high cost and, particularly, the challenges of finding an adequate substrate for depositing the diamond layer limit its wide-scale implementation.SnO 2 electrodes, on the other hand, are considered preferable for the oxidation of organic compounds with high OER and high activity.Due to its strong electrical conductivity and chemical stability, SnO 2 is recognized as a co-material for RuO 2 (Xu et al., 2015).
Several chemical deposition techniques have been used to create nanocrystalline SnO 2 thin films, including chemical vapor deposition, spray pyrolysis, sol-gel, and electrodeposition.Electrodeposition have been gained attention because of their benefits, including simplicity, cheap cost, low-temperature process, capacity to regulate characteristics and morphology by varied electrochemical factors, as well as deposition over vast and complicated regions (Daideche et al., 2017).Cathodic electrodeposition is an effective method for synthesizing nanocrystalline tin oxide.For the electrodeposition of metal oxides, OH ions or O radicals must be present on the electrode surface.Several kinds of oxygen sources are reported to be effective for the cathodic electrodeposition of oxides, including nitrate ions, hydrogen peroxide, and blown oxygen (Abdo et al., 2021).
The present work aimed to investigate the sonoelectrochemical process for organic pollutants removal from simulated wastewater over a SnO 2 electrode because several side reactions and the production of intermediates occurs when treating real refinery wastewater as well as the real wastewater with a variety of contaminants could take part in unknown reactions or serve as a catalyst to enhance unfavorable reactions.Phenol has been selected as a test compound, as it is one of the greatest challenging organic compounds to be treated by a sonoelectrochemical process, because oxidation of phenol changes it to large number of intermediates (other organic compounds) by sequence of reactions until the organic material entirely transformed to CO 2 and H 2 O.The prepared electrodes were characterized, and the performance of the best anode electrode (obtained by the best deposition on it) was tested by organic removal using a sonoelectrochemical process.

Chemicals
All chemicals utilized in the present study were of reagent grade; thus, no further purification was required.These chemicals were SnCl 2 .2H 2 O (with a purity of 97.0%, Thomas Baker), H 2 SO 4 (with a purity of 98.0%, Alpha Chemika), phenol (with a purity of 99.5%, LOBA Chemie), NaNO 3 (with a purity of 99.5%, Alpha Chemika), NaCl (with a purity of 99.9%, Central Drug House (P) Ltd-CDH), and HNO 3 (with a purity of 69-72%, Alpha Chemika).All aqueous solutions have been prepared by using distilled water.

Preparation of SnO 2 anode
Tin oxide electrodes were prepared by the cathodic electrodeposition method.Two graphite substrate plates of 6.5 cm*8 cm were firstly cleaned, polished by 2000-grit paper strips with water as a lubricant, and boiled in deionized water for 30 minutes.Then these substrates were electrochemically activated using CD of 14 mA/cm 2 at 90°C for 30 minutes in an electrolyte with 1.44 M of H 2 SO 4 .A hot plate magnetic stirrer (Nahita Blue, model 692/1) was used to maintain the electrolyte at the required temperature.The electrodes were washed using distilled water after the electrolysis.The cathodic deposition has been carried out with (25, 50, 75, and 100) mM SnCl 2 .2H 2 O, 100 mM NaNO 3 and (150, and 250) mM HNO 3 as shown in Table 1.The total volume of electrolytes was 500 ml.The solutions were stirred with 400 rpm at a temperature of 85 °C for 3 h to produce a stable solution (this is essential step for converting Sn 2+ ions into Sn 4+ ions).
A mercury thermometer was used to measure the contaminated solution temperature.After that, the prepared substrate was placed in the electrolytic solution with a 25 mm gap between the anode and cathode.The connection of electrodes to the power source (DC power supply, Maisheng, MS-605D) was attained by copper wires, the power supply was operated galvanostatically at 5 and 10 mA/cm 2 , and the applied current was measured by the current multimeter.Throughout the experiment, the temperature of the electrolyte must be maintained at roughly 85°C.Cathodic deposition occurred for a certain period (1hr), after that, the electrode was washed with deionized water, dried at ambient temperature, and then calcined at 400°C for 4 hours.

Structural characterization
The physical structures of SnO 2 film were examined by X-ray diffraction using X-ray diffractometer (XRD 6000, SHIMADZU, Japan) powder diffractometer (CuKα radiation, λ= 1.5418 Å), surface morphology was characterized using scanning electron microscopy (SEM) (SEM TES-CAN Vega III in College of Engineering/ Imir Kabir University/ Tehran) and chemical composition was analyzed using energy-dispersive X-ray spectroscopy (EDX) is attached with SEM

Sonoelectrochemical process
In the phenol removal experiments, a glass beaker containing a volume of 500 ml of simulated wastewater with 150 mg/l phenol concentrations was utilized.Figure 1 shows the electrochemical cell which was placed in a digital ultrasonic bath (Ultrasonic, model: 031S) at an ultrasonic frequency of 40 kHz (Zhang et al., 2020).Sonication was produced parallel to the liquid surface, and electrodes were fixed vertically in the solution.The prepared SnO 2 electrode was used as the anode and graphite as the cathode.The electrodes were connected to the DC power supply (MASHENG; MS-605D), and a constant CD was applied to the electrolytic cell for a specified time.Indirect oxidation was used in this study, so 3 g/l of NaCl (Divya et al., 2021) was added to the solution, several drops of H 2 SO 4 were added to make the pH equal to 3, and the temperature of the electrolyte was maintained at about 25°C ± 2.

Electrode performance measurement
The performance of the SnO 2 anode sonoelectrochemical oxidation was investigated through where: IPC -the initial phenol concentration in (mg/l), CPC -the final phenol concentration in (mg/l).
The chemical oxygen demand evaluated with a COD reactor (Lovibond Water Testing, MD 200 COD, tube tests, Germany).The COD removal efficiency is determined by the Equation 2, as follows (Heydari Orojlou et al., 2022): where: CODi and CODt -correspond to the initial and final COD concentration in (mg/l).

RESULTS AND DISCUSSION
The XRD results Figure 2 shows the graphite XRD pattern prior to electrodeposition, which is exactly identical to the standard card and has a distinct sharp peak at 26 ͦ of 2θ. Figure 3 illustrates the XRD patterns of the deposit SnO 2 on graphite for different electrodes in the range (5-80) of 2θ.The average crystalline size of the deposit SnO 2 was calculated using Scherrer's equation (Khan et al., 2020).
The results showed that electrodes number 4, 5, and 6 are the best electrodes for the electrodeposition process, where the peaks of tin oxide are evident and produce the smallest crystal size.It shows five peaks obtained at 2θ equal to 26 It was found that the crystal size of the SnO 2 deposit was determined to be 16.03 and 19.76 nm for 5 and 10 mA/cm 2 of applied current density, respectively; this means that the crystal size increases as the applied current density increases, a result that was supported by other studies (Salman et al., 2019), the crystal size for the two HNO 3 concentrations (150 and 250 mM) was 19.76 and 5.33 nm, respectively, so the crystal size decreases as the concentration of HNO 3 increases, and The crystal size was changed slightly from 3.79 to 5.43 nm when the SnCl 2 .2H 2 O concentration increased from 25 to 100 Mm.

The effect of the current density
The applied current density has a significant effect on the surface morphology of SnO 2 deposits.It seems that when the CD increases, the crystal size will enlarge with greater crystallinity (Raj et al., 2015).This effect is explained by the metallic ions diffusing to the substrate more quickly than usual and quick nucleation takes place during the electrodeposition process, as illustrated in EDX has been employed to detect the composition of the electrode elements.The EDX results indicated the presence of Sn and oxygen elements, which indicates that the SnO 2 was deposited successfully on the electrode surface.Table 3 shows that when the CD is 5 mA/Cm 2 (electrode number 1(, the weight percentage of Sn is 7.78 and oxygen is 20.59, while when the CD increases to 10 mA/Cm 2 (electrode number 2(, the weight percentage of Sn is 34.21 and oxygen is 18.30., this demonstrated that the deposition efficiency of SnO 2 increases along with CD, because more (Sn + ) and (OH -) ions (building blocks of film production) accumulated towards the cathode surface when increasing CD (Hessam et al., 2022).

The effect of HNO 3 concentration
It is well known that hydroxyl ions (OH -) or O radicals must be present close to the electrode surface in order to prepare metal oxide by electrodeposition (Abdo et al., 2021).Nitric acid was used as the source of oxygen in this work, and tin chloride dehydrate (SnCl 2 .2H 2 O) served as the Sn 2+ source and the prior treatment of the electrolyte is essential for converting Sn 2+ ions into Sn 4+ ions.The half-reaction of the cathode electrode surface is depicted below, as follows (Daideche et al., 2017): The deposition of the SnO 2 electrode was considered to involve the formation of OH − ions on the electrode surface, as shown in Equation 4.
The stannic ions from the bulk solution reacted with the OH − ions that had formed on the electrode surface to create nanocrystalline SnO 2 , as shown in Equation 5.
However, because of the system complexity, the next two reduction processes could coexist and lead to the deposition of Sn metal, as illustrated in Equations 6 and 7.
Sn 4+ + 2e -→ Sn 2+ (6) As a result, the deposition potential of reaction (4) shifts in the positive direction with an increase in NO 3 concentration and electrolyte acidity, whereas reaction (6) shifts in the opposite direction with a decrease in Sn 4+ concentration.Hence, the favored deposition of SnO 2 occurs whenever the ratio of [HNO 3 ] to [Sn 4+ ] is high (Chen et al., 2010(Chen et al., , 2017)).
Figure 5 shows the SEM morphology of SnO 2 electrodes.It appears that the crystal size increases along with the HNO 3 concentration  increases, , and it can be seen that small particles have been agglomerated and have the same spherical morphology as well as porous surface, which leads to a large surface area at different HNO 3 concentrations.EDX results (Table 4) showed that when the concentration of HNO 3 is 150 mM (electrode number 2(, the weight percentage of Sn is 34.21 and oxygen is 18.03, while when the concentration of HNO 3 is 250 mM (electrode number 3(, the weight present of Sn is 33.94 and oxygen is 24.63.This demonstrated that the deposit phases changed from Sn and SnO 2 to pure SnO 2 with the increase in HNO 3 concentration (Chen et al., 2010).

The effect of SnCl 2 .2H 2 O concentration
Figure 6 shows the SEM pictures, which demonstrate the dramatic impact of the SnCl 2 .2H 2 O concentration on the shape of the deposited film.Over the whole surface of the graphite substrate, irregular spherical granules can be seen.Large crystallites are created as the nanocrystals  It can be seen from Figure 6 b that some agglomerated deposits were obtained at a concentration of SnCl 2 .2H 2 O equal to 50 mM (electrode number 4(and illustrate a huge interconnected, uniform porous structure formed by the unevenly linked nanoparticles.No cracks appeared, but these cracks were observed at higher concentrations.When the salt concentration was 75 and 100 mM (electrode number 5 and 6) the surface covering with SnO 2 was homogeneous and excellent, but the electrode surface was distinguished by a "cracked-mud" morph.In line with this theory, through electrodeposition, it is more likely to produce pure SnO 2 with a lower concentration of SnCl 2 .2H 2 O.However, the deposition rate can be too sluggish if the SnCl 2 .2H 2 O concentration is too low (25 mM (electrode number 3)).Therefore, the deposition rate was excellent at 50 mM SnCl2.2H2O salt concentration, and 250 mM HNO3 concentration.This is confirmed by the results of the EDX, as in Table 5 (Chen et al., 2010).

Phenol and COD removal
With sonoelectro-oxidizing phenol (150 mg/l) in the presence of 3 g/l NaCl and several drops of H 2 SO 4 as a supporting electrolyte, 25 mA/cm 2 CD, and 40 kHz ultrasonic frequency, the performance of the best-produced SnO 2 electrodes numbers 4, In electrode number 4 for the SEM test, crystals were appeared as homogeneous and there were no cracks, but in electrodes 5 and 6, cracks appeared in the SEM test, and the phenol removal was lesser.This may be due to the presence of cracks that make the surface breakable and quick to separate from the electrode at the start of the process.Electrode number 4 demonstrated the maximum removal efficiency.This is further supported by the characterization testing at rest, allowing it to be employed in the subsequent removal processes.
To compare their impact on phenol removal from wastewater, sonoelectrochemical oxidation of phenol using NaCl and Na 2 SO 4 as the electrolytes was carried out in two distinct reactors at 25 mA/cm 2 CD, 40 kHz ultrasonic frequency, 150 mg/l phenol concentration, and at 25°C. Figure 9 shows the phenol removal by using the different employed electrolyte.The 76.87% degradation of phenol was achieved when using NaCl as the electrolyte.However, 64.68% of the phenol was removed when using Na 2 SO 4 .Figure 10 shows the COD removal efficiency was 69.68% during using NaCl as an electrolyte.However, 51.87% COD was removed when using Na 2 SO 4 .It is evident that NaCl as an electrolyte is more effective in the degradation of phenol because, in the wastewaters containing NaCl, active chloro-species like chlorine, hypochlorite, and  -2 can be transformed to obtain activated persulfate, and this generates sulfatefree radicals that can remove organic pollutants like phenol.In this investigation, the temperature in the reactors reached about 30°C, but this condition was insufficient to generate the theoretical amount of sulfate free radicals (Zambrano et al., 2019).Previous investigations have shown that CD is an important factor of the electrochemical process.Higher CD is beneficial to the combined system (Ang et al., 2022).The sonoelectrochemical procedure took place at various CD (10, 25, 40) mA/cm 2 , a pH of 4, an ultrasonic frequency of 40 kHz, a temperature of 25°C, and 3 g/l of NaCl.The findings of phenol concentration were recorded for up to 5 hours and plotted in Figure 11.The phenol removal percentage increased from 62.94 to 76.87 and 100%.As the CD increases from 10 to 25 and 40 mA/cm 2 , respectively, Figure 12 shows that the COD removal percentages were 45.57, 69.68, and 100% at 10, 25, and 40 mA/cm 2 , respectively, after 5h of electrolysis.This indicates that the increase in CD caused enhancement in phenol degradation, and this occurs because there is now more electrochemically produced hydrogen peroxide as a result of the electrolysis, and when the concentration of hydrogen peroxide rises, hydroxyl radical (OH • ) rises to a level sufficient to react with the organic pollutants already present in the treated solution, reducing the concentration of phenol.The obtained findings concur with those of other studies (Abbas et al., 2016(Abbas et al., , 2021;;Ahmed et al., 2023).

CONCLUSIONS
Successful preparation and testing of a SnO 2 electrode by cathodic electrodeposition for the process of phenol degradation were attained in the present study.According to the XRD data, tetragonal SnO 2 is the significant phase structure at all deposits.It was found that the applied current density, HNO 3 concentration, and SnCl 2 .2H 2 O concentration were essential for preventing the co-deposition of Sn and had a significant impact on the morphology of the deposits.These processing parameters could be precisely controlled to produce dense SnO 2 films that adhered well to the graphite substrate.The electrode which prepared at 10 mA/cm 2 , 50 mM of SnCl 2 .2H 2 O, and 250 mM of HNO 3 is the best electrode which produced the smallest crystal size, with no cracks, and gave the best phenol and other organic by products removal.Therefore, the current electrode is a promising method for sonoelectrochemical indirect oxidation to remove a variety of organic contaminants from wastewater.The combined sonochemical and electrochemical treatments combined display an advantageous effect.Using sodium chloride as the electrolyte leads to a higher degradation of phenol compared with sodium sulfate.The performance of the sonoelectrochemical removal of organic compounds is most affected by CD.The phenol and COD removal efficiency reached 100% at current density of 40 mA/cm 2 at temp.= 25°C, NaCl conc.= 3 g/l, pH = 3 and 40 kHz ultrasonic frequency.The results of current research showed that the SnO 2 electrode is very promising in the removal of organics by sonoelectrochemical oxidation.

Table 1 .
Experiments of SnO 2 preparation measurement of the phenol content and phenol can be converted into additional by product organic substances during oxidation, so it was better to estimate the chemical oxygen demand (COD).The phenol removal efficiency (PRE) is determined by Equation1(Hamad, 2021) : PRE = −  × 100 (1) Peak positions, relative intensity, and crystal size are shown in Table2.These peaks are all in strong agreement with the data provided for tetragonal SnO 2 (JCPDS no.41-1445) and previous studies (Daideche et al., 2017 b).

Table 2 .
Data analysis from XRD patterns in Figure3