TiO 2 surface modified electro-graphite nanocomposite as a novel nano photocatalyst for removal of furfural from aqueous solution

Abstract

In the present study, novel nanocomposites based on carbon electro-graphite modified with titanium dioxide (TiO₂) nanoparticles were prepared by a sol-gel method and used as a strong nano photocatalyst to increase the removal efficiency of Furfural from petrochemical saline solutions. The structural properties of the prepared nanocomposite EG@TiO₂ were fully characterized by different microscopic and spectroscopic analyses, such as SEM, TEM, XRD, FT-IR, BET, EDS, Raman and GC-MS. Under optimal conditions, the synthesized nanocomposite at acidic pHs increased the oxidation process efficiency of furfural by more than 99.5% in the presence of 50 mg/L of the pollutant, and the oxidation process was completed with the SPEK method in less than 3 h. Furthermore, the XRD results demonstrated that the formed layer at pH = 4 on the electrode surface had more crystallinity than that deposited at pH =8. Therefore, the obtained results for real samples showed that the prepared electrode at pH = 4 increased the Furfural oxidation efficiency and reduced the reaction time.

Keywords

nano technology electro-graphite sol-gel method furfural pollution environment

Introduction

The process known as “graphitization” involves heating a mixture of finely crushed coke in an electric furnace to a temperature of between 2500 to 3200 °C. The result is a relatively porous material called electro-graphite oxide. This process usually involves the use of a catalytic mixture (e.g., silicon dioxide and iron oxide), which is extruded into square or circular blocks.^1,2 The obtained material is widely used for photocatalytic applications due to its various advantages, which include non-toxicity, stability, high electrical conductivity, high durability, and superior chemical resistance.³ The employment of carbon-based materials to remove organic pollutants is widely investigated in recent years.^4–6 Miardan et al.⁵ investigated the performance of a graphene quantum dot-based composite and a carbon-cage adsorbent for removing methylene blue dye from wastewater and demonstrated that both adsorbents showed excellent adsorption performance for the pollutant. Ansari et al.⁴ used fullerene-oxide nanostructure as an adsorbent foe ammonia and proved that the nanocomposite is an affordable adsorbent for NH₃ substance with dual functions of removal and detection purposes. Despite these advantages, this material has some disadvantages over metal electrodes, including a memory effect in the composites and slow electron transfer rates. This slower rate of electron transport results in an increase in the excess potential needed to reduce or oxidize the chemicals on their surface. Nanoceramic coatings can be used to overcome some of the problems by increasing their practical application efficiency.^7,8

Biological systems are hampered by numerous industrial pollutants that are not biodegradable. These materials often decompose very slowly. Among the numerous organic pollutants, Furfural finds widespread application in the petrochemical and refinery industries (as the principal solvent), paper production, oil refining, and related processes.^9,10 Furfural serves as a raw material for the production of tetrahydrofuran and furfural alcohol, which are used to make organic solvents and resins, respectively. It is also used as a chemical intermediary in the production of herbicides, fungicides, and flavorings.¹¹

Furfural is a dangerous chemical that can have negative effects on the environment and individuals. The moreover, kidneys, spleen, and lungs can all suffer damage from prolonged exposure to Furfural. Tumours and genetic disorders may potentially result from it. It can stimulate the skin, eyes, mucous membranes, and respiratory tract in humans. Several employees have complained of red eyes, sore throats, and headaches following exposure to furfural concentrations of more than 14 ppm. Therefore, in order to avoid furfural's detrimental effects on both humans and the ecosystem, effluent containing it needs to be adequately treated before being discharged into the environment.¹² Traditional methods of treating organic pollutants, like separation, dilution, and toxin trapping, are frequently energy-intensive and insufficiently effective.^13–15 The disadvantage has prompted researchers to create effective and affordable methods.^16,17 Significant volumes of refractory organic matter can be decomposed into harmless minerals and carbon dioxide by semiconductor-based photocatalysts, such as titanium dioxide, zinc oxide, hematite, gallium phosphine, and cadmium sulphate.¹⁸ TiO₂ (in rutile, brookite, and anatase forms) is one of the widely used photocatalysts due to its high stability over a wide range of catalytic cycles. Titanium oxide nanoparticle-based photocatalytic systems are very popular in many domains, such as the removal of harmful bacteria and cells from contaminated water, the breakdown of dangerous chemical compounds, and more.^19,20 Aarich et al.²¹ synthesized a Bi₁₂TiO₂₀/Bi₄Ti₃O₁₂ heterostructure and investigated its photocatalytic activity in the MO degradation under UV irradiation. Their results showed that the heterostructure is a promising candidate as an UV-visible light photocatalyst. Attaf et al.²² studied the structural and optical properties of TiO₂ thin coatings applied on glass substrates via a sol-gel method. According to the results of the study a degradation efficiency of 94% was obtained at optimum conditions for methylene blue in a real wastewater. Beidaghdar et al.²³ found that UV/H₂O₂/O₃/TiO₂ combined system caused the complete removal of phenol with an efficiency of almost 100% from urban and industrial wastewaters.

TiO₂ has a wide range of uses due to its exceptional resistance to environmental pollutants and high temperatures. For instance, it can be used in fuel sensors that operate at temperatures above 400 °C. Among the three different crystalline forms of TiO₂, it is demonstrated that anatase is the most stable, while brookite is the most unstable.²⁴ The following is the mechanism of furfural (F) compound oxidation in the presence of titania-based photocatalytic (PC)²⁵:

PC + Hv ➔ h⁺_vb + e_eb⁻

OH⁻ + h⁺_vb ➔ OH.

H₂O + h⁺_vb ➔ OH + H⁺

O₂ + e_eb⁻ ➔ O₂⁻

O₂⁻ + e_eb⁻ + 2H⁺ ➔ H₂O₂

2O₂⁻ + 2H⁺ ➔ O₂ + H₂O₂

H₂O₂ + e_eb⁻ ➔ OH⁻ + OH.

F + h⁺_vb ➔ F⁺ (Final products)

F + OH. ➔ F. (Final products)

Based on the reactions, it can be observed that Furfural is removed in the presence of the photocatalyst by hydroxyl radicals. Titania nanoparticles are made by several methods, such as sol-gel, hydrothermal, ethanol-thermal, chemical solution decomposition, chemical vapour decomposition, wet two-stage chemical approach, and ultrasonic irradiation. One of the advantages of the sol-gel process is its ability to produce pure TiO₂ nanoparticles at relatively low temperatures.^26,27 The applying of TiO₂ nanoparticles onto electro-graphite substrates using the sol-gel method has not only enhanced the surface area and active sites but also improved the electron transfer efficiency, leading to higher photocatalytic performance.^28,29 The present study introduces a novel nanocomposite, TiO₂ surface-modified electro-graphite, as a robust photocatalyst for the efficient removal of furfural from saline petrochemical wastewater. This innovative approach addresses the limitations of conventional photocatalysts and demonstrates a significant advancement in the field, paving the way for future applications in environmental remediation.

The sol-gel method employed in this study ensures a uniform and compact deposition of TiO₂ nanoparticles, which is crucial for achieving high catalytic efficiency and stability.^30,31 Furthermore, the study highlights the importance of pH control in optimizing the photocatalytic performance, as demonstrated by recent research indicating that pH levels affect the crystalline of TiO₂ layers.³² These findings contribute to the growing body of knowledge on the use of TiO₂-based nanocomposites for environmental applications and underscore the potential of this approach for addressing the challenges associated with wastewater treatment.

The hypotheses of the present research study are: (i) the TiO₂ surface-modified electro-graphite nanocomposite will exhibit higher photocatalytic efficiency in the removal of furfural from aqueous solutions compared to conventional photocatalysts. (ii) The pH for the photocatalytic degradation of furfural affects the final properties of the composite. (iii) The sol-gel method will ensure a uniform and compact deposition of TiO₂ nanoparticles on the electro-graphite electrode, leading to improved catalytic performance. Therefore, by addressing the specific challenge of furfural removal from saline petrochemical wastewater, this study contributes to both scientific knowledge and practical solutions for wastewater treatment. The findings can be applied to improve the performance of industrial wastewater treatment systems, thus reducing environmental contamination.

Materials and instruments

All of the chemical compounds used in this study were of analytical grade and purchased from Merck company (Germany). Titanium tetra isopropoxide (TTIP), tri ethanol amine (TEOA), nitric acid, sodium hydroxide, acetone, and electrographites were used for the sol-gel synthesizing process. Titanium dioxide (Anatase phase) nanoparticles with a purity of over 99% with a particle size of 10–25 nm were purchased from Houston, USA. The pH of the solutions was adjusted using 0.1 M NaOH/ HCL. Double distilled water was used for the preparation of the solutions in the sol-gel process and for washing laboratory dishes.

UV-Vis absorption studies were conducted using the Perkin Elmer Lambda 35 type UV-Vis spectrophotometer. A pH meter, model pH METRE F-11, made by the HORIBA company, was used to measure the pH values. A 100 KV transmission electron microscope (TEM) (Zeiss-EM 1° c-100 KV model) was used to assess the microstructure of the manufactured nanocomposites. The panalytical company's X'pert pro model XRD instrument was utilized to examine the products’ crystalline structure. FTIR and Raman spectra were acquired with an XPLORA PLUS model machine from HORIBA firm and a Perkin Elmer machine, respectively. The German-made ZEISS company's sigma VP instrument was used for the FESEM study. A Microtrac-manufactured BELSORP Mini gadget was used to conduct the BET test.

Preparation of the samples

TiO₂ nanoparticles were synthesized by sol-gel method using titanium tetra isopropoxide (TTIP) and tri ethanol amine (TEOA) solutions as the gel layer. The prepared gel layer was calcinated on the surface of electrographites using a Teflon autoclave. During the mixing of the sol phase to prepare the nanocomposite, the pH of the solution was adjusted at 8 and the stirring process continued until a brown solution was obtained. In order to prepare the nanocomposite with acid concentration, the pH of the solution was adjusted at 4 (using drop wise addition of HCL) and a white solution was obtained.

In the first step, the surfaces of the electrographites were polished with a soft sandpaper, and then, the electrographites were immersed in 10% NaOH, HNO₃, and CH₃COCH₃ solutions for 5 Min. Next, 1 mol/l titanium tetra isopropoxide was mixed with 2 mol/l Triethanolamine and stirred for 30 Min using a magnetic stirrer. Then double distilled water was added to the solution in the presence of metal alkoxides and stirred for two hours at room temperature. Then the solution was stirred for 24 h at 100°C. Then the obtained gel was heated using a teflon autoclave for 72 h at 140°C. 0.005 g of TiO₂ nanoparticles were mixed with 10 mL of double distilled water and homogenized using an ultrasonic homogenizer for 30 min. The obtained solution was added to the salt solution. Then electrographites were coated with a film containing the synthesized nanoparticles. The coated electrographites were placed in an electric induction furnace at 500°C to 550°C for 2 h. After cooling, the final product was taken out of the electric induction furnace and prepared for the next analyses. As is shown in Figure 1, the surface-modified graphite electro cathode nanocomposite was prepared in two separate suspension with pH = 4 (acidic solution) and pH = 8 (alkaline solution).

Figure 1.

The surface modified graphite electro cathode nanocomposite with TiO₂ nanoparticles prepared in (a) acidic solution (white suspension) at pH = 4 and (b) alkaline solution (brown suspension) at pH = 8.

Results and discussion

Characterization of TiO₂ surface modified electrographite cathode

TEM and FE-SEM microscopic tests were used to investigate the size distribution and morphology of titanium dioxide particles deposited on the graphite electrode surface (EG@TiO₂) and the results are shown in Figure 2.

Figure 2.

TEM and FE-SEM micrographs of graphite electrodes deposited by a layer of titanium dioxide at (a and b) pH = 4 and (c and d) pH = 8.

We appreciate the reviewer's suggestion to provide a reference for the hypothesis regarding the conversion of titanium dioxide (TiO₂) into titanium hydroxide under alkaline conditions. The enlargement of agglomerated particle sizes at pH = 8 is indeed linked to the chemical behavior of TiO₂ in alkaline environments, where TiO₂ can partially transform into titanium hydroxide species (e.g., Ti (OH)₄ or related intermediates). These hydroxide groups can form hydrogen bonds, leading to increased agglomeration. This phenomenon has been documented in the literature, particularly in studies on TiO₂ synthesis via sol-gel methods under varying pH conditions. For instance, it was reported that in alkaline conditions, TiO₂ nanoparticles tend to form hydroxylated species, which enhance particle aggregation through hydrogen. We have added these references to support our hypothesis in the revised manuscript at the specified location (Page 11, Line 190).

Additionally, our TEM micrographs (Figure 2(a) and 2(c)) confirm this trend, showing agglomerated particle sizes of approximately 50–100 nm at pH = 4, increasing to 160–190 nm at pH = 8. The FE-SEM images (Figure 2(b) and 2(d)) further support this by showing a less compact coating at pH = 8, which is consistent with increased agglomeration.

According to Figure 2, TEM micrographs display the agglomerated particles. Based on the microscopic images, the size of the agglomerated particles in the prepared sample at pH = 4 is approximately 50 to 100 nm. However, when the pH is increased to 8, the size of the agglomerated particles increases to around 160 to 190 nm. The significant enlargement of agglomerated particle size may be attributed to the conversion of titanium dioxide into titanium hydroxide under alkaline pH conditions, wherein these hydroxide groups are joined together through hydrogen bonding to form bigger agglomerates. The process of agglomeration negatively impacts the structure and ultimate characteristics of the coating. In addition, the difference in coating density applied at different pHs is quite clear from FE-SEM micrographs. Accordingly, the coating layer applied at acidic pH (4) is more compact than the other (pH = 8). In other words, the acidic environment can enhance the rate of gelation in the sol-gel process, leading to the formation of a more compact and cohesive titanium dioxide layer on the surface of the electro-graphite. This dense coating is beneficial for improving the photocatalytic properties as it increases the surface area and active sites available for catalytic reactions.^33,34 As shown in these images, three particles were randomly measured on the coating surface and their dimensions were reported. According to the obtained dimensions, it is clear that the particle sizes for the sample with the coating applied at pH = 4 are smaller than the coating applied at pH = 8. For a more detailed study on the particle size distribution, the diameter of 100 particles was randomly measured by Image J processing software and the obtained histograms are shown in Figure 3.

Figure 3.

Histograms of the particle size distribution of the electo-graphite electrodes coated by a layer of titanium dioxide nano particles at (a) pH = 4 and (b) pH = 8.

According to the histograms, for the prepared sample at the acidic pH, the diameters of more than 50% of the measured particles were between 25 to 35 nm. While for the prepared sample at the alkaline pH it can be seen that, the diameter size of more than 60% of the measured particles was in the range of 30 to 50 nm. The obtained statistical data from the measurements are reported in Table 1.

Table 1.

The statistical results obtained from the particle size distribution histograms.

pH	Total number of measurements	Average (nm)	Standard deviation (nm)	The smallest particle (nm)	The largest particle (nm)
pH = 4	100	28.81	7.65	13.58	51.26
pH = 8	100	39.23	11.19	21.03	68.95

According to Table 1, the coating applied at the higher pH had a larger average particle size and a wider distribution than the sample prepared at the lower pH. Therefore, it can be concluded that the preparation of the modification layer in acidic pHs has positive effects on the final properties of the coating. In fact, at higher pH levels, the decreased concentration of hydrogen ions can lead to a reduction in the repulsive forces between TiO₂ nanoparticles. This reduction in repulsive forces facilitates the agglomeration of particles, resulting in larger agglomerates. Furthermore, the sol-gel process at low pH can accelerate hydrolysis and condensation reactions, leading to a reduction in the growth and agglomeration of TiO₂ particles.³⁵

We acknowledge the reviewer's concern regarding the sample size of 100 particles measured using ImageJ and the suggestion to use additional techniques like DLS (Dynamic Light Scattering) and SAXS (Small-Angle X-ray Scattering) for a more comprehensive particle size distribution analysis. We agree that increasing the sample size or complementing ImageJ measurements with other techniques can provide a more robust dataset.

To address this, we first clarify that the choice of 100 particles for ImageJ analysis was based on standard practice in TEM/SEM-based particle size analysis, where 50–100 particles are often considered sufficient for a representative distribution in nanomaterial studies (e.g., ISO 13322–1:2014, “Particle size analysis — Image analysis methods”). However, to strengthen our findings, we have conducted additional DLS analysis, which was already performed as part of our broader experimental characterization but not initially included in the manuscript due to space constraints.

The DLS data, includes 250 measurements for each sample (pH = 4 and pH = 8), offering a statistically significant dataset. The DLS results indicate:

Sample at pH = 4: Mean particle size of 27.76 nm, with a standard deviation of 20.27 nm, and a range from 0.105 nm to 69.23 nm.

Sample at pH = 8: Mean particle size of 28.23 nm, with a standard deviation of 21.72 nm, and a range from 0.1032 nm to 70.51 nm.

These DLS results complement the TEM/ImageJ analysis, which reported that at pH = 4, more than 50% of particles had diameters between 25–35 nm, while at pH = 8, more than 60% were between 30–50 nm. The DLS data aligns with this trend, showing a slightly larger mean size at pH = 8, though we note a discrepancy: the DLS sizes are generally smaller than the TEM measurements, which report agglomerated particle sizes (50–100 nm at pH = 4, 160–190 nm at pH = 8). This difference arises because DLS measures the hydrodynamic diameter of particles in solution, which may include smaller primary particles or loosely bound aggregates, whereas TEM captures the size of agglomerates in the dried, coated state on the electrode surface. To clarify this, we have added a discussion in the revised manuscript (Page 11, Line 203) explaining the complementary nature of DLS and TEM measurements and the reasons for the observed differences.

Regarding SAXS, while it is an excellent technique for analyzing particle size distributions and shapes, particularly for nanoscale materials, we did not have access to SAXS instrumentation during this study. However, we believe the combination of TEM/ImageJ (100 particles) and DLS (250 measurements per sample) provides a robust dataset for our conclusions.

In order to quantitatively study on the present elements in the prepared coatings and the distribution of the elements, EDS and elemental mapping analyses for the coatings applied at pH = 4 and 8 were performed and the obtained results are shown in Figure 4 and Figure 5, respectively.

Figure 4.

(a) EDS spectrum and (b-f) elemental mapping of the TiO₂ nanoparticles coated on electro-graphite electrode at pH = 4 (b) SEM micrograph, (c) Ti K_α, (d) C K_α, (e) O K_α, and (f) combination of the elements.

Figure 5.

(a) EDS spectrum and (b-f) elemental mapping of the TiO₂ nanoparticles coated on electro-graphite electrode at pH = 8 (b) SEM micrograph, (c) Ti K_α, (d) C K_α, (e) O K_α, and (f) combination of the elements.

According to Figure 4 (a), it is clear that in the coated sample at pH = 4, there was about 36% wt. of Ti element, while the concentration of carbon was only about 7% wt., confirming the presence of a layer of titanium dioxide nanoparticles on the electro-graphite surface. In these results, there was also about 56% wt. of oxygen element, due to the presence of two oxygen elements per titanium element in the structure of the coating. Furthermore, based on the elemental mapping analysis (Figure 4 (b-f)), it can be seen that the titanium dioxide blocks covered a large part of the graphite electrode and the carbon element related to the graphite substrate only was detectable in the small parts between the blocks. By increasing the pH value to 8 (Figure 5 (a)), the concentration of detected titanium and carbon elements decreased and increased to about 21% and 49% wt., respectively, indicating less coverage of the TiO₂ coating on the graphite substrate. In addition, the results of the elemental mapping analysis show that the titanium dioxide blocks on the surface of the sample are smaller than those that were observable for the coating applied at pH = 4. Therefore, the obtained results suggest that lower pHs results in achieving a denser coating on the electrographite surface. It is due to the fact that at higher pH levels, the rate of gelation during the sol-gel process slows down. This slower rate results in less compact and less cohesive gel formation, leading to a less dense coating on the surface.

XRD test was used to investigate the crystalline structure of the titanium dioxide-based coating on the electro-graphite surface and the obtained results for the coatings applied at pH = 4 and 8 are shown in Figure 6.

Figure 6.

X-ray diffraction patterns of EG@TiO₂ coatings applied at different pHs.

By matching the XRD patterns of the prepared EG@TiO₂ nanoparticles at different pHs with reference cards using X’Pert Highscore Plus software, it was found that the anatase phase (JCPDS Card no. 21–1272) with a tetragonal crystalline structure is formed for both coatings. The Miller indices for each crystalline plane were labelled on the related peak in Figure 6. By comparing the intensity of the peaks for the coatings, it is clear that the peaks for the coating applied at pH =4 had more intensities, indicating that the anatase phase formed on the electro-graphite surface has more crystalline degrees at lower pHs. Higher hydrogen ion concentrations at low pH levels can help mitigate the impact of impurities and contaminants during the crystal formation process. In addition, acidic conditions tend to stabilize the anatase phase of TiO₂ over other phases. Studies have shown that the formation and stabilization of the anatase phase are favored in low pH environments, as these conditions support the formation of a stable crystalline structure.^36,37 This results in purer and more well-organized crystals. The more crystallinity of the formed anatase phase can positively affect the final properties of the coating.

Debye-Scherer's equation (Eq. 1) is used to calculate the crystallite size of the formed TiO₂ nanoparticles in the coatings.³⁸

D = k λ / β \cos (θ)

(1)

where λ is the X-ray wavelength (1.54 angstroms), K is the shape factor (approximately equal to 0.9), β is the peak width at half of the maximum, and θ is the peak position. Accordingly, the average crystallite size for the coatings applied at pH = 4 and 8 was about 8.98 nm and 5.77 nm, respectively, indicating that the crystallite size decreased by increasing the pH values.

Raman spectroscopy provides more information about the formed coatings on the electro-graphite surface (Figure 7).

Figure 7.

Raman spectra for the TiO₂-based coatings applied at different pHs.

Based on Figure 7, characteristic Raman peaks of TiO₂ anatase phase can be seen in the spectra of both samples. In these spectra, the located peaks at 150.4 cm⁻¹, 277.7 cm⁻¹ and 645.6 cm⁻¹ are related to the E_g vibration mode of anatase. In addition, the peaks at 406.3 cm⁻¹ and 520.7 cm⁻¹ correspond to the vibration modes of B_1g and A_1g + B_1g of the phase, respectively.³⁹ The appeared peaks are in good agreement with those reported in previous studies and confirm the formation of pure anatase phase.^40,41 the small peak shift in the spectrum of the prepared sample at pH = 8 may be due to the reduction of crystallite size of TiO₂ nanoparticles deposited at the higher pH, as was demonstrated by XRD analysis. The scattering from the (110) face and the B_1g mode from the (001) are the underlying causes of the extremely intense E_g mode.³⁹ In addition, it is clear that the intensity of all Raman peaks in the spectrum of the coating applied at pH = 4 were more than those obtained for the coating deposited at the higher pH, which could be another confirmation of the formation of more Ti-O bonds in the coating applied at the lower pH. As previously mentioned, lower pH levels promote the formation of a more crystalline anatase phase of TiO₂. Crystalline structures have well-defined and more ordered atomic arrangements, which enhance the Raman scattering intensity. This results in stronger Raman peaks due to more coherent and efficient interaction of the incident light with the Ti-O bonds. At lower pH, the sol-gel process results in a denser coating. A denser TiO₂ layer means a higher concentration of Ti-O bonds within the same volume. The increased density of Ti-O bonds contributes to the higher intensity of Raman peaks, as more bonds are available to scatter the incident Raman light. Furthermore, in acidic conditions, the reduction in impurities and defects within the TiO2 structure leads to fewer disruptions in the crystal lattice. This cleaner and more perfect lattice structure enhances the Raman signal because there are fewer non-radiative decay paths for the excited vibrational states.

Another powerful test to identify the formed bonds and functional groups is FT-IR analysis. Figure 8 shows the FT-IR spectra of the coatings applied at two different pHs.

Figure 8.

FT-IR spectra for the TiO₂-based coatings applied at different pHs.

According to Figure 8, in the FT-IR spectra of the coatings applied at pH = 4 and 8, three absorption peaks can be seen, in well agreement with other studies on EG@TiO₂ nanocomposites.^42,43 Accordingly, the broad peak in the wavenumber range of 3000–3800 cm⁻¹ corresponds to the stretching vibration of the O-H bonds.⁴⁴ In addition to the adsorbed moisture, this bond can also be due to the hydroxyl bonds present on the electro-graphite surface (EG@TiO₂). The absorption peaks at 1630 cm⁻¹ and 650 cm⁻¹ are due to the stretching vibration of the C = C bond in electro- graphite (EG) and the stretching vibration of the Ti-O bond in TiO₂ nanoparticles on the electrode surface, respectively.^45,46 From the spectra, it is clear that the absorption peak intensity of stretching vibration of Ti-O bond in the FT-IR spectrum of the coating applied at pH = 4 was more than that applied at pH = 8, suggesting a higher number of titanium-oxygen bonds in this sample. So, the FTIR results confirm the results of other analyses and show that the lower pH leads to the formation of a denser coating on the electrode surface. On the other hand, by comparing the peak intensities of the C = C and O-H bonds, it is clear that the absorption peak intensity of the bonds increased by increasing pH due to lower compaction of the TiO₂ layer on the graphite substrate, as was seen in microscopic and elemental results.

We sincerely apologize for the error in our FT-IR interpretation regarding the assignment of the peak at 650 cm⁻¹. The reviewer is correct that the 450–800 cm⁻¹ range is typically associated with Ti-O-Ti stretching vibrations in TiO2, particularly for the anatase phase, and we incorrectly attributed the 650 cm⁻¹ peak solely to the C = C stretching vibration of the electrographite substrate.

Upon re-examination of the FT-IR spectra (Figure. 8) and the literature, we confirm the following:

The peak at 650 cm⁻¹ is primarily due to the Ti-O-Ti stretching vibration of the anatase TiO₂ phase, as this is a characteristic mode for TiO₂ in this wavenumber range. This assignment is consistent with our Raman spectra (Figure. 7), which show a peak at 645.6 cm⁻¹ corresponding to the Eg vibration mode of anatase TiO₂, supporting the presence of Ti-O bonds.

However, the electrographite substrate may also contribute to the spectrum in this region, as C = C stretching vibrations in graphitic materials can appear around 600–700 cm⁻¹ in some cases, particularly if the graphite is functionalized or defective. In our original interpretation, we overemphasized the C = C contribution due to the presence of the electrographite substrate.

Revised Interpretation: The 650 cm⁻¹ peak is a composite signal, with the dominant contribution from the Ti-O-Ti stretching vibration of TiO₂, and a minor contribution from the C = C stretching of the electrographite substrate, particularly at pH = 8, where the TiO2 coating is less dense (as shown in Figure 5, with 49% wt. carbon vs. 7% wt. at pH = 4). The higher intensity of the 650 cm⁻¹ peak at pH = 4 reflects the denser TiO₂ coating (more Ti-O bonds), while the increased carbon exposure at pH = 8 enhances the relative contribution of the C = C mode.

According to the results of the analysis of BET techniques in Tables 2 and 3 , it can be seen that the active absorption surface, volume and size of the holes in the nano sono –photo catalyst sample EG@TiO₂ is desirable and acceptable.

Table 2.

Characteristics of the case study sample based on the results of BET techniques.

Type Technique	Active Surface, m²/g	Pore Size, nm	Shapes & Types
BET	39.014 m²/g	25.737 nm	Plate-Shaped/ Mesoporous
BJH-Plot, Absorption	42.218 m²/g	1.22 nm	Bell-Shaped/ Small mesopores
BJH-Plot, Absorbed	185.89 m²/g	1.88 nm	Conical Shape / Small mesopores
t-Plot	38.925 m²/g	1–2 nm	Non-Porous sample/ Type I microporous
Langmuir-Plot	50.533 m²/g	1–2 nm	Langmuir Isotherm/ micropore
MP, the first category	36.817 m²/g	0.7 nm	Abundance Curve diagram/ micropore
MP, the second category	52.723 m²/g	1.1 nm	Abundance Curve diagram/micropore
Alpha	41.302 m²/g	1–2 nm	Isotherm diagram/micro holes
CI	33.198 m²/g	1.66 nm	Hole distribution Curve diagram / mesopore
DH	43.94 m²/g	1.22 nm	Hole distribution Curve diagram/mesopore

Table 3.

Findings and process capabilities of BET techniques.

Type Technique	Technique Ability	Technique Process	Findings
BET	Multilayer gas absorption	A developed theory of Langmuir monolayer absorption	Proof of monolayer absorption hypothesis
BJH-Plot, Absorption	Calculation of the active surface of the adsorption process	Calculate the volume and diameter of the hole	Holes with a hole diameter of 1–2 nm are the most abundant hole
BJH-Plot, Absorbed	Size distribution of holes	Determining the active surface, size and frequency of holes	Adsorbed molecules are more strongly desorbed
t-plot	Determining the volume of micro-holes	Measuring the amount of nitrogen gas absorbed in relative pressures holes	The studied sample is type I and non-porous sample
Langmuir-Plot	Calculations of the active surface of the adsorption process	Determining the amount of absorption in Cavities and coatings of two or more layers	Weak interaction BET ween adsorbent and adsorbate
MP, the first category	Determining the frequency of micro-holes	Determining the active surface and frequency of micropores	The largest amount of micro pore are in the first category of the MP Technique
MP, the second category	Determining the frequency of micro-holes	Determining the active surface and frequency of micropores	The abundance of micropores in the sample is of the second type of the MP Technique
Alpha	Calculation of the active surface of the sample with standard isotherm	Determining the isotherm of the sample using standard isotherms	The slope of the isotherm curve is calculated by alpha technique.
CI	Hole size distribution using the Kelvin equation	More accurate calculation of absorption and desorption branches than BJH technique	The results of the Technique are in relative agreement with the results of BET and BJH Techniques.
DH	Hole size distribution using the Kelvin equation	Simplicity of calculations without the use of complex assumptions	The results of the Technique are in relative agreement with the results of BJH, CI and BET Techniques.

We appreciate the reviewer's observation regarding the FT-IR spectra in Figure 8, where a positive peak at approximately 2250 cm⁻¹ is present in the spectrum of the sample prepared at pH = 8 but absent in the spectrum at pH = 4. These peak warrants further explanation, as it is not typically associated with the expected vibrational modes of TiO₂ or electrographite.

The peak at 2250 cm⁻¹ falls in the region typically associated with triple-bond stretching vibrations, such as those of nitrile groups (C≡N, 2200–2260 cm⁻¹) or carbon dioxide (CO₂, ∼2350 cm⁻¹) adsorbed on the surface. In our case, we attribute this peak to the presence of trace atmospheric CO₂ adsorbed on the sample surface during preparation or measurement, which is more pronounced at pH = 8 due to the chemical environment. At pH = 8, the alkaline conditions may enhance the adsorption of CO₂, forming carbonate-like species on the TiO₂ surface, as TiO₂ is known to interact with CO₂ under basic conditions. These carbonate species can exhibit vibrational modes in the 2200–2400 cm⁻¹ range, depending on their coordination with the TiO₂ surface.

In contrast, at pH = 4, the acidic environment likely inhibits CO₂ adsorption or carbonate formation, resulting in the absence of this peak. Additionally, the sample preparation process for FT-IR (e.g., exposure to air, drying conditions) may have contributed to this difference, as the pH = 8 sample may have a higher affinity for CO₂ due to its surface chemistry (e.g., more hydroxyl groups available for interaction).

To confirm this interpretation, we re-examined the FT-IR measurement conditions and noted that the samples were not measured under a controlled atmosphere (e.g., N₂ purging), which could have minimized CO₂ interference. We also suggest that future studies could use controlled atmospheres to eliminate such artifacts, but we believe this peak does not affect the primary conclusions regarding the Ti-O and O-H bonds, which are the focus of our analysis.

From the reported results, the BET active surface area for the under-studied sample was 39.014 m²/g. This value indicates a relatively high specific surface area for the material. A high surface area is beneficial in many applications, especially for photocatalysts, as it provides more active sites for adsorption and reactions. This can enhance the overall efficiency of the photocatalytic processes by facilitating better interaction between the catalyst and the target pollutants. The pore size of 1.22 nanometers (according to BJH model) falls within the microporous range (pores with diameters less than 2 nanometers). Microporous materials are advantageous for various applications, including adsorption and catalysis, because they offer high surface area and can effectively trap and interact with small molecules.^47,48

Effect of pH and initial concentration of pollutant on furfural removal

It can be expected that pH has a significant effect on the fluctuation of the electric charge of the catalyst surface, absorption of photons, and electron-hole transfer in the hydrolysis process of Furfural molecules. From Figure 9, it can be seen that by increasing the pH from 4 to 8, the Furfural decomposition efficiency of the refinery salt decreased slightly. Therefore, the maximum decomposition efficiency occurred at pH = 4. In fact, electron-hole transfer and hydrogen peroxide production have an increasing trend in acidic conditions, leading to an increase in the activity of TiO₂ nanoparticles at catalytic sites. In alkaline conditions, the low electrostatic attraction between negative ions on the surface of the catalysts and lower hydrogen peroxide production negatively affect the efficiency of the Furfural removal from the solution.

Figure 9.

The effect of ph and initial concentration of furfural on the removal efficiency of the pollutant.

In addition, according to Figure 9, after 240 min of reaction in the presence of 50 mg/L of the pollutant at pH = 4, the dye removal efficiency reached to 99.50%, which is 1.82% more than that obtained at pH = 8, it shows an increase up to 1.82%. Furthermore, the results of the pollutant removal from the effluent solution after 240 min of the reaction in the presence of 100 mg/L Furfural at pH = 8, show an efficiency of 94.80%, which is about 3.45% lower than that obtained at pH = 4. Therefore, it can be concluded that the optimal efficiency for Furfural removal from the wastewater using the synthesized nanocomposite is at the lower pH and pollutant concentration. As previously mentioned, lower pH levels lead to the formation of a more crystalline anatase phase of TiO₂ with higher surface area. A more crystalline structure provides more active sites for photocatalytic reactions, thereby enhancing the efficiency of furfural degradation. In addition, the sol-gel process at lower pH levels results in a denser TiO₂ coating. This denser coating increases the interaction between the photocatalyst and the pollutant, improving the overall degradation efficiency. A higher density of active sites means more opportunities for the furfural molecules to interact with the TiO₂ surface, leading to higher removal efficiency.^49,50

In Figure 10 and 11, shows that the absorption peaks caused by the compounds of the raw sample of Furfural solution and the metabolites resulting from its decomposition were identified using GC-MS analysis.

Figure 10.

Graph has been obtained from the sample (furfural) using a GC-Ms.

Figure 11.

Graph has been obtained from the sample (refined furfural) using a GC-MS.

The chromatogram of a hydrocarbon sample in a GC-MS system was the same as FID. Note the one second wide peak. This means that the mass spectrometer system must scan the peak of the gas chromatography system about several times per second in order to obtain a suitable mass spectrum. Flame ionization (FID) GC was used to identify hydrocarbons, the results of which have been provided as follows.

According to the observed peak, the presence of carboxaldehyde eruption was confirmed at the 5^th minute and the largest amount of sample 1 was assigned to itself.

According to the observed peak and the proximity of several cases, the presence of carboxaldehyde eruption was confirmed in minutes 4 to 6, and the largest amount of sample 5 was assigned to itself. According to the graphs above and the starting and end time of the device, it can be stated that the amount of Furan carboxaldehyde was effective and efficient.

In Tables 4 and 5, shows that the compounds of the raw sample of Furfural solution and the metabolites resulting from its decomposition were indentified using GC-MS analysis.

Table 4.

The results of GC-MS analysis of the sample of furfural salt solution in the refinery.

N0.	RT	Area%	Compounds	Ref
1	5.609	94%	2- Furancarboxaldehyde	6155
2	7.045	0.72%	Ethanone, 1-(2-furanyl)-	12009
3	8.493	4.49%	2-Furan carboxaldehyde,5-methyl-	11991
4	12.773	0.10%	FURFURYL ACROLEIN	19837
5	14.827	0.19%	Naphthalene	25273
6	27.747	0.16%	Heptadecane	174546
7	29.870	0.19%	Octadecane	193184
8	43.088	0.15%	1,2-Benzenedicarboxylic acid, 3-nitro-	326908

Table 5.

The results of GC-MS analysis of the raw furfural sample from the refinery.

NO.	RT	Area%	Compounds	Ref
1	5.174	85.89	2-Furancarboxaldehyde	6154
2	6.862	1.88	1-(1,1- dimethylethyl)-4-methy-	44729
3	8.436	2.31	2-Furan carboxaldehyde,5-methyl-	11997
4	11.926	1.21	Cyclotrisiloxane, hexamethyl-	76687
5	22.540	0.67	Cyclopropane, nonyl-1-Dodecanethiol	71520
6	27.742	0.51	Heptadecane	174553
7	29.870	0.73	Octadecane	193176
8	31.289	0.54	1,2-Benzenedi carboxylic acid, bis (2-methylpropyl) ester	223255
9	43.082	0.58	1,2-Benzenedicarboxylic acid, bis (2- methyl hexyl) ester	326902

Conclusion

In the present study, a sol-gel method was used for the deposition of TiO₂ nanoparticles (anatase phase) on an electro-graphite electrode to improve its efficiency for the removal of Furfural from wastewater. Microscopic tests and elemental analyses depicted that lower pHs led to the formation of a compacter titanium dioxide layer on the electrode. In addition, spectroscopic results demonstrated that the formed layer at pH =4 on the electrode surface had more crystallinity than that deposited at pH =8. In addition, real sample studies showed that the maximum efficiency for Furfural removal from the wastewater using the synthesized nanocomposite was at the lower pH and pollutant concentration. Specifically, the nanocomposite synthesized at pH = 4 exhibited superior performance, achieving over 99.5% removal efficiency within 3 h at an acidic pH. Additionally, BET surface area and BJH pore size analyses indicated a specific surface area of 39.014 m²/g and a pore size of 1.22 nm. These characteristics are advantageous for photocatalytic applications, as they provide a high surface area and microporous structure, promoting better interaction with furfural molecules. These results can open a new path for using the sol-gel method to improve the catalytic properties of the electrographite electrode modified by titanium oxide nanoparticles.

Footnotes

Abbreviations

Acknowledgements

This research is a main part of doctoral dissertation at the Department of Environmental Engineering of the Islamic Azad University (Ahvaz Branch).

Funding

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

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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TiO 2 surface modified electro-graphite nanocomposite as a novel nano photocatalyst for removal of furfural from aqueous solution

Abstract

Keywords

Introduction

Materials and instruments

Preparation of the samples

Results and discussion

Characterization of TiO2 surface modified electrographite cathode

Effect of pH and initial concentration of pollutant on furfural removal

Conclusion

Footnotes

Abbreviations

Acknowledgements

Funding

Declaration of conflicting interests

References

Characterization of TiO₂ surface modified electrographite cathode