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Synthesis and Investigation of TiO2/g-C3N4 Performance for Photocatalytic Degradation of Bromophenol Blue and Eriochrome Black T: Experimental Design Optimization and Reactive Oxygen Species Contribution

Fadimatou Hassan
Pierre Bonnet
Jean Marie Dangwang Dikdim
Nadege Gatcha Bandjoun
Christophe Caperaa
Sadou Dalhatou
Abdoulaye Kane
1,* and
Hicham Zeghioud
UniLaSalle-Ecole des Métiers de l’Environnement, Cyclann, Campus de Ker Lann, 35170 Bruz, France
Department of Chemistry, Faculty of Science, University of Maroua, Maroua 814, Cameroon
Institut de Chimie de Clermont-Ferrand (ICCF), Université Clermont Auvergne, 24 Avenue Blaise Pascal, 63178 Aubiere, France
Authors to whom correspondence should be addressed.
Water 2022, 14(20), 3331;
Submission received: 20 September 2022 / Revised: 17 October 2022 / Accepted: 18 October 2022 / Published: 21 October 2022


Graphitic carbon nitride (g-C3N4) based photocatalyst was synthesized and the photocatalytic performance was investigated for the removal of Eriochrome Black T (EBT) and Bromophenol Blue (BPB) under UV irradiation. The prepared materials were characterized by SEM-EDX, XRD, Raman, FTIR and DRS. Higher degradation efficiency for the same initial concentrations of EBT and BPB in presence of TiO2/g-C3N4 have been achieved within 160 min of irradiation. The kinetic study showed that the photodegradation of BPB by TiO2/g-C3N4 follows pseudo-first-order kinetics with an R2 value of 0.98. The addition of persulfate (PS) in BPB solution improved the degradation yield from 8.81% to 80.14% within 20 min of UV light irradiation. A Box-Behnken model was developed from three factors and Response surface methodology (RSM) was employed to identify the optimum conditions for the treatment of BPB solution by TiO2/g-C3N4. The experimental values of degradation of BPB agreed with predicted values obtained from central composite design (CCD) analysis with an R2 value of 0.9999. The scavenger study revealed that superoxide radical anion (O2•−) plays a key role (68.89% of contribution) followed by OH and h+ with 22.40% and 15.55% of contribution, respectively. This study has obviously exhibited the potential of TiO2/g-C3N4 composite as a promising catalyst for photocatalytic purposes.

1. Introduction

Water is a natural resource essential to human life. However, the scarcity of this precious resource has become a major problem faced by most countries in the world. This is usually due to the contamination of its sources by wastewater from human activities, mainly industry and agriculture [1]. Several pollutants found in water are organic in nature, such as dyes which are used in the textile and printing industries [2]. About 15–20% of dyes are lost during the dyeing process and therefore end up in the environment and can become a source of pollution [3]. Pharmaceutical pollutants such as drugs are also found in water when they are unmetabolized, and can be considered pollutants if their by-products are discharged into the environment [4].
The presence of pollutants in the water can be very toxic for living beings, causing health problems such as cancerogenic diseases, hepatitis, cholera and other diarrheal disease [5]. Therefore, the removal of these toxic organic pollutants from water is essential to protect public health. Reverse osmosis, ion exchange, electro-dialysis, electrolysis, coagulation, flocculation and adsorption are examples of some conventional methods commonly used for the removal of pollutants from water [6,7]. Adsorption is an effective technology for the removal of organic and pharmaceutical pollutants from water [8,9,10,11]. In adsorption, pollutants from water adhere to the surface of an adsorbent to purify the water, the pollutants are not eliminated but transferred to the surface the adsorbent which may be considered as another possible future source of pollution [12].
Advanced Oxidation Processes (AOPs) are also promising methods for removal of contaminants from wastewater effluents due to their fast reaction rate and strong oxidation capability [13]. There exists a variety of AOPs, such as Fenton [14], ozonation or catalytic ozonation [15], photocatalysis [16], electrochemical oxidation [17], discharge plasma [18], Sonolysis [19] and ionizing radiation [20]. Among AOPs, photocatalysis is widely studied for wastewater treatment. Thus, our interest in this present work to focus on the photocatalysis which has is an emerging technology for the degradation of organic pollutants in wastewater [21,22].
Part of photocatalysis is the use of catalysts to enhance removal of organic pollutants in wastewater. Due to the increase in demand for catalysts with high performance in photocatalytic degradation of organic compounds, researchers have been looking for means to synthesize new catalysts or modify the existing ones to increase their photocatalytic activities [23]. Various oxidants were used in order to intensify the degradation performance of photocatalyst as reported in different works such as such as H2O2, O2 and K2S2O8 [24,25]. Photocatalytic activated persulfates system was reported to have the merits of environmental protection, a great chemical stability and a sustainability [26]. For example, Sun et al. reported an increase of 75% in quinoline degradation efficiency with increasing persulfate concentration in WO3-CuFe2O4 Z-scheme system within 150 min [27].
Over these years, researchers have developed an interest in graphitic carbon nitride (g-C3N4) catalyst in the photocatalytic treatment of pollutants in water [28]. g-C3N4 is a low-cost metal-free polymer with properties such as facile synthesis, high stability, unique optical and electronic characteristics which makes g-C3N4-based catalysts suitable candidates for photocatalytic applications [29]. Semiconducting metal oxides such as TiO2 are known to be efficient in the degradation of organic pollutants and combining it with other compounds may increase their photocatalytic activities [30,31]. To promote the photocatalytic performance of TiO2, different strategies were used, such as surface modification, doping, morphology control, facet engineering, coupling different semiconductors and adding oxidants (i.e., H2O2, O3, and persulfate) [32,33]. Herein, the last two strategies were adopted to achieve these objectives. Indeed, coupling g-C3N4 with TiO2 can increase the reaction site on the surface of g-C3N4, and can also act as an active site to promote the transfer of radical species [34,35]. Heterojunction composites like TiO2/g-C3N4 help to reduce electron-hole recombination rate which leads to increase in photocatalytic reactions [36]. Such catalysts have been widely used in the photocatalytic hydrogen production [37], to enhance visible photocatalytic activity [38] and in the degradation of organic pollutants [39]. Eriochrome Black T (EBT) and Bromophenol Blue (BPB) used in printings, pharmaceutical and textiles industries as dyes, They are commonly used to evaluate the photocatalytic performance of semiconductor materials in aqueous media [16,40,41,42].
In this study, a simple synthesis route of g-C3N4 based catalysts (g-C3N4, 30% TiO2/g-C3N4) were tested and the degradation performances of obtained photocatalysts were studied on the removal of EBT and BPB solutions. The behavior of TiO2/g-C3N4 toward BPB degradation under in influence of a simultaneous different variable parameters was examined via Response Surface methodology (RSM). Furthermore, the potential of TiO2/g-C3N4 in the degradation of dye solutions under various conditions (catalyst dosage, dye concentration, pH values, presence of salt and some oxidants) were also studied. Kinetics of photocatalytic degradation was studied and the contribution of active species in the degradation process was investigated.

2. Materials and Methods

2.1. Materials

Chemicals used for this study were: Bromophenol blue was purchased from REACTIF RAL (Paris, France), Eriochrome Black T and sodium persulfate (Na2S4O8 ≥ 98.0%) were obtained from PROLABO (Paris, French), Hydrogen peroxide (H2O2, 30%, from Millipore, Darmstadt, Germany), sodium chloride (NaCl ≥ 99.0%, purchased from SIGMA-ALDRICH, Missouri, USA), hydrochloric acid (HCl ≥ 37%, from Honeywell, Seelze, Germany), sodium hydroxide (NaOH, 98% purity) were purchased from Sigma–Aldrich (Grenoble, French). Isopropanol (C3H8O ≥ 99.7%) and methanol (CH4O ≥ 99.8%) obtained from MERCK company (Darmstadt, Germany) and potassium dichromate (K2Cr2O7 ≥ 99.5%) purchased from Panreac (Barcelone, Spain). All the solutions were prepared using reverse osmosis water. Eriochrome black t and Bromophenol blue properties are shown in the Table 1:

2.2. Synthesis of g-C3N4 and TiO2/g-C3N4 Composites

The synthesis of bulk g-C3N4 was done by thermal polymerization based on the previous work [43,44]. Briefly, g-C3N4 was synthesized by thermal polymerization where 10 g of dicyandiamide was placed into an alumina crucible, and then heated at 550 °C in a muffle furnace during 4 h (20 °C/min). Afterward cooled to room temperature, the obtained solid was grinded into powders for further usage.
The synthesis of TiO2 was done by the sol-gel method inspired by the work of Askari et al. [45]. Briefly, titanium isopropoxide was dissolved in acetic acid and the mixture was maintained under stirring. The reverse osmosis water was added drop by drop for the hydrolysis. The resultant gel was dried at 200 °C for 2 h. The TiO2 nanoparticles were obtained after calcination of the sample at 400 °C during 4 h in order to improve the crystallinity of the nanoparticles.
However the TiO2/g-C3N4 composite was prepared by wet- impregnation according to the literature [46,47]. In which, a suitable quantity of g-C3N4 and TiO2 were dissolved separately in methanol and then, sonificated in an ultrasonic bath during 30 min. The two solutions were mixed and stirred at room temperature for 24 h. The resultant powder was preserved under room temperature for 12 h eliminate solvent by evaporation.
The schematic illustration of the materials preparation was regrouped in Figure 1.

2.3. Photocatalytic Treatment

EBT and BBP solutions used in the experiments were prepared at different concentrations (2, 5, 10 and 12 ppm). Photodegradation experiments were performed in the batch system of 1000 mL beaker and stirred at 400 rpm. The mass of the catalyst was dispersed into a volume of 500 mL solutions. The mixture was left in the dark and continuously stirred for 30 min to achieve the adsorption-desorption equilibrium before UV irradiation (UVA lamp of 24 W with wavelength of 365 nm as reported in Figure S1). After reaching the adsorption-desorption equilibrium, the dye solution was then exposed to UV-light for 180 min. At 20 min time intervals, 3 mL samples of the solution were collected from the photoreactor and filtered to remove the photocatalysts by a syringe filter (pore size 0.45 µm).
A dual-beam spectrophotometer (SHIMADZU UV-1800) was used to measure the absorbance at maximum wavelengths of 535 nm and 592 nm for EBT and BPB, respectively (example of BPB degradation in Figure S2). The photodegradation rate was estimated using the following equation:
R   ( % ) = C 0 C t C 0 × 100
where C0 is the initial concentration of pollutant in mg/L and Ct is the concentration of dyes in mg/L at time t. For all photocatalytic experiments the pH value was adjusted only at the beginning of the experiment. The pH of the solution was adjusted by HCl and NaOH (0.01 M).
All figures (at the exception of figures generated from RSM study) was drawn using OriginPro 2022, from OriginLab Corporation, Northampton, MA, USA.

2.4. Photocatalyst Characterization

The X-ray diffraction (XRD) patterns of as-prepared catalysts were made using a D8 Bruker spectrometer (Cu Ka radiation with wavelength λ = 0.15418 nm as a wavelength) the incident angle of 2θ, 5–130° using 0.017° each step, and the acceleration tension is 40 kV and current emission equals 30 mA.
Raman spectra were acquired by using Raman spectrometer of JobinYvon company model T64000. The wavelength of laser was 514.5 nm (2.41 eV) and the power was set at 100 mW. The measurement was carried out in solid state by dispersing the sample powder upon glass slide under air at room temperature.
The scanning electron microscopy (SEM) images of the photocatalysts were obtained using an (JEOL 5910 LV) apparatus with EDS elementary analysis using SDD detector (Bruker)
The UV–vis diffuse reflectance spectra (UV–vis DRS) of the photocatalysts were recorded by Cary 300 instrument with scan Rate of 600 nm/min in shifting range of 80 to 500 cm−1.

3. Results and Discussion

3.1. Characterization of Photocatalyst

The SEM images and EDX spectrum of the photocatalytic materials were shown in Figure 2. It seems that g-C3N4 and TiO2/g-C3N4 were composed of many stacked particles, which presented an irregular spherical shapes morphology with more homogeneity for g-C3N4. Moreover, g-C3N4 particles appear as platelets, which is in good agreement with the 2D structure however in TiO2/g-C3N4 images in addition to the observed platelets there are small particles which may be attributed to TiO2. Images presented also a porous surface with various sizes. EDX spectrum confirmed the presences of C, N and O elements in g-C3N4 sample, and the presence of C, N, O and Ti for TiO2/g-C3N4 composite sample. This may demonstrate that both photocatalyst composites were well synthesized.
The crystallinity structure of the synthesized materials g-C3N4 and g-C3N4 modified TiO2 nanocomposites were characterized and the results were shown in Figure 3a. The XRD pattern of TiO2/g-C3N4 revealed numerous peaks at 25.3°, 37.8°, 48.1°, 54.0°, 55.1°, 62.8°, 68.9°, 70.1°, 75.3° and 83.2°, corresponding to the diffraction planes of (101), (004), (200), (105), (211), (204), (116), (220) and (215) which corresponds to the anatase phase JCPDS 21-1272 [44,48]. The g-C3N4 shows the diffraction peaks at 12.8° and 27.57°, corresponds to the inter-plane structural packing motif (100) and the interlayer diffraction plane (002) of the hexagonal graphitic carbon nitride [49]. However, while adding the g-C3N4 to the TiO2 the intensity of g-C3N4 peaks was relatively narrowed compared to TiO2 peaks in the composite. The result confirmed that the crystallinity structure of TiO2 is highly reduced in TiO2/g-C3N4 composites [50].
Raman spectra of photocatalytic materials are shown in Figure 3b. Theses spectra were recorded to obtain more structural information on synthesized composite. Three characteristic Raman active modes of anatase crystalline form of TiO2 with symmetries B1g, A1g and Eg were detected at 396.7, 514.6 and 638.5 cm−1, respectively [51]. The presence of these characteristic vibrational frequencies and their relative intensity confirmed the phase pure anatase TiO2. These results are in an agreement with XRD analyses and confirm the formation of phase pure anatase TiO2 [52]. The spectrum of g-C3N4 did not present exploitable peaks in this zone.
FTIR spectra of g-C3N4 and TiO2/g-C3N4 composites with different g-C3N4 contents are presented in Figure 3c. Pure g-C3N4 shows characteristic IR peaks similar to those of the previous results [53,54]. The peak at 1640 cm−1 is assigned to C=N stretching vibration mode, while those at 1247, 1325 and 1408 cm−1 are associated with C−N heterocycle stretching of g-C3N4, fully condensed C–N moieties/partially condensed C–NH moieties and aromatic C−N heterocycles, respectively [55]. The peak at 808 cm−1 can be attributed to the characteristic bending mode of s-triazine ring system [54]. A broad band in the range of 3150–3300 cm−1 corresponds to the stretching vibration modes of terminal −NH2 and −NH− groups [56]. Pure TiO2 shows characteristic broad absorption band at 500–700 cm−1 [57]. It can be clearly seen that the main characteristic peaks of g-C3N4 and TiO2 appeared in TiO2/g-C3N4 sample suggesting the formation of a composite between g-C3N4 and TiO2 [58].
The UV–Vis DRS spectra of g-C3N4 and TiO2/g-C3N4 were presented in Figure 4. It can be seen that a combination of TiO2 and g-C3N4 could significantly change the absorption band of composites compared to that of g-C3N4 or TiO2 components. Accordingly, the presence of g-C3N4 in the composite gives to TiO2 the capacity for absorbing light in visible region rather than UV region. Based on these data, the band gap values for g-C3N4 and TiO2/g-C3N4 were calculated as 2.65 eV and 2.48 eV, respectively.

3.2. Catalyst Activity

The photocatalytic performances of the catalysts were evaluated by the photodegradation of 10 ppm solutions of EBT and BPB as pollutants using UV light and 200 mg/L of catalyst. For EBT, under UV light irradiation, we obtained a higher degradation efficiency after 180 min with TiO2/g-C3N4 followed by g-C3N4, for both dyes. From Figure 5, we observed that degradation efficiencies of EBT are 87.94% and 100% for g-C3N4 and TiO2/g-C3N4, respectively.
For BPB, we observe a degradation efficiency of 38.53% and 100% for g-C3N4 and TiO2/g-C3N4, respectively. There is a significant difference in the degradation efficiency of the dyes when using g-C3N4 with 87.94% and 38.53% for EBT and BPB, respectively. These results may be attributed to the fact that under UV light irradiation and in the absence of catalyst, we observe 36% degradation of EBT, while no degradation of BPB. From another hand, EBT and BPB belong to the Azo and sulphonephthalein families respectively and therefore differ in their physico-chemical properties. It is easier to break the unsaturated bonds of BPB than those of EBT (azo bonds).
A comparative table of photocatalytic degradation of dyes with g-C3N4 based photocatalyst with different light sources is shown in Table 2 below:

3.3. Effect of Photocatalyst Dosage

In views of the results obtained during photolysis were BPB showed no sensitivity to UV-light compared to EBT, it was preferable to continue our studies for more precise results with BPB. The effect of photocatalyst dosage was studied using 10 ppm of BPB solution. An increase in photocatalyst dosage from 50 mg/L to 200 mg/L increases BPB degradation efficiency from 58.17 to 100% during 180 min irradiation time as shown in Figure 6.The addition of catalyst results in an increase in the photocatalytic performance due to an increase in active sites which leads to an increase in radical species formation [66]. Excess dosage of catalyst into the reaction mixture may enhance light reflectance due to the increase of the solution turbidity, which in turn leads to preventing light absorption by the catalyst reducing the degradation rate [30,67,68]. The degradation efficiency increases rapidly with an increase in photocatalyst from 50 to 100 mg/L and increases slightly with an increase in photocatalyst from 100 to 200 mg/L after 180 min irradiation. In order to obtain high degradation efficiencies and avoid the use of excess catalyst, the quantity of photocatalyst chosen to continue our studies was 100 mg/L.

3.4. Effect of Initial Dye Concentration

The effect of initial BPB concentration on degradation was explored under concentrations from 2 to 20 ppm with 100 mg/L of catalyst. Figure 7 shows that an increase in dye concentration from 2 to 20 ppm decreases overall degradation, from 100% to 27.33%.
This can be attributed to the quantity of active sites available when using the same quantity of catalyst for different dye concentration [69]. With low initial concentration, TiO2/g-C3N4 provides a sufficient number of active sites for BPB molecules adsorption. With higher initial concentrations, the number of active sites provided by the TiO2/g-C3N4 reduces, thus decreasing the degradation efficiency of the dye [70].
The opacity of the solution at high concentration may also reduce the intensity of the light travelling through the system which leads to a decrease in the photocatalytic activity of the photocatalyst. Increasing dye concentration facilitates light adsorption by the dye molecules and reduces the amount of light reaching the surface of the catalyst, thus decreasing generation of active sites [71,72].

3.5. Kinetic Study

To determine BPB degradation kinetics, pseudo-first-order kinetic and Langmuir-Hinshelwood models were investigated. The kinetic parameters of the reaction were obtained using the pseudo-first-order kinetic model expressed as follows:
ln ( C 0 C ) = k 1   t
The terms k1, C and C0 are respectively the rate of pseudo-first-order reaction, expressed in min−1, BPB concentration at the time t, expressed in mg/L, and BPB initial concentration, also in mg/L.
The pseudo-first-order (k1) reaction rate was obtained from the linear plot of ln(C0/C) versus time as seen in Figure 8. From Table 3, it can be seen that k1 decreases with increase in initial concentration which is quite common in photocatalytic degradation of organic compounds in solution [73]. The correlation coefficients close to 1 (R2 > 0.95) confirmed that photodegradation of BPB by TiO2/g-C3N4 effectively follows a pseudo first-order kinetics model [74].
Most heterogeneous photocatalytic degradation reactions follow Langmuir–Hinshelwood (L-H) kinetics to describe the relationship between initial degradation rate and initial concentration [75,76].The L-H model used in this study can be expressed as follow:
1 r 0 = 1 k + 1 k K 1 C 0
where k denotes the reaction rate constant for the process (mg L−1 min−1), K is the adsorption coefficient of reactants (L mg−1), C0 is the initial concentration of dye (mg/L) and r0 is the initial rate of disappearance of dye (mg L−1 min−1).
A linear expression can be conveniently obtained by plotting the reciprocal initial rate against the reciprocal initial concentration. The slope is 1/(kK) and the intercept is 1/K (Figure 9). The linear transform of this expression yielded k = 0.108 mg L−1 min−1 and K = 0.0585 L mg−1.

3.6. Effect of Persulfate

The effect of persulfate (PS) on the performance of TiO2/g-C3N4 towards BPB degradation was evaluated by introducing 2.4 mmol/L of sodium persulfate (Na2S2O8) in the suspension. In the first 20 min we observed a removal efficiency of 80.14% and 98.83% after 180 min irradiation (Figure 10). This improvement in removal efficiency comes from the presence of PS which inhibits the recombination of electron/hole pairs in the catalyst by capturing photo induced electrons [77]. Addition of PS accelerates the generation of radicals, thus boosting decontamination rate [67]. This can be explained by recent studies of persulfate activation of g-C3N4 and TiO2 based catalysts.
A review on the activation of persulfate on g-C3N4-based catalyst for environmental remediation states that PS-captures photoinduced electrons of g-C3N4 and reduces electron-hole pairs recombination thereby increasing the formation of radicals which increases the photodegradation of the pollutant (bisphenol A; BPA) [78]. This was further explained through the following reactions:
g-C3N4 + hν → e + h+
e + O2 → O2•−
e + S2O8−2 → [ S2O8−2]
[S2O8− 2] + O2 → S2O8− 2 + O2•−
O2•−/h+ + organic pollutants → organic pollutants
From the recent advances in persulfate-assisted TiO2-based photocatalysis for wastewater treatment, TiO2 can be excited to produce electron-hole pair and persulfate is then activated by photo-induced e to generate SO4•− radicals [33]. The h+ can react with H2O to produce hydroxyl radicals (OH). Under UV light, persulfate can also be activated to form SO4•− radicals [79] which can also react with OH ions to form OH radicals [80]. SO4•− and OH radicals with strong oxidation ability can degrade organic pollutants in water, as explained in these reactions [33]:
TiO2 + hν → e + h+
h+ + H2O → OH + H+
S2O82− + hν → 2SO4•−
SO4•− + OH → SO42− + OH
SO4•−/OH + organic pollutants → degradation products

3.7. Effect of pH, H2O2 and NaCl

Three-level CCD (low, central and high) with three factors was used for create a model for the photodegradation of BPB in the presence of TiO2/g-C3N4 under UV-light. Barzegar et al., applied this design to express mathematical relationships among variables such as process time, catalyst mass and initial concentrations of methylene blue (MB) and rhodamine B (RhB) in the degradation process with TiO2/g-C3N4 [35]. The model examined was made of 13 experiments to determine the contribution of the factors (A: pH, B: CH2O2, C: CNaCl) on photocatalytic degradation of BPB and Table 4 represents the obtained BPB removal percentage for each experimental run.
The experimental results obtained were represented also as a spider or radar graph visualization to better highlight the effect of different parameters on the degradation yield of BPB as shown in Figure 11. To get a good presentation the three studied parameters (pH, NaCl, and H2O2) was reported with theirs coded values (−1, 0 and 1) however the degradation yield was reported from zero to one. I t seems that a basic pH and hydrogen peroxide are suitable for a high BPB molecules degradation, nevertheless, the NaCl presence in some case have a negative effect and, in another case, have no effect in the dye elimination.
Response surface methodology (RMS) was applied to determine the best conditions for BPB degradation. A Box-Behnken design model was used to investigate the effects of 3 independent variables at three pH levels (3, 7, and 11), concentration of H2O2 (0.4, 1.4 and 2.4 mmol L−1) and concentration of NaCl (5,10 and 15 mg L−1). The modified quadratic model equation was as follows:
%R degradation = +108.56630
−13.20569 * pH + 31.65992 * CH2O2 +8.74214 * CNaCl
−14.24516 * pH * CH2O2 − 2.76016 * pH * CNaCl
+ 0.916335 * pH2 − 0.370286 * CNaCl2
1.23617 * pH2 * CH2O2 + 0.063672 * pH2 * CNaCl
+ 0.100700 * pH * CNaCl2
The ANOVA results of BPB removal (%) by TiO2/g-C3N4 composite is presented in the Table 5. As seen, the p-value of <0.0001 for the model signifies that the model is statistically significant and indicate that the applied mathematical model better fitted to the experimental data as confirmed by lack of fit [81]. To be determined as significant, p-value for a model parameter (individual or interaction) must be < 0.05. In this case, all the model terms are significant [81].
The Predicted R2 of 0.9877 is in reasonable agreement with the Adjusted R2 of 0.9995 (Table 6). Adequate Precision measures the signal to noise ratio. A ratio greater than 4 is desirable. The obtained ratio of 122,161 indicates an adequate signal, which confirm that this model can be used to navigate the design.
A comparison of the experimental data to the predicted values by the refitted model was made through a predicted vs. actual plot, their closeness can be better observed in Figure 12. Data points were fragmented consistently on the diagonal line of 45° proving the goodness of fit. These results are similar to those obtained when using optimization approach in phosphoric acid-treated spent tea residue biochar for wastewater decoloring [82] and the sorption and removal of crude oil spills from seawater using peat-derived biochar [83].
The 3D surface response graph was plotted against two parameters to illustrate the combined effects of parameters on simultaneous photocatalytic degradation of BPB by TiO2/g-C3N4 as shown in Figure 13.
In Figure 13a,b, the degradation efficiency reveals that lower pH leads to more photodegradation efficiency with approximately 75–80% of BPB removal at pH 3 and CH2O2 (0.4–2.4 mmol L−1). At pH > 3 we observe a decrease in degradation and a gradual increase from pH 9 up to pH 11 where we observed a degradation efficiency of approximately 70% at CH2O2 ranging between 1.9 to 2.4 mmol/L.
Figure 13c,d showed similar results as the previous plot with photodegradation efficiency of approximately 75–80% of BPB removal at pH 3 and with CNaCl (5–15 mgL−1). At pH 11 there is a degradation efficiency of approximately 60% at CNaCl (5–15 mgL1).

3.8. Reactive Oxygen Species Contribution

To determine the contribution of Reactive Oxygen Species (ROS) in the degradation of BPB by TiO2/g-C3N4 under UV light illumination, different ROS trappers were used at sufficient concentrations to suppress the corresponding ROS. Isopropanol, methanol and potassium dichromate were used as specific trappers of hydroxyl radicals (HO), holes (h+) and superoxide ions (O2•−) respectively in bulk solutions [24,84].
The experiments were carried out by addition of 43.5 mM isopropanol (IsoPro), 50 mM methanol (MeOH), and 1.1 mM potassium dichromate (K2Cr2O7) into a 500 mL BPB (10 ppm) to examine their impact on photocatalytic degradation of BPB with 100 mg/L of catalyst.
From Figure 14, the presence of these trappers in the TiO2/g-C3N4 suspensions affected the BPB degradation, which suggested that all of the h+, HO•−and O2 contributed to the BPB degradation. Using IsoPro, MeOH and K2Cr2O7 we obtained a degradation percentage of 71.87, 78.22 and 27.89%, respectively. From this result, Figure 14 confirmed that O2•− was the primary ROS involved in the photocatalytic degradation of BPB with 68.89% contribution followed by HO and h+ with 22.40% and 15.55% contribution, respectively.
The amount of radicals in the system involved in the dye degradation can differ significantly due to electron-hole recombination and recombination reactions of radical species [24].
Considering the scavenging tests, the photodegradation of BPB with the TiO2/g-C3N4 catalyst could be described by following steps: TiO2/g-C3N4 was excited by UV-light to generate electron-hole pairs with photogenerated electrons flowing from g-C3N4 to the surface of TiO2. Part of the electrons produced by g-C3N4 are captured by the oxygen adsorbed on the surface, generating superoxide radicals (O2•−) with strong oxidizing properties, then decomposing the pollutant in the wastewater, as shown in reactions below [85]:
TiO2/g-C3N4 + hν → e + h+
O2 + e → O2•−
H2O + h+ → OH+ H+
h+ + OH → HO
HO + O2•− + pollution → degradation products
Researches on photocatalytic degradation of organic pollutants generally align with the fact that h+ and O2•− are the main radical species using g-C3N4-based catalyst [78] which is slightly different from the results obtained.This implies that the presence of g-C3N4 with TiO2 and UV irradiation can affect the physico-chemical properties of the photocatalyst and influence main radical species in the photocatalytic organic treatment process.

4. Conclusions

In this work, TiO2/g-C3N4 was synthesized by wet- impregnation and applied in the photodegradation of EBT and BPB dye solutions. The heterojunction phenomenon in TiO2/g-C3N4 facilitates the photoexcited charges transfer and simultaneously reduces photogenerated electron-hole recombination, which in turn markedly improves its photocatalytic performance which is higher than that of g-C3N4 alone. The degradation efficiency indicated that a decrease in initial dye concentration and increase in catalyst dosage positively affects photodegradation. The study of the degradation kinetics showed that the degradation of BPB follows both the pseudo-first-order kinetic and Langmuir-Hinshelwood model with an average value of R2 equal to 0.979 and 0.975 respectively. The addition of Na2S2O8 to the suspension increased the degradation rate in 20 min from 8.8% to 80% yield, an amplitude of 9.1. The optimization of the photodegradation of BPB was investigated using CCD and RSM methodologies where R% reached maximum values at pH 3 in the presence of NaCl and H2O2. Isopropanol, methanol and potassium dichromate were used as scavenger radicals and O2•− were identified to be the predominant radical species in the photodegradation using TiO2/g-C3N4 catalyst. The present work provides the efficient photocatalysts as a promising material for environmental remediation purposes.

Supplementary Materials

The following supporting information can be downloaded at:, Figure S1: UV lamp spectrum (24 W); Figure S2: Photoctalytic degradation of BPB with TiO2/g-C3N4 under uv light (C0: 10 ppm, Catalyst dose: 200 mg/L,V solution: 500 mL, natural pH).

Author Contributions

Conceptualization, H.Z. and A.K.; methodology, H.Z.; software, H.Z.; validation, H.Z., A.K., S.D., P.B. and C.C.; formal analysis, H.Z.; investigation, F.H., H.Z., P.B. and C.C.; writing—original draft preparation, F.H. and H.Z.; writing—review and editing, H.Z. and J.M.D.D.; visualization, H.Z., P.B. and C.C.; supervision, H.Z., A.K., S.D., P.B. and N.G.B. All authors have read and agreed to the published version of the manuscript.


This research received no external funding.

Data Availability Statement

Not applicable.


The authors would like to thank Ivane LELIEVRE (UniLaSalle Rennes) for their technical help and Jessica Wilson for her proofreading. The authors would like to acknowledge The Erasmus+ International Credit Mobility for the Grant (Grant agreement numbrer: 2019-1-FR01-KA107-060920) between Unilasalle Polytechnic Institute and University of Maroua. The authors would also like to acknowledge the financial support of the INTERREG NorthWest Europe ThreeC with the project number NWE 1010, under the umbrella of the European Regional Development Fund (ERDF).

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Schematic illustration of the preparation of materials.
Figure 1. Schematic illustration of the preparation of materials.
Water 14 03331 g001
Figure 2. SEM images with EDX spectra of g-C3N4 and TiO2/g-C3N4.
Figure 2. SEM images with EDX spectra of g-C3N4 and TiO2/g-C3N4.
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Figure 3. (a) XRD patterns of materials; (b) Raman spectra and (c) FTIR spectra of g-C3N4 and TiO2/g-C3N4 composites.
Figure 3. (a) XRD patterns of materials; (b) Raman spectra and (c) FTIR spectra of g-C3N4 and TiO2/g-C3N4 composites.
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Figure 4. Diffuse Reflectance Spectra (a) and plot of transferred Kubelka-Munk versus energy of the light absorbed (b) of pure g-C3N4 and TiO2/g-C3N4.
Figure 4. Diffuse Reflectance Spectra (a) and plot of transferred Kubelka-Munk versus energy of the light absorbed (b) of pure g-C3N4 and TiO2/g-C3N4.
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Figure 5. Photodegradation of 500 mL of dye solution (10 ppm) with 200 mg/L of catalyst under UV irradiation and natural pH.
Figure 5. Photodegradation of 500 mL of dye solution (10 ppm) with 200 mg/L of catalyst under UV irradiation and natural pH.
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Figure 6. Effect of catalyst dosage in the photodegradation of BPB solution (10 ppm), natural pH, under UV irradiation.
Figure 6. Effect of catalyst dosage in the photodegradation of BPB solution (10 ppm), natural pH, under UV irradiation.
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Figure 7. Effect of initial dye concentration with 500 mL of BPB solution, 100 mg/L of catalyst, natural pH, under UV irradiation.
Figure 7. Effect of initial dye concentration with 500 mL of BPB solution, 100 mg/L of catalyst, natural pH, under UV irradiation.
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Figure 8. Pseudo-first-order kinetics for BPB at different initial concentration with 100 mg/L of catalyst under UV irradiation.
Figure 8. Pseudo-first-order kinetics for BPB at different initial concentration with 100 mg/L of catalyst under UV irradiation.
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Figure 9. Linear correlation of 1/r0 versus 1/C0.
Figure 9. Linear correlation of 1/r0 versus 1/C0.
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Figure 10. Effect of persulfate on the photodegradation of 500 mL BPB solution (10 ppm) with 100 mg/L of catalyst, natural pH, under UV irradiation.
Figure 10. Effect of persulfate on the photodegradation of 500 mL BPB solution (10 ppm) with 100 mg/L of catalyst, natural pH, under UV irradiation.
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Figure 11. Spider graph visualization of experimental design results for the mutual effects of pH, NaCl, and H2O2 on photocatalytic degradation BPB by TiO2/g-C3N4.
Figure 11. Spider graph visualization of experimental design results for the mutual effects of pH, NaCl, and H2O2 on photocatalytic degradation BPB by TiO2/g-C3N4.
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Figure 12. Predicted vs. actual measurements for R%.
Figure 12. Predicted vs. actual measurements for R%.
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Figure 13. Response surface and contour plots for the mutual effects of pH, CNaCl, and CH2O2 on photocatalytic degradation BPB by TiO2/g-C3N4.
Figure 13. Response surface and contour plots for the mutual effects of pH, CNaCl, and CH2O2 on photocatalytic degradation BPB by TiO2/g-C3N4.
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Figure 14. (a) Effect of additives in the degradation of 10 ppm of BPB and (b) Contribution of various active species in the photodegradation (500 mL of BPB solution, natural pH, under UV irradiation).
Figure 14. (a) Effect of additives in the degradation of 10 ppm of BPB and (b) Contribution of various active species in the photodegradation (500 mL of BPB solution, natural pH, under UV irradiation).
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Table 1. Physico-chemical properties of Eriochrome black t and Bromophenol blue.
Table 1. Physico-chemical properties of Eriochrome black t and Bromophenol blue.
DyeMolecular FormulaStructureλ Max
Eriochrome black tC20H12N3O7SNaWater 14 03331 i001535 nm
Bromophenol blueC19H10Br4O5SWater 14 03331 i002592 nm
Table 2. Comparison of photocatalytic activity of g-C3N4 based photocatalysts.
Table 2. Comparison of photocatalytic activity of g-C3N4 based photocatalysts.
PhotocatalystPollutantDegradation Efficiency (%)Light SourceReference
40 wt% g-C3N4/Ag3VOBasic Fuchsin95.0Visible[59]
g-C3N4/TiO2(NT)Rhodamine B96.7Visible[60]
g-C3N4-TiO2Rhodamine B99.0Visible[61]
9 wt% Bi2O3/porous g-C3N4Reactive Black 584.0UV-vis[62]
NP-GQDs-90/g-C3N4Methyl Orange96.0UV[63]
g-C3N4/TiO2-1.5Methylene Blue95.3UV[64]
Nanosheet g-C3N4/CNMBGtMethylene Blue98.2UV[65]
g-C3N4EBT87.9UVPresent work
TiO2/g-C3N4EBT100.0UVPresent work
g-C3N4BPB38.5UVPresent work
TiO2/g-C3N4BPB100.0UVPresent work
Table 3. Variation of the pseudo-first-order rate constant (k1) at various initial concentration of BPB (C0).
Table 3. Variation of the pseudo-first-order rate constant (k1) at various initial concentration of BPB (C0).
C0 (mg/L)k1 (min−1)R2
Table 4. Experimental design for the mutual effects of pH, NaCl, and H2O2 on photocatalytic degradation BPB by TiO2/g-C3N4.
Table 4. Experimental design for the mutual effects of pH, NaCl, and H2O2 on photocatalytic degradation BPB by TiO2/g-C3N4.
RunFactor 1
A: pH
Factor 2
B: CH2O2 (mmol/L)
Factor 3
C: CNaCl (mg/L)
Degradation %
Table 5. ANOVA Table and goodness-of-fit coefficients for response variables.
Table 5. ANOVA Table and goodness-of-fit coefficients for response variables.
A217642.96< 0.0001
Table 6. Fit Statistics.
Table 6. Fit Statistics.
Adjusted R20.9995
Predicted R20.9877
Adequate Precision122.1613
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Hassan, F.; Bonnet, P.; Dangwang Dikdim, J.M.; Gatcha Bandjoun, N.; Caperaa, C.; Dalhatou, S.; Kane, A.; Zeghioud, H. Synthesis and Investigation of TiO2/g-C3N4 Performance for Photocatalytic Degradation of Bromophenol Blue and Eriochrome Black T: Experimental Design Optimization and Reactive Oxygen Species Contribution. Water 2022, 14, 3331.

AMA Style

Hassan F, Bonnet P, Dangwang Dikdim JM, Gatcha Bandjoun N, Caperaa C, Dalhatou S, Kane A, Zeghioud H. Synthesis and Investigation of TiO2/g-C3N4 Performance for Photocatalytic Degradation of Bromophenol Blue and Eriochrome Black T: Experimental Design Optimization and Reactive Oxygen Species Contribution. Water. 2022; 14(20):3331.

Chicago/Turabian Style

Hassan, Fadimatou, Pierre Bonnet, Jean Marie Dangwang Dikdim, Nadege Gatcha Bandjoun, Christophe Caperaa, Sadou Dalhatou, Abdoulaye Kane, and Hicham Zeghioud. 2022. "Synthesis and Investigation of TiO2/g-C3N4 Performance for Photocatalytic Degradation of Bromophenol Blue and Eriochrome Black T: Experimental Design Optimization and Reactive Oxygen Species Contribution" Water 14, no. 20: 3331.

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