SQU Journal for Science, 2021, 26(2), 86-97 DOI:10.53539/squjs.vol26iss2pp86-97 Sultan Qaboos University 86 Photocatalytic Degradation of Bisphenol A (BPA) Present in Aqueous Solution Using g-C3N4 Nanosheets Under Solar Light Irradiation Faisal Al-Marzouqi 1,2 and Rengaraj Selvaraj* 1 1 Department of Chemistry, Sultan Qaboos University, P.O. Box 36, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman; 2 Department of Process Engineering, International Maritime College Oman, P.O. Box 532, PC 322, Falaj Al Qabail, Suhar, Oman.,*E-mail: rengaraj@squ.edu.om. ABSTRACT: A graphite-like carbon nitride (g-C3N4) nanosheet sample was synthesized from a melamine precursor by a method of simple direct heating in a semi-closed system followed by thermal oxidation etching at 550 °C for 12 h. The sample was labelled as (g-C3N412h) and was systematically characterized. Moreover, the results were then compared with a pristine g-C3N4 sample for the degradation of Bisphenol A (BPA) present in water. Bisphenol A (BPA) is an endocrine disruptor. It is known that the BPA is one of the most harmful organic materials and that it does not degrade easily in the environment. It was therefore selected as a target to test the photocatalytic activity of prepared carbon nitride nanosheets under direct solar irradiation. The results showed the structure of the g-C3N4 nanosheets when the sample had been treated for a longer time compared to the regular treatment time. The optical band gap results remained the same, indicating the existence of a g-C3N4 backbone structure. However, the XPS and FTIR spectra showed some modification on g-C3N4 after longer etching treatment time such as the C-H, CO and N pyridinic structure. The photocatalytic degradation of Bisphenol A by the g-C3N4 nanosheets under solar irradiation was much better (around 60%) than that with the g-C3N43h bulk sample (around 30%). This enhanced photocatalytic activity can be attributed to multiple factors such as the smaller particle size, rich carbon surface and high surface area exhibited by the g-C3N4 nanosheets. This further indicates that g-C3N4 can be used with solar irradiation to treat wastewater containing endocrine disruptor chemicals. Keywords: Bisphenol A; Carbon Nitride; Graphite; Nanosheet; Photocatalyst. إشعاع ضوء تحت g-C3N4( باستخدام شرائح نانومترية من الكربون المنترد BPA)ئي للبيسينفول أ الضوالتحفيزي التكسير الشمس المباشر فيصل المرزوقي ورنجراج سلفراج شبه بوتقة الميالمين في لمركبمباشرة ال الحرارية معالجةال باستخدام الشبيهة بالجرافين g-C3N4شرائح الكربون المنترد من تحضير عينات :صلخمال العينات تم دراسة لتحويل المادة الى شرائح نانومتريه صغيرة. ساعة 21مئوية لمدة 555األكسدة الحرارية عند درجة بمتبوًعا ه. كان التحضيرمغلق Bisphenol A بيسفينول أيم مركب غير معالجة باألكسدة الحرارية من حيث قوتها لتحط g-C3N4 تمت مقارنة النتائج مع عينةحيث بشكل منهجي. (BPA) .من المعروف أن مادةو بيسفينول أ هو مادة معطلة لعمل الغدد الصماءالالموجود في الماء BPA تحللهي من أكثر المواد العضوية ضرًرا وال ت ان العينات مباشر. أظهرت النتائج الشمسي اإلشعاع مساعدةنومترية بالنا هذة الشرائحتم اختياره كهدف الختبار النشاط التحفيزي ل لذلك،بسهولة في البيئة. حيث اكتسب العينة القدرة على فصل نواقل الشحنات لمدة .اكتسبت خصائص تحفيزية أكثر فعالية عندما عولجت لفترة أطول مقارنة بوقت العالج المعتاد -C وجود لكل منبعد وقت معالجة أطول مثل g-C3N4تركيب ال التعديل علىبعض FTIR و XPS أظهر أطياف كما،أطول مقارنة بالعينة الغير معالجة H و CO و N بيريدين. كان التحلل الضوئي لـ Bisphenol A بواسطة الصفائح g-C3N4 ( من ذلك 05تحت اإلشعاع الشمسي أفضل بكثير )حوالي٪ يُعزى هذا التحسين على نشاط التحفيز الضوئي إلى عوامل متعددة مثل حجم ٪(. يمكن أن 05)حوالي لغير معالجة باألكسدة الحرارية مع العينة ا -g م شير إلى أنه يمكن استخداالشرائح النانومترية. ان هذه الدراسة تالكربون ومساحة السطح العالية التي تظهرها بالجسيمات األصغر وسطح الغني C3N4 اإلشعاع الشمسيبمساعدة هذه المحفزات الضوئية وؤدي إلى اختالل الغدد الصماء لمعالجة مياه الصرف التي تحتوي على مواد كيميائية ت. .المحفزات الضوئية ، الكربون المنترد، الجرافيت، شرائح نانومترية ،البيسفينول أ :مفتاحيةالكلمات ال PHOTOCATALYTIC DEGRADATION OF BISPHENOL A (BPA) 87 Graphical Abstract 1. Introduction owadays, a significant awareness has been established with regard to the impact on human health of the organic and inorganic contaminants in the water and environment. These toxic wastes are growing yearly in our environment due to both increased development and energy demand. Recently there have been more and more studies highlighting the presence of these chemical pollutants in water. There are many ways in which these contaminants can reach water bodies, as emphasized in much research 1-4 . Many organic pollutants have been found in the environment in levels of µg/L to ng/L. Amongst all the organic contamination, the pharmaceuticals and personal products and the endocrine disrupting chemicals have attracted more attention due to their potential risk to human health . 5 Furthermore, the polycarbonate resins and antioxidant materials used to stabilize plastics mainly depend on the use of Bisphenol A (BPA). BPA has been identified as an endocrine disrupting pollutant that changes the function of the endocrine system and consequently causes adverse effects in the health of an organism. 6 Thus developing efficient techniques to remove emerging contaminants from water and wastewater is a very urgent need. The degradation of such pollutants using active photocatalysts is a promising technique to treat wastewater. 7 Several photocatalysts, such as metal oxides and metal sulphides have been applied for this purpose 8 . There is a growing awareness of visible active photocatalysts. 9,10 Unfortunately, most metal based material shows a weak light absorption, low stability and is of high price. 11-14 Recently, carbon based photocatalysts have emerged as promising conducting materials due to their high charge mobility. Carbon nitride (g-C3N4) is an interesting photocatalytic material. Wang et al and his co-worker have published regarding the use of bulk carbon nitride as an active photocatalyst for hydrogen development under irradiation with visible light. 15 A great deal of attention has focussed on the use of g-C3N4 for pollutant degradation and chemical synthesis. Most of these studies used bulk carbon nitride material. The carbon nitride (g-C3N4) constructed of nano sheets has a 2D polymer semiconductor structure that is very similar to that of graphene (Figure 1a). The band gap of this graphite like carbon nitride (g-C3N4) photocatalyst was measured to be around 2.7 eV 16-17 . The g-C3N4 has shown interesting properties including a suitable redox potential, a band structure located in the visible light region unlike the band structure of TiO2 which is located in UV region (Figure 1b), thermal and chemical stability, and an ease of preparation that would allow for large-scale production from low-cost precursors. 18,19 N FAISAL AL MARZOUQI and RENGARAJ SELVARAJ 88 Figure 1. (a) The chemical structure of graphite-like carbon nitride (g-C3N4) sheet and (b) the band gap structure comparison of graphite-like carbon nitride (g-C3N4) with titanium dioxide. The carbon nitride (g-C3N4) semiconductor can be prepared via a thermal polycondensation process either in a low vacuum system or under high pressure. 20-22 Moreover, several research groups have successfully prepared carbon nitride (g-C3N4) under ambient pressure in a semi-closed system, which would be more convenient from an industrial point of view 23,24 . Unfortunately, the resulting product was a bulk carbon nitride material and its activity was considered insignificant. The transformation of g-C3N4 bulk material to nanosize can be achieved by different methods such as acidification and sonication 25 . The thermal oxidation process has been less investigated and therefore, enhancement of g-C3N4 photocatalytic performance is targeted to transfer the g-C3N4 bulk material to nanosize materials via this process. To enhance the degradation of Bisphenol A via g-C3N4 nanosheets obtained from g-C3N4 bulk materials via thermal oxidation process. The report here, pointed out an enhancement of the degradation of Bisphenol A with help of g-C3N4 nanosheets obtained from g-C3N4 bulk materials via thermal oxidation process under solar light irradiation. 2. Experimental section 2.1 Materials and preparation The nitrogen rich precursor (melamine) was supplied by Sigma-Aldrich and was used in the synthesis of g-C3N4 without further purification. The preparation took place in a muffle furnace, where melamine powder was subjected to direct heating in a semi-closed system. A small amount of melamine powder (1g) was placed in a crucible with a cover. The direct heating increased the temperature from room temperature to 550 °C at a heating rate of 20 °C/min. the thermal etching process was conducted at 550 °C for 12 h, and the g-C3N4 nanosheets were obtained. (For comparison, 1 g of melamine powder was heated at 550 °C for 3 h to prepare the g-C3N4 bulk material.) The product was collected for further analysis. 2.2 Characterization Crystal property was investigated by X-ray diffraction (XRD) analysis, using a benchtop X-ray diffractometer (MiniFlex600). The morphology was examined and elemental surface analysis (EDX) performed with a Transmitted Electron Microscope (TEM) of model (JEM-1400-JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were conducted using multi-probe X-ray photoelectron spectroscopy (XPS) (Omicron Nanotechnology, Germany). The results were analysed using the Casa XPS software (Casa Software Ltd). UV-vis diffuse reflection spectroscopy (UV-vis DRS) was performed on a UV-Vis Spectrometer Lambda 650S (Perkin Elmer), and the infrared spectra were obtained using CARY 600 FTIR (Agilent Technologies). Optical properties were investigated via photoluminance (PL) with the help of a PerkinElmer LS55 spectrometer. The particle sizes were measured using a Microtrac particle sizer. The surface analysis obtained by Brunauer Emmett Teller (BET) surface area analysis using ASAP2010 under liquid N2 with 50 P/Po points, Micromeritics, USA. 2.3 Photocatalystic activity test Bisphenol A (BPA) is an endocrine disruptor which was selected to be used for testing the photocatalytic activity of prepared nanosheet and pristine carbon nitride samples. All photoreaction experiments were carried out in a photocatalytic reactor batch system consisting of a cylindrical borosilicate glass reactor vessel with an effective volume of 500 ml. The activity studies were conducted in an open atmosphere with an air diffuser fixed at the reactor to PHOTOCATALYTIC DEGRADATION OF BISPHENOL A (BPA) 89 uniformly disperse the air into the solution. The reaction suspensions were prepared by adding 0.1 g of as -prepared g- C3N4 into 250 ml of aqueous bisphenol A (BPA) solution with an initial concentration of 10 mg/L. Prior to illumination, the reaction suspensions were magnetically stirred for 30 min in darkness to ensure adsorption-desorption equilibrium between the photocatalyst and the bisphenol A (BPA). During illumination (under solar light of average 1400 W/m 2 ), about 6 ml of the suspension solution were taken from the reactor at scheduled interval. The samples were centrifuged at 8000 revolutions per minute for 5 min and were then filtered to remove the catalyst. The filtrate was analysed using a Shimadzu1800 UV-vis spectrophotometer. 3. Results and discussion 3.1 Characterization of g-C3N4 nanosheets The EDX analysis (Fig. 2a) revealed the presence of carbon (C) and nitrogen (N) as the main elements. A weak signal was detected from the platinum coating. This result indicates the high purity of the samples. The morphology of the samples showed a sheet structure, which is a characteristic shape of g-C3N4. TEM analysis (Fig2b and 2c) showed the clear transformation of the multilayer bulk structure (dark colour) to nanosheet structure (transparent colour). Figure 2. (a) EDX spectrum of g-C3N4 nanosheet and SEM image (b) and (c) TEM images of g-C3N4 bulk and nanosheet respectively. The XRD results for the g-C3N4 samples are shown in Fig. 3. Carbon nitride materials exhibit two diffraction peaks at 27.90 o corresponding to the (002) plane typical for the interlayer stacking of the C-N conjugated aromatic systems and a peak at 13.05 o for the (100) plane and representing the interplanar separation of tri-s-triazine units. 28 The position of the diffraction peaks was retained during the treatment process, showing the existence of the main structure of g-C3N4. However, the overall diffraction intensity was reduced by increasing the etching treatment from three to up to 12 hours. The (001) plane peak disappeared after 12 h of treatment, and this can be considered to be a consequence of the reduced structural correlation length prompted by a decrease in the number of layers. This result reflects the formation process of the g-C3N4 nanosheets. FAISAL AL MARZOUQI and RENGARAJ SELVARAJ 90 10 15 20 25 30 35 40 (100) g-C 3 N 4 12hrs In te n s it y ( a .u ) 2 g-C 3 N 4 3h (002) Figure 3. XRD patterns of g-C3N4 samples obtained after 3 and 12 h of heating. Figure 4 presents the Fourier transform infrared (FTIR) spectrum of the as-prepared samples to identify the specific interaction of the functional groups. The result indicates the presence of the graphite-like structure of carbon nitride. The N-H stretching modes and the O-H from water absorbed on the surface are present in the broad peak observed in the range of 3000-3500 cm -1 (area 1). The bands around 1200-1600 cm -1 are characteristic of a typical stretching mode of CN heterocycles (area2). In addition, the s-triazine ring mode was observed at 801 cm -1 (area3). However, there was some broadening in the peak at 3000-3500 cm -1 , which is indexed to CO vibration. 4000 3500 3000 2500 2000 1500 1000 791 898 11181630 3074 3175 3247 32 g-C 3 N 4 12hrs T ra n s m it ta n c e Wavenumber (cm -1 ) g-C 3 N 4 3h 1 Figure 4. FTIR spectrum of g-C3N4 samples obtained after 3 and 12 h of heating. 3.2 Optical properties of nanosheets The UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) was conducted to assess the optical properties of the g-C3N4 samples. Fig. 5a illustrates the main absorption results for g-C3N4 heated for 3 and 12 h. The optical absorption edge of g-C3N4 heated for 3 occurs at 390 nm and then red shifts to a longer wavelength for g-C3N4 heated for 12 h, to 410 nm. The effect of a longer thermal oxidation treatment is consistent with the reported results. 29,30 Moreover, the g- C3N412h sample was more efficient than the g-C3N43h sample in absorbing near UV and visible range up to 470nm. The spectra exhibit a long absorption tail for the samples treated for 12 h, and this can be attributed to structural defects PHOTOCATALYTIC DEGRADATION OF BISPHENOL A (BPA) 91 formed in the sample treated for a longer time. The Tauc plot was used to calculate the optical band gap. 31 The Tauc plots (the curve of (αhυ) 2 versus E where E is the energy in eV and A is the absorption) are shown in Fig. 5b and 5c. The band gap value of g-C3N4 was calculated to be 2.7 eV for g-C3N4 heated for 3 and 12 h. The analysis of the band gap values shows that the main structure of g-C3N4 was present and dominant in all samples. Figure 5. (a) The UV-Vis absorption spectra for g-C3N4 samples. (b) and (c) represent the Tauc graph with estimated band gap value for g-C3N4 samples. Figure 6a showsthe chemical composition of the constituent elements of the g-C3N4 , obtained by XPS analysis. The XPS spectra reveal that the main elements present were carbon, nitrogen and some oxygen. The high resolution spectra of carbon and nitrogen peaks are shown in Fig. 6b. The binding energies at 286.7, 288.4 and 289.2 eV are indexed to carbon from C-NH2, C-N and C=N respectively, while the binding energies at 397.1, 397.1 and 400.3 eV are attributed to nitrogen from C=N- C, N-(C)3 and C-N-H respectively. Moreover, some changes occurred for the carbon and nitrogen peaks after treating the sample up to 12 hours. Low intensity core level peaks were detected indicating the surface modification of the sample. Two peaks appear at binding energies 284.27 and 290.2 eV which could be attributed to C-C or C-H carbons while the other peak at 290.2 eV can be assigned to CO resulting from the etching process, and adsorbed on the sample surface 32-33 . This result is compatible with broadening observed in the FTIR result. For the nitrogen, the peak at binding energy 393.6 eV can be assigned to edge type N pyridinic which was reported previously for the sample prepared at higher energy 34-36 . The amounts of each element and the carbon to nitrogen ratio (C/N) (wt%) are shown in Table 1. There was a slight increase in the C/N ratio. This enhancement may indicate a carbon-rich surface which may enhance the photocatalytic activity by promoting charge carrier separation 37 . FAISAL AL MARZOUQI and RENGARAJ SELVARAJ 92 Figure 6. (a) The XPS survey of g-C3N4 and (b) comparison of high resolution spectra of C1s and N1s for the g-C3N4 3h and g-C3N4 12h . Table 1. The amount of C, N, and O in different g-C3N4 samples. XPS surface elemental analysis Content (%) C N O C/N g-C3N4 3h 42.23 56.92 0.65 0.74 g-C3N4 12h 43.68 55.54 0.78 0.78 PHOTOCATALYTIC DEGRADATION OF BISPHENOL A (BPA) 93 The photoluminescence emission (PL) spectra of as-obtained g-C3N4 samples are shown in Fig. 7. The PL measurement was performed at room temperature for the g-C3N4 3h and g-C3N4 12 h samples at excitation wavelengths of 360 nm. The main emission peak is located at about 450 nm for both samples. The emission peak resulted from the photo-generated electron-hole pairs process in the g-C3N4. Moreover, the emission intensity indicates the rate of photo generated electron-hole pairs. The spectra show that the g-C3N4 heated for 12 h had a higher intensity compared to that for the g-C3N4 heated for 3 h. The decrease in the intensity can be explained by the electron trapping process that occurs within the bulk material due to a crystal mismatch, which prevents electron-hole mobility. Moreover, the formation of nanosheets promoted the charge mobility in the g-C3N4 heated for 12 h. 38 400 450 500 550 600 650 0 50 100 150 200 250 300 350  Em =450 nm In te n s it y ( a .u ) Wavelenghth (nm) g-C 3 N 4 3hrs g-C 3 N 4 12hrs  EX =370 nm Figure 7. The comparison of photoluminescence (PL) spectra of g-C3N4 prepared at different thermal oxidation times. 3.3 Particles size and BET analysis The reduction of g-C3N4 multilayered structure (bulk) to few-layered g-C3N4 nanosheets was followed by size particles analysis with the help of a particle sizer. The measurements were performed for both the sample heated for 3 hours and the sample heated for 12 hours. The particle size distributions are shown in Fig. 8a and 8b, the average particle sizes being 4967 and 1783 nm for g-C3N4 3h and g-C3N4 12h respectively. This decline in the particle size is attributed to the thermal oxidation etching process. Moreover, the BET surface area and porosity of the samples was analysed, with the result showing a type 3 shape of isotherms according to the IUPAC classification (Fig. 8c). This result indicates the occurrence of slightly mesoporous-like structure due to the aggregation of sheets to form the bulk g- C3N4, leaving pores in between. 39 The BET surface area was found to be 94.2435 and 94.2435 m 2 /g, where the BJH adsorption surface area of pores was found to be around 123.0573 and 37.8759 m 2 /g for g-C3N4 3h and g-C3N4 12h respectively. This decrease indicates the transformation of the multilayers of g-C3N4 3h (bulk) to a few layers of g- C3N4 nanosheets after 12h, where a significantly low amount of surface area was available for nitrogen adsorption due to the overlap of the single nanosheets 40 . FAISAL AL MARZOUQI and RENGARAJ SELVARAJ 94 Figure 8. Histogram of corresponding particle size distribution (a) g-C3N4 3h, (b) g-C3N4 12h (c) BET surface area plot of g-C3N4 samples 3.4 Evaluation of the photocatalytic activity Bisphenol A was chosen as a pollutant to determine the photocatalytic activity of the as-obtained catalysts on endocrine disrupting chemicals. Bisphenol A is often used as a model for hard degraded endocrine disrupting chemical pollutants. The performance of the g-C3N4 nanosheets was studied with the help of UV-Vis absorption spectra changes where the maximum absorptive energy is at 278 nm. Fig. 9a shows that the band intensity at 278 nm gradually declines with an increase in the irradiation time. These results indicate that Bisphenol A underwent degradation under the catalysis of g-C3N4. Fig. 9b shows a comparison of the photocatalytic degradation between g-C3N4 bulk and g-C3N4 nanosheets. Bisphenol A degraded faster using g-C3N4 heated for 12 h (around 60%) than with g-C3N4 bulk material (around 30%). 4. Conclusion In this work, photocatalytic enhancement has been achieved for degradation of Bisphenol A (BPA) through g- C3N4 nanosheet photocatalysis, the nanosheet samples being obtained by direct heating of melamine followed by thermal oxidation etching for 12 h. The g-C3N43h bulk material was obtained to conduct a comparison by performing the etching process for only three hours. Moreover, the g-C3N412h nanosheet samples show very good photocatalytic activity under solar irradiation when compared to the bulk samples. This enhancement of the photocatalytic activity can be attributed to multiple factors such as the smaller particle size, carbon-rich surface and high surface area exhibited by the g-C3N4 nanosheets. Such g-C3N4 nanosheets have good potential for use in advanced water and wastewater treatment to eliminate endocrine disrupting chemicals under solar light irradiation or reduce them to a very low level. PHOTOCATALYTIC DEGRADATION OF BISPHENOL A (BPA) 95 Figure 9. (a) Time-dependent UV-vis absorption spectra of Bisphenol A (10mg/L) degradation with g-C3N4 12 h nanosheets under solar light irradiation [catalyst dosage 100 mg, solution volume 250 ml, source of light solar] (b) Photocatalytic degradation of Bisphenol A with g-C3N4 prepared at different thermal oxidation time under solar light irradiation. Conflict of interest The authors declare no conflict of interest. Acknowledgment Faisal Al Marzouqi acknowledges the International Maritime College Oman, Sultanate of Oman. Rengaraj Selvaraj acknowledges Dr. Myo Tay Zar Myint from Surface Science Lab, Department of Physics, College of Science, Sultan Qaboos University and The Central Analytical and Applied Research Unit (CAARU) College of Science, Sultan Qaboos University, Oman. References 1. Ribeiroa, A.R., Nunesb, O.C., Pereiraa, M.F.R. and Silvaa, A.M.T. 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