Organic Pollutants Photo-Degradation by Steady g-C3N4 Nanorods for Wastewater Treatment Applications


 The photo-catalysis is a promising method for the degradation of harmful organic pollutants existing in the wastewater. In this research work, degradation of organic pollutants (dyes) have been done proficiently for the wastewater treatment applications. The synthesized steady graphitic carbon nitride (g-C3N4) nanorods have been examined by various sophisticated characterization techniques such as XRD, SEM, EDS, TEM, UV-Vis and PL. The hexagonal phase of g-C3N4 has been showed in XRD analysis. SEM shows that the morphology of g-C3N4 consists of aggregated nanorods. The EDS depicting only two peaks which are carbon and nitrogen and nitrogen without impurities. According to the study of UV-Visible and PL, 2.77 eV band gap has been observed showing consistency. The steady g-C3N4 nanorods have been used for the degradation of prominent organic dyes such as: methylene orange (MO), methylene blue (MB) and rhodamine-B (Rh-B) for treatment of wastewater which is a dire need. The effect of catalyst mass loading and time have been studied for this purpose. This research depicted that the anion (•O2−) and holes (h+) are attributed to the degradation of organic pollutants. whose Rh-B, MB and MO dyes. Research based a tentative photocatalytic mechanism has also been proposed for waste treatment purpose.


Introduction
Graphitic carbon nitride has been explored as a metal-free photo-catalyst for the water purification. It has extensive applications for organic compounds and also degradation of organic pollutants under the visible light. The band gap of graphitic carbon nitride (g-C 3 N 4 ) has been calculated 2.7-2.8eV. It has superior thermal and chemical stability, useful optical features, fast charge carrier recombination rate and low solar energy utilization. Various researchers have manipulated the g-C 3 N 4 by various methods including hetero atom doping, thermal treatment, hybridization with other semiconductor and also by using different precursors to improve their structure and properties [1][2][3]. The improvisation of g-C 3 N 4 played a vital role to improving the photo degradation activity. It has deployed the electronic structure that may act as active sites for reactive molecules and central gap states can also be created when the structure of the semiconductor is defective. The overlapping of valence or conduction band with mid gap states of the semiconductor extends optical response and electrons-hole pair excitation that act as the active sites. These trapping sites can be improved through quantum efficiency of photo-induced carriers [4,5]. The g-C 3 N 4 has prepared by using ammonium thiosulfate through hydrothermal method. The process of photo-catalytic enhanced its efficiency due to lack of terminal amino species.
The g-C 3 N 4 is prepared with concentrated hydrochloric acid having nitrogen vacancies and big surface area which exhibit outstanding N 2 photo fixation activity [6][7][8][9]. Nevertheless, the structure of g-C 3 N 4 can be ruined through usage of acid and reaction conditions such as the concentration of acid. It is difficult to control the nitrogen vacancies due to the high synthesis temperature of g-C 3 N 4 . Consequently, it is interesting and exhilarating to discover the new concrete and facile treatment for formulating nitrogen-deficiency from g-C 3 N 4 and the activity of photo-catalytic can be improved in limited way [10,11].
The graphitic carbon nitride has vast applications. It is generally used as an organic dye degradation agent in different industries such as textiles, tannery, food and cosmetics etc [12][13][14][15].
Having all these benefits, the removal of pollutants has attracted growing consideration to worldwide, although it utilization maybe limited due to its high solubility and atmospheric risk.
Commonly, methylene blue (MB) is a common constituent of industrial wastewater contaminants. Its constant use burns to humans and animals' eyes, nausea, vomiting, profuse sweating, nervous system disorder, and methemoglobinemia [16][17][18][19][20][21][22][23][24]. In the recent century, photo-catalytic treatment has increased the abundant concerns because of its ease, low cost, easy operation and ease of being reused after completion of the reaction (because catalyst can be easily separated) [25,26]. To improve the photo-catalytic activity, performance-based nano composites have been established by combination of g-C 3 N 4 with extra types of inorganic photocatalysts [27][28][29][30]. However, utmost of these variations possess some further disadvantages, such as tedious artificial steps, etc [31,32]. Therefore, it is essential to accept a novel metal-free photo-catalytic material and easily recoverable degradation of organic pollutants [33][34][35][36].
Herein, novel metal-free g-C 3 N 4 nanorods have been synthesized by facile and simple calcination process. Various sophisticated analysis techniques have been employed to analyze its proper synthesis, its purity and its effective utilization as an effective photo-catalyst. The two main factors such as catalyst mass loading and time have been comprehensively studied in this research work for effective use in the industries for waste water treatment against methylene orange (MO), methylene blue (MB) and rhodamine-B (Rh-B). This study showed the improvement of the photocatalytic properties of the steady g-C 3 N 4 which may be used effectively in the industries for waste water treatment.

Prepared of g-C 3 N 4
Graphitic carbon nitride (g-C 3 N 4 ) nanorods have been synthesized by a simple calcination method through urea. Six gram (6 g) urea was taken and placed in crucible china dish, which covered with an alumina sheet later on. This product is kept in a furnace and heated at the temperature of 550°C for 2 hours. After that sample was cooled down to room temperature and then the product was annealed at 450 °C for 1 hour. Finally, the product was obtained as yellow-pale color in powder form as predicted in Fig.1. The whole synthesis process has been depicted in the

Materials Characterizations
The prepared samples (g-C 3 N 4 ) were characterized by diverse techniques. X-rays powder diffractometer have been used to examine the X-ray diffraction (XRD) pattern with Cu-Kα radiation source (λ = 0.15418 nm). The surface morphology of g-C 3 N 4 , nanorods have been analyzed by scanning electron microscopy (SEM), ZESIS-V80 which is equipped with the energy dispersive X-rays spectroscopy (EDX) technique toward study the fundamental elemental conformation of the prepared samples. For insight study TEM was performed with JEOL JEM-2100 model. The photoluminescence (PL) measurements were studied by using JASCO model number FP-8200 spectrometer.

Photo-catalytic performance of organic dye
The photo-degradation of g-C 3  Degradation efficiency (η) of photo-catalytic of three organic dyes have been obtained by using following equation 1: Where C o is representing the solution initial concentration with respect to dyes, C is the concentration of the dyes solution at reaction time t [37,38]. For finding the apparent rate constant (K), the equation of pseudo-first-order kinetic energy is used as following equation 2:

FESEM, TEM and EDS Analysis
The morphology of the synthesized sample has been studied through scanning electron microscope as depicted in Fig. 2(a). The FESEM confirmed that the nano fibers like structures are present with almost uniform diameter with a smooth surface. In the FESEM some agglomeration has also been observed in the nanorods. The dimeter of the nanorods is  whereas inset is the weight and atomic percentage.

XRD Analysis
XRD analysis has been done to observer the structure and size distribution of prepared sample as depicted in Fig. 3. One strong peak has been observed at 27.9° with split head at high intensity which are corresponding to the JCDPS card number (00-003-0401) with miller indices (002). The observed phase group of (P61/mmc) and space group number 192 has hexagonal phase structure. Lattice constants are a =b = 3.096Å, c=5.1300Å and its cell volume is 92.93 g/cm 3 . The sharp peaks also confirmed the crystalline nature of the sample.   Optical Properties and Bandgap Calculation of g-C 3 N 4 nanorods.
Optical properties are crucial for photocatalytic study as it accredits the number of incident photon absorbed during the photo catalytic process [27][28][29][30]. The optical properties of prepared samples were analyzed by UV-vis spectroscopy. The absorption spectra and Tauc plot of the prepared samples were shown in the Fig. 4. The UV-vis absorption spectra show that the higher absorption is in the range of 250-400 nm as shown inset of the Fig. 4.

Fig: 4 : UV-visible spectrum of g-C 3 N 4 and inset is the tauc plot.
The band gap of the prepared samples was measured by the following Tauc Eq as depicted in equation 4.

Photoluminescence (PL)
The emission properties of g-C 3 N 4 nanorods have been studied by photoluminescence (PL) spectroscopy technique at room temperature. Photoluminescence spectrum of g-C 3 N 4 nanorods with excitation wavelength of 325 nm is shown in Fig. 5. A strong and broader emission peak has been observed at 447 nm for g-C 3 N 4 nanorods which is attributed to band edge emission resulted from the valence band and conduction band recombination of electrons and holes [27][28][29][30]. The band gap obtained from these g-C 3 N 4 nanorods is 2.77eV which is consistent with the literature as shown in Fig 5 [39]. Moreover, it is analyzed that there are not any other defects related peaks in synthesized g-C 3 N 4. It is well-known that in the bulk phase defects are commonly determined on the surface of the material, which are missing in these g-C 3 N 4 nanorods.

Photo-catalytic Performance
Photo-catalytic degradation of g-C 3 N 4 nanorods under the visible light irradiation (λ≥400 nm) of organic dyes, methylene orange (MO), methylene blue (MB) and rhodamine-B (Rh-B) dye has been studied comprehensively as depicted in Fig. 6(a-f) with respect to catalyst concentration at fixed time. The catalysts concentration has varied the efficiency of the photo-catalysis as shown in Fig. 6(d, e, f). The prepared g-C 3 N 4 nanorods has degraded 99.6% methylene orange dye (MO) in 50 minutes. It has also degraded 98% of methylene blue dye (MB) in 100 minutes, 97.8% of rhodamine-B (Rh-B) in 75 minutes. The graph has been plotted between the C 0 /C versus time t for degradation of these organic dyes as depicted in Fig. 6 (d, e, f).
For all these photo-catalysts the graph is also plotted between ln C/C 0 versus time t after finding the slope of a straight line. The Pseudo-first-order reaction has been showed as in the linear curve of all samples. The degradation efficiency of g-C 3 N 4 nanorods for MO dye has been verified by the photo-catalytic activity, which is higher because of higher first-order constant.
The graph plotted between C 0 /C versus time t in these organic dyes is shown in Fig. 10 (b).
Photo-catalysts of g-C 3 N 4 has been degraded 99.9% in the methylene orange dye (MO) in 70 mints, 96 % methylene blue (MB) in 70 mints and 98% rhodamine-b (Rh-B) in 70 mints. After comparing the efficiency of these dyes, it could be concluded that g-C 3 N 4 nanorods has degraded MO is the best way with respect to time. The g-C 3 N 4 nanorods catalyst can be effectively used for degradation of 99% MO for the industrial wastewater treatment. The graph has been plotted between ln C/C 0 versus time t of these dyes for the time variation as depicted in Fig.10 (c). The apparent rate constants value (K) of these dyes for the time variation as shown in Fig 10(d).   [40], the 62.5% efficiency in MB has been studied at the temperature 600 and 650°C respectively. Here, the g-C 3 N 4 nanorods has shown, 99 % degradation efficiency for MO, 96% efficiency in MB (time factor) in 70 minutes and 98% efficiency in MB (catalyst factor) in 100 minutes has been observed at the temperature of 500 to 550°C, which indicates comparatively better performance than Xu and Mo et al [40]. The photocatalytic process for the degradation of organic dyes as depicted in Figure 12. Their corresponding reactions are demonstrated from equations 5 to 16 which are consistent with the literature [39].
As sunlight falls on the photocatalyst and absorbed then following reactions are possible.
With the absorption of irradiated light, in the presence of the g-C 3 N 4 nanorods as photocatalyst, electrons (e−) will be excited from valence band (VB) to the conduction band (CB) with instant production of holes in the VB in equal number. The photogenerated e− will reacts with surrounding oxygen to generate super oxide radical anion (O 2 •−) in Eq (10). The photogenerated holes in the conduction band react with water and •OH to form photogenerated hydroxyl radical (•OH) in Eqs (11) and (12). These reactive oxygen species such as •OH and O 2 •− involved in degradation of dye to form degradation products in Eq. (11) and (12).

Conclusion
In research work, successful synthesis of g-C 3 N 4 nano-rods have been performed by using urea through a simple calcination method which has been confirmed by sophisticated techniques such as XRD, TEM, SEM, EDX, PL and UV. The g-C 3 N 4 nano-rods have enhanced photocatalytic properties due to greater active sites which are crucial for photo degradation and waste water treatment. The superior photo-activity of g-C 3 N 4 has been studied in its two factor degradation which are: time variation and catalyst loading. The performance of g-C 3 N 4 towards the three primary pollutants revealed noble constancy as a photo-catalytic performer. In the comparison of these dyes Rh-b, MO and MB, it is concluded that the MO is the best dye to be employed for industrial and societal purposes because it can degrade up to 99.9% in time variation and 99.7% in catalyst loading.