A Laboratory-based Experiment on the Efficacy of Graphitic Carbon Nitride Doped with Tungsten Chloride in the Purification of Industrial Wastewater through Degradation of Organic Dye Pollutant

Water is an essential commodity whose quantity and quality needs to be secured for easier accessibility at both the industrial, public and household levels. However, its availability in adequate quality and quantity has continued to decline worldwide. Indeed, rise in human population coupled with the climate change phenomena have greatly impacted on the quality of water resources through increased organic and inorganic pollution. Rhodamine B (RhB) dye is a common organic pollutant majorly in industrial wastewater and with numerous environmental and human health effects. The application of graphitic carbon nitride (G-C3N4) in the purification of industrial wastewater to enhance the removal of RhB is a technology of interest to most environmental quality regulators and agencies. The study was therefore Environmental Management and Sustainable Development ISSN 2164-7682 2019, Vol. 8, No. 2 http://emsd.macrothink.org 63 aimed at investigating the performance of graphitic carbon nitride doped with tungsten chloride in the degradation of organic dye pollutant rhodamine B dye from industrial wastewater. The study showed that the as-prepared hybrid photo catalyst exhibits an improved photo degradation performance because of its synergetic effect. Indeed, the photo excited electrons from g-C3N4 were able to efficiently separate and are injected to the conductive band of WO3. The optimum photo activity occurred at the optimum ratio of 0.25WO3/gC3N4. There was also stability and efficiency within the hybrid catalyst within the photo degradation process. Indeed, the composite indicates a high activity for degradation of RhB under visible light irradiation. The presence of g-C3N4 proved to be beneficial for enhancement in photo catalytic activity of the g-C3N4-WO3 composite and proved to be one of the best alternative modes of n the degrading organic dye pollutant Rhodamine B dye from wastewater.


Introduction
Water is an essential commodity whose quantity and quality needs to be secured for easier accessibility at both the industrial, public and household levels (Boniface and Mwangi, 2002). The availability of water in adequate quality and quantity have continued to decline worldwide. Indeed, rise in human population coupled with the climate change phenomena have greatly impacted on the quality of water resources (U.S. Global Change Research Program, 2016). Most drinking water sources are increasing getting polluted by both the organic and inorganic materials (Rop et al., 2016). The organic pollutants can be in chemical, biological or physical (Rop et al., 2014;Ekevwe et al., 2017). The pollutants not only degrade the quality of water but also cause consequential impact on human health as well as other animals that are consumed by man . The increased pollution of drinking water sources has created a worldwide clean water shortage and quality catastrophe, a situation that needs advanced and efficient water treatment techniques for improved safety of water for domestic use (Pang et al., 2016;Donde et al., 2018). Most chemical methods for removal of organic pollutants in waste water have proved to have an ecological and an overall environmental significance (REF). Conventional methods such as absorption, coagulation and use of membranes for depleting organic pollutants in wastewater have proved to be less effective considering that they still leave remnants, generate secondary pollutants and suffer from a high operation cost (Sielechi et al., 2010).
Rhodamine B (RhB) is an organic pollutant common in industrial wastewater, it contains four N-ethyl groups at either side of the xanthene ring. It can be in the form of liquid, dry powder or wet solid. It is always reddish violet powder or green crystals and strongly fluorescent when diluted in water. It has a molecular weight of 479.01034g/mole, molecular formula of C 28 H 31 CIN 2 O 3 and a covalent-bonded unit count of 2 (Ping et al., 2017). The organic RhB is an important representative of xanthene dye, widely used as a colorant in textiles and food stuffs, and is also a well-known water tracer fluorescent, which has the property of carcinogenicity, reproductive and developmental toxicity, neurotoxicity, and chronic toxicity towards humans Environmental Management and Sustainable Development ISSN 2164-7682 2019 and animals (Sun and yang, 2017). The use of graphitic carbon nitride (G-C3N4) has been applied in the purification of wastewater to enhance the removal of RhB in wastewater (Zhang et al., 2015). G-C 3 N 4 has a large band gap which makes it suitable for photolytic reactions and photo catalysis. It is one of the most stable allotropes that is preferred in the splitting water, disintegrating organic pollutants and photosynthesizing under visible light (Zhang et al., 2015). It is also a semiconductor and moreover highly stable under light irradiation in water solutions as well as acidic ph. (0) and alkaline solution ph. (=14) solutions due to strong covalent bonds between carbon and nitride atoms. When doped with metal precursors such as (Co, Mn, Cu, Zn or W), it has been proven to obtain a high photo catalytic activity and good stability. It also has strong oxidative ability and high quantum efficiency. The functional organic metal hybrid material also exhibited its high modified electronic properties. The study was aimed at investigating the performance of graphitic carbon nitride doped with tungsten chloride in the degradation of organic dye pollutant rhodamine B dye from wastewater.

Materials and Methods
Tungsten chloride (analytical reagent), urea and ethanol (chemical pure) were purchased from Shanghai Chemical Corp. All the chemicals were of reagent grade and used without further purification. The tungsten chloride, urea and ethanol were accurately measured, mixed and grounded and heated ( Table 1). The mixture was then transferred into a petri dish and maintained at room temperature for 4-6 hours to evaporate the ethanol. Thereafter, the powders were collected and placed into a semi-closed alumina crucible with a cover, then constantly heated at 550°C for 3 h in the muffle furnace with a heating rate of 15°C/min. After that, the product was then grounded into fine powder and labeled appropriately as xWO 3 /g-C 3 N 4 (xrefer to the amount of tungsten chloride and x = 0, 0.05, 0.15, 0.25, 0.4, 0.5 mmol). Characterization of the structural properties of the obtained WO 3 /g-C 3 N 4 composites was done by X-ray powder diffraction (XRD, Bruker D8 Advance) through Cu Ko radiation at voltage of 40kV and 40mA over a temperature range of 10-80°. Nitrogen adsorption-desorption was done on a nitrogen physical adsorption (Beishide 3H-2000PS4).
The morphology and structure of the samples were further observed using a high-tech Transmission Electron Microscopy (TEM) (JEOL JEM-2010). The measuring of the photoluminescence spectra was done through a fluorescence spectrophotometer (F-7000, Japan), with Xe lamp as excitation source with optical filters. The recording of the UV-vis diffuse reflectance spectra (DRS) of the samples was done through n a Shimadezu UV-2401 spectrophotometer. ISSN 2164-7682 2019 The evaluation of the photo catalytic activity of the WO 3 /g-C 3 N 4 composites was done by observing the degradation of organic pollutant RhB, and this was done under visible light. For each of the experiment, 30 mg sample was dispersed into 50 mL of 5 mg/L RhB solution with magnetic stirring. Prior to being exposed to the visible light irradiation (500 W xenon lamp) having a cutoff filter (Z N420 nm), stirring of the solution was done in a dark condition for a n approximate duration of 30 min. The aim of doing that was to obtain an adsorption/desorption equilibrium between the catalyst and Rh B solution. Sampling of the two milliliters of the solution was then done after every 10 min. To separate the catalyst, it was then centrifuged at a speed of 10000 revolutions per minute for approximately five minutes. Measurement of the concentration of RhB solution was then done using a UV-vis spectrophotometer at 554 rims. To help separate the catalyst (WO 3 /G-C 3 N 4 ) from the RhB organic dye solution, the Rhb dye and WO 3 /g-C 3 N 4 sample mixture were taken to the centrifuge. Thereafter, the calculation of the degradation rate was calculated using the spectrophotometer.

Improvement on Photo-activity
The improvement on the photo activity was observed through the formation of a unique hetero-junction-like interfacial structure which occurred between g-C3N4 and WO 3 semiconductors ( Fig. 1). Technically, under the visible light exposure, both g-C 3 N 4 and WO 3 can be excited. This is due to the fact that the band gaps of both of them lies within the visible region. Based on that, the photo-excited electrons from the conduction band (CB) of g-C 3 N 4 would be injected into the CB of WO 3. The injection is due to a higher favoring over potential between the CB edges of g-C 3 N 4 and WO 3. On the other hand, the movement of the photo generated holes from the valance band (VB) of WO 3 towards the VB of g-C 3 N 4 occurred because of the more positive VB edge of WO 3 (3.0eV, vs. NHE) than that of g-C 3 N 4 (1.7 eV, vs. NHE). Additionally, the great reduction of the electron-hole recombination, which promote the photo catalytic reactions to decompose RhB occurred through the redistribution of electrons on one side of the junction (WO 3 ) and holes on the opposite side (g-C 3 N 4 ). This resulted to a higher photo catalytic activity of WO 3 /g-C 3 N 4 composites than that of WO 3 and the pure g-C 3 N 4 . Based on this outcome, it indicated a high relation between the enhanced photo catalytic activity and the hetero-junction structure and the synergetic effect at the interface between g-C 3 N 4 . and WO 3 . (Gondal et al., 2015).

The X-ray Diffraction Patterns
There was evidence of the difference in the structures of the sample based on the X-ray diffraction patterns indicated. This indicated that samples of g-C 3 N 4 and WO/g-C 3 N 4 have different peaks (Fig. 2). The difference may be a a result of the different ratios of WO 3 ranges (high to low), with the highest peak of WO 3 being from the 0.05 WO 3 /g-C 3 N 4 . Based on the X-ray diffraction patterns and the 28 corresponding characteristic values of the diffraction peaks, it was shown that only the peaks attributed to g-C 3 N 4 and WO 3 , and peaks attributed to other components were not detected. Such output provided indications of the purity of the obtained samples on degradation. The original g-C 3 N 4 had two peaks which were within the 13.1˚ and 27.6˚. These can be indexed as (100) and (002) diffraction planes (JCPDS no. 87-1526). A 27.6°pick, which was attributed to the (002) of the g-C3N4, was found within the X-ray diffraction patterns of all the samples except the 0.5WO 3 /g-C 3 N 4 Sample. On the other hand. peaks which were attributed to the WOW (JCPDS no.83-0950) could also be observed in all the WO 3 /g-C 3 N 4 composites. Based on such outcome, there was indication that the different WO 3 concentrations have great impact on the phase composition. Moreover, the low-angle reflection peak which was observed at 13.1° appeared less pronounced and the corresponding crystallization degree of WO 3 /g-C 3 N 4 composites was observed to slightly decreased with the increase in the WO 3 concentration. This occurred due to the introduced ion which had an alleviation on the crystallization process during the process of calcination. This finding was in agreement with studies by (Zhang et al., 2015;Wang et al., 2018) which indicated that the enhanced photocatalytic activity should benefit from the narrow band gap, fast separation rate of photogenerated carriers and enhanced absorption in visible light range. These proved that the work provides a promising approach to improving the performance of visible light-driven catalysts in the fabrication of other photocatalysts for improved wastewater purification. ISSN 2164-7682 2019 Figure 2. X-ray diffraction patterns of the WO 3 /g-C 3 N 4 samples with different WO 3 concentration

Morphological Analysis
The investigation of the samples morphology was done by SEM and the outcome presented in Figure 3 (a) bare g-C 3 N 4, which indicated a difference in particle size from sample in figure (b) which represented 0.25WO 3 /g-C 3 N 4 . Indeed, the treated figure (b) sample exhibited some particle agglomeration to a certain degree. Moreover, it is evident that in figure (b) 0.25 WO 3 /g-C 3 N 4 sample, the occurred well deposition of the WO 3 on the sheet-like g-C 3 N 4 . Going by such revelation, there was good inter-facial contact between g-C 3 N 4 and WO 3 which will be beneficial in separately transferring the photo excited carriers within the photo cata1ysis. The observation of the morphology of the bare g-C 3 N 4 and the representative 0.25WO 3 /g-C 3 N 4 sample was also undertaken through TEM. The result showed that the bare g-C 3 N 4 formed a sheet like structure that consisted of wrinkles that formed due to the aggregation and gathering of the g-C 3 N 4 sheets. The representative of the 0.25WO 3 /g-C 3 N 4 sample had a particle size hat ranged between 40-50 nm, as well as a worm-like pore structure. Furthermore, the WO 3 particles were showed to cover the g-C 3 N 4 surface, which were almost in direct contact with the g-C 3 N 4 surface which lacked the agglomeration. The interaction between the WO 3 and g-C 3 N 4 was so strong that these particles could not peel off from the g-C 3 N 4 surface. This outcome could not only improve the dispersion property of the materials but also offer additional photocatalytic reaction sites. Indeed, there was further confirmation of the elemental information of the as-prepared 0.25WO 3 /g-C 3 N 4 nano-composite of the EDX spectrum that the introduced W species have been doped in the lattice of g-C 3 N 4 Sample. This provided a reliable indication that the successful formation of the nano-composite with high purity had been achieved. Indeed, the finding was in concurrence with what oher studies has reported (Zhang et al., 2015;Wang et al., 2018). ISSN 2164-7682 2019 Figure 3. TEM images of (a) bare g-C 3 N 4 and (b) the representative 0.25WO 3 /g-C 3 N 4 ; (c) EDX spectrum of the 0.25WO 3 /g-C 3 N 4 Sample

The Nitrogen Adsorption-desorption Isotherms on the Brunauer-Emmett-Teller Surface Area
The Nitrogen adsorption-desorption isotherms was used to calculate the Brunauer-Emmett-Teller surface area (SBET) of the WO 3 /g-C 3 N 4 samples and the results are presented in Fig. 4. The classification of the Nitrgen adsorption-desorption isotherms of the WO 3 /g-C 3 N 4 was done and grouped as 4 in the International Union of Pure and Applied Chemistry classification (Wang et al., 2018). This suggested the presence of mesopores, which is consistent with the result observed by transmission electron microscopy. There was an association between the hysteresis loop in the low-pressure range (0.4<P/P 0 < 0.9) and the intra-aggregated pores. Additionally, there was a relation between the high-pressure hysteresis loop (0.9<P/P 0 < l) with the to the larger pores formed between secondary particles. There was an indication that all the samples possess pore size distributions centered between 30 and 40 nm as a result of the obtained pore size distribution curves from an analysis of desorption branch of the isotherms. The Brunauer-Emmett-Teller analysis run under this study was intended to explain the physical adsorption of nitrogen gas molecules on surface of the obtained composites. From that occurrence, it served as the basis for an important analysis technique for the measurement of the specific surface area of the g-C 3 N 4 composites. Moreover, it was found that the specified surface area of the samples was not directly proportional to the observed pore size increment. Despite such occurrence, a little effect on photo catalytic activity occurred, furthermore, the entire band gap energies were all shown to be within the same range (Table 2). ISSN 2164-7682 2019

The UV-vis Diffuse Reflectance Spectroscopy on the Optical Properties
The examination of the optical properties of the g-C 3 N 4 and WO/g-C 3 N 4 composite samples in range of 200-700 nm was done using UV-vis diffuse reflectance spectroscopy. The results are presented in Fig. 5 (a) and (b). Based on this examination run, all the samples were observed to exhibit absorbance within the visible light region. Moreover, the estimation of the visible absorption range was done and found to be at around 480nm. The UV-vis spectrum of the WO/g-C 3 N 4 was able to exhibit a red shift as well as a significant adsorption at within 250 -700 nm. The shift and adsorption wavelength also increased with the increase in WO 3 concentration. Such a shift and adsorption wavelength were an indication of the existence of the interaction between the g-C 3 N 4 and WO/g-C 3 N 4 composite samples. Such finding provided a reliable prove that the WO/g-C 3 N 4 composites is indeed a very good photocatalyst for the visible light driven applications with enhanced absorption ability. This result was matching the findings that were achieved and reported by Lin et al., (2017). ISSN 2164-7682 2019 Figure 5. The Ultraviolet-vis diffuse reflectance spectroscopy on the optical properties The room temperature photoluminescence spectra of the bare g-C 3 N 4 and the representative 0.25 WO 3 /g-C 3 N 4 samples are provided in Fig. 6. The photoluminescence emission spectra are significant in the determination of the efficiency of charge carrier trapping, migration, and separation. This also help in the investigation of the fate of photogenerated electron-hole pairs. From the study, both the samples indicated a single kind of broad luminescence peak. The luminescence peak was found to be centered at approximately 450 nm with the peak position having some shift. The observed emission peak was directly originating from the band-to-band transitions and the shift of the peak position was consistent with variation of band gap energy of both the samples. Indeed, there was an observed decrease in the photoluminescence intensity of 0.25 WO 3 /g-C 3 N 4 as compared to the decrease within the bare g-C 3 N 4 . This observation was an indication the recombination of the electron-hole pairs was moderately inhibited or the charge separation has a significant accelerated on the 0.25WO 3 /g-C 3 N 4 sample. This further implied that the nanocomposite had lower recombination rates of electrons and holes under visible light irradiation. This finding further proves that the photo-induced electrons flow from g-C 3 N 4 to WO 3 , causing photoluminescence quenching of g-C 3 N 4 with WO 3 , thereby preventing the direct recombination of electron-hole pairs. Indeed, Lin et al., (2017) identified the need to for the design and synthesis of high-performance photocatalysts with highly-efficient charge separation.

RhB Degradation Through Photocatalytic Activities
The result on photocatalytic activities for RhB degradation over the WO 3 /g-C3N 4 composites samples with different WO 3 content were carried out under visible light irradiation the result presented in Fig. 7. The normalized concentrations are provided as C/Co (where C and Co are the concentration at the reaction time t and the initial RhB concentration). The blank test experiment indicated that the concentration of RhB experience little decrease (removal of only 2%) without any photocatalyst under visible-light irradiation. This proved that the photoinduced self-decomposition can be neglected in comparison with the photocatalysis that is brought about by various existing catalyst particles. Furthermore, prior to the visible-light irradiation, the mixture containing the catalyst and Rhb was thoroughly mixed by being stirred in the dark for 30 minutes until an adsorption equilibrium was achieved. All the materials had a relatively similar adsorption capacity which was in the range of 10% -12% of C/Co in the dark. In comparison to the blank test, it was observed that there was difference in the decrease in the C/Co of WO 3 /g-C 3 N 4 composites samples under visible light. Furthermore, the best photocatalytic activity of WO 3 /g-C 3 N 4 was obtained when the content of WO 3 was at 0.25. Through experimentally increasing the WO 3 content from 0 to 0.25, there was a consequential increase in the photocatalytic activity of WO 3 /g-C 3 N 4. Such increase may be attributed to the which may be due to the presence of g-C 3 N 4 in the materials. Through further increase in the content of WO 3 , the photocatalytic activity of WO 3 /g-C 3 N 4 also decreased to a level that was even poorer than the bare g-C 3 N 4. Such manifestation may be related with the aggregated WO 3 particles at the surface of g-C 3 N 4 that leads to the decrease in the photocatalytic capacity. Therefore, such occurrence shows that the optimal WO 3 content was 0.25 in the WO 3 /g-C 3 N 4 samples, where the best activity was displayed and nearly 100% of RhB decomposition and degradation within 2h of visible-light irradiation was realized. Indeed, Shivatharsiny et al., (2015) suggested that the periodic mesoporous materials can be used as adsorbents and photocatalysts for the removal of dye molecules from aqueous solutions with high efficiency, a suggestion that also agree with the findings from the present study.

Conclusion
Based on the findings from this study, the as-prepared hybrid photo catalyst has proved to having a greater ability to improve the photo degradation performance because of accompanying synergetic effect. Indeed, the photo excited electrons from g-C 3 N 4 were noted to be efficiently separated and were injected to the conductive band of the WO 3 . The highest photo activity was achieved for the optimum ratio of 0.25WO 3 /g-C 3 N 4 . The hybrid catalyst equally showed stability and high efficiency in the photo degradation process. Moreover, the composite further demonstrated a high activity for degradation of RhB under visible light irradiation. The presence of g-C 3 N 4 proved to be beneficial for enhancement in photo catalytic activity of the g-C 3 N 4 -WO 3 composite and provided one of the best alternative modes of degrading organic dye pollutant Rhodamine B dye from wastewater.