Dual functioning TiO2-brilliant yellow composite for efficient removal of methylene blue from water

A novel composite of titanium dioxide and brilliant yellow dye (Ti-BY) was synthesized using one-pot synthesis technique under solvothermal conditions. The composite was stable in both acidic and basic conditions and also Ti-BY demonstrated dual functionality in the removal of methylene blue dye from water, in which it was found to be active as a photocatalyst as well as an adsorbent. Remarkably, the composite was 56% efficient in the dark and 97% under sunlight irradiation within 2 h at pH 11 without H2O2.


INTRODUCTION
Water pollution is one of the main environmental crises that we face today as it limits the consumable volume of water, space available for aquatic systems and increase health risks in living beings. One significant method of water pollution occurs due to the release of untreated synthetic dyes from industries such as paper production, leather tanning, food and textile (Lellis et al., 2019;Yaseen and Scholz, 2019) The degradation products of these dyes are toxic or even carcinogenic to human and aquatic life and also increase the toxicity and chemical oxygen demand (COD) and biological oxygen demand (BOD) levels of aquatic sources (Crini, 2006;Lellis et al., 2019 ). In order to remove these dyes from effluents many physicochemical methods such as filtration, flocculation, electrocoagulation, photocatalytic degradation and adsorption have been investigated (Sharma et al., 2018;Jamee and Siddique, 2019;Shindhal et al., 2021). Photocatalytic degradation is one of the preferred techniques used due to the ability of degrading toxic materials in to non-toxic and environment friendly compounds. Biomass degradation (Al Zoubi et al., 2021;Hussain et al., 2021 ;Sriramoju et al., 2021) Fenton process (Babuponnusami and Muthukumar, 2014), degradation by semiconductors such as TiO 2 , ZnO and their derivatives (Miranda-García et al., 2011) are few of the commonly utilized methods of catalytic degradation (Forgacs et al., 2004). However, degradation of some of the synthetic dyes is challenging due to their high stability under light and oxidation processes (Sing, 1998). Adsorption technique is widely practiced for removal of dyes which are resistant for degradation and due to its high efficiency, comparatively low cost, reusability and facile regeneration of the adsorbent (K. Ramakrishnan et al., 2021;Yu et al., 2021). Porous materials such as alginate beads, activated carbons, zeolites, metal organic frameworks, polymeric resins, metal matrix composites and clays are some of the examples for materials used in adsorptive removal (Haque et al., 2011). Recently, we reported a one-pot synthesis technique to impregnate a high amount of N719 dye on to TiO 2 nanoparticles (Herath, Perera and Hettiarachchi, 2020). This composite of TiO 2 / N719 demonstrated an outstanding rapid photocatalytic activity on removal of methylene blue (MB) under visible light which was stable and reusable. However, N719 dye is very expensive and it desorbed from TiO 2 under strongly basic conditions. Therefore, much cheaper and stable dye based composite was investigated to develop a new visible light photocatalyst.
Herein we report the synthesis of a new TiO 2 composite with brilliant yellow dye (C 26 H 18 N 4 Na 2 O 8 S 2 ) following the same solvothermal one-pot synthesis technique. The uniqueness of this composite (Ti-BY) is that it exhibits not only the photocatalytic activity but also it acts as an adsorbent in dye removal process.
MB was used to demonstrate the dual functionality of the Ti-BY composite. MB is a highly neurotoxic commonly used synthetic cationic dye in various industries (Oz et al., 2011).

Chemicals
All chemicals and reagents were purchased from Sigma Aldrich except brilliant yellow dye (C 26 H 18 N 4 Na 2 O 8 S 2 ) (HOPKIN & WILLIAMS Ltd) and were used as received without further purification.

Synthesis of Ti-BY composite
Brilliant yellow (1.00 mmol) dissolved in 5.00 mL of DMF was added drop-wise to 2.00 mL of acetic acid while stirring. Then, 0.30 mL (1.00 mmol) of titaniumisopropoxide was added to this solution. The solution was transformed into a 23 mL Teflon lined autoclave reactor and heated in an oven at 145 °C for 84 h. The brown product was filtered under vacuum and soaked in ethanol for three days by replacing the solvent each day with fresh solvent. Finally, the product was air dried.

SHIMADZU IR Prestige 21 spectrometer with a KBr
Beam splitter and a DLaPGS detector was used to record the FT−IR absorbance spectra using KBr pellets with 1wt% sample.
UV−1800 SHIMADZU spectrophotometer was used to record UV-visible spectra at slow scan rate in auto scan mode. Compound was characterized qualitatively using SIEMENS D5000 diffractometer equipped with a Cu target metal and a Ni K β filter in the scan range of (2θ) 5°-40° with the step size of 0.02° operated at 25 kV and 40 mA.

Adsorption and photocatalytic experiments
Aliquots of 10.00 mL MB solution (0.025 mmol dm −3 ) were taken into 20 mL beakers and 0.020 g of finely powdered Ti-BY was added to each MB soltuion. Solutions were then kept in the dark for 1 h to achieve the equilibrium. The adsorption experiments were conducted under dark, sunlight (0.7321 W m -2 ), white LED light (power-12 W, Luminous Flux-1000 lm, colour-6500 K ± 450K) and ambient conditions. Absorbance measurements were recorded after 2 h at λ max = 664 nm (Completion of the adsoption/photocatalysis at 2 h was determined by checking the absorbance of the test solution in every 20 min for 3 h). All experiments were carried out in triplicates. One beaker containing only TiO 2 (0.020 g) with MB was kept under the dark condition and another beaker with TiO 2 (0.020 g) and MB was irradiated with sunlight as controls.
The percentage removal of dye was calculated using equation (1). (1) where, A i is the initial absorption of the dye and A t is the absorption of the dye at time "t".

Optimization of pH
Aliquots of 10.00 mL MB solution (0.025 mmol dm −3 ) were taken into 20 mL beakers and 0.020 g of finely powdered Ti-BY was added to each MB solution. The pH (2 to 12) of the solutions was adjusted using HNO 3 and NaOH. Solutions were then kept in the dark for 1 h to achieve the equilibrium. The absorbance of each solution was recorded at λ max = 664 nm after 2 h irradiation with sunlight.

Desorption experiments
Dye adsorbed Ti-BY composite (0.020 g) was taken into 20 mL beaker and 10.00 mL of 5.00 mol dm −3 HCl was added. The solution was kept for three days while stirring. The UV-Vis absorption data of the desorbed solution was measured on each day. Then, on the third day, the compound was washed with a mixture of water and 1.00 mol dm −3 NaOH solution. Concentrations of desorbed MB dye in HCl washings were determined using UV Visible spectroscopy at the λ max value of 664 nm.

Characterization
Using the solvothermal one-pot synthesis technique, a brown thermally stable TiO 2 /brilliant yellow composite (Ti-BY) having a melting point above 350 ºC was obtained. Figure 1 (Anandhi et al., 2016). Further, the additional peak at 2θ value of 32.1° in the PXRD pattern of Ti-BY corresponds to the major peak in BY dye which confirmed the presence of BY dye as well, and thereby the formation of a composite. However, broader PXRD peaks with some noises indicate the presence of TiO 2 and dye with poor crystallinity. FT-IR spectra of Ti-BY and BY dye and the chemical structure of BY are shown in Figure 2 (a) and (b) respectively. In both spectra, the broad band maximising at 3471 cm −1 corresponds to the vibration frequency of O-H bonds providing a direct evidence of unaffected O-H groups of BY remaining in the composite. Bands appearing at 1508 cm −1 in both spectra can be assigned to the vibrations of the C=C bonds in benzene rings (Abedi et al., 2015) ZnO/GAC, and TiO2-ZnO/GAC combined with non-thermal plasma (NTP. Moreover, the asymmetric stretching vibration of S=O bond is responsible for the prominent absorption band appearing at 1477 cm −1 in BY, while less noticeable in Ti-BY. Further, the peak at 1080 cm −1 in the BY corresponds to the symmetric stretching vibration of S-O bond which disappeared in the spectrum of Ti-BY. These evidences clearly imply the formation of a metal-ligand bond between group in BY and Ti(IV) in TiO 2 particles. The broad band appearing at 667 cm −1 in Ti-BY spectrum may prove the formation of S-O-Ti bonds. Also the strong peak appearing around 1643 cm −1 in BY spectrum has shifted to the 1635 cm −1 in Ti-BY as a weak peak indicating the presence of N=N bond, but, shifted to a lower value due to the formation of S-O-Ti bond (Moussa, 2000).

Photocatalytic and adsorption activity
Initially, to optimize the mass of the Ti-BY needs to be used in photocatalytic and adsorption activity efficiently, the amount of composite was changed from 5.0 mg to 25.0 mg upon sunlight irradiation. Accordingly, dye removal efficiency increased from 74% to 86% in 5.0 mg to 20.0 mg respectively, whereas after 20.0 mg, the efficiency was not changed significantly. Thus, 20.0 mg of Ti-BY was used as the optimum mass for further dye removal experiments.   removal of MB. Irradiation with LED white light has increased the efficiency from 45% to 79% which must be due to the photocatalytic removal by Ti-BY along with the adsorption. The photocatalytic activity of Ti-BY was further confirmed by ion chromatography data recorded for the dye solution after 2 h. Appearance of peaks corresponding to nitrate and sulfate confirmed clearly the degradation of some amount of MB photocatalytically as well (Herath, Perera and Hettiarachchi, 2020).
The presence of brilliant yellow dye in the composite allows TiO 2 particles to absorb visible light and the mechanism is as indicated in the Figure 4. In the composite, dye and the semiconductor work together but as two units. Dye molecules absorb visible light and due to photoexcitation, they inject electrons to the CB of the semiconductor. These photogenerated species will initiate the photo degradation of the pollutant. Conversely, when the reaction mixture was exposed to sunlight, the dye removal efficiency further increased to 86% (Figure 3) due to the presence of UV component in the sunlight. Thus, other than the adsorptive removal and photocatalysis initiated by the visible radiation, TiO 2 nanoparticles themselves have undergone the photocatalysis upon UV irradiation. The spectrum in the LED bulb covers only the visible region without UV component. Thus, the efficiency under LED light is lower than that of under sunlight. Hence, all three phenomena; adsorption, photocatalysis by both visible and UV energies were observed under sunlight demonstrating the highest dye removal efficiency.
The dye removal efficiency was further optimized by conducting the experiments from pH 2-12 upon sunlight irradiation ( Figure 5). Accordingly, high efficiencies were observed in basic pHs and the maximum performance of 97% removal was observed at pH 11. In basic pHs, the deprotonation of terminal OH groups of BY which were unaffected by the composite formation and surface OH groups of TiO 2 could be enhanced. This facilitated a better adsorption of cationic MB molecules to the composite leading to higher dye removal efficiency in the basic medium. When the bare TiO 2 was used, only 82% of the MB dye was removed at pH 11 upon sunlight irradiation. This clearly indicates the improvement gained by utilizing the Ti-BY composite for MB removal. In the adsorption studies, the colour of the Ti-BY composite changed from brown to green and the original brown colour was regenerated by desorbing the MB from the composite using a 5.00 mol dm −3 HCl solution. This allowed the Ti-BY composite to be reused in dye removal process. The efficacies in reusability of the composite tested for three dye removal cycles were found as 97%, 94% and 91% for cycle 1, 2 and 3 respectively. Even in the third cycle, the dye removal percentage by Ti-BY remained above 90% implying the excellent reusability. Further, the total average desorbed amounts of MB were calculated based on UV-Visible spectroscopic data as 0.014 ± 0.004 mmol dm -3 for three trials. This further proved thoroughly that ~56% of the MB has been adsorbed to the composite at pH 11 and the remaining dye amount must have been photocatalytically degraded. This was further confirmed with the support of IC data collected for the reaction mixture after 2 h on white light irradiation. Accordingly, MB molecules have also photocatalytically been degraded to sulfates and nitrates by the composite.
According to the literature, only a limited number of reports have been published on dye-sensitized TiO 2 as a photocatalyst for the synthetic dye degradation process. On the other hand, the degradation of sensitizing dye gradually via a self-sensitized photocatalytic process, the desorption of the sensitized dye at high pHs and a slow photocatalytic degradation process were the main drawbacks reported in those systems (Chatterjee et al., 2006;Jiang et al., 2008). Comparingly, Ti-BY composite is stable even at high pH values and free from self-degradation. Most importantly, these efficiencies were obtained in the absence of H 2 O 2 which indicates that when H 2 O 2 is used, 100% removal of MB can be achieved within less than 2 h.

CONCLUSIONS
In conclusion, Ti-BY composite was successfully synthesized under solvothermal conditions using onepot synthesis method. This composite was found to be active under dark conditions as well as under irradiation with sunlight owing to the excellent dual functionality. Two dye removal processes, adsorptive removal and photocatalytic dye degradation were observed using the Ti-BY composite on methylene blue. Optimized system of Ti-BY demonstrated a dye removal percentage of 97% at pH 11 after 2 h of irradiation with sunlight without H 2 O 2 . Further, the Ti-BY composite exhibited more than 90% dye removal capability even in the third dye removal cycle. Thus, Ti-BY may be an outstanding composite suitable to be utilized in removal of methylene blue from industrial effluents with an excellent reusability.

ACKNOWLEDGEMENT
Authors would like to thank the University of Peradeniya Grant, URG/2017/46/S, for the financial assistance.

DECLARATION OF CONFLICT OF INTEREST
The authors declare no competing interests.