Synthesis and Characterization of Ag@C-TiO 2 Nanocomposite for Degradation of Sasirangan Textile Wastewater

Carbon-titanium oxide nanocomposite (denoted as @C-TiO 2 ) was successfully synthesized via hydrothermal method at 150°C for 24 h. The C-TiO 2 nanocomposite was furtherly modified by adding an Ag metal dopant (denoted as Ag@C-TiO 2 ) to improve and applied to the photocatalytic degradation of Sasirangan textile wastewater. The composite photocatalysts were characterized by XRD and UV – Vis DRS spectroscopies. XRD patterns showed that TiO 2 in @C-TiO 2 mainly consisted of a brookite phase, as indicated by a series sharp diffraction peak at 2 θ = 27.2° (111), 31.5° (121) and 55.9° (241). The calculated band gap energy ( E g ) derived from UV-Vis DRS spectra for TiO 2 , @C-TiO 2 , and Ag@C-TiO 2 were 2.95 eV, 2.54 eV, and 2.74 eV, respectively. Ag@C-TiO 2 photocatalyst was found to be active for the photocatalytic degradation of Sasirangan textile wastewater, as indicated by the change of wastewater color from dark to clear. The quantitative photocatalytic activity of Ag@C-TiO 2 was evaluated in the degradation of methylene blue, whereas the conversion of methylene blue was 41.3%. The addition of Ag to @C-TiO 2 is believed to play an essential role in the enhancement of photocatalytic activity.


Introduction
Photodegradation is a common method for the treatment of textile dyes before it disposed into the environment using photocatalyst materials, where organic dyes are broken down into smaller compounds in the presence of light (photons), and the reactions are accelerated by using photocatalysts [1,2].One of the most widely used photocatalyst material is TiO2 because it has high photoactivity in the UV region (gap energy of TiO2 = 3.0 eV (rutile), 3.2 eV (anatase)) and had high chemical stability.TiO2 performance can be significantly improved by adding additional material (dopant), both metal and non-metal, which will reduce the energy gap so that the response to UV rays or appears to be increased [3].There are several studies have been reported on the addition of non-metal dopants such as nitrogen (N), sulfur (S), and carbon (C) to improve the sensitivity of TiO2 towards visible-light resources [4,5].For example, carbon-doped-TiO2 (C-TiO2) with the amount of carbon-doped around 5.2 %mol has the lowest bandgap energy 2.3-2.8eV [4].Several previous reports have also shown that C-TiO2 catalysts were effective for the photodecomposition of acid orange 7 (AO7) with 99% conversion [6].Teng et al. [7] also reported the conversion of Rhodamine B using a C-TiO2 catalyst (with a bandgap energy of 2.91 eV) and 95% conversion Rhodamine B was achieved under visible-light irradiation.
Utilization of palm oil shell has a big challenge as a raw material in the synthesis of activated carbon since Indonesia is the biggest country in the production of crude palm oil (CPO) with around 38.2 million tons of CPO and 3.05 million tons of palm kernel oil (PKO).Every ton of CPO production, around 6.5% of palm kernel oil shells, will be generated.Several previous reports have shown that palm kernel oil shells can be transformed into active carbon using the carbonization method at a relatively mild temperature [8,9].The performances of activated carbon can be improved both in economic value and usedvalue through combination with other active substances, such as semiconductor materials (TiO2, ZnO, Fe2O3) produce photocatalyst composite materials that can be used for dye degradation, clean water treatment, and anti-bacterial with the better performance [10].In addition, active carbon derived-palm oil kernel shell that obtained via chemical activation of Na2CO3/ZnCl2 mixture has high specific surface area BET (SBET) 743.71 m 2 g −1 and demonstrated high adsorption capacity for the adsorption of hydrogen sulfides (H2S) 247.33 ppm [11].
Sasirangan is a traditional cloth that originally comes from South Borneo, it is similar to Batik, but it has different making process.The Sasirangan industry in South Kalimantan is generally a home industry with traditional processing.Seeing the nature of these industrial activities, most of the craftsmen have not made efforts to treat the generated waste (e.g., wastewater after the coloring process) and directly dispose of it into water bodies.Therefore, it has an impact on the disruption of the life processes of aquatic organisms, and at the same time, can threaten the sustainability of aquatic ecosystems [11].Moreover, the presence of textile waste in waters can interfere with sunlight penetration and oxygen diffusion into water bodies.
In this report, we describe the synthesis of silverdoped carbon-titanium oxide nanocomposites (denoted as Ag@C-TiO2) using a straightforward hydrothermal method which are producing particles with high crystallinity, high purity, homogeneous particle size distribution, and also use low temperatures below <300°C for the reaction.The addition of Ag aims to increase the photocatalyst's response to both UV and visible light and to increase the stability of the photocatalyst, while activated carbon is added to increase the active surface area of the photocatalyst.Ag@C-TiO2 nanocomposites that obtained will be compared in their characteristics and photocatalytic activity with pure TiO2 and @C-TiO2.The photocatalytic reaction will be carried out on the degradation of Sasirangan wastewater in a batch reactor system.As a comparison, photocatalytic reactions of commercial dye c.a. methylene blue (MB) under the same reaction conditions.

Synthesis of palm shell charcoal
The crushed palm shell charcoal is acidified first using FeCl3•6H2O overnight.Giving acid is one of the chemical activation methods of charcoal to enlarge the pores of activated charcoal.The acidified charcoal is then filtered and reactivated by heating at 500°C for 3 hours to get activated charcoal.

Preparation of TiO2 photocatalyst
TiO2 photocatalyst was prepared using a 99% TiCl4 solution as a precursor diluted to 2M TiCl4 by mixing 100 mL of 99% TiCl4 and 1 mL of 37% HCl in 350 mL of distilled water.This mixing produces a 2M TiCl4 solution which is turbid white and white precipitate is formed, which is thought to be TiO2 deposition:

Synthesis of @C-TiO2 and Ag@C-TiO2 nanocomposite
The @C-TiO2 nanocomposite was prepared by mixing activated charcoal into a 2 M TiCl4 solution using a 24-hour hydrothermal reduction method at 150°C.Ag@C-TiO2 nanocomposites are made by mixing synthesized @C-TiO2 powder and adding AgNO3.The method used is also the same as the synthesis of @C-TiO2 nanocomposites.

Catalyst characterization
Powder X-ray diffraction (XRD) measurements were filed on a Mac Science M18XHF instrument using monochromatic CuKα radiation ( = 0.15418 nm).The XRD equipment operated at 40 kV and 200 mA with a step width of 0.02° and a scan speed of 4° min -1 (α1 = 0.1540 nm, α2 = 0.1544 nm).
Analysis ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) was performed on a UV-Vis Shimadzu 2450 with a dual-beam system at Affiliation Laboratory of Chemistry, Faculty of Mathematics and Natural Sciences (FMIPA), University of Indonesia Jakarta BaSO4 powder was used as standard material.The calculation of bandgap energy (Eg) was derived from the obtained spectra using a formula of Eg = h.C/λC, whereas h is Planck constant (4,136.10 -1 eV.s), C is light velocity (2,997.10 8 m.s -1 ), λC is cut-off wavelength (nm).λC was derived from plotted data of absorbance versus wavelength with linear cross-section on the spectra.

Photocatalytic degradation of Sasirangan wastewater and methylene blue
A typical procedure for the photodecomposition of methylene blue (MB) over the Ag@C-TiO2 catalyst is described as follows.A solution of MB (5.0 ppm, 30 ml) and 0.15 g Ag@C-TiO2 were mixed in the glass-tube reactor (50 ml) then immersed in water batch at 60°C under UV or visible irradiations (UV or visible lamp sources are 4 x 8 W).The reaction mixture was stirred at 310 rpm for 30 min to reach the equilibrium point of adsorption-desorption.The reaction mixture was irradiated for 120 min at a reaction temperature of 60°C and sampled every 30 min and analyzed by using Perkin Elmer UV-Vis double beam spectroscopy.

Result and Discussion
The synthesized TiO2, @C-TiO2, and Ag@C-TiO2 photocatalysts were characterized by using X-ray diffraction, UV-Vis Diffuse reflectance spectroscopy, and the physicochemical properties are summarized in Table 1.

UV-Vis DRS Analysis
Figure 3 displays the UV-Vis DRS spectra of each synthesized catalysts.The bandgap energy (Eg) was estimated from the plotting of absorbance (A) versus wavelength (nm) using Tauc equation .h.c/c= Bd(h.c/c -Eg) n for allowed transitions (n = 2 for indirect transition, n = ½ for direct transition), h is Planck's constant, c is light velocity, and Bd is the absorption constant [14,15].The reflectance spectrum profile shows that TiO2, @C-TiO2, and Ag@C-TiO2 have absorption regions in visible light areas (λ> 400nm), indicating the doping process that has been carried out was successfully achieved.The values of λc for each TiO2, @C-TiO2, and Ag@C-TiO2 are 420 nm, 487 nm, and 451 nm.The bandgap energy of Ag@C-TiO2 (2.74 eV) produced is relatively small compared to the pure TiO2 (2.95 eV).These results are a good agreement with the previous report of Wang and Lewis [16], who reported the synthesis of @C-TiO2 photocatalysts, which have Eg of 2.3 to 2.8 eV.Hence, it can be confirmed that the addition of C material and cocatalyst in the form of Ag can shift the gap energy of the TiO2 band to the visible light region.These results indicate that the addition of dopant Ag significantly shifted the absorption band of TiO2 to the visible region, as indicated by the Eg.

Photocatalytic degradation
The Ag@C-TiO2 nanocomposite photocatalytic activity test was carried out through the photocatalytic reaction of waste Sasirangan.The reaction is accomplished in a reactor that has been designed and carried out under UV light and visible light for 120 minutes with stirring at 60°C.Stirring is done so that both UV and visible light can hit all parts of the photocatalyst evenly.Photocatalyst samples were weighed at 0.15 g, then added to 30 mL of Sasirangan waste.Before irradiation is done (dark), the photocatalyst mixture and the Sasirangan waste are stirred for 30 minutes so that the molecules of the substances in the waste can be absorbed on the surface of the catalyst.Then the test is carried out for 120 min.The results of the reaction are then compared with Sasirangan waste before being reacted.Methylene blue was also tested by catalytic activity to see the comparison of the degradation results.The photo images of the de-coloration of Sasirangan and methylene blue waste before and after photocatalytic degradation are shown in Figure 4.The color produced after the reaction is quite bright.This indicates that the concentration in the solution decreases.After irradiation, the solution is centrifuged to separate the solution with a photocatalyst and analyzed using a UV-VIS spectrophotometer.Based on UV-Vis analysis data, spectra results (Figure 5) show a decrease in absorbance, which indicates that there is a change in the concentration of the MB solution when reacted with Ag@C-TiO2 photocatalysts and the results are summarized in Table 1.The results of the qualitative analysis showed that the addition of Ag dopants to the @C-TiO2 nanocomposite could degrade Sasirangan waste and methylene blue waste.While the results of quantitative analysis can only be done on a blue methylene solution and cannot be done on Sasirangan waste, as there are many Sasirangan wastes dyes and organic substances, so it is less supportive in quantitative analysis using UV-Vis.Photocatalytic reactions also produce better degradation when done under visible light.This can be seen from the more transparent color changes in the blue methylene solution.This result is also due to the bandgap energy decreasing, where Ag@C-TiO2 has a smaller Eg value compared to TiO2.This can be seen from the results of UV-DRS characterization.

Conclusion
Based on the results of research and data processing, it can be concluded that the Ag@C-TiO2 nanocomposite has been successfully synthesized using the hydrothermal method at 150°C for 24 hours.The XRD pattern of the TiO2 and @C-TiO2 nanocomposite shows that the TiO2 that appears is dominated by the brookite phase (121) while in Ag@C-TiO2, the primary identified TiO2 phases were anatase.The bandgap energy values of TiO2, @C-TiO2, and Ag@C-TiO2 are 2.95 eV, 2.54 eV, and 2.74 eV, respectively.Ag@C-TiO2 nanocomposite is active in degrading Sasirangan and methylene blue waste under visible light with a conversion of 41.28%.

Figure 1 .
Figure 1.Schematic diagram of the reactor for the photodegradation of methylene blue Ag@C-TiO2nanocomposites[12].

Figure 4 .
Figure 4. Photo images of photocatalytic reactions of Sasirangan wastewater and methylene blue (MB) using Ag@C-TiO2 under irradiation of (a) UV light and (b) Visible light.

Figure 5 .
Figure 5. UV-Vis spectra of methylene blue under (a) UV light and (b) visible light after 0 minutes and 120 minutes.

Table 2 .
Results of Photodegradation of MB over Ag@C-