Photodegradation of aqueous eosin yellow dye by carbon-doped TiO2 photocatalyst

In this study, a novel photocatalyst, carbon-doped TiO2 was prepared via a sol-gel technique with titanium (III) chloride as a precursor. The characterization of C-doped TiO2 was obtained by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and BET surface area analyses. The prepared catalyst’s photocatalytic activity was investigated using UV irradiation for the photo-oxidation of eosin yellow (EY) dye. The photocatalysis of EY dye was performed under various experimental parameters such as solution pH and oxidant dosage (H2O2). The prepared photocatalyst dosages were also taken separately to optimize process efficiency. About 77.43% of EY decolorization was obtained at an optimum pH of 3, and the performance was also observed by varying the oxidant concentration ranging from 5-10 mM. The % decolorization was higher for C-doped TiO2-200 photocatalyst (81.88%) compared to carbon-doped TiO2-400 photocatalyst (75.55%). It can be concluded that the carbon-doped TiO2, calcined at 200°C, can effectively decolorize EY dye in an aqueous medium.


Introduction
The removal of toxic compounds from water bodies is a major ecological concern in controlling water pollution. Water pollution by industrial effluents containing dyes and phenolic compounds is a grim issue for human beings and aquatic life [1]. Even small traces of colors are noticeable and detrimental because of their higher amount of toxicity and carcinogenicity. Thus, the remediation of dyes from the National Conference on Challenges in Groundwater Development and Management IOP Conf. Series: Earth and Environmental Science 597 (2020) 012010 IOP Publishing doi: 10.1088/1755-1315/597/1/012010 2 industrial effluents is of utmost importance. About 100,000 commercial dyes are accessible, with 70,000 tons of annual production, and 15% of total dye production is used in the dyeing process [2]. Dye containing wastewater is not easy to treat since the refractory dye compounds are highly resistant to the bio-degradation and show stability to the heat energy, light, and various oxidizing species [3]. It has been observed that neither the secondary wastewater treatment such as biological processes nor chemical treatment gives a significant amount of percentage decolorization and the reduction in the chemical oxygen demand (COD) [4]. The advanced oxidation processes (AOPs) are supposed to be promising technologies, treat the recalcitrant compounds found in industrial wastewater. AOPs are completely based on hydroxyl (•OH) radical species' production during treatment [5]. In conjunction with some new dye effluent treatment processes, the various advanced oxidation technologies have been widely studied elsewhere [5]- [9]. Semiconductor materials such as titanium dioxide (TiO2) and ZnO are generally used in the photocatalytic processes to remove organic and inorganic pollutants [10]. The photocatalyst, TiO2 with the large bandgap 3.2 eV, can only be activated in the presence of UV light, which is a major limitation. Therefore, doping has been modified, enabling to work under visible light range [11], [12]. The doping of TiO2 notably improves the visible-light response of titanium dioxide. The doping of TiO2 with p-block elements such as nitrogen, carbon, sulphur, boron, and fluorine is a famous method for preparing visible-light catalysts [13]. The doped photocatalysts have different band gap electronic structure than undoped photocatalysts. The non-metallic elements like carbon (C) and nitrogen (N) may occupy the cationic and anionic sites in the lattice of titanium dioxide. The dopants like carbon (C) and nitrogen (N) can reduce the band-gap during the addition and combination of electrons (e -) in higher energy levels than valence band or the addition of energyorbitals below the energy of the conduction band [14]. The carbon doping has gained intense research interest since carbon can be immobilized in the lattice in diverse ways: as an anion by replacing the oxygen atom; as a cation by inhabiting interstitial sites or even substituting titanium atoms [15]. The doping of photocatalyst is carried out for the effective use of the UV-irradiation, either in the form of any mercury lamps or solar energy) [16]. In the present study, C-doped catalyst was prepared via a sol-gel technique by using titanium (III) chloride as precursor. Further, the synthesized photocatalysts was characterized for the SEM/EDX and BET surface area analysis. Prepared catalyst was also calcined at different temperatures (200 o C and 400 o C) to enhance its effectiveness for the final application. The photocatalytic activity of modified photocatalysts were reported with the decolorization of eosin yellow dye using low pressure mercury vapour lamp of 125 W intensity. Photocatalysis of EY dye was carried out under various operating conditions such as pH and oxidant dosage (H2O2). The photocatalyst dosages for the photocatalysis of EY dye were taken individually to optimize the process efficiency. Table 1 shows the physico-chemical properties of eosin yellow dye.

Catalyst synthesis.
The modified carbon-doped TiO2 catalyst was synthesized using a sol-gel technique. The starch powder was used as a carbon resource in the reaction. 5 mL, 12% titanium (III) chloride solution was added in a beaker having 0.5 L of starch solution (1%) with continuous stirring by magnetic stirrer at 720 rpm. In this aqueous medium, the surplus amount of ammonia (51.7 mL, 0.88 gm/cm 3 ) was added in a dropwise manner with continuous stirring. After 20-25 min of this treatment, titania precipitate of white color was obtained. The titania precipitate was further washed 3 to 4 times with the de-ionized water so that an excessive amount of ammonia was washed off from the titania solution and dried at room temperature. This was a prepared C-doped TiO2 catalyst [17]. Further, the prepared photocatalyst samples were divided into two parts and calcined for 6 hr at 200 oC (C-TiO2-200) and 400oC (C-TiO2-400), individually.

Characterization of the prepared photocatalysts.
The study of the modified catalyst's physicochemical properties is challenging, and many analytical techniques were used to characterize the prepared materials. The prepared C-doped photocatalysts were characterized by SEM utilizing the model of ZEISS EVO 18 series to study catalyst morphology. The EDX analysis performed a detailed analysis of elements in the photocatalyst. The prepared photocatalysts (C-doped TiO2-200 and Cdoped TiO2-400) were also characterized for their surface area/ pore volume and their average diameters using the BET surface area analyzer (Smart instruments, India and single point).

Experimental.
Experiments were carried out in a cylindrical shaped photochemical reactor with an effective reactor volume of 300 mL under batch flow condition. The reactor set up consists of inside reflecting surface and UV irradiation source (125 W, low-pressure mercury-vapor lamp) surrounding glass/quartz cylinder, which was placed centrally in the reactor. The feed tank of the photochemical reactor was made up of glass material. 250 mL of 100 mg/L solution of EY dye was prepared and fed to the reactor. The reactor set up was switched on along with its stirrer. The solution was continuously stirred for 10 min at 300-400 rpm.
Further, 1000 mg/L of titanium dioxide (TiO2) was added, and an appropriate amount of hydrogen peroxide (H2O2) was added to start TiO2 photocatalysis for the oxidation of dye wastewater. The reaction begins immediately after the addition of H2O2 in the solution. After every 5 min of a time interval, the samples were taken out from the reactor and tested under a UV-vis spectrophotometer (UV 1800, Shimadzu) for absorbance using a quartz cell of path length 1 cm. The calibration curve equation of an aqueous eosin yellow dye was YEY = 0.117XEY -0.011, R 2 =0.998; where, YEY is the absorbance value obtained at maximum wavelength of 517 nm and XEY is the dye concentration (eosin yellow) in mg/L. The EY dye, percentage decolorization was calculated by Eq. 01. (1) Where, C0: initial concentration of eosin yellow dye and Ct being the dye concentrations at time t.

Results and discussion
The prepared photocatalysts from titanium (III) chloride were initially characterized by different analytical techniques for its suitability in the final application. Further, the trends in the decolorization efficiency of eosin yellow dye using photocatalysis has been reported by varying different experimental parameters on percentage decolorization.

Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis of photocatalysts.
The carbon-doped photocatalysts have been tested under SEM for its morphological study and for the EDX technique to identify the major components in the photocatalysts. Figure. 1 displays the SEM images of the prepared photocatalysts and results in the presence of the agglomeration of carbon particles on the titania surface [17]. The particle size cannot be easily resolved with the SEM analysis. The EDX analysis confirmed that the surface of C-doped photocatalysts is fully packed with the carbon element. In contrast, the nanoparticles' surface becomes apparent with the increment in the calcination temperature from 200 o C to 400 o C. The exterior of the catalysts was enclosed by the carbon, strongly supported by EDX analysis. The presence of carbon elements on the surface of titania nanoparticles is depicted in Figure. Figure. 2 shows that the prepared sample of photocatalyst majorly contains carbon (C), titanium (Ti), and oxygen (O) elements, in addition to a few traces of calcium (Ca), silicon (Si), and phosphorous (P) elements.

Analysis of photocatalysts with BET surface area analyzer.
BET, a single-point surface area analyzer, characterized the prepared photocatalysts (C-doped TiO2-200 and C-doped TiO2-400). Table  2 shows the data for their surface area values, pore-volume, and the modified photocatalysts' average diameters. This analysis's obtained results indicate that the prepared samples' porosity decreases because of the heat treatment from 200 o C to 400 o C, which ultimately shows the decrease in value of surface area (m 2 /gm) [17].

Assessment of photocatalytic activity
The photocatalytic activity of carbon-doped catalysts was assessed for the decolorization studies of eosin yellow dye using a tubular type photochemical reactor.

Effect of initial dye concentration on its percentage decolorization in photolysis.
The photolysis using UV-irradiation is a better way for the generation of free radicals via cleavage of a sigma bond. The produced radicals exist as precursors, which can generate more free radicals [18], [19]. Certain steps are involved in the photochemical reaction of the dye molecules. The dye molecule gets excited with the absorption of one photon. Again, the excited molecules take part in the chemical reaction and result in the dye molecule oxidation in the effluent. In this study, the initial dye concentration was considered in the range of 50-200 mg/L to get the trend of % decolorization for EY dye. Figure. 3 reports the percentage decolorization of EY dye at its varying initial concentrations. EY dye was efficiently decolorized to 43.32% for 100 mg/L of its initial concentration in just 60 min of treatment time. At higher concentrations ranging from 150-200 mg/L, the percentage decolorization was decreased to 29.11 and 26.92%, respectively. The percentage decolorization for eosin yellow dye was reduced at higher concentrations (150-200 mg/L) because of the required quantity of free radicals' unavailability in the aqueous medium. The % decolorization of EY dye was found to be maximum at

Influence of pH in TiO2 photocatalysis.
The influence of pH value on the decolorization of EY dye was studied by varying it from 3 to 9. Figure. 4 shows that the percentage decolorization of EY dye reduces severely when the solution pH increases from a value of 3 to 7 and further to 9 (acidic to neutral and then to alkaline). The actual amount of solution pH can change the distribution of various protonation stages of the dye molecule in an aqueous medium. The pH value strongly influences the dye adsorption onto the TiO2 surface [20], [21]. Stronger adsorption of the dye molecules to be oxidized is generally considered as a requisite for effective decolorization of dye molecule in photocatalysis. In this study, the percentage decolorization for EY dye started to decrease continuously, when the solution pH was changed to a value of 7 and 9 from 3, with the decolorization efficiencies of 77.43%, 43.32%, and 33.87%, respectively. The solution pH might also affect the nature of the photogenerated species like hydroxyl radicals and trapped holes.