Photocataytic Degradation of Phenol Using TiO2-Fe Under H2O2 Presence by Visible and Sunlight Irradiation

Phenol is one of the essential organic pollutants released into the environment because of its high stability and toxicity. It is harmful to organisms, environment, and posing a serious threat to human health at low concentration. This research investigated the photocatalytic degradation process of phenol using a TiO2-Fe catalyst under visible light irradiation and additional H2O2. The effect of various conditions process was applied, including different catalyst doses (0.2, 0.4, 0.6, and 0.8 g/L), pH (3, 6, 8, and 11), irradiation times (60, 90, 120, 150, and 210 minutes) and the presence of H2O2. The degradation process was studied at an initial concentration of phenol 5 mg/L. This study has been decreasing phenol content (90.51%) with catalyst doses 0.6 g/ L sample solution, pH solution 11, reaction time 210 minutes and H2O2 concentration 30%. This final phenol concentration after photodegradation under halogen light was 0.18 mg/L, while sunlight irradiation was 0.11 mg/L. This result is below government regulation as per Permen LH RI No. 5/2014 i.e. 0.5 mg/L. Therefore, this process possible to remove phenol in aqueous such as industrial wastewater or other resources.


INTRODUCTION
Phenols are listed in the US EPA as dangerous substances abandoned in the aquatic environment. It is one of the essential organic pollutants released into the environment because of its high stability and toxicity. Phenol is harmful to the organism, environment, and posing a severe threat to human health at low concentration. According to Indonesia's government regulation, Permen LH RI No. 5/2014, minimum concentration of phenol compounds in wastewater must be below 0.5 mg/L (Menteri et al., 2014). The pharmaceutical industry, petrochemical industry, pesticides, polymeric resin, mine sewage, coal conversion are the primary sources of phenolic pollution (Sufian, 2020). It is a substantial issue to remove them in order to minimize the risks to environments. Several treatments such as adsorption (Luo et al., 2015), advanced oxidation (Bethi et al., 2016), biodegradation and activated sludge, membrane filtration, and photocatalytic degradation have already been established (Aryani, 2011 ;Zul et al., 2020). However, photocatalytic degradation and adsorption are considered the most effective treatment techniques (Heydaripour et al., 2018;Linda J Kusumawardani et al., 2020). These are simple, high performance for the elimination of phenol, and available at low cost.
TiO 2 has the ability to oxidize large amounts of harmful organic pollutants into non-toxic products and promote as a potential photocatalyst. Because of its ability to oxidize large amounts of harmful organic pollutants into non-toxic products, TiO 2 has a relatively moderate to low bandgap energy between the valence and conduction bands. In addition, it has been promoted as a potential photocatalyst that can be used for the degradation of phenol compounds. However, TiO 2 works at the wavelength of UV light to activate photocatalysts. Gota et al., 2014 reported 0.2 g TiO 2 in a 1-liter sample solution gives the optimum effect for phenol reduction (Sundaramurthy, 2014). Kusumawardani and Syahputri (2019) reported that the structural and optical properties of TiO 2 differ from the addition of Fe(III) in the photocatalytic process under visible light (L J Kusumawardani & Syahputri, 2019). Kusumawardani and Syahputri (2020) have reported the high degradation for paraquat dichloride as an organic compound using TiO 2 -Fe, which reached 98.4% by catalyst TiO 2 -Fe from 30 mg/L to 0.2 mg/L only for 75 minutes (Linda J Kusumawardani et al., 2020). Iron (III) is a promising dopant into TiO 2 crystal lattice due to ionic radius, where the Fe radius is 0.64Å and Ti is 0.745 Å (Sood et al., 2015).
The present work aims to develop conditions process for phenol removal using TiO 2 -Fe catalyst and the presence of H 2 O 2 . The TiO 2 -Fe catalyst, with the amount of Fe 3+ dopant by 10% (w/w) , was prepared by the sol-gel method, as a previous report by our group research (L J Kusumawardani & Syahputri, 2019). The various operating conditions including catalyst doses (0.2, 0.4, 0.6, and 0.8 g/L), pH (3, 6, 8, and 11), and reaction times (60, 90, 120, 150, and 210 minutes). Phenol removal will be evaluated under halogen light and measured by spectrodirect Lovibond. Then, the optimum operating process will be applied under sunlight irradiation.

Materials and Instruments
Materials used are phenol GR for analysis and H 2 O 2 30% were purchased from Merck, NaOH and HCl for pH adjusting, Aquadest, and Halogen Lamps (Philips, 1000 watt) as a visible light source. Spectrodirect from Lovibond was used for determining the concentration of phenols.

Procedures
In this experiment, catalyst TiO 2 -Fe was prepared by the sol-gel method, with the amount of Fe 3+ dopant by 10% (w/w) as reported by our previous research (L J Kusumawardani & Syahputri, 2019). Photocatalytic degradation was carried out by adding TiO 2 -Fe to the sample solution with a concentration of 5.00 mg/L phenol compound. The light sources used are halogen and sunlight. The reaction vessel is equipped with magnetic stirring. Photocatalytic degradation of phenol was evaluated with the presence of 2.5 ml H 2 O 2 and measured by Spectrodirect Lovibond. The effect of various condition process was applied i.e., different catalyst dosages (0.2, 0.4, 0.6, and 0.8 g/L), pH (3, 6, 8, and 11), reaction times (60, 90, 120, 150, and 210 minutes). The optimum condition for phenol removal under halogen light was selected as the optimum goal for photocatalytic degradation under sunlight exposure. Figure 1. shows a photocatalytic degradation reactor model. The percentage of photocatalytic activity was calculated using Eq. (1), where % D is the percentage of degradation, C o is the initial concentration of the sample before halogen and sunlight irradiation and C t is the final concentration after irradiation under halogen and sunlight at various condition process (Catalyst Dosages, pH, and time reaction) under H 2 O 2 presence as an independent variable.  (1)

RESULTS AND DISCUSSION
Prior to the photodegradation study at various operation conditions, the effect of 2.5 ml H 2 O 2 addition on photocatalytic degradation of phenol was investigated. This study was carried out at an initial phenol concentration of 5.00 mg/L, under halogen light for 60 minutes. The phenol removal is illustrated in Fig. 3  The catalyst dose is one of the important factors affecting the photocatalytic degradation process (Mohamed et al., 2019). In this study, the effect of catalyst dosages was carried out by using catalyst amounts 0.2, 0.4, 0.6, and 0.8 g/L at an initial concentration of 5 mg/L. The result was shown in Fig. 4. It shows the highest degradation efficiency at 0.6 g/L during 1-hour halogen lamp exposure. This catalyst dose demonstrated the sufficient number of active sites could be provided. We also reported that the maximum catalyst dose will increase the photocatalytic efficiency due to the increasing number of surface active sites of the catalyst. Meanwhile, the excessive amount of catalyst dose will cause turbidity in the solution and agglomeration between particles. Hence, it will reduce the light absorption on the catalyst surface, thus decreasing the light penetration and photo absorption efficiency. Hence, in the next operating conditions study, the addition of H 2 O 2 was used as an independent variable. The effect of pH on reducing phenol concentrations was carried out at various pH (3, 6, 8, and 11). The results are shown in Fig.  5. It shows that the removal in phenol levels will increase with increasing pH due to the presence of OHions. Degradation efficiency of phenol was improved during 1 hour, respectively at pH 3 <pH 6 <pH 8 <pH 11, which was 33.62; 39.08; 41.19; and 51.49%. pH alkaline causes more degradation of phenol oxides than phenol. Meanwhile, an acidic pH causes the phenol to break down slightly. This is consistent with Gota et al. (2014), who reported that increasing the pH value can increase the efficiency of removing phenol in solution (Sundaramurthy, 2014). Phenol was converted to phenoxide ion that more degradable than phenol. A simple scheme is reported in Fig. 6 to represent phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. This seems contrary to (Borji et al., 2014) which reported the highest degradation efficiency occurred at acidic condition. However, it also stated the higher phenol degradation occur at alkaline condition due to the presence of phenol molecules as negatively charged phenolate species which more reactive than phenol molecules and can be cause of more degradation of phenol. According to the result, this study showed the high concentration of OHin the solution was not prevent the penetration of visible light to reach the catalyst surface.  Fig. 7 shows the relative changes in phenol removal with TiO 2 -Fe with respect to time was monitored by increasing the reaction time under the same condition, i.e., catalyst dose 0.6 g/L , at pH 11 and the addition of H 2 O 2 over 210 minutes period under a halogen lamp. The photolysis process achieves the degradation efficiency during 210 minutes' radiation time. The phenol compounds were successfully degraded 96.25%. This result shows that radiation time has a significant effect and an important aspect of the overall photocatalytic process. It can be observed that the longer exposure time will degrade more phenol, and the efficiency degradation will increase. The final concentration of phenol after a degradation process for 210 minutes reached 0.18 mg / L. This is in accordance with the Permen LH RI No. 5/2014, which states that the maximum phenol levels in wastewater allowed is 0.5 mg / L. The optimum operating process, such as pH, catalyst dose and time reaction applied under sunlight irradiation as the final stage of this research. Fig.8. shows that degradation product with halogen and sunlight irradiation for 210 minutes has been degraded by 96-97% from 5 mg/L to 0.11 -0.11 mg/L. It implies the photon energy generates a stronger oxidizing agent, •OH radical, which will assist in phenol compounds degradation.