Spent Bleaching Earth Supported CeFeO3 Perovskite for Visible Light Photocatalytic Oxidation of Methylene Blue

Introduction

Nowadays, the treatment of wastewater as a source of clean water is vital for society's lives, both from an economic and environmental perspective. Thus, making the development of the wastewater treatment process develop rapidly. One of the most effective methods today is the advanced oxidation process (AOP) [1]. The AOP process relies heavily on the production of highly reactive radical species such as hydroxyl radicals (OH●) which have high oxidation potential for the degradation of organic compounds selectively [2]. One process that is relatively attractive for researchers to study is photocatalytic for removing methylene blue (MB) from aqueous solution become less toxic substances, CO2, and H2O [3-5].

The solid waste generated in vegetable oil processing, such as spent bleaching earth (SBE), has properties and structures similar to zeolites, which has the main elements of Al2O3 and SiO2 [6]. The bleaching earth is widely used to purify crude oil from undesirable color and impurities [7]. SBE contains more than 20% of oil by weight; it considers hazardous substances and dangerous pollutants to the environment [8,9]. Disposing of SBE without proper handling is harmful to the environment due to the degradation of residual oil in SBE and associated with emissions of greenhouse gases in its dumping. The nature of SBE, which has a size of nano/micropores and also zeolite like composition. Therefore, SBE has potential usage after the reactivation process, such as adsorbents, filler, and support catalysis [10].

Over the past decade, the development of semiconductor catalysts for the process of photocatalysis has been increased in wastewater treatment technology. One catalyst that often used is TiO2 because this catalyst has several advantages, such as inertness, non-toxic, and high chemical stability. However, TiO2 also has a severe weakness, such as a high energy bandgap (3.0-3.2 eV), thus requiring UV radiation and inhibiting the effectiveness of TiO2 [11,12]. This deficiency is overcome by modifying the structure and chemical composition of semiconductor material by emerging new favorable properties. One of the impressive materials that have been widely examined is perovskite. Perovskite, with the general AMO3 formula, has cations A and/or M, which can be exchanged with foreign cations without changing their structure, but changing the oxidation status of the M cation and enter a new oxygen vacancy [13-15]. CeFeO3, which is a ferrite spinel nanoparticle material with a narrow bandgap (<1.9 eV), is suitable for photocatalysts. Besides, the stable, non-toxic and magnetic nature of CeFeO3 makes it easy to separate, making CeFeO3 suitable for use in wastewater treatment [16,17].

In this paper, we reported the application of low cost and eco-friendly novel semiconductor catalyst support from SBE, the waste of palm oil industry, for synthesizing CeFeO3/SBEe and CeFeO3/SBEc perovskite catalysts. The photocatalytic performances of both catalysts were compared under visible irradiation. Their physico-chemical properties were also characterized. Furthermore, some critical parameter such as initial pH and the concentration of MB solution was carefully considered in this study. The application of this supports in perovskite catalysts shows excellent performance in degrading MB from wastewater and has a great potential to develop further to be applied in the chemical industry.

Experimental Section SBE Support

SBE sample was obtained from a crude palm oil refinery plant in Riau Indonesia, which has contained ~ 20 % of oil. Before using it as a catalyst supports, two types of SBE were pretreated with two methods, namely solvents extracted and calcined. One SBE sample was extracted by hexane and collected from suspension by filtration, then desiccated in an oven at 120oC for 24 h and represented as SBEe. The other support was attained by calcined at 500 oC for three h and denoted as SBEc.

Preparation of CeFeO<sub id="sub-8b0f226bcc2f274e7b854031ce2471da">3</sub>/SBE Composite Catalyst

The preparation of the CeFeO3/SBEe perovskite catalyst was prepared using a modified method [18]. The reactant precursors of the compound are 2.20 g of Ce(NO3)3.6H2O (Sigma Aldrich), 2.02 g of Fe(NO3)3.9H2O (Merck), and 2.10 g of C6H8O7.H2O (Merck), while the solvent is 30 mL of mixed H2O/C2H5OH with the ratio of 1:2. Furthermore, the solution was stirred homogeneously at room temperature for 30 min. Subsequently, 2 g SBEe were added and stirred at 70oC until the gel was formed. Next, the gel dried at 100oC for 24 hours and calcined at 500oC for 6 h in air. Then the catalyst was put in storage in a desiccator while waiting for usage. The same procedure was also carried out for CeFeO3/SBEc.

fig-86d8e49462bbaeeb2e23902be05d4f7b Schematic of the preparation of CeFeO3/SBEe and CeFeO3/SBEc perovskite catalyst

Catalyst Characterization

Two materials were characterized by X-ray diffraction (XRD) analysis using SmartLab SC-70, with Cu-Ka Radiation of λ = 1.54059Å. Accelerating Voltage and current was 40 kV and 30 mA. The scanning rate was 0.01s-1 and 2θ range of 10-90. The morphologies of the catalyst were observed using the field scanning electron microscope (A JEOL JSM-6300F, USA). EDS, energy-dispersive X-ray spectroscopy were also utilized to detect metal particles on supported catalysts. N2 adsorption-desorption isotherm with the BET (NOVA 4200e, Quantachrome). A thermal gravimetric analyzer was used to collect the thermal stability of materials using a TGA/DSC1 STARe system-METTLER TOLEDO.

Photocatalyst Process

The catalytic photodegradation has taken place in a 500 ml beaker glass, in a thermo-controlled water bath, containing 10-40 mg/L of MB solution (250 mL). After that, as much as 0.25 g/L of the catalyst was added. The solution was stirred at 400 rpm in room temperature (30oC ± 2oC) and illuminated with a Mercury Lamp 250 W (Phillips). The top of the reactor was located at a distance of 25 cm away from the light source. The solution was a place in darkness for 20 min to reach absorption and adsorption equilibrium. Next, the photocatalytic process started when turning on the light. Every interval time, samples were taken using a 0.45 µm syringe filter from the reactor, and then were 1 mL of suspension liquid collected. For adjusting the pH of the solution, HCl and NaOH solution of 1N was utilization. The concentration of samples was analyzed using UV-Vis absorption Shimadzu 2600i. Analysis of chemical compositions of the collected SBE was carried out by an X-ray Fluorescence Spectroscopy (XRF, PW 2400, Philips).

 Results and Discussion Characterization of Catalysts

The composition of the collected spent bleaching earth (SBE) are listed in Table 1 . As can be observed, SBE mostly consist of silica and alumina with particular iron and calcium oxide.

table-wrap-316d06b8af78897589c775278eb70b52 Tabel 1. Composition of SBE

Composition SBE (wt%)
MgO 3.56
Al2O3 10.23
SiO2 61.51
K2O 1.12
CaO 4.23
TiO 0.64
FeO 4.32
L.O.I 5.40
pHa 5.10

apH was decided by blending 0.1 g solid with 10 mL water.

CeFeO3/SBEe perovskite catalyst was studied by TGA and DSC under the air atmosphere (Figure 2). The TGA patterns of catalyst show a substantial changed up to 53.5% of weight loss between 30 oC and 200 oC. It corresponds to the loss of water in the crystallization process and thermal transformation (decomposition) of organic, volatile compounds on precursor substances and a trace amount of oxygen. The exothermic peak occurred at 200 oC on the DSC curve due to the vaporization of the volatile component [18,19].

fig-75ecd88d7bc5f1e92d0d6db76f549cca TGA dan DSC profiles of CeFeO3/SBEe

The Following, at around 250 oC to 500 oC, weight loss of about 21.5% relates to the loss of oxygen from precursor resulting in the phase transformed into perovskite structural compounds CeFeO3, as shown in the following reaction.

\begin{equation} \begin{split} Ce\left ( NO_{3} \right )_{3}.6H_{2}O + Fe\left ( NO_{3} \right )_{3}.9H_{2}O + C_{6}H_{8}O_{7}.H_{2}O \\ \rightarrow CeFeO_{3} + 6CO_{2} + 2N_{2} + 2NO_{2} + 20H_{2}O \end{split} \tag{1} \end{equation}

Three of the exothermic peaks appeared between 250 oC and 850 oC (about 320, 420, 850 oC) due to the thermal transformation of citric acid and gradual crystallization of CeFeO3. Thus, the calcination temperature of 500 oC was chosen to synthesize CeFeO3 perovskite.

Figure 3 presents the XRD spectra of supported CeFeO3 perovskite catalysts, which are the utilization of two different treatments of supports. Overall, two modes of catalysts displayed a weak crystalline phase, as indicated by the observance of a relatively broad peak. CeFeO3 oxide peaks are observed with diffraction peaks occurred 21.04°, 26.24°, 28,81°, 33.41°, 39.98°, 48.30°, and 57.50°, corresponding to JCPDS standard 00-022-0166 for perovskite crystalline structures. Those XRD results show a successful synthesis of CeFeO3 perovskite from a sol-gel approach.

fig-32d6d2d53fea997950f56704fac9e33a XRD patterns of spent bleaching earth and their supported CeFeO3/SBEe and CeFeO3/SBEc perovskite catalyst

N2 adsorption/desorption of ​​the perovskite catalyst CeFeO3/SBEe and CeFeO3/SBEc, as shown in Figure 4. While the results of the analysis of the two catalysts are shown in Table 2. The SBET of the CeFeO3/SBEe perovskite catalyst has a higher 20 m2/g than CeFeO3/SBEc, this caused by differences in the treatment of the SBE. Moreover, both catalysts have a pore radius of less than 20 Å, indicating their microporous nature.

fig-f35d2a0f0601fdc5c713e646eb53a3ec N2 adsorption isotherm of ​​the perovskite catalyst CeFeO3/SBEe and CeFeO3/SBEc

table-wrap-012d901aea56cf3e33def6a2f0ee5a71 Tabel 2. BET Characterization of Catalyst

Catalist Type SBET (m2 g-1) Pore Volume (cm3 g-1) Pore Radius (Å)
CeFeO3/SBEe 59.44 0.133 19.114
CeFeO3/SBEc 39.17 0.129 19.175

Figure 5 and Figure 6 present SEM images, EDS spectrum, and mapping images of supported CeFeO3 catalysts. Most CeFeO3/SBEe and CeFeO3/SBEc perovskite catalysts exhibited agglomeration of the particles, and asymmetrical shapes with the average size of CeFeO3/SBEe particles are 1-30 μm, while CeFeO3/SBEc are 10-70 μm (Figure 5 a and Figure 6 a). It was apparent that the diameter of CeFeO3/SBEe particles was less then CeFeO3/SBEc particles. The EDS spectrum of two catalysts is shown in Figure 5 b and Figure 6 b. Both samples showed the attendance of Ce, Fe, Si, Al, Ca, and Mg. Consequently, the EDS spectra implied the presence of Ce and Fe on two catalysts, conforming XRD patterns. The elemental mapping images of the particle is shown in Figure 5 c, d, e, and Figure 6 c, d, and e, and it indicates the presence of two elements such as Ce, Fe. Both elements are equally distributed.

fig-69a999dd5d6a5c321393bcbe3d7dd274 Morphologies of the CeFeO3/SBEe perovskite catalyst. SEM images of samples (a), the inset demonstrates the particle magnification, (b) EDS spectrum, and (c, d, and e) the elemental mapping images

fig-be4dc008b8d60f37d55da337f859e47c Morphologies of the CeFeO3/SBEc perovskite catalyst. SEM images of samples (a), the inset demonstrates the particle magnification, (b) EDS spectrum, and (c, d, and e) the elemental mapping images

Degradation of MB by SBE Supported Cerium Orthoferrite Photocatalytic

Figure 7 displays the photocatalytic performance of catalysts in the removal of MB under visible light irradiations. In general, there is no MB degradation in photolysis using visible light minus the presence of the catalyst. Equally, catalysts showed about 40 % MB removal in dark conditions. UV-vis light could drastically degrade MB. It shows that the efficiency of photocatalytic degradation present of the CeFeO3/SBEe catalyst reached a peak of 99.5% for 120 minutes, while the CeFeO3/SBEc catalyst is reached 90.68% efficiency at the same time. It shows that the efficiency of CeFeO3/SBEe in photocatalytic activity is better than CeFeO3/SBEc. That was attributed to several factors, such as surface area, pore size, and the particle size of the catalyst [20]. CeFeO3/SBEe is demonstrated to have better efficiency than CeFeO3/SBEc. This is directly related to the SBET of ​​the catalyst used where CeFeO3/SBEe has a larger surface area than CeFeO3/SBEc. Specifically, CeFeO3/SBEe had a surface area of 59.44 m2/g while CeFeO3/SBEc had only 39.17 m2/g. The highest surface area will give the more active sites of the catalyst. Therefore that the formation of hydroxyl free radicals (OH•) is more due to the electron-hole recombination process on the surface of perovskite catalyst [21]. Also, smaller particle size tends to disperse more evenly onto the solution and lead to an increase in the activity of the photocatalytic process [22]. The formation of hydroxyl free radicals will direct an oxidation-reduction reaction (redox) of long carbon chains organic compounds (aromatic) (MB, C16) into smaller molecular products such as CO2 and H2O [23].

fig-d1d3a841a988c540102a1ccb48c6344a MB decreased with time in photocatalytic and photolysis. Reaction condition: [MB concentration] = 10 mg/L, [Catalyst concentration] = 1.0 g/L, [pH] = 7

The effects of Reaction Parameters on MB Degradation on CeFeO<sub id="sub-a47c48cccddb0764cc64b22d7e0439cd">3</sub>/SBEe

Owing to the high-activity of CeFeO3/SBEe, additional investigation on CeFeO3/SBEe was carried out to comprehend the effect of operational conditions. The stability and activity of the catalyst under acidic, neutral, and basic conditions, were investigated under various pH conditions. The effect of pH at 5, 7, and 9 on the photocatalytic degradation is presented in Figure 8. Overall, the pH of the Methylene Blue (MB) solution affects the photocatalytic degradation process due to the relationship between the stability of the catalyst nature and the pH solution [24]. At pH 5, MB was degraded 89.60% at 120 min, while at higher pH, which is 7.0 and 9.0, removal would be achieved at 99.5 and 94.45% in 120 min, respectively. Under acidic conditions, OH• radical ions produced by the catalysis process will react with H+ to produce H2O compounds, and reduce the efficiency of degradation. Under higher pH conditions, OH- ion might be more available to be converted to OH• radicals. Thus, it will increase the efficiency of MB decolorization. However, at high pH value, H2O2 produced from the water catalyzed process would be unstable and could be decomposed into H2O and O2 compounds, which cannot perform degradation activities [25-27]. This phenomenon can be seen at pH 9.0; there is a slight decrease in degradation activity compared at pH 7.0. Thus, the efficiency of BM removal toward the effect of pH followed the order of pH 7 > pH 9 > pH 5. The CeFeO3/SBEe perovskite catalyst has high activity at the pH range of 7-9. So, the catalyst that has been synthesized has good activity properties under various pH conditions [28].

fig-a9b493b40f57dceb91b5f1efe89200b8 Effect of pH on the photocatalytic degradation of MB. Reaction condition: [MB concentration] = 10 mg/L, [CeFeO3/SBEe catalyst concentration] = 1.0 g/L.

The effect of initial MB concentration at 10, 25, and 40 mg/L on MB removal is presented in Figure 9. In broad terms, the MB efficiency of degradation reduced with increasing MB concentration. At a high MB concentration, it means that more number molecules of MB are adsorbed on the surface of the CeFeO3/SBEe, and the necessity of reactive species such as OH• for the degradation of MB also increases, meanwhile the amount of catalyst remains constant [29]. Moreover, increasing the initial concentration of MB will inhibit the intensity of the light that will be absorbed by the catalyst [30]. Because the amount of catalyst used is the same, the rate of formation of hydroxyl radicals (OH •) at each concentration of methylene blue is also estimated to be the same. So it will require more time to reach the same degradation rate, therefore reducing MB deterioration in the efficiency [17].

fig-b3de44d17a185e05ecd76ba754ba275f Effect of MB concentration on the photocatalytic degradation of MB. Reaction condition: [CeFeO3/SBEe catalyst concentration] = 1.0 g/L, [pH] = 7

Conclusion

Spent bleaching earth (SBE) was inactive for photocatalytic degradation of MB. However, it could be used as effective support for the CeFeO3 perovskite catalyst. CeFeO3/SBEe perovskite catalyst demonstrated a high activity of MB removal in a visible light photocatalytic oxidation. CeFeO3 was found to be presented on the catalyst, and the dispersion was higher on treated SBE. CeFeO3/SBEe produced a higher efficiency in MB removal than CeFeO3/SBEc. The efficiency of MB removal toward the effect of pH followed the order of pH 7 > pH 9 > pH 5. The CeFeO3/SBEe catalyst can be chosen as a catalyst that is valuable for industrial applications for water purification.

 Acknowledgment

This work was supported by Universitas Riau, through the LPPM-Universitas Riau under the grand number of 1001/UN.19.5.1.3/PT2019. The authors are grateful to acknowledge this financial support.

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