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Synthesis of Titanium Dioxide/Silicon Dioxide from Beach Sand As Photocatalyst for Cr and Pb Remediation

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Synthesis of Titanium Dioxide/Silicon Dioxide from Beach Sand As Photocatalyst for Cr and Pb Remediation catalysts Article Synthesis of Titanium Dioxide/Silicon Dioxide from Beach Sand as Photocatalyst for Cr and Pb Remediation Diana Rakhmawaty Eddy 1,* , Soraya Nur Ishmah 1, Muhamad Diki Permana 1 and M. Lutfi Firdaus 2 1 Department of Chemistry, Faculty of Mathematics and Sciences, Universitas Padjadjaran, Jl. Raya Bandung-Sumedang km. 21 Jatinangor, Sumedang, Jawa Barat 45363, Indonesia; [email protected] (S.N.I.); [email protected] (M.D.P.) 2 Graduate School of Science Education, Bengkulu University, Jl. W.R. Supratman Kandang Limun, Bengkulu 38371, Indonesia; lutfi@unib.ac.id * Correspondence: [email protected]; Tel.: +62-8132-273-1173 Received: 30 September 2020; Accepted: 27 October 2020; Published: 29 October 2020 Abstract: Heavy metals are non-biodegradable and have a high toxicity effect on microorganisms which makes their presence in the environment extremely dangerous. The method of handling heavy metal waste by photocatalysis techniques using TiO2/SiO2 composite showed a good performance in reducing harmful pollutants. In this study, SiO2 from Bengkulu beach sand, Indonesia, was used as a support material for TiO2 photocatalyst to remove Cr(VI) and Pb(II). SiO2 was obtained through leaching techniques using NaOH as a solvent. The TiO2/SiO2 composite photocatalyst was synthesized using a solvothermal method at 130 ◦C and then characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and a particle size analyzer (PSA). Based on the XRD diffractogram, the synthesized TiO2 showed the anatase structure while the SiO2 showed the amorphous structure. The Ti–O–Si bond is defined in the infrared (IR) spectra, which indicates that the relationship between TiO2 and SiO2 is a chemical interaction. The results of SEM and PSA characterizations show agglomerated spherical (round) particles with a mean particle size of 616.9 nm. The TiO2/SiO2 composite of 7:1 ratio showed the highest photocatalytic activity after 180 min of ultraviolet (UV) irradiation, with a concentration-decrease percentage of 93.77% and 93.55% for Cr(VI) and Pb(II), respectively. Keywords: photocatalyst; TiO2/SiO2 composite; solvothermal; silica sand 1. Introduction The rapid growth of technology and manufacturing results in an increase in waste pollution to the environment. Heavy metals from industrial waste are one of contributors to water pollution [1]. These toxic metals cannot be broken down by microorganisms, and when accumulated in the body cause severe illness and even death [2]. Chromium or Cr is a metal that is commonly found in the environment because it is widely used in large industries such as metal coating, petrochemicals, mining, fertilizer, tanning leather, batteries, pesticides and the paper industry [2]. Chromium commonly comes in two forms, trivalent chromium Cr(III) and hexavalent chromium Cr(VI). Chromium in hexavalent form is 500 times more carcinogenic and very toxic compared to its trivalent form because it penetrates cell membranes and causes harmful effects. The maximum acceptable environmental threshold values for Cr(III) and Cr(VI) are 5 mg/L and 0.05 mg/L, respectively [1]. Besides chromium, water pollution due to the presence of lead is another global concern. Lead or Pb(II) is among the dangerous contaminants found in industrial waste. This compound causes Catalysts 2020, 10, 1248; doi:10.3390/catal10111248 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1248 2 of 11 mutations and is carcinogenic. Besides human activities that produce Pb, it is also naturally present in water. Pb undergoes crystallization in the air and is subsequently dissolved in rainwater that enters water bodies such as rivers and lakes [3]. Indonesian government regulations stipulate that the maximum acceptable of Pb level for conventional drinking water treatment is 0.1 mg/L[4]. ≤ Several methods for removing heavy metals from the environment have been actively investigated through various techniques such as adsorption, cross-flow microfiltration, precipitation and reverse osmosis [1,2]. However, these methods are relatively expensive and often inefficient when working at low concentrations. The method of photocatalysis using semiconductor material has the potential to reduce harmful pollutants and is also effective when working at low concentrations of Cr [5] and Pb [6]. Titanium dioxide (TiO2) is a semiconductor material widely used as a photocatalyst in the purifying wastewater process because it has many advantages such as high photocatalytic activity, high chemical stability when exposed to acidic and basic compounds, high oxidizing power, and low toxicity and it is easy to obtain [7]. Anatase is a form of photocatalyst claimed to be the best for photocatalytic activity, but its transformation into rutile is a weakness that limits its photocatalytic activity. Several studies have been conducted to overcome this problem by modifying the surface of TiO2 and synthesizing composite catalysts such as TiO2/SiO2, TiO2/ZrO2, TiO2/A12O3/SiO2, and TiO2/ZrO2/A12O3 [8]. Furthermore, the modification of TiO2 using SiO2 material to improve photocatalytic activity shows a high thermal stability and mechanical strength, and increases the active side of the photocatalyst surface. Another advantage of TiO2/SiO2 mixed oxides is that it inhibits agglomeration of TiO2 and transformation from anatase to rutile [7]. Silica is obtained from both organic and inorganic natural materials, by synthesis and extraction. Based on the types of natural materials that are non-biological, silica is obtained from sand and coal waste such as flying ash sludge. Beach sand is one of sources of natural silica which is very abundant. Eddy et al. [9] extracted silica successfully from Palangkaraya beach sand, Indonesia with a benefit of 91.19% and Munasir et al. [10] extracted silica successfully from Tuban beach sand, Indonesia, reaching 98.9% gain. In addition, Ishmah et al. [11] and Firdaus et al. [12] have succeeded in extracting silica from Bengkulu beach sand, Indonesia, by 97.3% and 99.5%, respectively. In this research, we evaluated the extraction of silicon dioxide (SiO2) from beach sand precursors and subsequently composite with titanium dioxide (TiO2) as a photocatalyst for decreasing the concentrations of Cr(VI) and Pb(II). 2. Results and Discussion 2.1. Characterization of Catalyst The solvothermal method is widely used for the manufacture of oxides and is usually performed at temperatures below 400 ◦C. The basic principle of this method is the growth of crystals based on material solubility in solvents under high-pressure conditions. Medium temperature conditions are generally used to increase chemical diffusion while high pressure results in lesser energy consumption compared to temperature. Calcination temperature in the synthesis process has a significant impact on the TiO2 crystallinity phase, as it also affects the band gap energy value, which has a significant effect on photocatalytic performance. The anatase TiO2 phase is the best TiO2 crystal phase to be used in the photocatalytic process with 3.23 eV band gap energy. Based on research by [13], structures with optimal anatase phases are formed at temperatures of 400–600 ◦C. At temperatures above 600 ◦C, i.e., at 700 ◦C, the anatase structure phase begins to be transformed into rutile, the surface area decreases, and the photocatalyst activity is drastically weakened [14]. X-ray diffraction (XRD) analysis of TiO2/SiO2 composites was carried out to determine the characteristics of the TiO2 formed. The resulting diffractogram showed conformity to Inorganic Crystal Structure Database (ICSD) 98-015-5245 for amorphous SiO2 structures (tetragonal, space group P41/212), ICSD 98-017-2916 for TiO2 anatase structures (tetragonal, space group I41/amd) and ICSD 98-016-7961 for TiO2 rutile structures (tetragonal, space group P42/mnm). The TiO2 P25 Degussa crystal showed a peak pattern with the peaks at 2θ = 25.3◦, 37.8◦, 48.0◦, 53.9◦, 55.0◦, 62.7◦, 68.8◦, 70.3◦, and 75.1◦ that Catalysts 2020, 10, 1248 3 of 11 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 12 andmight 75.1° belong that tomight (011), belong (004), (020),to (011), (015), (004), (121), (020), (024), (015), (116), (121), (220), (024), and (125)(116), crystal(220), and planes (125) of anatasecrystal planes of anatase phase respectively. Moreover, the peaks at 27.4° and 36.1° were ascribed to the (110) phase respectively. Moreover, the peaks at 27.4◦ and 36.1◦ were ascribed to the (110) and (011) planes and (011) planes of rutile phase. For TiO2/SiO2 composites there was an additional peak at 24.1° that of rutile phase. For TiO2/SiO2 composites there was an additional peak at 24.1◦ that was ascribed to was ascribed to the (011) planes of amorphous SiO2 phase. Figure 1 shows that the composition of the (011) planes of amorphous SiO2 phase. Figure1 shows that the composition of TiO 2:SiO2 affects TiO2:SiO2 affects the peak intensity and the sharpness of the peak produced, while the higher the the peak intensity and the sharpness of the peak produced, while the higher the content of TiO2 causes contentthe sharpness of TiO2 of causes the peak the intensity.sharpness of the peak intensity. (a) (0 1 1) Anatase Rutile Amorphous SiO2 (0 2 0) 2 (0 (0 0 (0 4) (1 1 0) 1 (1 (0 1 5) 1 (0 1) 2 (1 (0 2 (0 4) (0 1 1) 1 (0 (1 2 (1 5) (1 1 (1 6) 2 (2 0) (b) (0 1 1) (0 1 1) 1 (0 (0 0 (0 4) 2 (0 0) (0 1 5) 1 (0 1) 2 (1 (0 1 1) 1 (0 (0 2 (0 4) 2 (2 0) (1 1 (1 6) (1 2 (1 5) (c) (0 1 1) (0 1 (0 1) (0 2 (0 0) (0 0 (0 4) (1 2 1) 2 (1 Intensity (arb.u.) Intensity (0 2 (0 4) (0 1 (0 5) (1 2 (1 5) (0 1 (0 1) (2 2 0) 2 (2 (1 1 6) 1 (1 (d) (0 1 1) (0 1 1) (0 (0 2 (0 0) (0 0 4) (0 (0 1 (0 5) (1 2 (1 1) (1 2 5) 2 (1 (0 1 1) 1 (0 (0 2 4) 2 (0 (1 1 (1 6) (2 2 (2 0) 20 30 40 50 60 70 80 2 theta (o) Figure 1.
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