The Use of TiO2-SiO2 in Photocatalytic Process to Degrade Toxic and Dangerous Waste

This study was conducted to investigate the use of TiO2 immobilized on SiO2 (TiO2-SiO2) in a photocatalytic process to degrade toxic industrial waste, phenol, linear alky lbenzene sulfonate (LAS), and Cr(VI), which is dang erous for humans and the environment. Titanium dioxide (TiO 2), as a photocatalyst, can make the solution become turbid. Thus, TiO2SiO2 was used to increase the possibility of ultraviole t (UV) transmission. The phenol and LAS levels were m asured with the Indonesian National Standard (INS) while t h Cr(VI) level was determined with the colorimetri c method. The activity test for the catalyst in suspension and im mobilization against phenol showed that TiO 2-SiO2 was more active than TiO2. By using the photocatalytic process with the TiO 2-SiO2 photocatalyst for 8 h, degradation of phenol and L AS reached 50% as a single compound and 12% as a mixtu re. However, TiO2-SiO2 did not decrease Cr(VI).


Introduction
Photocatalysis is a technology for treating toxic and dangerous waste. Photocatalysis, an effective and inexpensive process, is a combination of a photochemical process and catalysis. The photocatalytic process treats wastewater and removes toxic and dangerous substances [1]. Tjahjanto and Gunlazuardi [2] and Kajitvichyanukul et al. [3] observed that catalyst immobilization in the substrate could increase photocatalytic efficiency. The photocatalytic process is inefficient if TiO 2 is used in powder form. This form disperses homogenously in water and produces milky dispersion. Therefore, the catalyst is difficult to recover. To measure the residue from the photocatalytic process, the solution must be clear. Ultraviolet light cannot activate a catalyst in a turbid solution. Consequently, catalyst immobilization in a substrate, in granule form, should produce a clear, easy-to-handle solution.
When the light (hv) from photon light interacts with a catalyst particle, electrons and holes form according to the reaction TiO 2 + hv → e -+ h + . Electrons exist in the conduction band, and the hole is in the valence band. Holes produced by TiO 2 are very active and oxidize many compounds. Electrons in the presence of oxygen can also produce an active form of oxygen that runs in the reduction process. Joshi and Shrivastava [4] stated that the charges can react directly with adsorbed pollutants, but reactions with water are more likely since the water molecules are far more numerous than contaminant molecules. Oxidation of water or OHby the hole produces the hydroxyl radical (OH*), an extremely powerful oxidant ( Figure 1).
In this study, a photocatalytic reactor was designed in which the type of catalyst used in the degradation process varied. Thiruvenkatachari et al. [5] showed that the photocatalytic process was directed by the catalyst, source of light, and reactor configuration. Construction of Photocatalytic Reactor. The photocatalytic reactor was made in the batch system. Six 10 W lamps with ultraviolet black light as the photon source were place around the tube. The radiation process occurred in a closed system (the wall of the reservoir was made from stainless steel) and equipped with a blower. The sample tube in the center was made from quartz; the distance between the tube and the lamp was 5 cm. The tube was placed on the magnetic stirrer plate ( Figure 2).
Catalyst Preparation. The catalyst was prepared by dip coating TiO 2 into SiO 2 thermally [6,7]. First, Degussa TiO 2 P25 was added by aquades, then sonicated for 20 m. Second, SiO 2 was mixed to TiO 2 solution, sonicated again for 20 m. Third, the mixture was heated in 100 o C for 2 h. The last step, dried TiO 2 -SiO 2 was furnaced in 500 o C for 5 h, followed by washing the TiO 2 -SiO 2 using aquades. The ratio of TiO 2 :SiO 2 were 1.6, 2.4, and 3.2 % (w/w), respectively.

Determination of Catalyst Activity-Preliminary
Experiment. Catalyst TiO 2 (as positive control), TiO 2 -SiO 2 (three types of composition), SiO 2 (as negative control) with 1 g/L loaded inside the designed reactor were tested to degrade 100 mg/L phenol [7]. The photocatalytic process was carried out for 3 h, and then the concentrations in the initial and final processes were measured. The data were compared, and a t-test was calculated to find the significance of the photocatalytic process for each catalyst. The most active catalyst was then applied in the following process and was characterized by using XRD and SEM.

Figure 2. The Photocatalytic Reactor Measurements of the Use of TiO 2 -SiO 2 in the Photocatalytic Process
Samples that contained phenol, LAS, Cr(VI), or a mixture of the three (phenol-LAS-Cr(VI)) were degraded and measured. The previously prepared catalyst was placed in the sample tube and stirred under UV radiation. The most active catalyst was applied in this process. The initial and final concentrations of the samples were measured each hour for 8 h.
Determination of Cr(VI). Cr(VI) was measured with the colorimetric method. The λ maximum of the potassium dichromate solution was measured using the UV-Vis spectrophometer. Then, the absorbance of the Cr(IV) in the samples was measured in the λ [3].
Parametric Tests. The following parameters were analyzed: limit of detection, precision, accuracy, and linearity range [10].

Results and Discussion
The sample tube was made of quartz glass. Quartz glass transmits UV light. Six 10W UV-A lamps were used as the photon source and placed around the tube [11]. The wavelength of the lamps ranged from 315 to 400 nm, whereas the peak appeared at 352 or 368 nm with E bg 3.37 and 3.52eV.
The irradiation process was carried out in a closed system (enclosed in containers made of stainless steel).
The closed system kept the radiation inside the reactor, to make catalyst absorb the maximum UV light. Irradiation for 8 h increases the system temperature. Therefore, the reactor was equipped with a blower. The catalysts were put in a sample tube that contained the sample solution and constantly stirred to produce a homogenous catalyst system.
Immobilization of TiO 2 on SiO 2 followed the slurry precipitation method/dip coating method [5] with a specific composition. The catalyst was mixed with silica gel, and then the mixture were sonicated. After the water evaporated, calcination was carried out at 500 °C for 5 h. Based on the literature, at higher temperature, rutile is formed and decreases the catalyst's activity [2]. Figure 3 shows that the photocatalytic process decreased 2.1% of the phenol within 3 h by using silica gel (SiO 2 ). The decrease was due to the capacity of the silica gel-adsorbing analyte. The TiO 2 catalyst shows the highest percentage of phenol decomposition of the catalysts. The decomposition was caused by the photocatalyst activity activated by UV light and followed by the oxidation process.
For the three types of prepared catalysts, the ability to degrade phenol was about 10-12%. Adding TiO 2 increased phenol degradation. Although the TiO 2 catalyst gave the highest result for the percentage of decomposition of phenol, TiO 2 -SiO 2 also worked well. A small amount of TiO 2 on SiO 2 can reduce 10% of phenol, a fourth than 100% TiO 2 (reduce 36.6% of phenol). This study showed that 100% TiO 2 was not entirely activated by photons. This shows that turbid solution prevents photons from activating the catalyst.
In the phenol decomposition for SiO 2 , 1.6% TiO 2 -SiO 2 , 2.4% TiO 2 -SiO 2 , and 3.2%TiO 2 -SiO 2 differed statistically from the TiO 2 catalyst (t-test). These data indicated that 100% TiO 2 gave the best result for phenol decomposition. In addition, phenol decomposition produced by the three catalysts did not differ significantly. Thus, the ability of the catalysts was similar. Catalytic decomposition of phenol by immobilized catalysts were significantly different from that SiO 2 . These results show that TiO 2 , not SiO 2 , has the main role in the photocatalytic process.
At the end of the procedure for preparing the catalyst, the immobilized catalyst was washed with aquadest. The filtrate that contained TiO 2 was weighed quantitatively, while the residue, TiO 2 -SiO 2 , was used for the degradation process. Based on the data shown in Table 1, to test the use of TiO 2 -SiO 2 in the photocatalytic process in the reactor, 2.4% TiO 2 -SiO 2 was used.
The rutile crystal on TiO 2 Degussa P25 disappeared in the immobilized catalyst due to the calcination process. Then a stable anatase crystalline phase appeared in these conditions. According to Tjahjanto and Gunlazuardi [2], this situation indicates that calcination techniques can change amorphous TiO 2 to become crystalline anatase, not rutile crystals [3]. Figure 5 shows that morphology of the pure TiO 2 catalyst are nano-sized particles that have a wide surface. The size of the SiO 2 particles is not uniform in the profile obtained. The existence of immobilized TiO 2 on SiO 2 is obvious, even in small quantities.
Photocatalytic degradation of organic materials, namely, phenol and LAS, as well as inorganic materials, that is, metal ions Cr(VI) in aqueous solution as a model of industrial waste samples, was performed. Before being used to degrade the mixture samples, the photocatalytic process was used to process each type of waste (phenol, LAS, or Cr(VI) individually).
The measurement results for TiO 2 -SiO 2 in 100 mg/L phenol oxidation either alone or in a mixture in the designed reactor are shown in Figure 6. In the figure, the phenol concentration tends to decrease the longer photocatalytic process lasts. The decrease in the phenol concentration for a single sample was better than that for simultaneous samples (merging with LAS and chromate), 19% and 9%, respectively.
In addition to phenol oxidation, the use of TiO 2 -SiO 2 in the photocatalytic process was tested to degrade LAS. Immobilized TiO 2 on the SiO 2 catalyst was used for oxidizing 100 mg/L LAS either alone or in a mixture. The result is shown in Figure 7. In the figure, the LAS concentration continues to decline the longer photocatalytic duration process lasts. The decrease in the LAS concentration for single samples was better than that for simultaneous processing (merging with phenol and chromate), 52% and 12.5%, respectively.   Phenol and LAS are organic materials and thus have similar properties. The phenomenon of the degradation process through the oxidation reaction is initiated by a hole (h+) as shown in Figure 1. The results of the phenol and LAS processing (Figures 6 and 7) can be explained as the relation of the catalyst mass versus the sample concentration. The larger the sample concentration, the larger the catalyst mass needed. Therefore, C/Co of the degradation process for a mixed sample for 8 h is proportional to a single sample for 4 h.
The sample concentration affects the ability of TiO 2 -SiO 2 during the photocatalytic process, as reported by Slamet et al. [12].
The role of reduction of TiO 2 -SiO 2 was not comparable to its oxidation ability. Slamet and Daryanto [13] observed that TiO 2 catalysts reduced metals in Cr(VI) and oxidized phenolic compounds. In contrast, in this study, TiO 2 -SiO 2 did not reduce Cr(VI). Figure 8 shows that by increasing the duration of the photocatalytic processes, the Cr(VI) concentrations in solution remained constant.
In addition, when TiO 2 -SiO 2 could not reduce Cr(VI), Cr(VI) degradation when pure TiO 2 was used. The result shows that 40 mg/L bichromate solution was reduced up to 53.6% for 4 h. Joshi and Shrivastava [4] eliminated the toxic hexavalent form of Cr(VI) from wastewater, using TiO 2 , ZnO, and CdS. They concluded that photocatalysis is a promising technique for removing heavy metals from industrial effluents. In addition, Qiu et al. [14] proved that Cr(VI) and phenol could be degraded simultaneously by using TiO 2 induced with visible light.
Therefore, TiO 2 -SiO 2 cannot reduce Cr(VI). This result might be caused by specific properties of the catalyst in metal reduction. Additional research on TiO 2 -SiO 2 in decreasing other metals is required.
The analytical parameters used to measure phenol, LAS, and K 2 Cr 2 O 7 at the 95% confidence level are shown in Table 2. Table 2 shows the data for each measurement. The chosen standard concentration fulfilled the range of curve linearity. The precision and accuracy were acceptable. The limit of detection for phenol, LAS, and  Cr(VI) determination was 0.038, 0.237, and 4.036, respectively. The sample concentration was higher than the limit of detection. To sum up, the measurement methods were verified in this research.
Titanium dioxide immobilized on silica gel can be used in a photocatalytic process to degrade toxic and dangerous waste. Titanium dioxide can reduce phenol and LAS levels using the proposed reactor. Degradation reached 50% and 12% alone and in the mixture, respectively. The catalyst cannot be used to degrade Cr(VI).