Degradation of Semiconductor Manufacturing Wastewater by Using a Novel Magnetic Composite TiO 2 / Fe 3 O 4 Photoreactor Design

The purpose of this research is to develop a photocatalytic TiO2 that can be activated by visible light and can be conveniently recollected for reusing. This research synthesizes the 20 to 40 nm TiO2/Fe3O4 particles with magnetization of 5.8 emu/g using the modified sol-gel method followed by 500◦C calcinations. The experiment verified that visible fluorescent light (VFL, contains no UV-A) could activate the photocatalytic activity of TiO2/Fe3O4 particles as did ultraviolet A light (UV-A, 360 nm). Regular magnets can be used to separate TiO2/Fe3O4 particles from solution. The results indicate that VFL-sirradiated TiO2/Fe3O4 particles could decompose isopropanol (IPA) in the absence of UV-A and the issue of TiO2/Fe3O4 recollection from water for reusing is also resolved.


Introduction
In the typical semiconductor manufacturing processes, a large amount of IPA is used for wafer cleaning.The wafer cleaning processes invariably generate a large quantity of wastewater that contains refractory and complex organic compounds.Not only do these organic compounds pose direct or indirect harm to the liver, kidney, central nervous system, and skin, some of them have already been verified as carcinogen, teratogenic agent, and gene mutagen for humans.Traditional wastewater treatment approaches involving physical-chemical decomposition or microorganisms have shown only limited efficiencies [1].Many advanced oxidation processes (AOPs) have been extensively utilized for the decomposition of hazardous or recalcitrant pollutants in the environment.In recent years, TiO 2 has attracted much attention because of the low price and steady and high photocatalytic activation [1][2][3][4].However, requirement of using UV light to activate TiO 2 photocatalyst and difficultly to recover the photocatalyst are still major limitationly for degradation of environmental pollutants [5,6].There are many types of conventional methods to modify quality of photocatalyst.One of them is TiO 2 photocatalyst doped by the transition metal [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].It makes the defect of the structure of TiO 2 and produces the lower energy gap.Therefore, just less energy reacts with O 2 and H 2 O in the environment to form the excited electron and holes.As a consequence, the absorbed spectrum of TiO 2 can shift into the region of visible light.V, Fe, Cr, Ni, Cu, and other transition metals are well-known to be doped into TiO 2 thin film to shift the band gap to visible light.Producing visible-light-activated TiO 2 powder for easy recovery can be beneficial.Iron enhances TiO 2 visible light absorption and thus increases its photocatalytic activity.Previous researches have reported that iron-based nanoparticles have been superparamagnetic [1,18,23,24] and have been coated with organic polymers such as porphyrin and other dyes [6,25,26] or other inorganic elements such as N and H [27][28][29] and other methods [29,30].This research uses sol-gel method to add Fe 3 O 4 during TiO 2 synthesis and then uses visiblelight-irradiated TiO 2 /Fe 3 O 4 to conduct photodegradation effect on IPA.The photodegradation efficiency of IPA and

Preparation of Photocatalyst.
In this study, the TiO 2 / Fe 3 O 4 complex was prepared using the modified sol-gel method [4].The sol-gel process is one of the versatile methods to prepare high-pure and nanosize materials shaped as powders or coatings.This approach does not need complicated instruments such as chemical vapor deposition.It provides a simple and easy method to synthesize nanosize photocatalysts.First, soluble TiO 2 precursor powder was prepared using a mixture of tetraisopropyl orthotitanate (Ti(OC 3 H 7 ) 4 ) (TTIP, >98%, Merck Co.), 2-propanol (>99.7%,JT.Baker), and acetic acid (>99.9%,JT.Baker).Then, the mixed solutions were stirred for 24 h and dried at 105 • C followed by 150 • C for 1 h, respectively.The dried gel was then pulverized into powder and calcinated in the high-temperature oven.The parameters employed to control the preparation of the catalyst include 2-propanol, acetic acid (TTIP was fixed), calcinations time, and calcinations temperature.The orthogonal arrays in the design of the experimental method proposed by Taguchi were adopted to conduct the multiple-factor experiment.Second, the above-prepared TiO 2 powder was directly mixed with iron oxide (Fe 3 O 4 , >98%, Aldrich) and Fe 3 O 4 /TiO 2 at the designed molar ratio of 0.1.Then, the mixed solutions were stirred for 1 h and dried at 105 • C for 1 h.The conversion rate of IPA in this experiment was employed to identify the optimal conditions for assembly.The reactor containing TiO 2 /Fe 3 O 4 complexes was irradiated with 419 nm fluorescent lamps to undergo the batch photocatalytic degradation experiment.

Photoelectrocatalytic
Procedure.The liquid-phase photoelectrocatalytic system consisted of 16 VFLs (Rayonet Model RPR 100 Photochemistry Reactor, the Southern New England Ultraviolet Company, fluorescent lamps λ max = 419 nm, intensity = 724 lx), a completely mixing reactor, and an air compressor for aeration and stirring use, and a peristaltic pump was used to draw sample into the reactor for the continuous refluxing batch experiments and the flow rate controlled at 300 mL min −1 .The apparatus is illustrated in Figure 1.The experiment used the batch reaction system with the aqueous sample of 850 mL, and the controlled parameters were pH values and the amount of photocatalyst.A gas chromatography equipped with flame ionization detector (GC-FID; Agilent Co., Model HP6890 Series) and chromatography column (60 m × 0.32 mm × 0.5 μm, Agilent HP-INNOWax) were employed to determine the concentration of IPA.Total organic carbon instruments (TOC; Shimadzu, Japan, Model TOC-VCSN) were employed for the analysis of the experiment to investigate the variations of the residual rates and the mineralization rates under the parameters described above.Besides, all the other chemicals were of either analytical or reagent grade.The deionized water (≥18 MΩcm −1 ) used for preparing the chemical solutions was processed by the Milli-Ultrapure R/O System (Millipore, USA) to ensure the quality of the prepared solution.

Optimal Preparation of Photocatalysts Using Taguchi
Method.The controlling factors, which include 2-propanol (0.08, 0.16, and 0.32 mol), acetic acid (0.08, 0.16, and 0.32 mol), calcinations temperature (400, 500, and 600 • C), and calcinations time (90, 120, and 180 min), were chosen according to the work of Cheng et al. [4].The orthogonal arrays in the experimental design proposed by Zhang [31] were adopted to conduct the multiple-factor experiment and obtain the best combination of conditions for experimental photocatalytic conversion of IPA.The four parameters of various levels yield 9 different combinations as summarized in the L 9 orthogonal array table.Different photocatalysts were prepared according to different combinations of controlling factors for conducting the heterogeneous photocatalytic reactions to obtain the conversion rates of IPA.The IPA conversion rates thus obtained were then statistically analyzed by the F test to determine the significance and variability of each factor.The result showed that, among the factors, calcinations temperature had the greatest effect on conversion rate of IPA in the photocatalysis system (42.06%),followed by calcinations time (27.14%), the amount of 2propanol (14.86%), and, finally, the amount of acetic acid (14.67%).In general, the effect of calcination temperature was more significant due to its function in removing impurities and increasing crystal strength.Table 1 shows the contributions and major effects of different parameters.The optimal preparation condition of TiO 2 catalyst was achieved under water 2-propanol of 0.08 mol, acetic acid of 0.32 mol (i.e., TTIP/2-proanol/acetic acid with a molar ratio of 1 : 2 : 8), calcinations temperature of 500 • C, and calcinations time of 90 min.The TiO 2 was prepared at the above optimal experimental conditions.Then, the above prepared TiO 2 powder was directly mixed with Fe 3 O 4 and Fe 3 O 4 /TiO 2 at the designed molar ratio of 0.1, and the mixed solutions were stirred for 1 h and dried at 105 • C for 1 h.The solid was calcinated in an atmospheric oven heated to 200 • C at

Properties Analysis of Photocatalysts.
The photocatalyst prepared by the modified sol-gel method under the optimal conditions described above and the magnetism was tested and is shown in Figures 2, 3 6, and they clearly displayed that the particle was little irregular and the size was about 20-40 nm.Anatase TiO 2 containing Fe 3 O 4 was obtained and analyzed by the X-ray diffractometer (XRD) and compared with the JCPDS database (no.84-1286 and 89-4319) (Figure 7).The X-ray diffractometer (XRD, MAC MXTIII) was used to examine the crystals form.

Adsorption Effect.
In this study, the adsorption ability of the TiO 2 /Fe 3 O 4 composite catalyst has been studied at varied pH (4, 7, and 10) with 30 mg L −1 IPA without any irradiation at temperature of 25 • C in the reaction chamber for 240 minutes.The adsorption ability for the TiO 2 /Fe 3 O 4 composite catalyst was weak under alkaline and neutral conditions but not significantly different.Because all of the adsorption rates were less than 1%, the effect of   From the experimental data, we could find and understand that the TiO 2 and TiO 2 /Fe 3 O 4 composites had good performance under UV irradiation because UV light was with higher energy, but this experiment verified that visible fluorescent light could activate the photocatalytic activity of TiO 2 /Fe 3 O 4 particles as did ultraviolet A light.The result showed that the TiO 2 /Fe 3 O 4 composite catalyst also had good destruction rate for IPA under visible light, and this evidence revealed that the system of the TiO 2 /Fe 3 O 4 composite catalyst could be used to treat organic pollutants in practical wastewater treatment factories.were obvious variations in the photocatalytic reactions.In general, the yield of hydroxyl radicals in the photocatalytic reaction increases with increasing pH of the aqueous solution.However, the distribution ratios of the intermediates produced in the solution were directly related to the pH values.Moreover, the pH value alters the electrical behavior on the surface of the photocatalyst leading to changes in the absorption and desorption properties and capabilities of  the intermediates produced by the catalyst.Therefore, the control of pH during the operation had significant impact on the efficiency of the overall treatment.
The pH zpc value of TiO 2 is 6.4.Therefore, the TiO 2 surface carries a positive charge at pH below 6.4.However, more and more oxygen ions dissociate from the TiO 2 surface at pH above 6.4,causing a negative charge to develop on the catalyst surface.The substances formerly attached to the catalyst surface as a result of electrostatic adsorption begin to leave the catalyst under the influence of electric repulsion between the substances and the catalyst, thus lowering the oxidation/reduction rate of the reactants.Therefore, the ionic reactants are more affected by the pH of the environment due to electrostatic interaction.Figure 9  be seen, the removal rate of IPA was higher under basic conditions, and the result coincided with previous studies [32].

Effect of Addition of Photocatalyst.
The objective of the present study was to identify the optimal quantity of the TiO 2 /Fe 3 O 4 composite catalyst added according to economic consideration.This part of the experiment was carried out under the following conditions: initial IPA concentration of 30 mg L −1 , pH 7, temperature of 25 • C, and VFL for 240 min.The variations in DRE thus obtained are illustrated in Figure 10.As can be seen, when the photocatalyst added was increased from 1 g to 5 g, the DRE was improved from 46% to 88%.However, when the amount of the TiO 2 /Fe 3 O 4 composite catalyst added was increased from 5 g to 10 g, the DRE was reduced.It was speculated that excessive amount of the TiO 2 /Fe 3 O 4 photocatalyst added to the solution caused shielding of the light.Thus, both the light path and amount of TiO 2 /Fe 3 O 4 photocatalyst available for visible light irradiation were reduced, resulting in a decrease in overall treatment efficiency.At the same time, the recovery rate of photocatalyst could reach to 99.8% as we recollected easily from recycled water using magnet (shown in Figure 11).It is also illustrated that the system of TiO 2 /Fe 3 O 4 has significant efficiency of photodegradation under visible light irradiation and can be used and recycled in the practical wastewater treatment factories in the future.

Conclusions
Various dopants have been used for TiO 2 to absorb visible light.Fe 3 O 4 was chosen and added during the preparation of TiO 2 photocatalystsusing sol-gel method that made TiO 2 photocatalytic activity be activated by visible light.In this study, the synthesized TiO 2 /Fe 3 O 4 can be activated by irradiation of UV-A and VFL.No significant photocatalytic activity was lost after being recollected from water using magnet for reusing.

, and 4 .
As can be seen, the pure Fe 3 O 4 particles and TiO 2 /Fe 3 O 4 composite catalyst also had good magenetism.The magnetization curve of TiO 2 /Fe 3 O 4 composite catalyst was measured with Superconducting Quantum Interference Device Magnetometer (SQUID; SQUID-PMS5, Quantum Design Corp.), and Figure 5 illustrates the saturation magnetization intensity of TiO 2 /Fe 3 O 4 composite catalyst was 5.8 emu/g.The field-emission scanning electron micrographs (FE-SEM, JSM-6700F, JEOL, Japan) of TiO 2 /Fe 3 O 4 composite catalyst were shown in Figure

3. 4 .
Photoelectrocatalytic Tests.Because the photolytic efficiency of UV irradiation treated with IPA was less than 2% in 3 replicates test, therefore the part experiment was not taken into comparing in the subsequent experiments.The photoactivity indicator of the TiO 2 /Fe 3 O 4 composite catalyst was investigated by the conversion rate of IPA.The 2.0 g TiO 2 /Fe 3 O 4 composite catalyst was used to react with 30 mg L −1 IPA under 3.0 mW cm −2 , 365 nm UV light (UV-A), 724 lx VFL at pH 7 for 240 min.The results are illustrated in Figure 8, and the destruction removal efficiency (DRE) of the TiO 2 /Fe 3 O 4 composite catalyst on the conversion of IPA was higher than the TiO 2 catalyst alone.Evidence reveals that the system of the TiO 2 /Fe 3 O 4 complexes has significantly high efficiency of photocatalytic degradation than TiO 2 only.

3. 5 .
pH Effect.Although there was no significant variation of adsorption experiments under different pH values, there field B a ≡ μ 0 H(T) 300 K Fe 3 O 4 mass: 172.35 mg

Table 1 :
Response of parameters in L 9 table.
TiO 2 /Fe 3 O 4 also demonstrated the DRE of IPA under VFL and resolved TiO 2 /Fe 3 O 4 recovery problem when it was used in the practical wastewater treatment factories.In additional, The prepared TiO 2 /Fe 3 O 4 photocatalyst was identified as anatase, and particle size was about 20∼40 nm formed by XRD and FE-SEM.In the photodegradation experiments, the results indicated that 40∼88% DRE reached at different amounts of TiO 2 /Fe 3 O 4 , and the best addition amount was 5 g TiO 2 /Fe 3 O 4 under pH 10.