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Ind. Eng. Chem. Res. 2007, 46, 6566-6571
SEPARATIONS Photocatalytic Treatment of Wastewater Contaminated with Organic Waste and Copper Ions from the Semiconductor Industry Shuai-Wen Zou, Choon-Wai How, and J. Paul Chen* DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Simultaneous decontamination of organic compounds and copper ions in the wastewater discharged from semiconductor manufacturing facilities by a UV/TiO2 photocatalytic degradation was investigated in this study. Two organic compounds of ethyl lactate and phenol and copper ions were studied due to their common applications in various wafer fabrication processes and higher toxicities. The optimal TiO2 dosage and initial pH for the photocatalytic oxidation of ethyl lactate and phenol were 0.1 g/L and 3.0-4.0. Photocatalytic degradation under these optimal conditions was able to simultaneously mineralize those solvents and remove copper(II) in the synthetic wastewater. Under anoxic condition, oxygen inhibited copper reduction and copper(II) ions were removed through precipitation. Under anaerobic conditions, the removal rate of copper(II) ions and the rate of reduction of the organic solvents content were lower than that in the aerobic condition. 1. Introduction The semiconductor industry is one of the most important industry sectors in Singapore, China, Japan, and United States. Wafer Fab is a highly complex manufacturing process that consists of over 100 steps such as photolithography, etch, strip, diffusion, ion implantation, deposition, planarization, and cleaning.1,2 Various organic and inorganic compounds are involved in the manufacturing process. A large amount of ultrapure water is used in different kinds of washing and cleaning steps. Consequently, a huge quantity of wastewater is generated. Although considerable efforts have been made through process modification or chemical substitution to reduce environmental contamination, the wastewater still contains heavy metals, solvents, acids, bases, additives, and other organic compounds.3 Presently, most wastewater treatment processes for semiconductor manufacturing facilities are intended to remove inorganic compounds such as acids and bases as well as heavy metals such as copper, cobalt, and nickel.4 In contrast, the potential problems associated with the waste organic compounds have not been properly addressed. There are a number of waste disposal methods currently in practice with various degrees of success. Unit operations and processes are grouped together to provide various levels of treatment known as primary, advanced primary, secondary, and advanced treatment systems. An ideal waste treatment process should completely mineralize all the toxic species present in the waste stream without leaving behind any hazardous residues. It must be cost-effective. Most of the conventional treatments require subsequent treatment which results in high cost. For example, biological degradation is not applicable to waste streams with very toxic organics. Copper, cobalt, and nickel are heavy metals present in the semiconductor wastewater. As they are toxic, the treatment of these metals is required. Concentration of copper ions discharged * To whom correspondence is addressed. Fax: (831) 303-8636. E-mail:
[email protected],
[email protected].
into the public sewer, and watercourse should not exceed 5 and 0.1 ppm in Singapore. The conventional treatment processes to remove heavy metals from industrial waste stream are adsorption, ion exchange, electroreduction, precipitation, and membrane processes. However, most of these technologies have their own limitations. For example, precipitation creates a significant amount of hazardous sludge which requires a further treatment (e.g., dewatering). In this study, simultaneous treatment of organic solvents and toxic heavy metals was investigated. The aim of this work was to evaluate treatment technologies for wastewater contaminated with several organic solvents commonly used in the wafer fabrication process. The selected organic chemicals were ethyl lactate (EL) and phenol. The two organic compounds were vastly different in their chemical structures. Copper was studied as a model heavy metal. The important parameters affecting the treatment were investigated, which included TiO2 loading, pH, and presence/absence of oxygen. The decontamination mechanisms were studied. It is anticipated that the concepts and approaches developed in this study can provide a better alternative for treatment of highly concentrated metal-organic contaminants in the key units within the semiconductor manufacturing process. 2. Experimental Section 2.1. Chemicals. All chemicals were analytical reagent grade. All experimental solutions were prepared with water from a Millipore Direct-Q water purification system. The titanium dioxide catalyst employed in this study was supplied by Merck (mainly anatase) with a surface area of 7.83 m2/g determined from the BET nitrogen adsorption. The ethyl lactate, phenol, copper(II) sulfate anhydrous, and other chemicals used for experiments were purchased from Merck. Solution pH was adjusted by addition of hydrogen chloride and/or sodium hydroxide solutions.
10.1021/ie070478c CCC: $37.00 © 2007 American Chemical Society Published on Web 08/30/2007
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Figure 2. Effect of TiO2 dosage on the photooxidation. Conditions: UV exposure time ) 120 min; [phenol]0 ) [ethyl lactate]0 ) 1 mM; O2 flowrate ) 300 mL/min; [Cu2+] ) 0.
Figure 1. Schematic diagram of experimental setup: 1. water-cooling inlet; 2. water-cooling outlet; 3. UV lamp; 4. sampling port; 5. purging gas; 6. Teflon cover; 7. magnetic stirrer; 8. quartz tube; 9. suspended TiO2.
2.2. Methods and Procedures. The adsorption of organic compounds onto the catalyst in the absence of UV light was first studied. In the experiments, 1 g/L TiO2 was added to a 500-mL organic solution, of which the initial concentration was 0.92 mM at nature pH. The solution was then stirred at a constant rate. Samples were taken at different time intervals, and the concentrations were determined. Photodegradation reactions were then conducted in a batch reactor (volume of 450 mL) constructed of Pyrex glass and Teflon. The experimental apparatus is shown in Figure 1. A 30-W low-pressure mercury lamp with a wavelength of 254 nm and an intensity of 12.0 mW/cm2 from Aquafine Corporation was installed inside the annular reactor as the illumination source. This vessel was thermostated at 25 °C by a water jacket. Oxygen or nitrogen gas was bubbled at a rate of 300 mL/min through a gas tube located in the reactor. In all TiO2 photocatalytic experiments, a magnetic bar was used to ensure the TiO2 particles mixed with the solution completely. All synthetic solutions were ultrasonicated before irradiation during 10 min for homogenization; the solution pH was adjusted to different levels using HCl or NaOH prior to the addition of catalyst, and the suspension was stirred for 30 min in the dark to ensure sufficient contact of the catalyst with the organic compounds. 2.3. Analysis. The phenol concentration was measured by high performance liquid chromatography (HPLC) (10A series, Shimadzu, Japan) with a fluorescence detector. A ZORBAX SB-C18 LC column (4.6 mm × 25 cm, HP) was employed for the compounds separation. The fluorescence detector was set with excitation at 260 nm and emission at 305 nm.5 The mobile phase, distilled water/methanol (15/85 by volume), was delivered at a flow rate of 1 mL/min at ambient temperature. Measurement of ethyl lactate concentration was followed by gas chromatography (GC) (GC-17A, Shimadzu, Japan) using a 60-m HP-Voc capillary column (i.d. of 0.32 mm; film thickness of 1.8 µm; HP). Helium was used as carrier gas at 3.0 mL/min. The temperature program was set as follows: injector, 200 °C; detector, 225 °C; column initial temperature, 60 °C; hold 2 min;
ramp 10 °C/min to 170 °C; hold 2 min. All samples were filtered using a Whatman Autovial Syringeless 0.45 PTFE filter (Clifton, NJ).15 The pH of solutions was measured by an ATI Orion 525A pH meter. Inductively coupled plasma-emission spectroscopy (ICP-ES, Optima 3000DV, Perkin-Elmer) was used to analyze the metal ion concentrations. An initial determination of the unknown solids obtained from experiments was performed by using an energy dispersive X-ray spectrometer system (EDX) which was linked to scanning electron microscopy (SEM). The investigation of the morphology of the solids was carried out using the SEM, performed on a JEOL JSM-5600LV scanning electron microscope. Confirmation of the identity of the solid was conducted using X-ray photoelectron spectroscopy (XPS). The XPS spectra obtained were then curved fitted using the software XPSPEAK. 3. Results and Discussion 3.1. Effect of TiO2 Loading on Photocatalytic Degradation of Organic Solvents. The organic compounds may be adsorbed onto the TiO2. In order to find out the adsorption capacity, phenol and EL adsorption experiments were conducted. The removal percentages of phenol and EL were found to be 0.6% and 1.2%, respectively. It is thus concluded that the adsorption of organic compounds onto the catalyst is negligible. In slurry photocatalytic processes, catalyst dosage is an important parameter. Photocatalytic degradation of 1 mM ethyl lactate and phenol solutions was carried out with TiO2 catalyst loading of 0-1.0 g/L under the UV-irradiation. The degradation of phenol and ethyl lactate as a function of TiO2 dosage is shown in Figure 2. A dimensionless concentration of C/C0 is used to demonstrate the treatment results, where C and C0 represent concentration after reaction and initial concentration. As shown, in the absence of TiO2, the removal percentages of phenol and EL are 58% and 9%, respectively. The addition of a catalyst enhances the removal of both contaminants. As its concentration is increased, the removal of both contaminants increases. An optimal result is achieved at a TiO2 dosage of 0.1 g/L. The removal then decreases with a further increase in the catalyst dosage. The observation of lower decontaminations of organics at a higher catalyst dosage can be explained as follows. When the TiO2 loading exceeds the optimal dosage, the presence of excess photocatalyst in the aqueous solution increases the turbidity in the solution, which reduces the penetration of UV light. The reason for the retardation in the decontamination at the TiO2
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Figure 4. Effects of oxygen on removal of copper ions. Conditions: UV exposure time ) 120 min; [phenol]0 ) [ethyl lactate]0 ) 1 mM; TiO2 dosage ) 0.1 g/L; [Cu2+]0 ) 1 mM; pH ) 4.5.
Figure 3. Effects of initial pH on photocatalytic oxidation phenol and EL: (a) concentration vs initial pH and (b) pH vs time. Conditions: UV exposure time ) 120 min; [phenol]0 ) [ethyl lactate]0 ) 1 mM; TiO2 dosage ) 0.25 g/L; O2 flowrate ) 300 mL/min; [Cu2+] ) 0.
dosages beyond the optimum value is due to its lower dosage, leading to the generation of a less chemically active OH• radical. As a result, a less satisfactory result is achieved. Ethyl lactate is a kind of ether. It is more difficult to oxidize than phenol, as shown in the figure. 3.2. Effect of pH on Photocatalytic Degradation of Organic Solvents. Solution pH strongly affects both the surface properties of TiO2 and the dissociation of phenol and ethyl lactate. Figure 3a illustrates the change in dimensionless concentration of phenol and ethyl lactate versus initial pH. The pH has a significant effect on the degradation of phenol and ethyl lactate. At a lower pH of 2.0, the organic removal efficiencies are 50% for EL and 90% for phenol. As the pH is increased to 3, the removal efficiency slightly increases. At pH between 3 and 4, the degradation of phenol and ethyl lactate reaches their best removal efficiencies. At pH > 4.0, increased pH retards the removal. This observation is consistent with the results reported in the literature,13,14 which reported the optimum pH value of 3.5. Figure 3b demonstrates the change in solution pH vs time in the experiment. As shown, the pH decreases during the oxidation. This is due to the formation of a series of simple organic compounds (e.g., acids) and carbon dioxides. At a very low pH such as pH 2 in the figure, the TiO2 particle surfaces are occupied by H+, which hinders the production of OH• radicals. An increase in pH (e.g., in the pH range of 3-4) increases the OH- concentration on the TiO2 surfaces, which accelerates the OH• production in the solution.15-17 The oxidation is thus facilitated. A further increase in initial pH (e.g., pH 8-10 in this case) has a negative impact on the removal of both organics as shown,
which can be explained as follows. The zero point charge (ZPC) of TiO2 is 6, while the pK value of phenol is 10.18 In the pH range that was studied, the initial pH was below 10. Under such aqueous conditions, adsorption of phenol onto the surface of catalyst can occur due to electroneutralization. As a result, the production of OH• is retarded, and thus the removal of organics decreases. 3.3. Simultaneous Decontamination of Copper-Organic Waste. Copper ion is one of the important components in the wastewater discharged from the semiconductor industry. Our preliminary study shows that only 1.6% of copper ions can be adsorbed onto the TiO2 at pH 4.5, in agreement with the findings from other researchers.11,19 The low adsorption may be due to the fact that the solution pH is below the pHzpc of the TiO2, which inhibits the adsorption of cationic copper ions. The simultaneous copper-organic decontamination was studied. As the presence of oxygen may affect the redox reactions in the photocatalytical system, the study was performed in the presence/absence of oxygen. It can be seen from Figure 4, that the copper(II) ion concentration in the solutions decreases under both aerobic and anoxic conditions. The decontamination of copper(II) in the synthetic wastewater in the anoxic condition is lower than that in the aerobic condition. After the reaction for 2 h under aerobic conditions, green solids were found in the solution. The solids were collected by using membrane filters. An analysis of the green solid was performed by the SEM-EDX. Figure 5 shows the SEM micrographs with the morphology of the precipitated copper green solid and the bare TiO2, respectively. The surfaces of solids become less porous, indicating a coating by precipitates. An elemental analysis of both solids was conducted by the EDX. The copper content cannot be detected on the bare TiO2. It is found that there is a significant copper content in the green solid. Titanium is also found in the solid, which is due to the presence of TiO2. Meanwhile, carbon is one of the main constituents of the solid. The color of solid turns to black after it is heated. The green solid could possibly be CuCO3. The green CuCO3 can be easily decarbonated to form the black CuO. Figure 6a-b gives a wide scan XPS spectrum of both solids under aerobic and anaerobic conditions. The presence of TiO2 is clearly demonstrated. The copper species are further examined, as shown in Figure 7a. The peak of the green solid is found to be at 935.0 eV (Cu 2p). This corresponds to copper carbonate. Hence it can be confirmed that the Cu2+ ions are removed in the form of copper carbonate. After the reaction for 2 h under anoxic conditions, brown solids were found in the synthetic solution, and the solids were
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Figure 5. SEM images of the surface of green precipitate and bare TiO2. Table 1. Effects of Oxygen Concentration on Photocatalytic Oxidation of Phenol and ELa aerobic condition C/C0
) 0.5 mM
[Cu2+]0 )0
[Cu2+]0 ) 0.5 mM
[Cu2+]0 )0
phenol EL
0.109 0.574
0.083 0.475
0.668 0.782
0.583 0.778
a
Figure 6. The wide scan of XPS spectra for (a) the green solid (solid from aerobic condition) and (b) the brown solid (solid from anaerobic condition).
Figure 7. XPS spectra after curve fitting for (a) the green solid (solid from aerobic condition) and (b) the brown solid (solid from anaerobic condition).
collected using a membrane filter. The XPS analysis was conducted to identify the chemistry of the solid. As shown in Figure 7b, two peaks of 933.6 and 932.6 eV are found, which correspond to copper oxide (CuO) and elemental copper (Cu0), respectively.7 Copper ions (Cu2+) initially present in the synthetic wastewater can photocatalytically be reduced to
anoxic condition
[Cu2+]0
Condition: [phenol] ) [EL] ) 1 mM; UV exposure time ) 3 h.
elemental copper (Cu0). CuO is formed due to the oxidation of elemental copper during the drying process prior to the analysis using the XPS. The results given in Table 1 show that the presence of 0.5 mM copper(II) ions leads to slightly lower decontamination of the organic solvents in both aerobic and anoxic conditions. Anions would inhibit both the adsorption and photocatalytic degradation of organics.6,8,20 This inhibition could be due to the presence of anions (SO42-) as CuSO4 was used as a heavy metal salt. Fox et al.20 reported sulfate, chloride, and phosphate can be rapidly adsorbed onto the catalyst and reduce the oxidation rate. These inorganic anions may compete with the organics for surface active sites or can form a highly polar environment near the TiO2 particle surface thus blocking the diffusion of organics to the activate site. Moreover, the HCO3and CO32- formed from the photogenerated CO2 in the solution can contribute additional inhibition. As the copper concentration is increased, its impact on the oxidation of organics is reversed. Figure 8a shows the dimensionless concentration of Ct/C0 versus time, where C(t) and C0 represent concentration after reaction time (t) and initial concentration. It is found that greater decontamination of the phenol is achieved with an increasing copper(II) ions concentration. This observation is in agreement with what was reported in the literature.21 It has to be pointed out that the negative influence from copper ions occurs when its concentration is lower (e.g., less than 1.1 mM as reported in ref 22). The enhancement in photodegradation by copper ions can be explained as follows. Firstly, the photogenerated electrons are trapped at the copper ions according to eq 1. The undesirable electron-hole recombination can then be prevented, which results in an increased rate of hydroxyl radicals formation.
Cu2+ + e- f Cu+
(1)
Secondly, the Cu+ produced according to the above reaction can act as a catalyst in the Fenton reaction below.9-11,13,19
Cu+ + H2O2 f Cu2+ + OH• + OH-
(2)
The H2O2 in the reaction is generated by the following reactions:
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Ind. Eng. Chem. Res., Vol. 46, No. 20, 2007 Table 2. Apparent Rate Constant at Different Cu2+ Concentrations [Cu2+] (mM)
k′ (min-1)
r2
1 2 5
0.0106 0.0141 0.0161
0.98 0.99 0.99
the reaction time of 120 min, as shown in Figure 8b. The apparent first-order rate constants as a function of copper(II) concentrations and the corresponding linear regression coefficients are tabulated in Table 2. As shown a higher copper(II) concentration would lead to a higher reaction kinetic rate of photodegradation of phenol. 4. Conclusions
Figure 8. Effects of the initial copper(II) concentrations on the photocatalytic oxidation of phenol: (a) concentration vs time and (b) kinetic modeling. Conditions: UV exposure time ) 120 min; [phenol]0 ) 1 mM; TiO2 dosage ) 0.1 g/L; aerobic condition.
TiO2 + hν f ecb- + hvb+ + TiO2
(3)
ecb- + O2(ads) + H+ f HO2•
(4)
•
•
HO2 + HO2 f H2O2 + O2
(5)
The kinetics of photocatalyzed oxidations can be described by the following Langmuir-Hinshelwood equation12
R)-
dCt KAdkCt ) dt 1 + KAdCt
(6)
where KAd is the equilibrium adsorption constant, k is the reaction rate constant, and Ct is the concentration at reaction time t. The integration of eq 6 yields the equation below
t)
()
C0 1 1 ln + (C0 - Ct) KAdk Ct k
(7)
Equation 7 for a low initial concentration C0 can be written as follows
ln
()
Ct ) -KAdkt ) -k′t C0
(8)
where k′ is the apparent reaction rate constant. The plots of ln(Ct/C0) versus t give approximately straight lines over the range of copper(II) concentrations studied up to
In the slurry photocatalytic process, the removal efficiency of organic solvents is affected by catalyst dosage, initial pH, oxygen concentration, and heavy metals. It is found that the optimal TiO2 dosage and initial pH are 0.1 g/L and 3.0-4.0, respectively. When the ethyl lactate and phenol contaminated wastewater contains copper ions, heterogeneous photocatalysis is able to simultaneously remove both copper and organic compounds. Under aerobic conditions, electron scavenging by O2 is a thermodynamically favored process which interferes with copper reduction. As a result, reduction of Cu2+ to Cu0 is not observed. Cu2+ ions are removed through precipitation in the form of a green copper carbonate. Higher concentration of copper enhances the organic decontamination. Under anoxic condition, the reduction from Cu2+ to elemental Cu0 is observed. The copper removal in the anoxic condition is lower than that in the aerobic condition. Acknowledgment Support of this work by the National University of Singapore (R-288-000-023-112 and R-279-000-175-112) is gratefully acknowledged. Literature Cited (1) McGuire, G. E. Semiconductor Materials and Process Technology Handbook; Noyes Publications: Park Ridge, NJ, 1988. (2) Kern, K. Handbook of Semiconductor Wafer Cleaning Technology; Noyes Publications: Park Ridge, NJ, 1993. (3) Lin, S. H.; Jiang, C. D. Fenton Oxidation and Sequencing Batch Reactor (SBR) Treatments of High-strength Semiconductor Wastewater. Desalination 2003, 154, 107. (4) Den, W.; Ko, F. H.; Huang, T. Y. Treatment of Organic Wastewater Discharged From Semiconductor Manufacturing Process by Ultraviolet/ Hydrogen Peroxide and Biodegradation. IEEE Trans. Semicond. Manuf. 2002, 15, 540. (5) Niwa, T. Phenol and p-Cresol Accumulated in Uremic Serum Measured by HPLC with Fluorescence Detection. Clin. Chem. 1993, 39, 108. (6) Halmann, M. M. Photodegradation of Water Pollutants; CRC Press: Boca Raton, FL, 1996. (7) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy: a Reference Book of Standard Spectra for Identification and Interpretation of XPS data; Perkin-Elmer Corporation, Physical Electronics Division: Waltham, MA, 1992. (8) Chen, H. Y.; Zahraa, O.; Bouchy, M. Inhibition of the Adsorption and Photocatalytic Degradation of an Organic Contaminant in an Aqueous Suspension of TiO2 by Inorganic Ions. J. Photochem. Photobiol., A1997, 108, 37. (9) Fujihira, M.; Satoh, Y.; Oas, T. Heterogeneous Photocatalytic Reactions on Seminconductor Materials. III. Effect of pH and Cu2+ Ions on the Photo-Fenton Type Reaction. Bull. Chem. Soc. Jpn. 1982, 55, 666. (10) Foster, N. S.; Noble, R. D.; Koval, C. A. Reversible Photoreductive Deposition and Oxidative Dissolution of Copper Ions in Titanium Dioxide Aqueous Suspensions. EnViron. Sci. Technol. 1993, 27, 350.
Ind. Eng. Chem. Res., Vol. 46, No. 20, 2007 6571 (11) Beydoun, D.; Tse, H.; Amal, R.; Low, G.; McEvoy, S. Effect of Copper (II) on the Photocatalytic Degradation of Sucrose. J. Mol. Catal. A: Chem. 2002, 177, 265. (12) Lu, M. C.; Roam, G. D.; Chen, J. N.; Huang, C. P. Factors Affecting the Photocatalytic Degradation of Dichlorvos over Titanium Dioxide Supported on Glass. J. Photochem. Photobiol. A: Chem 1993, 76, 103. (13) Okamoto, K. I.; Yamamoto, Y.; Tanaka, H.; Tanaka, M. Heterogeneous Photocatalytic Decomposition of Phenol over TiO2 Powder. Bull. Chem. Soc. Jpn. 1985a, 58, 2015. (14) Okamoto, K. I.; Yamamoto, Y.; Tanaka, H.; Tanaka, M. Kinetics of Heterogeneous Photocatalytic Decompositon of Phenol over Anatase TiO2 Powder. Bull. Chem. Soc. Jpn. 1985b, 58, 2023. (15) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (16) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A.; Draper, R. B. Kinetic Studies in Heterogeneous Photocatalysis: TiO2Mediated Degradation of 4-Chlorophenol Alone and in a Three-component Mixture of 4-Chlorophenol, 2,4-Dichlorophenol, and 2,4,5-Trichlorophenol in Air-Equilibrated Aqueous Media. Langmuir 1989, 5, 250. (17) Wei, T. Y.; Wan, C. C. Heterogeneous Photocatalytic Oxidation of Phenol with Titanium Dioxide Powders. Ind. Eng. Chem. Res. 1991, 30, 1293.
(18) Yang, J. K.; Davis, A. P. Competitive Adsorption of Cu(II)-EDTA and Cd(II)-EDTA onto TiO2. J. Colloid. Interface. Sci. 1999, 216, 77. (19) Butler, E. C.; Davis, A. P. Photocatalytic Oxidation in Aqueous Titanium Dioxide Suspensions: the Influence of Dissolved Transition Metals. J. Photochem. Photobiol., A 1993, 70, 273. (20) Fox, M. A.; Dulay, M. T. Heterogeneous Photocatalysis. Chem. ReV. 1993, 93, 341. (21) Sykora, J.; Pado, M.; Tatarko, M.; Lzakovic, M. Homogeneous Photo-oxidation of Phenols: Influence of Metals. J. Photochem. Photobiol., A 1997, 110, 167. (22) Brezova, V.; Blazkova, A.; Borosova, E.; Ceppan, M.; Fiala, R. The Influence of Dissolved Metal Ions on the Photocatalytic Degradation of Phenol in Aqueious TiO2 Suspensions. J. Mol. Catal. A: Chem. 1995, 98, 109.
ReceiVed for reView April 3, 2007 ReVised manuscript receiVed July 10, 2007 Accepted July 17, 2007 IE070478C
