PHOTOCATALYTIC TREATMENT OF WASTEWATER CONTAMINATED WITH ORGANIC WASTE AND HEAVY METAL FROM SEMICONDUCTOR INDUSTRY ZOU SHUAIWEN NATIONAL UNIVERSITY OF SINGAPORE 2004 PHOTOCATALYTIC TREATMENT OF WASTEWATER CONTAMINATED WITH ORGANIC WASTE AND HEAVY METAL FROM SEMICONDUCTOR INDUSTRY ZOU SHUAIWEN (B Eng., Tsinghua University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 ACKNOWLEDGEMENT I would like to thank my supervisor, Dr J Paul Chen for his extensive guidance, interests and helpful suggestions throughout the project I would also like to express my appreciation to my fellow postgraduate students, Mr Sheng Pingxin and Mr Yanglei for many helpful discussions I would also like to acknowledge the National University of Singapore for funding this research i TABLE OF CONTENTS Page ACKNOWLEDGEMENTS i TABLE OF CONTENTS ii SUMMARY v NOMENCLATURE vii LIST OF FIGURES viii LIST OF TABLES xi CHAPTER INTRODUCTION 1.1 Problem statement 1.2 Objectives 1.3 Organization of the thesis CHAPTER LITERATURE REVIEW 2.1 Background of wafer fabrication processes 2.2 Physical properties of UV light 2.3 Advanced oxidation process 10 2.4 Basic principles of the TiO2/UV process 13 2.5 Kinetic models 18 2.6 Effects of temperature 19 2.7 Effects of initial solution pH 19 2.8 Photocatalytic recovery of metals 20 ii 2.9 Effects of the electron donor on photocatalytic reductions 20 2.10 Role of oxygen 20 2.11 TiO2 as a stationary phase 22 2.12 Model of metal ion adsorption on TiO2 particles 23 CHAPTER MATERIALS AND METHODOLOGY 26 3.1 Materials 26 3.2 Methodology 26 3.2.1 Photocatalysis reactor 26 3.2.2 Encapsulation equipment 28 3.2.3 Experimental procedure 29 3.3 Analysis CHAPTER RESULTS AND DISCUSSION 31 33 4.1 Adsorption of organic solvents on TiO2 suspensions 33 4.2 Turbidity 35 4.3 Effects of the TiO2 loading 36 4.4 Effects of initial solution pH 46 4.5 Effects of oxygen concentration 52 4.6 Effects of different brands of TiO2 53 4.7 Simultaneous removal of copper-organic waste 55 4.8 Effects of encapsulation of TiO2 as a photo-oxidant 66 iii CHAPTER CONCLUSIONS 70 REFERENCES 73 iv SUMMARY Treatment of dilute organic-copper wastewater discharged from semiconductor manufacturing facilities using photocatalytic degradation mediated by illuminated TiO2 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 fabrication processes as well as their seriously negative environmental impacts Photocatalytic experiments showed that the removal efficiency of ethyl lactate and phenol were dependent on TiO2 catalyst loading, initial pH, oxygen concentration and TiO2 catalyst properties The optimal TiO2 dosage of 0.1 g/L and initial pH of 3.0 were determined The photocatalytic process had much better removal efficiency under pure oxygen conditions Kinetic experiments on ethyl lactate and phenol photodegradation illustrated that the photodegradation processes agreed with first-order rate reaction under the experimental conditions in this study It was found that the removal of ethyl lactate and phenol due to adsorption onto TiO2 particles could be neglectable Simultaneous removal of the copper ions and two organic compounds was investigated under aerobic and anaerobic conditions Under aerobic conditions, oxygen inhibited copper reduction and copper was removed through precipitation; while under anaerobic conditions, it can be reduced to elemental Cu The removal rate of copper and the rate of v reduction of ethyl lactate and phenol concentrations are lower than those in the aerobic conditions It has been a major obstacle that TiO2 particles are difficult to separate from the treated water stream due to their lower settling velocities In order to overcome the problem, the TiO2 particles were encapsulated by a novel electronic spraying technology The spraying involves extruding a liquid at a constant flow rate and subjecting the liquid to an electric field In this manner, a charge induced on the surface of the liquid results in a mutual charge repulsion that disrupts the liquid surface, breaking it up into a charged stream of fine droplets By the technology, the encapsulated titanium dioxide with calcium alginate was prepared Such parameters as particle size and setting velocity were investigate in this study It was found the settling capacity of the encapsulated TiO2 was significantly enhanced More importantly the photo-oxidation properties of ethyl lactate and phenol by the TiO2 were still maintained and the secondary organic pollution was negligible vi NOMENCLATURE Description Symbol c light speed, m/s λ wavelength, m v frequency, Hz h Planck’s constant, J·s E photon energy, J T temperature, K P total energy emitted by source matter, W·cm-2 S Stefan-Boltzmann constant, W·cm-2·K-4 K Langmuir adsorption constant t reaction time, k reaction rate constant, min-1 vii LIST OF FIGURES Figure Title Page Figure 2.1 Flow diagram for a typical sequence of wafer fabrication process Figure 2.2 Electromagnetic Spectrum Figure 2.3 Schematic illustration of two-pK triple-layer surface Complex formation model 25 Figure 3.1 Schematic of photoreactor used in this study 27 Figure 3.2 A schematic of the equipment layout of the microencapsulation process 28 Figure 4.1 The adsorption of phenol onto TiO2 suspensions 34 Figure 4.2 The adsorption of ethyl lactate (EL) onto TiO2 suspensions 34 Figure 4.3 The turbidity in solution with changing TiO2 dosage 35 Figure 4.4 Effect of TiO2 dosages on the photooxidation phenol and EL 37 Figure 4.5 Phenol concentration in solution vs time under different TiO2 loading 37 Figure 4.6 Ethyl lactate (EL) concentration in solution vs time under different TiO2 loading 38 Figure 4.7 Intermediate detected during the degradation of 1mM phenol solution under different TiO2 loading 38 Figure 4.8 pH in solution vs time under different TiO2 loading 40 Figure 4.9 Oxidation reduction potential (ORP) in solution vs time under different TiO2 loading 41 viii Figure 4.26 XPS Spectrum after curve fitting for the brown solid Two of the peaks after adjustment with respect to the peak position of carbon are 933.6eV and 932.6eV shown in Figure 4.26, which corresponds to CuO and elemental copper Cu respectively (Moulder et al., 1992) Hence Cu2+ initially present in the synthetic wastewater was photocatalytically reduced to elemental copper CuO and two other impurities were detected as well possibly due to the oxidation of elemental copper during the drying process prior to the analysis using XPS 61 0.8 no copper(II) 0.7 with copper(II) C/C0 0.6 0.5 0.4 0.3 0.2 0.1 nitrogen oxygen C/C0 Figure 4.27 Effects of different oxygen conditions on photocatalytic oxidation of phenol (Experimental conditions: UV exposure time=120min; Initial conc [Phenol]0=1mM; TiO2 dosage=0.1g/L; [Cu2+] =1mM) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 no copper(II) with copper (II) nitrogen oxygen Figure 4.28 Effects of different oxygen conditions on photocatalytic oxidation of ethyl lactate (EL) (Experimental conditions: UV exposure time=120min; Initial conc [Ethyl lactate]0=1mM; TiO2 dosage=0.1g/L; [Cu2+] =1mM) 62 From Figures 4.27 and 4.28, the presence of the copper (II) ions would lead to a reduction in the organic solvents content in the solution It was found to be hindered in both aerobic and anoxic conditions This inhibition may be due to the presence of anions like SO42- In addition to the HCO3- and CO32- formed from the photogenerated CO2 in the solution, the inhibition was worsed with the introduction of anions like SO42- All these anions would inhibit both the adsorption and photocatalytic degradation of organics In this study, CuSO4·5H2O was used The inhibition caused by the SO42- anions is coherent with what was reported in literatures (Chen et al., 1997; Halmann, 1996) It was reported that SO42- ions may reduce the photocatalytic oxidation rate of organic solutes by up to 70% HCO3-/CO32-, and NO3- ions had a smaller effect comparatively on the rates of photooxidation Regeneration of photocatalysts with the adsorbed SO42- is possible by rinsing with 0.1M NaHCO3 (Halmann, 1996) However it should be noted from Figure 4.29 that the Cu2+ in solution could have enhanced the rate of TiO2 photocatalytic oxidation Electrons could have been trapped by the copper ions and hence preventing the undesirable electron-hole recombination (Serpone, 1994) If the copper ions did not adsorb onto the TiO2 surface, enhancement could still be possible through a homogeneous pathway mechanism proposed by Beydoun et al (2002) 63 1.2 5mM copper(II) 2mM copper(II) 1mM copper(II) C/C0 0.8 0.6 0.4 0.2 0 30 60 90 120 Time (min) Figure 4.29a Effect of the initial copper(II) concentration on the photocatalytic oxidation of phenol (Experimental conditions: UV exposure time=120min; Initial conc Phenol=1mM; Ethyl lactate=1mM; TiO2 dosage=0.1g/L; aerobic ) From Figure 4.29a, greater reduction of the phenol content was observed with increasing copper(II) ions concentration The role of copper ions in solution in enhancing the photoactivity of titanium dioxide is generally believed to be via electron capturing at the semi-conductor surface by the copper(II) ions, as shown in Eq (2-11) This has been said to prevent electron-hole recombination resulting in an increased rate of hydroxyl radicals formation 64 ln(C/C 0) -0.5 -1 5mM copper(II) -1.5 2mM copper(II) -2 1mM copper(II) -2.5 30 60 90 120 Time (min) Figure 4.29b Test of pseudo-first order kinetics according to eq(4-3) at different copper (II) concentrations The plots of ln(C/C0) versus t gave approximately straight lines over the range of copper (II) concentrations studied up to 120 minutes of the reaction time as shown in Figure 4.29b The apparent first-order rate constants determined as a function of copper (II) concentrations and the corresponding linear regression coefficients were tabulated in Table 4.6 Figure 4.30 indicates that higher copper(II) concentration has higher reaction rate of photodegradation of phenol Table 4.5 Apparent rate constant obtained from Eq (4-3) at different Cu2+ concentrations 2+ Cu concentration (mM) k’(min-1) r2 0.0106 0.9937 0.0141 0.9957 0.0161 0.9978 65 k' (min-1) 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 [Cu2+] (mM) Figure 4.30 Effect of copper (II) concentration on k’(first-order rate constant) 4.8 Effects of encapsulation of TiO2 as a photo-oxidant The encapsulation of titanium dioxide was explored The encapsulation of titanium dioxide powder into beads may serve as an enhancement in separation slurry reactors in wastewater treatment With an increase in the settling velocity of the encapsulated titanium dioxide beads, the ease of separation of titanium dioxide from the treated wastewater may be enhanced significantly Due to their small size and long settling times, small particles cannot settle naturally Mechanical sand filtration can remove some larger particles Membrane processes, such as nonofiltration and reverse osmosis, can effectively deal with most of small particles However, membranes can easily be damaged by too much material, as is typically present in solution 66 Figure 4.30 shows the different size with different TiO2 Due to a much larger size of the encapsulated TiO2 beads compared to the raw TiO2 powder, the encapsulated TiO2 beads had a significantly higher settling velocity compared to its powdered form (Figure 4.31) This made the encapsulated TiO2 beads a promising form compared to its powdered form, since this would allow significantly faster and easier separation with the treated wastewater, in its application to the photooxidation of organics in wastewater treatment 350 Particle size (um) 300 305.1 250 200 150 79.8 100 50 3.1 TiO TiO2 TiO2(dry) TiO2(wet)TiO2(wet) TiO2(dry) Figure 4.31 Particle size with different TiO2 (TiO2: raw TiO2 powder; TiO2 (wet): encapsulated TiO2; TiO2 (dry): encapsulated TiO2 using vacuum dryer) 67 Setting velocities m/s 0.002 0.00193 0.00142 0.0015 0.001 0.0005 0.00002 TiO2 TiO2(dry) TiO2(wet) Figure 4.32 Comparing setting velocity of different forms of TiO2 0.7 phenol 0.6 ethyl lactate C/C0 0.5 0.4 0.3 0.2 0.1 TiO2 TiO2(dry) TiO2(wet) Figure 4.33 The effect of different forms of TiO2 on the photocatalytic oxidation phenol and EL (Experimental conditions: UV exposure time=120min; Initial conc Phenol=1mM; Ethyl lactate=1mM; TiO2 dosage=0.25g/L; aerobic ) 68 It can be seen from Figure 4.30, compared with pure TiO2 powder, the encapsulation Total organic carbon (mg/L) TiO2 has similar removal efficiency of phenol and ethyl lactate 16 14 12 10 0 50 100 150 200 250 Time (mins) Figure 4.34 The TOC leaching of encapsulation TiO2 (Experimental conditions: UV exposure time=240min; Initial conc Phenol=0mM; Ethyl lactate=0mM; pH=5.0, aerobic) When the encapsulation TiO2 was illuminated by UV lamp, as shown in Figure 4.32, the TOC leaching reached the top content about 15mg/L at 120minutes 69 CHAPTER CONCLUSIONS The adsorption of ethyl lactate and phenol onto TiO2 particles was investigated in this study Results from adsorption experiments indicate that the removal of ethyl lactate and phenol due to adsorption onto TiO2 can be neglectable It is therefore reasonable to assume the photooxidation process is the dormant mechanism for the removal of the organic solvents In slurry photocatalytic processes, the removal efficiency of organic solvents was affectd by catalyst dosage, initial pH, oxygen concentration and TiO2 catalyst form The optimal TiO2 dosage and initial pH were determined under the experimental conditions in this study, which were 0.1 g/L and 3.0 respectively Oxygen has a very important role in the photocatalytic process In the presence of oxygen, the electrons can be captured by the molecular oxygen rapidly, hence the photocatalytic process had much better removal efficiency under pure oxygen conditions The TiO2 (Merck) used in this study was found to be predominantly of the anatase form and has a BET surface area of 7.83 m2/g Compared with TiO2 (Merck), Degussa P25 TiO2 has much more BET surface area, and perform better removal efficiency of organic solvents Kinetic experiments on ethyl lactate and phenol photodegradation illustrated that the photodegradation process agreed with first-order rate reaction under the experimental conditions in this study 70 Heterogeneous photocatalysis was found to be able to simultaneously remove the Cu2+ Under aerobic conditions, electron scavenging by O2 is a thermodynamically favored process which interfered with copper reduction As a result, there was no observable reduction of Cu2+ to Cu Cu2+ ions were removed through precipitation in the form of a green solid The green solid was identified to be copper carbonate For the reaction of hour, under aerobic conditions and it was found that 63.2% of the initial Cu2+ was 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