title page

Abstract

Contents

Chapter 1. Introduction 16

1.1. Background 16

1.2. Objectives 18

1.3. Organization of dissertation 19

1.4. References 20

Chapter 2. Literature review 23

2.1. As(III) oxidation by UV-A/Fe(III) 23

2.2. As(III) oxidation by UV-A/TiO₂ 25

2.3. As(III) oxidation by VUV irradiation(λ=185 ㎚) 26

2.4. References 28

Chapter 3. Hypothesis 31

Chapter 4. Photochemical oxidation of As(III) in Fe(III)-citrate solutions 34

4.1. Introduction 34

4.2. Experimental 35

4.3. Results and discussion 37

4.3.1. Investigation of main oxidant for As(III) by UV/Fe(III)-citrate 37

4.3.2. Effects of [H₂O₂] and [Fe(III)] 41

4.3.3. Comparison of UV-A/Fe(III)-citrate/H₂O₂ and UT-C/H₂O₂ and UA/TiO₂ for As(III) oxidation 43

4.4. Conclusions 45

4.5. Relation of this chapter to hypotheses 45

4.6. References 45

Chapter 5. Combined use of photochemical reaction and activated alumina for As(III)oxidation and As(V) removal 47

5.1. Introduction 48

5.2. Experimental section 50

5.2.1. Materials 50

5.2.2. Experiments 51

5.2.3. Analyses 52

5.3. Results and discussion 53

5.3.1. Oxidation of As(III) by UV/Fe(III) ; effects of pH, Fe(III) concentration, and H₂O₂ concentration 53

5.3.2. Oxidation of As(III) by UV-illuminated TiO₂ 57

5.3.3. Oxidation of As(III) in the Ju-Am water by UV-A/Fe(III)/H₂O₂ and UV-A/TiO₂ 59

5.3.4. As(III) oxidation and As(V) removal by the combined use of photochemical reaction and activated alumina 61

5.4. Conclusions 63

5.5. Relation of this chapter to hypotheses 64

5.6. References 65

Chapter 6. Oxidation mechanism of As(III) in the UV/TiO₂ system : Evidence for a Direct Hole Oxidation Mechanism 68

6.1. Introduction 69

6.2. Experimental section 71

6.2.1. Materials 71

6.2.2. Experiments 72

6.2.3. Analyses 73

6.3. Results and Discussion 74

6.3.1. Oxidation of As(III) by UV-C/H₂O₂ 74

6.3.2. Effects of oxalate, formate, and Cu(II) on the As(III) oxidation by UV/TiO₂ 77

6.3.3. Effect of As(III) on the oxidation of organic compounds by UV/TiO₂ 79

6.3.4. Effects of methanol, iodide, and bromate on the oxidation of As(III) by UV/TiO₂ 82

6.3.5. Coagulation/precipitation process for the simultaneous removal of As(V) and TiO₂ 86

6.4. Conclusions 87

6.5. Relation of this chapter to hypotheses 87

6.6. References 89

Chapter 7. Mechanism establishment of TiO₂-photcatalysed As(III) oxidation 94

7.1. Introduction 94

7.2. Experimental Section 98

7.2.1. Materials 98

7.2.2. Experiments 99

7.2.3. Analyses 100

7.3. Results and Discussion 100

7.3.1. Effects of competitive substrates on As(III) PCO with O₂ as a sole electron acceptor 100

7.3.2. Dismissing the superoxide mechanism through monitoring of photo-generated H₂O₂ 103

7.3.3. Refutation against superoxide-mediated As(III) PCO mechanism 107

7.3.4. Oxidation of As(III) by KO₂ 110

7.3.5. Effects of competitive substrates on anoxic As(III) PCO 111

7.3.6. As(III) PCO in TiO₂-coated ∞il 112

7.4. Conclusions 114

7.5. Relation of this chapter to hypotheses 114

7.6. References 115

Chapter 8. Vacuum-UV (λ=185㎚) irradiation for the oxidation of arsenic(III) 121

8.1. Introduction 122

8.2. Experimental section 125

8.2.1. Materials 125

8.2.2. Experiments 126

8.2.3. Analyses 127

8.3. Results and discussion 127

8.3.1. Comparison of VUV irradiation with other photochemical methods for the oxidation of As(III) 127

8.3.2. Photo-generation of H₂O₂ and its role in the VUV-induced As(III)oxidation 131

8.3.3. Effects of Fe(III), humic acid(HA), and H₂O₂ on the VUV-induced As(III)oxidation 134

8.3.4. VUV-induced As(III) oxidation in natural water 137

8.3.5. As(III) oxidation and As(V) removal 138

8.3.6. Oxidation of As(III) in a flow reactor 140

8.4. Conclusions 141

8.5. Relation of this chapter to hypotheses 142

8.6. References 142

Chapter 9. Summary 149

국문초록

Table 3.1. Connectivity between hypotheses and chapters 33

Table 5.1. Effect of Fe(III) concentration on the treatment of As(III) by the UV/Fe(III) process 56

Table 6.1. Remaining concentration of total arsenic after preoxidation and coagulation 88

Table 9.1. Summary of thesis 150

Figure 4.1. Schematic illustration of batch reactor 37

Figure 4.2. Effect of ethanol on As(III) oxidation by UV-A/Fe(III)-citrate at different pHs. Experimental conditions; ultra-pure water(18.2MΩ·㎝), [Fe(III)]=50 μM, [Citric acid]=100 μM, [As(III)]=50 μM, UV-A(λmax=352 ㎚). (a) pH=3 and pH=6(0.5mM HCO3-); (b) PH=7...(이미지참조) 39

Figure 4.3. Effect of methanol on As(III) oxidation by UV-C/H₂O₂. Experimental conditions; ultra-pure water(18.2MΩ·㎝), [H₂O₂]=110 μM, [As(III)]=50 μM, pH=7(0.5mM HCO3-), [CH₃OH]=0~200 mM, UV-C (λmax=254 ㎚).(이미지참조) 40

Figure 4.4. Effect of [H₂O₂] on As(III) oxidation by UV-A/Fe(III)-citrate. Experimental conditions: synthetic water, [Fe(III)]=20 μM, [Citric acid]=100 μM, [As(III)]=30 μM, [H₂O₂]=0~120 μM, pH=8 UV-A (λmax=352 ㎚).(이미지참조) 42

Figure 4.5. Effect of [Fe(III)] on As(III) oxidation by UV-A/Fe(III)-citrate/H₂O₂. Experimental conditions; synthetic groundwater, [Fe(III)]=0~80 μM, [Citric acid]=100 μM, [As(III)]=30 μM, [H₂O₂]=40 μM, pH=8 UV-A (λmax=352 ㎚).(이미지참조) 42

Figure 4.6. Comparison of UV-A/Fe(III)-citrate/H₂O₂ with UV-C/H₂O₂ and UV-A/TiO₂ Experimental conditions : synthetic groundwater, [As(III)]=30 μM, pH=8 44

Figure 5.1. Schematic illustration of batch reactor 52

Figure 5.2. As(III) oxidation by UV-A/Fe(III) process. [As(III)]=200 μM, [FeCl₃]=200 μM, ultra-pure water. For the pH 7 condition, 2 mM NaHCO₃ was added as buffer component 54

Figure 5.3. Effect of [H₂O₂] on the oxidation of As(IIl) by UV/Fe(III). [As(III)]=200 μM, [Fe(III)]=20 μM, pH=7 (buffered with 2 mM NaHCO₃), ultra-pure water 57

Figure 5.4. Oxidation of As(III) by UV/TiO₂. [As(III)]=200 μM, pH=3 or 7 (buffered with 2 mM NaHCO₃), ultra-pure water 59

Figure 5.5. Oxidation of As(III) in the Ju-Am water by photo-Fenton and TiO₂ photocatalysis. (A) [As(III)]=200 μM, pH=3; (B) [As(III)]=10 μM, pH=7 (buffered with 2 mM NaHCO₃ 61

Figure 5.6. As(III) oxidation and As(V) removal by the combined use of photochemical reaction and activated alumina. [As(III)]=200 μM, Ju-Am water, pH=3 63

Figure 6.1. Effects of methanol and formate on the oxidation of As(III) by UV-C/H₂O₂. [H₂O₂]i=50 μM, [As(III)]i=100 μM, λmax=254 nm, molybdenum blue detection method, 1000ml solution pH=3 or 7 (buffered with 1 mM NaHCO₃(이미지참조) 75

Figure 6.2. Effects of oxalate, formate, and Cu(II) on the oxidation of As(III) by UV-A/TiO₂. [TiO₂]i=10 mg/L, [As(III)]i=100 μM, pH=3, λmax=352 nm, molybdenum blue detection method, 1000 ml solution.(이미지참조) 79

Figure 6.3. Effect of As(III) on the hydroxylation reactions of benzoate and terephthalate by UV-A/TiO₂. [TiO₂]i=100 mg/L, λmax=352 nm, air-equilibrium, fluorescence detection method, 1000 ml solution. (A) [sodium benzoate]i=1 mM, pH 3: (B) [disodium terephthalate]i=1 mM,...(이미지참조) 80

Figure 6.4. Effect of As(III) on the degradation of formate by UV-A/TiO₂. [TiO₂]i=100 mg/L, [HCOOH]i=100 μM, pH=3, λmax=352 nm, air-equilibrium, IC detection method, 1000 ml solution.(이미지참조) 81

Figure 6.5. Effects of CH₃OH, r, and BrO3- on the oxidation of As(III) by UV-A/TiO₂. [TiO₂]i=100 mg/L, [As(III)]i=100 μM, λmax=352 nm, IC detection method, 1000 ml solution. (A) pH 3; (B) pH 7 (buffered with 1 mM NaHCO₃); (C) pH1](이미지참조) 84

Figure 7.1. Effects of competitive substrates on the PCO of As(III). The experimental conditions were [TiO₂]=0.5 g/L, [As(III)]0=500 μM, pH=3, UV-A illumination λmax=352 nm). volume=200 ml, and air-equilibrated. As(V) was quantitatively measured by the molybdenum blue method.(이미지참조) 102

Figure 7.2. Effect of FA on the evolution of H₂O₂ and As(V). H₂O₂ and As(V) were measured by the DMP and molybdenum blue method, respectively. (a) [TiO₂]=25 mg/L, [As(III)]0=50 μM, pH=3, UV-A illumination (λmax=352 nm), volume=200 ml, and air-equilibrated. (b) [TiO₂]=10 mg/L,...(이미지참조) 104

Figure 7.3. Effects of FA and methanol on the evolution of H₂O₂ and As(V) under VUV irradiation. H₂O₂ and As(V) were measured by the DMP and molybdenum blue method, respectively. The experimental conditions were [As(III)]0=100 μM, pH=3, VUV irradiation (λmax=185 nm),...(이미지참조) 107

최근, 독성 발암물질인 비소 오염에 대한 관심이 전세계적으로 고조되고 있다. 수중의 비소는 3가 비소 및 5가 비소로 존재하는데, 비소의 효과적인 제거를 위해서는 3가 비소의 산화가 필수적이다. 3가 비소의 효과적인 산화를 위한 수처리 기술로서, 광-펜톤 반응, TiO₂광촉매 반응, 185㎚의 자외선 조사 기술에 대한 연구가 수행되었다.

광-펜톤 반응에서의 3가 비소의 산화 메커니즘은 과량의 메탄올과의 경쟁반응을 통하여 조사되었다. 그 결과, 광-펜톤에 의한 3가 비소의 산화 메커니즘은 용액의 pH에 크게 영향을 받는 것으로 드러났다. 산성 pH 영역에서는 수산화 라디칼이 주요 산화제로 작용하였으나, 용액의 pH가 높아짐에 따라 4가 철이 주요 산화제로 작용하게 되었다. 3가 과산화수소의 농도를 증가시켰을 경우, 광-펜톤 반응에 의한 3가 비소의 산화 효율이 증가함을 알 수 있었다. 광-펜톤 반응과 활성 알루미나를 함께 첨가한 경우, 3가 비소의 산화 및 5가 비소의 동시 제거가 가능하였다.

TiO₂ 광촉매 반응에 의한 3가 비소의 산화는 현재 학자들간에 논쟁이 진행중인 주제이다. 광촉매 반응에 의한 3가 비소의 산화 메커니즘을 경쟁 물질의 첨가 효과를 통하여 규명하는 연구를 수행하였다. 그 결과, 3가 비소는 광촉매의 정공(positive hole)과 직접 반응하여 5가 비소로 산화되는 단순한 반응 메커니즘이 작용하고 있음이 명확하게 확인되었다. 광촉매 반응에서 3가 비소가 수퍼옥사이드 라디칼 (HO₂ㆍ/O₂-ㆍ)에 의하여 산화된다는 일부 연구팀의 주장은 근거가 희박한 것으로 조사되었다. 유리 기판에 코팅된 TiO₂ 필름은 3가 비소의 효과적인 산화방법이 될 수 있음을 확인하였다.

185㎚ 파장의 자외선을 3가 비소의 산화에 적용하는 연구를 수행하였다. 이 반응에서 3가 비소는 거의 수산화 라디칼의 작용으로 산화됨을 알 수 있었다. 한편, 염기성의 pH 영역에서는, 185㎚의 자외선에 의하여 생성되는 H₂O₂가 3가 비소의 산화제로 작용할 수 있는 가능성이 일부 확인되었다. 185㎚의 자외선에 의한 3가 비소의 산화는 별도의 산화제 및 촉매가 필요치 않으나, 효율면에서 광-펜톤 및 TiO₂ 광촉매 반응을 능가하였다. 또한, 3가 철 및 휴믹산은 185㎚의 자외선에 의한 3가 비소의 산화 효율을 감소시키지 않는 것으로 드러났다. 따라서, 185㎚의 자외선 조사는 3가 비소의 효과적인 산화법임을 확인하였다. 185㎚ 자외선 조사와 활성 알루미나를 함께 첨가한 경우, 3가 비소의 산화 및 5가 비소의 동시 제거가 매우 빠르게 일어났다.