Studies on the advanced oxidation and adsorption processes for the removal of hazardous dyes in wastewater

다국어 초록 (Multilingual Abstract)

Textile dye wastewater generally contains a substantial amount of dye and as such make its coloration unattractive and its discharge to streams degrades water ecosystem leading to inhibition of microorganisms self-cleansing protocol. Due to increasing stringent restrictions on the organic content of industrial effluents, it is necessary to eliminate dyes from wastewater before it is discharged to natural waters. Therefore, this work investigated the effective treatment of dye in wastewater using ozone oxidation process. And evaluation of treatment by adsorption process was performed to find relatively cheaper adsorbents to replace activated carbon by using coffee beans and chicken feathers as alternative adsorbents for removal of dye in aqueous solution. Also, photocatalyst with a high activity under visible-light irradiation prepared, and degradation of dye by nitrogen-doped TiO2 photocatalyst under solar and visible light was investigated.
The kinetics of the decolorization of Acid Red 27 by ozone oxidation was studied. Batch kinetic tests were conducted to determine effects of ozone dosage, reaction temperature and catalysts on azo dye Acid Red 27 decolorization. The decolorization rate constant of Acid Red 27 followed the first-order reaction with occurrence of complete decoloration in 20 min. Acid Red 27 decay rate increases as ozone dosage increases and at reaction temperature range of 0.1–37 ºC, the rate of reaction decreases with time but vice-versa as from 43–65 ºC. With Mn, Zn and Ni used as transition metal catalysts during the ozone oxidation process, Mn displayed the greatest catalytic effect with significant increase in rate of reaction. The solution pH was observed to decrease during oxidation reaction, which was considered to be due to the generation of several organic and inorganic acids as the result of the decomposition of azo compound. Also, the structural change of Acid Red 27 by oxidation with ozone was investigated using FT-IR and 1H-NMR. The FT-IR analysis showed that changes in specific absorption bands of Acid Red 27 were observed after ozonation. When azo dye was oxidized by ozone, several new peaks were shown to appear by 1H-NMR analysis and the peaks were generally shifted up field. This was presumably due to the broken of the benzene ring contained in the molecular structure of Acid Red 27 by ozone oxidation and the shape of the peaks were shown to change according to the reaction time.
Employing the ozone oxidation process, the decomposition behavior of Malachite Green - a dye mostly used in textile industries was investigated. To determine the optimum conditions for complete decolorization, operating parameters such as initial dye concentration, color removal, reaction temperature and COD reduction were studied. Complete removal of the initial green color of the dye was achieved within 10 min of reaction with ozone. The decolorization rate constant of Malachite Green followed the first-order reaction. The oxidation reaction time increased as concentration of Malachite Green increases, inferring, there would be increase in dye removal as concentration decreases. COD (25%) reduction was observed after 5 min of aqueous Malachite Green oxidation with ozone and the rate of reaction monotonically increased as temperature increases from 0–46 ºC, which was different to the reaction of Acid Red 27 with ozone. From the spectroscopic analysis of the intermediates and the by-products of Malachite Green reaction with ozone, a tentative mechanistic approach of Malachite Green degradation was postulated based on results from UV-vis, FT-IR and 1H-NMR.
The applicability of the coffee beans and chicken feathers as alternative adsorbent for activated carbon in treatment of dye in wastewater was investigated. The adsorption characteristic of Malachite Green onto coffee beans and chicken feathers as adsorbents was compared with activated carbon. With increase in degreased coffee beans used as adsorbent amount, adsorption increased. The adsorption features of Malachite Green on activated carbon, coffee beans and chicken feathers was investigated with adsorbent dosage, initial concentration of adsorbate, temperature and pH as experimental variables. Adsorption of dye on coffee beans improved significantly after surface modification. With regard to the pre-treatment of chicken feathers with ethanol, the adsorbability of Malachite Green increased. The adsorbed amount of Malachite Green increased as initial Malachite Green concentration increase. Degreased coffee beans and chicken feather have lower adsorption efficiency than activated carbon in the range of Malachite Green concentration studied. Kinetic studies indicated that the sorption tends to follow pseudo second-order kinetics for the range of concentrations studied for the entire sorption period. Freundlich isotherm gave a better fit than Langmuir isotherm for adsorption Malachite Green onto activated carbon, but Malachite Green adsorption onto degreased coffee beans and chicken feather followed Freundlich and Langmuir models, respectively. Thermodynamic studies showed that the adsorption processes were spontaneous and endothermic since ∆G° value was negative and ∆H° value was positive. Increase in Malachite Green color removal was observed with corresponding increase in the pH of the dye wastewater.
Nitrogen-doped TiO2 particles have been successfully prepared using titanium tetraisopropoxide as the Ti source and urea as the nitrogen source. As-prepared nitrogen-doped TiO2 was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller method (BET) and ultraviolet-visible light (UV-vis) absorption spectra techniques. Photocatalytic degradation of Methylene Blue has been carried out in both solar light (UV-vis) and the visible region (λ = 420 nm). Nitrogen-doped TiO2 exhibits higher activity than the commercial TiO2 photocalyst, particularly under visible-light irradiation because band gap of nitrogen-doped TiO2 becomes remarkably decreased.

국문 초록 (Abstract)

염색폐수는 일반적으로 탈색이 잘 되지 않는 염료가 다량 함유되어 있으며 수중에 아주 소량의 염료가 존재하여도 육안으로 보여 미관상 좋지 않을 뿐 아니라 하천에 방류될 경우 수중에서 햇볕을 차단함으로 수중식물의 성장에 악영향을 미치며 미생물에 의한 자연정화작용을 방해하여 하천의 오염도를 증가시키므로 이에 대한 처리가 필수적이다. 따라서 본 연구에서는 오존산화공정을 적용하여 효율적인 염색폐수 처리에 대한 연구를 수행하였으며, 폐기물을 재활용하는 방안을 모색함과 동시에 활성탄의 대체흡착제로서 가능성을 검토하기 위하여 커피콩과 닭깃털을 이용하여 수중 염료의 흡착특성을 살펴보았다. 나아가 가시광 활성 광촉매를 개발하고자 질소 도핑 TiO2를 준비하여 이를 수중 염료 분해에 적용하였다.
상업용으로 널리 사용되는 아조염료 중 Acid Red 27을 선택하여 오존주입량, 온도, 촉매 등 다양한 변수를 설정하여 수용액에서 오존산화 반응을 수행하였다. 모든 반응은 반응시작 후 20분 이내에 완결되었으며 1차 반응 속도를 따르는 것으로 나타났다. 오존주입량이 증가함에 따라 Acid Red 27의 제거 속도는 증가하였고, 반응온도 0.1~37 ℃에서는 반응온도가 증가함에 따라 반응속도가 감소하였으나 반응온도 43~65 ℃에서는 반응온도가 증가함에 따라 반응속도가 증가함을 보였다. 전이금속 촉매로는 Mn, Zn, Ni을 이용하였으며 이 금속들 중 Mn을 이용하는 경우 가장 큰 촉매효과를 확인할 수 있었다. 또한 오존 산화 반응 동안 반응물의 pH가 감소함을 보였으며 이는 오존 산화 반응 중 발생한 유기산과 무기산의 영향으로 사료되었다. 오존산화반응 후 Acid Red 27의 구조변화를 예측하기 위해 FT-IR과 1H-NMR을 이용하여 분해산물을 분석하였다. FT-IR 스펙트럼을 보면 오존 산화 반응 후 특정 band에서의 변화가 관찰되었다. 1H-NMR을 측정한 결과 산화반응시간에 따라 새로운 peak의 생성이 관찰되었으며 peak가 전체적으로 upfield쪽으로 이동되는 것을 관찰할 수 있었다. 이는 Acid Red 27 구조내의 benzene ring의 일부가 오존 산화 반응에 의해 분해되었기 때문이라고 사료되었다.
양식 및 직물산업에서 널리 사용되고 있는 Malachite Green을 오존 산화 공정을 적용하여 분해 양상을 살펴보았다. Malachite Green 분해의 최적 조건을 찾고자 염료의 초기농도와 반응온도 등을 변수로 설정하였다. 염료의 색도는 반응 개시 후 10분 이내에 완전히 제거되었다. 오존산화에 의한 Malachite Green의 분해는 1차 반응 속도를 따르는 것으로 나타났다. Malachite Green의 농도가 증가함에 따라 산화반응시간은 증가하였으며 농도가 감소함에 따라 염료 제거는 증가하였다. 5분 오존 산화 반응 후 25%의 COD 감소를 보였으며, Acid Red 27과는 달리 0~46 ℃ 구간에서 반응온도가 증가할수록 반응속도가 증가하는 것으로 관찰되었다. UV-vis, FT-IR 그리고 1H-NMR을 이용한 분광학적 분석을 통하여 Malachite Green의 가능한 분해 경로를 제안하였다.
폐기물을 수중 염료 제거를 위한 활성탄의 대체 흡착제로서의 적용 가능성을 검토하고자 커피콩 그리고 닭깃털에 대한 Malachite Green의 흡착특성을 활성탄을 흡착제로 이용했을 때의 흡착특성과 비교하여 보았다. 활성탄, 커피콩 그리고 닭깃털에 대한 Malachite Green의 흡착 거동을 살펴보기 위하여 흡착제의 양, 흡착질의 초기농도, 반응온도 그리고 pH를 변화시켜가며 그 특성을 조사하였다. 커피콩의 표면개질 후 Malachite Green의 흡착은 향상되었으며, 에탄올로 닭깃털을 전처리 한 결과 전처리 전에 비해 닭깃털에 대한 Malachite Green의 흡착이 역시 향상되는 것으로 나타났다. 흡착제로 사용한 degreased 커피콩의 양이 증가함에 따라 흡착은 증가하였다. Malachite Green의 초기농도가 증가함에 따라 흡착제에 흡착된 Malachite Green의 양이 증가하였으며, degreased 커피콩과 닭깃털에 대한 Malachite Green의 흡착은 활성탄보다 낮은 흡착 효율을 보였다. 또한 Malachite Green의 흡착반응은 유사이차반응속도를 잘 따르는 것을 알 수 있었다. 흡착평형을 표현하는데 널리 이용되는 Freundlich와 Langmuir 흡착 모델에 평형흡착 결과를 적용한 결과, 활성탄에 대한 Malachite Green의 흡착은 Freundlich isotherm 모델에 적합한 것으로 나타났으며 커피콩과 닭깃털에 대한 Malachite Green의 흡착은 Freundlich와 Langmuir isotherm을 동시에 잘 따르는 것으로 조사되었다. 열역학 변수를 도출 한 결과 △Go는 음의 값을 △Ho는 양의 값을 가지는 것으로 조사되었으며 이는 본 흡착반응은 자발적 경향을 나타내며 흡열반응으로 진행된다는 것을 의미한다. 또한 pH가 증가함에 따라 Malachite Green의 색도 제거는 증가하는 것으로 나타났다. 커피콩과 닭깃털에 대한 Malachite Green의 흡착은 활성탄을 흡착제로 사용했을 때에 비해 크게 좋지 않았지만 커피콩과 닭깃털은 수중 Malachite Green을 제거하기 위한 저렴한 흡착제로 사용 가능할 것으로 사료된다.
Ti source로 titanium tetraisopropoxide를 N source로 urea를 사용하여 질소가 도핑된 TiO2를 제조한 후, XRD, SEM, BET, DRS를 이용하여 분석하였다. 준비된 질소 도핑 TiO2를 태양광과 자외선 영역의 빛을 이용하여 Methylene Blue의 광촉매 분해에 적용한 결과, 질소 도핑된 TiO2는 일반 광촉매보다 특히 가시광 영역에서 높은 활성을 띄는 것으로 나타났는데, 이러한 결과는 질소가 도핑된 TiO2의 밴드갭이 크게 감소하였기 때문으로 사료된다.

목차 (Table of Contents)

• 1. Introduction = 1
• 1.1. Background = 1
• 1.2. Research goal = 3
• 2. Literature review = 5
• 2.1. Characteristics of dyes = 5
• 2.2. Dye wastewater treatment processes = 7
• 2.2.1. Biological treatment process = 8
• 2.2.2. Physical treatment process = 9
• 2.2.2.1. Adsorption = 10
• 2.2.3. Chemical treatment process = 14
• 2.2.3.1. Ozonation = 15
• 2.2.3.2. Photocatalysis = 19
• 3. Experimental Section = 23
• 3.1. Materials = 23
• 3.2. Ozone oxidation process = 25
• 3.2.1. Apparatus = 25
• 3.2.2. Procedure and analyses = 25
• 3.3. Adsorption process = 27
• 3.3.1. Preparation of adsorbents = 27
• 3.3.2. Characterization of adsorbents = 28
• 3.3.2.1. Specific surface area measurement = 29
• 3.3.2.2. Surface modification and iodine value = 30
• 3.3.3. Batch adsorption experiment = 31
• 3.3.4. Calculation = 32
• 3.4. Photocatalysis = 33
• 3.4.1. Preparation of nitrogen-doped TiO₂ = 33
• 3.4.2. Characterization of photocatalysts = 33
• 3.4.3. Photocatalytic activity tests = 34
• 4. Results and Discussion = 35
• 4.1. Ozone oxidation process = 35
• 4.1.1. Decolorization of Acid Red 27 by ozone oxidation process = 35
• 4.1.1.1. Effect of ozone dosage = 37
• 4.1.1.2. Effect of temperature = 41
• 4.1.1.3. Effect of catalyst = 43
• 4.1.1.4. Structural variations = 45
• 4.1.2. Decolorization of Malachite Green by ozone oxidation process = 49
• 4.1.2.1. Decolorization of MG by ozone = 49
• 4.1.2.2. COD removal = 50
• 4.1.2.3. Kinetics and effect of initial MG concentration = 52
• 4.1.2.4. Effect of temperature = 55
• 4.1.2.5. FT-IR and ¹H-NMR spectroscopic analysis = 56
• 4.2. Adsorption process = 62
• 4.2.1. Characterization of adsorbents = 62
• 4.2.1.1. Coffee beans = 62
• 4.2.1.2. Chicken feathers = 66
• 4.2.2. Adsorption studies = 67
• 4.2.2.1. Effect of adsorbent surface modification = 67
• 4.2.2.2. Effect of adsorbent amount = 70
• 4.2.2.3. Effect of initial MG concentration = 71
• 4.2.2.4. Adsorption kinetics = 74
• 4.2.2.5. Adsorption isotherms = 79
• 4.2.2.6. Effect of temperature = 84
• 4.2.2.7. Activation energy and thermodynamic parameters = 86
• 4.2.2.8. Effect of pH = 91
• 4.3. Photocatalysis = 93
• 4.3.1. Photocatalyst characterization = 93
• 4.3.2. Photocatalytic activity tests = 98
• 5. Conclusion = 100
• References = 104
• ABSTRACT (KOREAN) = 125