Application of electroactive carbon nanotube membrane for wastewater treatment

다국어 초록 (Multilingual Abstract)

Humanity is suffering from an increase in non-decomposable pollutants and a decrease in clean water resources as it faces the most rapid development in history. To solve this problem, in environmental engineering, conventional water treatment processes should be improved. General water treatment processes consisted of physical, biological, and chemical treatment processes. These processes constitute the entire sewage treatment plant according to their purpose and target of treatment.
The physical processing process, including precipitation and filtration, separates various pollutants from raw water. In particular, the membrane separation process increases the efficiency of the treatment plant field by the production of uniform permeate water and reducing the volume of treatment processes. However, as pollutants are separated, fouling occurs on the surface of the membrane and inside the pores, and a physical and chemical membrane cleaning process is required periodically. This is one of the tasks that must be fundamentally solved because of operating costs. Various bacteria and other microorganisms participate in the biological process. It is a method of decomposing complex organic matter into simple and stable substances. Biological methods include anaerobic methods, aerobic methods, and anaerobic methods. The biological treatment method using microorganisms has the advantage of low operating cost and high organic treatment efficiency. However, there is a problem that microorganisms are sensitive to sudden changes in temperature or influent water composition, making it difficult to maintain constant activity. Chemical treatment methods include the injection of chemicals for removal of suspended substances, turbidity, heavy metals, phosphorus by precipitation, water disinfection, and advanced oxidation process (AOP). This approach is effective in improving treatment water quality, but it causes an increase in consumption operating costs due to continuous chemical injection.
These unit processes were researched to improve the process efficiency, and CNT was applied to various processes as one of the promising nanomaterials. CNT has been evaluated as a nanomaterial that can be applied to environmental applications due to its electrical, physicochemical, and mechanical complex properties. CNT has been applied to the membrane separation process by utilizing its high mechanical and chemical strength and fast water molecule transport properties for improvement of water permeability. By using CNTs’ compact and entangled porous structure and high electrical conductivity, CNTs were applied to identify the effect of direct interspecies electron transfer mechanism for biogas production in the anaerobic digester. As a catalytic characteristic of CNT, it was studied in the advanced oxidation process for the reduction of non-degradable organic matters. Further, CNTs have been applied as an electrically conductive material for the membrane fabrication to introduce electroactivity as an additional membrane function, called an electroactive membrane or electrified membrane. This approach could achieve not only the traditional membrane fiction of solute separation, but also electro-based phenomena, including electrochemical oxidation and reduction, electrostatic adsorption and rejection, electrophoresis, and electroporation. Among various conductive materials including carbon, metal or metal oxide, and conductive polymer, CNT is the most extensive material for the electrified membrane, because of its high electrical conductivity and ease of fabrication porous and compact structure. In this way, CNTs are being tried for various environmental applications by utilizing their conductivity. In short, application of electrically conductive membrane has a potential to environmental applications: (i) the degradation of organic compound, (ii) the decontamination of heavy metal ions, and (iii) electro active-based antifouling membranes. Therefore, the potential to address these applications was evaluated in this thesis.
First, the recent progress in the synthesis of CNT and fabrication of CNT membrane methods, with particular emphasis on improving water permeability and anti-biofouling properties was reviewed. Then, potential applications of CNT membranes other than water purification (e.g., conductive membranes, electrodes in proton exchange membrane fuel cells, and solar electricity–water generators) have been introduced. Also, future outlooks are provided to overcome the limitations of commercialization and desalination currently faced by CNT membranes.
Second, singlet oxygenation and mediated electron transfer as plausible nonradical mechanisms for organic degradation by carbon nanotube-activated peroxymonosulfate were explored by introducing CNT membrane that inhibits ionic substance exchange. The 1O2 scavenging effect was ascribed to a rapid PMS depletion by l-histidine and azide. Also, a comparison of CNT/PMS and photoexcited Rose Bengal (RB) excluded the possibility of singlet oxygenation during heterogeneous persulfate activation. concomitant PMS reduction and trichlorophenol oxidation were achieved when PMS and trichlorophenol were physically separated in two chambers using a conductive vertically aligned CNT membrane.
Third, chromium compound could be biologically and sustainably reduced through the vertically aligned carbon nanotube composite without growth inhibition by using electron transfer from an electron shuttle secreted by P. aeruginosa to Cr(VI) even in the microbial inhibitory concentration of Cr(VI). The bioelectrochemical reactor achieved a Cr(VI) removal rate of 99.6 % at 100 mg/L, which is the highest reduction rate using P. aeruginosa. Our developed microbial Cr(VI) reduction system allows microbial Cr(VI) reduction to overcome hurdles of biological treatment on toxic materials and advance our understanding of microbial electron transfer.
Next, the electrically conductive CNT membrane was fabricated for anti-biofouling property using electrical repulsion in MF process. The carbon nanotubes on commercial polyvinylidene fluoride microfiltration membrane was fabricated for biofouling control. The fabricated CNT membranes exhibited few anti-bacterial property and transmembrane pressure retardation under applied cathodic voltage. The CNT membrane under cathodic voltage plays critical role by electrical repulsion of bacteria, and a 50.0% of reduction of transmembrane pressure and 58.9 % of reduction of extracellular polymeric substances was achieved on the membrane surface. The irreversible and reversible biofouling resistance were diminished to 32.6 and 3.8 %. This study thus shows a electrical repulsion effect of the CNT membrane can be a useful strategy for controlling biofouling during the MF process without bacterial growth inhibition.
Finally, electrified membranes were fabricated using CNT and graphene for self-cleaning through bubble generation on cathodic membrane. The fabricated CNT/graphene membrane had low toxicity to microorganism and exhibited high stability even under overvoltage (i.e., 15 V). The self-cleaning effect was investigated under the applied voltages and application times and evaluated by water flux recovery. The developed self-cleaning membrane achieved more than 4 log viable cell removal, and 95.2% total EPS removal in 10 minutes. Additionally, it improved the irreversible biofouling removal effect by 30.9% in short time than the conventional physical cleaning method. Through filtration and self-cleaning cycle, 100% water flux recovery was achieved and the stability of the CNT/graphene membrane was verified. In this study, it was confirmed that the insufficient recovery of self-cleaning through conventional water electrolysis is due to the residual biofilm caused by EPS and improved the water flux recovery efficiency using high voltage, suggesting that this can be overcome by a combination of various strategies including physical cleaning.

국문 초록 (Abstract)

역사상 유래 없는 산업화와 도시화는 새로운 난분해성, 독성 오염물질의 출현을 야기하였고 이에 노출된 환경으로부터 오염물질들을 제거하기 위한 연구들이 수행되어왔다. 수계내 오염물질을 제거하기 위한 기술로써 수처리기술들이 개발되었으며, 기존의 수처리시설은 물리적, 생물학적, 화학적 처리공정으로 구성되어 있다. 하지만 새로운 오염물질의 출현으로 인해 이러한 단위 공정들의 개선이 필요하게 되었다.
탄소나노튜브는 전기적, 물리화학적, 기계적인 특성으로 인해 수처리시설의 단위공정들을 개선하기위해 심도 깊게 연구되어온 나노 물질이다. 물리학적 처리공정의 일환으로 탄소나노튜브의 물 분자 수송 특성을 활용하여 투수성이 향상된 막 분리 공정에 연구되었다. 또한 탄소나노튜브의 다공성 구조와 높은 전기 전도성을 활용해 혐기성 소화조에서 바이오 가스 생산 속도를 개선하기 위한 생물학적 처리 기술 연구에 적용되었다. 뿐만 아니라 난분해성 유기오염물질을 제거하기 위한 고도산화공정에서도 탄소나노튜브의 촉매적 특성을 활용한 연구가 실시되었다.
최근 분리막에 촉매적 특성을 부여하는 전기활성막 연구들이 시도되고 있다. 전기활성막은 기존 분리막에 전도성 재료를 도입하여 전통적인 오염물질의 분리 뿐만 아니라 다양한 전기 기반 현상을 달성할 수 있다. 탄소나노튜브는 타 전도성 재료 대비 높은 전기 전도성과 다공성 구조체 형성의 용이함으로 인해 가장 널리 사용되는 재료이다. 이러한 전기활성막은 (1) 유기 오염물질의 분해, (2) 중금속 이온의 무해화, (3) 전기활성 기반 항오염성 막 분야에 응용 가능성을 가질 것으로 평가받는다. 따라서 본 학위 논문에서는 이러한 응용분야에 대한 적용가능성에 대한 연구를 실시하였다.
첫째, 난분해성 유기오염물질에 대한 분해 메커니즘의 규명하고 둘째, 독성 중금속 이온의 생물학적 처리 공정을 평가하였다. 셋째, 막분리 공정에 적용을 위해 정전기적 반발을 통한 생물막오염 저해 막을 평가하였고 넷째, 생물막오염의 자가세정이 가능한 전기활성막 연구를 실시하였다. 본 학위 논문을 통해 전기활성막의 응용분야에 대한 적용가능성을 평가하였고 현재 개발된 전기활성막의 한계점과 개선방법에 대해 제시하였다.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Abstract 1
• 1.2 Introduction 2
• 1.3 Synthesis of VACNTs 5
• 1.3.1 Arc discharge 6
• 1.3.2 Laser ablation 7
• 1.3.3 Chemical vapor deposition (CVD) 8
• 1.4 Fabrication of the VACNT membranes 16
• 1.4.1 Interstitial space content 16
• 1.4.2 Channel opening 24
• 1.5 Applications of VACNT membranes 28
• 1.5.1 Highly permeable membranes 28
• 1.5.2 Anti-biofouling membranes 33
• 1.5.2 Salt rejection 38
• 1.6 Applications other than water purification 40
• 1.6.1 Electrical-conductive membrane 41
• 1.6.2 Electrode in proton exchange membrane fuel cell (PEMFC) 44
• 1.6.3 Solar electricity-water generator 45
• 1.7 Future outlook 46
• 1.8 Conclusion 51
• 1.9 References 52
• Chapter 2. Identifying the nonradical mechanism in the peroxymonosulfate activation process: singlet oxygenation versus mediated electron transfer 71
• 2.1. Abstract 71
• 2.2 Introduction 72
• 2.3 Materials and Methods 76
• 2.3.1 Chemicals and Materials 76
• 2.3.2 Preparation and Characterization of Vertically Aligned CNT Membrane 76
• 2.3.3 Experimental Procedure and Analytical Methods 78
• 2.4 Results and Discussion 80
• 2.4.1 Oxidation of furfuryl alcohol by the activated PMS 80
• 2.4.2 Dependence of reactivity on pH and substrate type 86
• 2.4.3 EPR study 92
• 2.4.4 PMS reduction and TCP oxidation in two chambers separated by CNT membrane 94
• 2.5 Conclusion 97
• 2.6 References 98
• 2.7 Appendix. Supporting material 105
• Chapter 3. Chromium(VI) Reduction by the Two-Chamber Bio-electrochemical System with an Electrically conductive Wall 127
• 3.1 Abstract 127
• 3.2 Introduction 128
• 3.3 Materials and Methods 129
• 3.3.1 Chemicals and Bacterial Strains 129
• 3.3.2 Preparation and Characterization of VACNT composite 130
• 3.3.3 Minimum inhibitory concentration 131
• 3.3.4 Two-chamber bioelectrical system 131
• 3.3.5 Analytical Methods 131
• 3.3.6 Electrochemical analysis 132
• 3.4 Results and Discussion 133
• 3.4.1 Microbial Cr(VI) Reduction Performance 133
• 3.4.2 Effect of pH and Concentration of Cr(VI) on Cr(VI) reduction 137
• 3.4.3 Identification of Reduced Products 140
• 3.4.4 Current Generation of Cr(VI) Reduction 140
• 3.5 Conclusion 141
• 3.6 Reference 144
• 3.7 Appendix. Supporting material 153
• Chapter 4. Biofouling mitigation using electrical repulsion through electroactive CNT membrane 161
• 4.1 Abstract 161
• 4.2 Introduction 162
• 4.3 Material and methods 163
• 4.3.1 Fabrication of the CNT membrane and synthetic wastewater solution 163
• 4.3.2 Bacteria strain and chemostat device 164
• 4.3.3 Anti-biofilm on CNT membrane tests 165
• 4.3.4 Open-loop lab scale electroactive MF setup 165
• 4.3.5 MF biofouling and anti-biofouling tests 166
• 4.3.6 Biofilm characterization 166
• 4.3.7 Biofouling resistance measurement 167
• 4.4 Results and discussion 168
• 4.4.1 Membrane characterization 168
• 4.4.2 Antibacterial property of the CNT membrane 169
• 4.4.3 Effect of applied voltage on biofouling in MF process 170
• 4.4.4 Effect of applied voltage on biofouling resistance 173
• 4.5 Conclusion 174
• 4.6 Reference 175
• Chapter 5. Carbon nanotube/Graphene composite membrane for self-cleaning of biofouling via bubble generation 180
• 5.1 Abstract 180
• 5.2 Introduction 181
• 5.3 Materials and methods 183
• 5.3.1 Materials 183
• 5.3.2 Fabrication of electrically conductive membranes 183
• 5.3.3 Membrane characterization 184
• 5.3.5 Biofouling and self-cleaning tests 185
• 5.3.6 Biofilm characterization 186
• 5.3.7 Resistance measurement 187
• 5.4 Results and discussion 188
• 5.4.1 Membrane characterization 188
• 5.4.2 antibacterial property of modified membranes 191
• 5.4.3 Effects of applied voltage on self-cleaning 193
• 5.4.4 Effects of treatment time on self-cleaning 194
• 5.4.5 Analysis of self-cleaning effect on biofilm removal on the CNT/graphene membrane 196
• 5.4.6 Effect of self-cleaning on irreversible biofouling resistance 199
• 5.5 Conclusion 201
• 5.6 References 202
• 5.7 Appendix. Supporting material 212
• Chapter 6. Concluding remarks - 217 -
• 6.1 Summary of the thesis - 218 -

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