Micromotors for Environmental Applications

Jemish Parmar

Dr. Samuel Sánchez

IBEC Smart nano-bio-devices’ at the Max Plank Institute for Intelligent Systems

This thesis was carried out to study, design and synthesize autonomously moving structures of the order of micrometers, also called micromotors, for the removal of contaminants in water. The micromotors are designed using advanced nanofabrication technologies, which allow the integration of various functional materials. These microstructures use the chemical energy present in their environment to be propelled through the expulsion of bubbles, while their active surface allows the removal of contaminants. The movement of the micromotors in fluids, together with the generated bubbles, provides a much faster mixing of the fluids, also increasing the mass transfer between the active material and the contaminant at the microscale. Our results demonstrate that the movement of the micromotors promotes a significant improvement in the removal of contaminants compared to that of these same active materials in the absence of movement. In fact, the increase in activity is even better than the mixing or external agitation generally used during the contaminant removal process. These results can be translated into an improvement in the activity of active catalytic and absorbent materials used both commercially and industrially for water treatment, while avoiding the energy cost associated with conventional mixing methods. The micromotors that we have synthesized in this thesis are driven by the use of hydrogen peroxide as a fuel source, or they contain magnesium, which acts as an on-board fuel by reacting directly with water. These active materials are used for the removal of organic contaminants (ink or pharmaceutical residues) and heavy metals (lead, mercury, cadmium, zinc), as well as for their bactericidal activity. We use advanced oxidation, photocatalytic and adsorption methods driven by the active materials present in the structure to eliminate the contaminants of interest. All the micromotor designs present in this thesis contain magnetic materials that allow their subsequent recovery through the use of magnets once the elimination of contaminants has been carried out. Similarly, magnetic fields are also used to guide micromotors in a controlled manner. In our efforts to develop multifunctional micromotors and scalable synthesis methods, we have managed to develop fabrication techniques for micromotors that use inexpensive materials such as metal oxides, while considering simple and low-cost chemical reactions. We have used these results to design prototypes for small water treatment units that use micromotors to perform efficient water filtration without the need for pressure, vacuum or electric pumps thanks to the chemical reactions that take place in the presence of the micromotors. This section is not mentioned in the thesis due to possible conflicts with patent application processes. The results obtained in this thesis have been published in prestigious journals and cited more than 280 times, while they have been highlighted in various media outlets, such as La Vanguarda, RTVE, Discovery Channel, Scientific American and Business Insider. We hope that this method of micromotor synthesis, based on relatively simple methods, will promote the implementation of micromotors in commercial applications for water treatment. Overall, our results show that multifunctional self-propelled micromotors can become an effective tool for water decontamination in the future.

This thesis was carried out to study, design and synthesize micrometer sized motile structures, dubbed as micromotors to remove pollutants from water. The micromotors are designed using advanced nanofabrication technologies and contain various combinations of materials. They use the chemical energy present in their environment to propel via ejection of bubbles and the active surface to remove pollutants. The motion of micromotors in fluid along with the bubbles generated contribute with rapid mixing of the fluid and enhance the mass transfer between active material and pollutant at the microscale. The micromotors that we synthesized during the research work for this thesis can propel using either hydrogen peroxide as their fuel source or contain magnesium as an on-board fuel, which reacts with water for propulsion. They are fabricated with active materials to remove organic (dye waste, pharmaceutical waste) and heavy metal (lead, mercury, cadmium, zinc) pollutants, as well as to exhibit bactericidal activity. We utilized advanced oxidative, photocatalytic and adsorption based methods driven by the active materials present in the structure to remove the target pollutants. All the designs of micromotors presented in this thesis contain magnetic materials and thanks to that, they can easily be recovered from the water after the pollution removal using magnets. Taking advantage of the same, magnetic guidance system is also used demonstrated to guide the direction of micromotors from one point to another. In our efforts to develop multifunctional micromotors and scalable synthesis methods, we achieved fabrication techniques for micromotors using inexpensive materials such as metal oxides and that use simple chemical approaches. Our results consistently demonstrated that the motion of the micromotors leads to significant improvement in the removal of the pollutants compared to the same active materials without motion. The increase in the activity is even better that external mixing or agitation used during the process of pollution removal. These results can be translated to the improve the activity of active catalytic and adsorptive materials used in the commercial and industrial practice for water treatment and the energy cost of the mixing can possibly be avoided as well. We exploited these results to design prototypes for a small water treatment unit that utilizes micromotors and can also efficiently carry out filtration of water without need of pressure, vacuum or electric pumps thanks to the chemical reactions carried out by the micromotors. This part in not mentioned in the thesis due to possible conflicts with patent processing. The technology of decentralized water treatment unit that we applied for patent has potential to be used as miniature water purification systems and later can be scaled up to treat large volumes of water. To start with, the results of the thesis work can directly be utilized for the synthesis of motile materials to enhance the pollution removal rate in treatment plants where adsorption processed, and advanced oxidative chemical processes are used in conjugation with heterogeneous catalysts. Some of the commercial applications in which, instead of incorporating micromotors into the existing infrastructures, new use cases can be developed such as cleaning water within the water transportation pipelines or to recover precious metals from water during mining processes. Some of the more advance applications which are not quite yet possible but certainly possible with further research include large scale water cleaning. Currently there are no efficient methods available to clean water after large scale oil spill, mining spill or to clean already heavily contaminated lakes and rivers due to industrial waste water. With sufficient advancement in the micromotors based technology in terms of propulsion, guidance and mass fabrication, swarms micromotors could clean up water bodies in future. Our current results serve as the pioneer work for the technology and has already sparked huge interest for further research in this area demonstrated by more than 280 citations of research articles published during this work. Due to very early and pioneering nature of the proposed technology, assessing viability is a difficult task. Immediate technological application of incorporating micromotors in existing infrastructures and development of decentralized treatment unit are feasible options and require strong collaboration with industrial partners for the development of the technology for specific use cases in order achieve economic benefit from enhanced pollution removal. Standalone applications in which micromotors are used to clean pipes and water during transportation in pipes or in mining processes require clear problem analysis, for the identification of the niche where micromotors technology can be commercially and economically viable. Nonetheless, the deployment of micromotor swarms for cleaning large water bodies requires further research into the topic to address many engineering challenges before the technology can commercially viable. In any case, micromotors based water cleaning technology is an example of high-tech nanotechnology based approaches for solving real word problems with strong commercialization potential. Upon successful implementation this technology can have huge impact in the lives of billions of people that lack access to clean water. Not only in the developing countries, but also in developed nations, such technology can help decrease the energy cost of water treatment, and especially with more and more stringent water quality criteria from regulators, new sustainable technologies are needed. The proposed technology is of special importance and can contribute towards global societal challenge of provide clean and safe water to the entire pollution of the word in a cost effective and sustainable way. The timeline for commercialization of proposed technology would depend on the level of advancement warranted in the specific commercialization approach. Incorporation of micromotors into existing water treatment infrastructure can be achieved within 3 years or less with dedicated and collaborative efforts with industrial partners. However, development of commercial pipe cleaning micromotors or precious metal scavenger micromotors may take 5 years depending on the potential economic and investment interest in the technology. Large scale water cleaning micromotors are extremely interesting for the perspective of both commercial and environmental interest but requires advancement in the scientific and engineering challenge which may take up to a decade for commercial micromotors that can clean lakes and rivers but very attractive opportunity in terms of its societal, economic and commercial benefits. The results presented in this thesis work propose and initiate the development of an advanced nanotechnology based approach for

Micromotors; Autonomously Moving Structures; Nanofabrication Technologies; Functional Materials; Chemical Energy; Bubble Expulsion; Contaminant Removal; Water Contamination; Fluid Mixing; Mass Transfer; Catalytic Materials; Absorbent Materials; Water Treatment; Energy Cost; Hydrogen Peroxide Fuel; Magnesium Fuel; Organic Contaminants; Pharmaceutical Residues; Heavy Metals; Lead; Mercury; Cadmium; Zinc; Bactericidal Activity; Advanced Oxidation; Photocatalytic Methods; Adsorption Methods; Magnetic Materials; Micromotor Recovery; Magnets; Magnetic Fields; Multifunctional Micromotors; Scalable Synthesis Methods; Inexpensive Materials; Metal Oxides; Simple Chemical Reactions; Low-Cost Chemical Reactions; Water Treatment Units; Water Filtration; No Pressure Pumps; No Vacuum Pumps; No Electric Pumps; Patent Application Processes; Scientific Publications; Prestigious Journals; Media Outlets; Commercial Applications; Self-Propelled Micromotors; Water Decontamination.