Pick up and dispose of pollutants from water via temperature-responsive micellar copolymers on magnetite nanorobots

Water pollution poses a significant global challenge, demanding innovative and efficient remediation technologies. Existing static water treatment methods often suffer from limitations in mass transport and mixing efficiency, resulting in incomplete pollutant removal. Nano/micromotor technology offers a dynamic alternative, enabling faster mass transport and uniform mixing, thus enhancing pollutant degradation and removal. While various light-powered and magnetically driven nano/micromotors have shown promise in environmental and biomedical applications, challenges remain in areas such as propulsion efficiency, stability, and cost-effectiveness. Many magnetically driven nanomotors rely on metal catalysts (e.g., Pt, Au) that are expensive, potentially toxic, and prone to oxidation. This research aims to address these limitations by developing cost-effective, environmentally friendly, and reusable magnetic nanorobots for efficient pollutant removal. The specific focus is on removing arsenic and atrazine, both highly toxic contaminants, utilizing a novel approach based on temperature-responsive micellar copolymers.

Existing research on nano/micromotors for water treatment highlights the advantages of dynamic systems over static methods. Light-powered micromotors have been explored for water purification, but further improvements are needed in propulsion, stability, and directionality. Magnetically driven micromotors have gained considerable traction due to their precise controllability and potential for cost-effective, long-lasting operation. However, many existing designs rely on potentially harmful metal catalysts. Previous studies have investigated the use of polymeric materials to modify the surface of nanomotors, aiming to reduce costs, enhance remediation efficiency, and minimize adverse effects. However, these polymeric nanomotors often involve complex manufacturing processes and exhibit poor mechanical characteristics, limiting their reusability and practical applicability. This study builds upon this existing work by proposing a novel design that overcomes the limitations of previous approaches.

The research employed a facile method for preparing thermosensitive magnetic nanorobots (TM nanorobots). The synthesis began with the preparation of Fe₃O₄ nanoparticles through coprecipitation of iron chloride salts. These nanoparticles were then functionalized with oleic acid to introduce carboxyl groups, enhancing the attachment of a hydrophilic/hydrophobic block copolymer, namely a pluronic triblock copolymer (PTBC). The PTBC coating provides the thermosensitive properties. The resulting TM nanorobots were characterized using various techniques including FTIR spectroscopy (to confirm PTBC attachment), XRD (to analyze the crystal structure), TEM (to examine morphology), and zeta potential analysis (to assess surface charge). The motion of the TM nanorobots under a transversal rotating magnetic field was evaluated by optical microscopy and high-speed camera. Pollutant removal experiments were conducted using solutions of arsenic and atrazine at various concentrations. The efficiency of pollutant pickup and disposal was determined by measuring the residual pollutant concentration using UV-visible spectroscopy and ICP-OES. The reusability of the TM nanorobots was assessed through multiple cycles of pickup and disposal. Experiments were conducted in ultrapure water, tap water, and river water to evaluate performance under realistic conditions. The Langmuir adsorption isotherm model was used to analyze the pollutant adsorption behavior. The characterization was performed via FTIR, XRD, TEM, and Zeta potential, followed by the motion evaluation of TM nanorobots.

The synthesized TM nanorobots exhibited excellent performance in removing arsenic and atrazine from contaminated water. The temperature-responsive nature of the PTBC allowed for efficient pollutant pickup at room temperature (above the CMT) through intermicellar aggregation, and subsequent disposal at low temperatures (below the CMT). The nanorobots showed high arsenic and atrazine removal efficiency, achieving >65% removal at optimized conditions. The efficiency depended on the concentration of the nanorobots, the adsorption time, and the applied magnetic field strength. The magnetic propulsion of the TM nanorobots significantly enhanced pollutant removal compared to static controls. The TM nanorobots demonstrated good reusability, maintaining significant removal efficiency even after ten cycles of pickup and disposal. Analysis of the adsorption behavior using the Langmuir isotherm model revealed a substantial maximum pickup capacity of 3.4 mg/g for arsenic and 3.3 mg/g for atrazine. Real-world testing in tap and river water samples showed effective pollutant removal, although performance was slightly reduced compared to ultrapure water due to the presence of other interfering substances. High concentrations of pollutants (100 mg/L) were also successfully removed by increasing the concentration of TM nanorobots. Experiments confirmed that the TM nanorobots' motion was affected by ionic strength but their aggregation was not significantly impacted.

The findings demonstrate the potential of TM nanorobots as an efficient and reusable technology for water remediation. The combination of temperature-responsive aggregation/separation and magnetic propulsion offers a significant advantage over existing methods. The high removal efficiency for both arsenic and atrazine, coupled with the reusability of the nanorobots, suggests a cost-effective and environmentally friendly approach to water treatment. The successful application in tap and river water samples further validates the practical applicability of this technology. The slight decrease in removal efficiency observed in real water samples highlights the importance of further research to optimize the nanorobot design for complex matrices. Further research could focus on exploring different types of PTBCs with varying molecular weights or functionalities to enhance removal efficiency and address the impact of ionic strength.

This study successfully demonstrates the development and application of thermosensitive magnetic nanorobots for efficient water purification. The temperature-responsive nature of the nanorobots, combined with their magnetic propulsion, offers a novel and effective strategy for removing toxic pollutants. The high removal efficiency, reusability, and simple fabrication method highlight the potential of this technology for practical water treatment applications. Future research could focus on optimizing the nanorobot design for diverse environmental conditions and exploring the removal of a wider range of pollutants.

The study's primary limitation is the slight decrease in pollutant removal efficiency observed in real water samples compared to ultrapure water. This reduction may be attributed to the presence of other interfering substances in the real water samples. Furthermore, the effect of increased ionic strength on the nanorobot's motion warrants further investigation. The long-term stability and potential environmental impact of the nanorobots over extended periods of use also require further assessment.