Date Published: July 14, 2017
Publisher: Public Library of Science
Author(s): Mohamed Ateia, Christian Koch, Stanislav Jelavić, Ann Hirt, Jonathan Quinson, Chihiro Yoshimura, Matthew Johnson, Yogendra Kumar Mishra.
Current methods for preparing magnetic composites with carbon nanotubes (MCNT) commonly include extensive use of treatment with strong acids and result in massive losses of carbon nanotubes (CNTs). In this study we explore the potential of taking advantage of the inherent magnetic properties associated with the metal (alloy or oxide) incorporated in CNTs during their production. The as-received CNTs are refined by applying a permanent magnet to a suspension of CNTs to separate the high-magnetic fraction; the low-magnetic fraction is discarded with the solvent. The collected MCNTs were characterized by a suite of 10 diffraction and spectroscopic techniques. A key discovery is that metallic nano-clusters of Fe and/or Ni located in the interior cavities of the nanotubes give MCNTs their ferromagnetic character. After refinement using our method, the MCNTs show saturation magnetizations up to 10 times that of the as-received materials. In addition, we demonstrate the ability of these MCNTs to repeatedly remove atrazine from water in a cycle of dispersion into a water sample, adsorption of the atrazine onto the MCNTs, collection by magnetic attraction and regeneration by ethanol. The resulting MCNTs show high adsorption capacities (> 40 mg-atrazine/g), high magnetic response, and straightforward regeneration. The method presented here is simpler, faster, and substantially reduces chemical waste relative to current techniques and the resulting MCNTs are promising adsorbents for organic/chemical contaminants in environmental waters.
Over the past decade, carbon nanotubes (CNTs) have found diverse applications because of their remarkable mechanical, thermal and electrical properties . More specifically, carbon nanotubes functionalized with magnetic particles have attracted researchers’ interest in many fields including environmental science and waste management [2–4]. Applications of ‘magnetic CNTs’ (MCNTs) include catalysis , biomedical imaging , supercapacitors, biomanipulation , drug delivery , data storage  and environmental remediation [11, 12]. In particular, attention has focused on CNTs adsorption characteristics, due to their extremely large ratio of surface area to mass relative to most natural materials (e.g., clays and hydroxide minerals) and synthetic materials (e.g., activated carbon) [13–15]. Further, it has been proposed that MCNTs could be used to reduce the cost of environmental remediation as they could be easily separated from a solution using magnets without the need for centrifugal separation or filtration .
This study shows a simple approach to utilize the inherited magnetic properties of CNTs to obtain magnetic CNTs. The approach consists of simple magnetic fractionation of commercial CNT’s. By eliminating commonly used acid treatment, our approach is considered to be environmentally friendly, scalable and inexpensive. This method upgrades the CNT without affecting their superior surface area and chemical composition of external nanotube walls. The resulting MCNT fractions shows saturation magnetization up to 10 times the value of the as-received materials. We employed these MCNTs to repeatedly remove atrazine from water in a cycle of dispersion into a water sample, collection by magnetic attraction, and regeneration by ethanol. In addition, the separated nonmagnetic fraction of CNTs can be used in applications that require minimal metal content (e.g., electrochemical applications, or in reactions that are sensitive to the presence of metal oxides).The resulting MCNTs show high magnetic response, high adsorption capacities (> 40 mg/g) as described well with Langmuir, Freundlich and RPM isotherms and straightforward regeneration. The adsorption properties of CNT’s did not change even after 10 times of recycling. It is also recommended that sorption should be tested in continuous-flow reactor systems with magnetic separation, regeneration, and recycling of MCNTs as adsorbents. On the whole, this method of producing MCNTs is greener compared to methods reported in literature and results in materials with superior characteristics that will contribute to reducing the cost for water treatment applications.