Synthesis of magnetic multi-walled carbon nanotubes via facile and solvent-free direct doping method for water remediation

Background: Owing to the requirement of additional processing steps to separate the spent adsorbents from treated water, magnetic multi-walled carbon nanotubes (MMWCNTs) have recently been hailed as novel adsorbent. Methods: In this study, MWCNTs doped with iron oxide (Fe3O4) nanoparticles (n-MMWCNTs) and MWCNTs doped with polydiallyldimethylammonium chloride (PDDA) coated Fe3O4 nanoparticles (PDDA-MMWCNTs) were synthesized via solvent-free direct doping method. Prior to the doping process, the pristine MWCNTs were pre-treated with nitric acid. Significant findings: Characterization results such as FTIR, EDX, XRD and zeta potential analysis confirmed the deposition of Fe3O4 onto the MWCNTs. The stability of the synthesized MMWCNTs under different pH was examined. Detachments of Fe3O4 from n-MMWCNTs and PDDA-MMWCNTs was found at pH 9 and 3, respectively. Both types of MMWCNTs were applied to the removal of methylene blue (MB). It was found that the MB adsorption capacity of n-MMWCNTs was 11% higher than PDDA-MMWCNTs, due to significantly higher specific surface area of n-MMWCNTs. n-MMWCNTs was found to perform better in alkaline condition in which the adsorption capacity increased about 87% from pH 3 to pH 11. The adsorption of MB on n-MMWCNTs was found to obey the Langmuir isotherm (indicating monolayer MB molecules adsorption on the n-MMWCNTs) with the calculated maximum adsorption capacity of 20.37 mg/g. Moreover, the adsorption of MB followed the pseudosecond order kinetic model.

Automated summary

In this study, two different carbon nanotube adsorbents were synthesized using a solvent-free direct doping method, and their adsorption performance for methylene blue was investigated. The results showed that carbon nanotubes doped with iron oxide had a higher specific surface area and better adsorption capacity. Moreover, the stability of the doping materials under different pH conditions was also studied.