Adsorption of methylene blue onto humic acid-coated Fe3O4 nanoparticles

Humic acid-coated Fe3O4 (HA-Fe3O4) nanoparticles as magnetic adsorbents were prepared with co-precipitation of humic acids and Fe3O4 nanoparticles. TEM analysis indicated that the average diameter of the spherical HA-Fe3O4 core was about 15 nm. TGA characterization showed that the HA-Fe3O4 nanoparticles contained about 50% (w/w) HA. The characteristic absorption of HA at 1604/cm and 1701/cm was observed in the FIRT spectra of HA-Fe3O4 nanoparticles. The HA-Fe3O4 nanoparticles exhibited a typical superparamagnetic characteristic with a saturation magnetization of 77 emu/g, which resulted in an easy solid–liquid separation with an external magnet. The HA-Fe3O4 nanoparticles were applied for methylene blue (MB) adsorption and results showed that the HA-Fe3O4 nanoparticles possessed much higher adsorbed amount of MB than the bare Fe3O4 nanoparticles and HA powders. The HA-Fe3O4 nanoparticles remained stable in a broad pH range of 3–11. The adsorption kinetics can be described by a pseudo-second-order equation, and the time when 50% of the MB was adsorbed (t1/2) was 7 min. The adsorption isotherm of the HA-Fe3O4 nanoparticles agreed well with Langmuir adsorption equation, and the maximum adsorbed amount of MB was 0.291 mmol/g. Desorption of the saturated HA-Fe3O4 nanoparticles was easily carried out with a mixture of methanol and acetic acid with a volume ratio of 9:1.