Reduction and removal of methylene blue from aqueous solutions via recyclable magnetic gold nanomaterials

Nano-sized iron oxide particles displaying superparamagnetism (SPIONs), were synthesised using the co-precipitation method. Gold-coated SPIONs stabilised by phosphine ligands (1,3,5-triaza-7-phosphaadamantane (PTA) or triphenylphosphine (PPh3)), were immobilised on the surface of these SPIONs. A total of 19 nanomagnetic systems were synthesised, with varying Au loading, stabilisers and reducing agents employed. The obtained nanomaterials were fully characterised by ultraviolet visible spectroscopy (UV–Vis), Fourier-transform infrared spectroscopy (FT-IR), powder X-Ray diffraction (PXRD), high-resolution transmission electron microscopy (HRTEM) and inductively coupled plasma spectroscopy (ICP). The PTA ligand was a better stabiliser than the PPh3 ligand, resulting in more monodisperse nanoparticles for the former. These nanomaterials were subsequently used for the treatment of wastewater from the textile industry, specifically methylene blue (MB) reduction. The kinetics of this reaction using our range of nanomaterials are discussed. It was found that the adsorbents not only reduce the MB, but also remove it completely from the aqueous solution when extracting the nanomaterial by applying an external magnetic field. The MB was desorbed from the nanoparticle surface by acetonitrile, after which the adsorbent could easily be recycled up to 5 times. This reaction was also possible without using NaBH4 as a reducing agent, albeit to a slower extent. The PTA-stabilized nanomaterials prepared with NaBH4 as opposed to sodium citrate, proved to be the best adsorbent for the removal of MB from aqueous solution.