Combined Tween 20-Stabilized Gold Nanoparticles and Reduced Graphite Oxide-Fe3O4 Nanoparticle Composites for Rapid and Efficient Removal of Mercury Species from a Complex Matrix

This study describes a simple method for removing mercuric ions (Hg2+) from a high-salt matrix based on the use of Tween-20-stabilized gold nanoparticles (Tween 20-Au NPs) as Hg2+ adsorbents and composites of reduced graphite oxide and Fe3O4 NPs as NP collectors. Citrate ions adsorbed on the surface of the Tween 20-Au NPs reduced Hg2+ to Hg-0, resulting in the deposition of Hg-0 on the surface of the NPs. To circumvent time-consuming centrifugation and transfer steps, the Hg-0-containing gold NPs were collected using reduced graphite oxide-Fe3O4 NP composites. Compared with the reported NP-based methods for removing Hg2+, Tween 20-Au NPs offered the rapid (within 30 min), efficient (>99% elimination efficiency), durable (>10 cycles), and selective removal of Hg2+, CH3Hg+, and C2H5Hg+ in a high-salt matrix without the interference of other metal ions. This was attributed to the fact that the dispersed Tween 20-Au NPs exhibited large surface-area-to-volume ratio to bind Hg2+ through Hg2+-Au+ metallophilic interactions in a high-salt matrix. The formation of graphite oxide sheets and reduced graphite oxide-Fe3O4 NP composites was demonstrated using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectrometry, and transmission electron microscopy. The mechanism of interaction between Tween 20-Au NPs and Hg2+ was studied using visible spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy.