Magnetic solid-phase extraction based on magnetic three-dimensional graphene for determination of nitrite in water samples

Graphene (G), a one-atom-thick planar carbon material patterned in a honeycomb lattice form, is a two-dimensional (2D) carbon nanomaterial and has attracted enormous research interest in recent years owing to its intriguing properties. However, due to the strong π-π stacking interaction, hydrophobic interaction and van der Waals forces, restacking and aggregation between individual graphene sheets may occur, which may greatly decrease the intrinsic specific surface area of G. In order to overcome the above problems, very recently, Chen et al. have synthesized a three-dimensional interconnected graphene (3D-G) with a foam-like network structure. Since 3D-G has a high surface area, large mesopore volume and three-dimensional nanoporous structure, it might be a promising candidate as an efficient adsorbent for some organic compounds [1]. Nitrite is one of the pollutants found in the atmosphere and natural water and is an important intermediate in biological nitrogen cycle. Traces of nitrite in drinking water may lead to mathemeglobmenia in infants and with long term exposure is a possible cancer risk [2]. Magnetic solid-phase extraction (MSPE), as a new mode of SPE, can be carried out directly in the crude sample solution with the magnetic solid absorbent being added to it and the phase separation can be realized simply by using an external magnet without the need of using additional filtration or centrifugation procedures, which makes separation easier and faster [3]. In this study, a three-dimensional graphene-based magnetic nanocomposite (3D-G-Fe3O4) was synthesized. Performance of the 3D-G-Fe3O4 was evaluated in magnetic solid-phase extraction of nitrite from water samples. For this purpose, 25 mL of a standard solution (0.2 mg L-1 of nitrite) or sample was placed into a 50 ml falcon tube. Next, 1 ml HCl (1M) and 1 ml 4-nitroaniline (10-3 M) were added to the tube in order to form a diazonium salt. After 5 min, 1 ml naphth-1-ol (10-3 M) and 2 ml NaOH (2M) were transferred into the tube to make the diazonium ion be coupled with naphth-1-ol to form an azo dye that was purple in alkaline solution (λmax= 620 nm). Then, 100 mg adsorbent (3D-G-Fe3O4) was used for magnetic solid-phase extraction of the product (azo dye). Finally, 0.5 mL acetone was used for desorption of azo dye from 3D-G-Fe3O4. Several experimental parameters affecting the reaction and extraction efficiencies, such as reagent concentration, the amount of the 3D-G-Fe3O4, reaction and extraction time, sample pH, salt addition and desorption conditions were optimized. Finally, the proposed method was successfully utilized for the pre-concentration and determination of nitrite ions from different water samples using colorimetric method.