Synthesis and application of fluorescent N-doped carbon dots/hydrogel composite for Cr(VI) adsorption: Uncovering the ion species transformation and fluorescent quenching mechanism.

A fluorescent composite material fabricated from nitrogen-doped carbon dots with polyvinyl alcohol (PVA)/polyvinylpyrrolidone (PVP)/citric acid (CA) hydrogel was synthesized using a microwave-assisted hydrothermal method. The composite was used as a metal ion sensor and adsorbent to remove chromium (Cr(VI)) from water. The chemical structure and Cr(VI) removal performance of the fluorescent composite films were also characterized. Fluorescent quenching upon Cr(VI) adsorption showed that Cr(VI) binding was attributed to the N-doped carbon dots. The results were confirmed by several analytical techniques, including X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and X-ray absorption spectroscopy (XAS). The mechanism of Cr(VI) removal from water by the fluorescent composite film was based on the adsorption and subsequent reduction of N-doped carbon dots within the 3D porous composite film. XPS measurements showed that 53.2% Cr(III) and 46.8% Cr(VI) were present on the composite surface after Cr(VI) adsorption. Moreover, XAS revealed a change in the oxidation state of Cr(VI) to Cr(III) after adsorption and in the Cr-O bond length (1.686 Å to 2.284 Å) after reduction. The Cr(VI) adsorption capacity of the composite film was 4.90 mg g at pH 4 and fit the pseudo-second-order kinetic and Freundlich models. The results of this study could be used as a platform to further apply CDs/HD composites to remove Cr(VI) from water sources.