Microwave-assisted synthesis of amphoteric fluorescence carbon quantum dots and their chromium adsorption from aqueous solution

The chromium adsorption behavior from aqueous solution by the amphoteric Janus nitrogen-doped carbon quantum dots (AJ–N–CQDs) was investigated. The pseudo-first-order and the second-order adsorption kinetics models were employed to analyze the experimental data; the second-order adsorption kinetics model presented a better correlation to the experimental data, suggesting a chemisorptions process. The values obtained in the pseudo-first-order are still suitable for describing the Kinetics of Cr(VI) sorption. These values elucidate the surface processes involving chemisorption and physisorption in the adsorption of Cr(VI) by AJ–N–CQDs. The R(2) of the Boyd model gave a better fit to the adsorption data of AJ–N–CQDs (i.e., external diffusion), which means the surface processes involving external Cr(VI) adsorption by AJ–N–CQDs. The higher value of α may be due to the greater surface area of the AJ–N–CQDs for the immediate adsorption of Cr(VI) from the aqueous solution. AJ–N–CQDs have fluorescence spectra before and after Cr(VI) adsorption, indicating they are promising for chemical sensor applications.