Electro-enhanced adsorption of lead ions from slightly-polluted water by capacitive deionization

Capacitive deionization (CDI) technology is an effective method for the removal of Pb²⁺ from slightly-polluted water. In this study, reduced graphene oxide/nitrogen-doped carbon quantum dots (rGO/NCDs) composites with different NCDs mass ratios were successfully synthesized by hydrothermal method and the preparation process of CDI electrode was optimized by electrospinning method to obtain monolithic mesh-like structure. The results showed that the doping of NCDs improved the structural and electrochemical properties of the composites. rGO/25%NCDs composite possessed well-developed three-dimensional porous structure, high specific surface area (347.65 m²/g) and excellent specific capacitance (284.0 F/g). The prepared CDI electrodes had a fish scale-like graded interfacial structure, and this unique structure made the electrodes exhibit superhydrophilic property. The results of CDI tests showed that the electrosorption capacity of rGO/25%NCDs electrode could reach 19.26 mg/g which was higher than that of the pure rGO electrode and many reported electrodes, when the initial concentration of Pb²⁺ was 5 mg/L and the voltage was −1.0 V. The adsorption process was consistent with the Langmuir adsorption isotherm and the pseudo-first-order kinetic model. In addition, the study on the competitive adsorption in a co-existed system revealed that the electrosorption capacity was strongly dependent on the ionic valence rather than hydrated radius.