An electrochemically-switched BPEI-CQD/PPy/PSS membrane for selective separation of dilute copper ions from wastewater

•A novel BPEI-CQD/PPy/PSS membrane was successfully synthesised and used for the removal of Cu2+.•Sorption-diffusion model was proposed for the ESIP system.•3D BPEI-CQD/PPy/PSS membrane exhibited excellent permselectivity for Cu2+.•The current efficiency of the ESIP system reached 39.9%.

The continuous separation of dilute Cu2+ from wastewater was performed by using a branched poly(ethylenimine) (BPEI)-functionalized carbon quantum dots (BPEI-CQD)/polypyrrole (PPy)/ polystyrenesulfonate (PSS) membrane in an electrochemically switched ion permselective (ESIP) process. The membrane with a high selectivity for Cu2+ separation was prepared by pressure filtering BPEI-CQD/PPy/PSS solution through a polytetrafluoroethylene (PTFE) membrane. In this ESIP system, the directional uptake/release of Cu2+ was realized by modulating the redox state of the membrane coupling with an external electric field. The effects of the operating parameters on the flux of Cu2+ and membrane permselectivity were investigated. The BPEI-CQD/PPy/PSS composite electrode showed excellent permselectivity for Cu2+ with a flux of 0.108 mg·cm−2·h−1, a current efficiency of 39.9% and an excellent cycling stability. The Cu2+ concentration in the solution was reduced from 30 to 0.82 ppm with a removal efficiency of 97.2%. Furthermore, the effects of BPEI-CQD content and the membrane thickness on the separation properties of the membrane from a mixed nitrate solution containing Cu2+, Ni2+ and Cd2+ were investigated. It is expected to understand the sorption-diffusion mechanism of such a membrane and provide more information on the design of it for a practical metal ion separation process.

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