A superhydrophilic battery sponge with positive and negative electrodes for the demulsification of oil-in-water emulsions

A battery sponge (BS), with the superhydrophilic positive and negative electrode, was modified for demulsifying various oil-in-water emulsions without power device. Two mechanisms of BS, same charge-BS (SC-BS, BS electrode first contact with same charged emulsion) and opposite charge-BS (OC-BS, BS electrode first contact opposite charged emulsion), were explored. The positive electrode PEI@PDA@PU (PPP) and negative electrode OA@PEI@PDA@PU (OPPP) were singularly compared with BS to elucidate the demulsification performance and mechanism. Among them, OC-BS achieved the best effect: it reduced the emulsion zeta potential to below 20 mV (absolute value); separation efficiency exceeded 99.9 %; the demulsified oil droplets coalesced inside the sponge; and residual oil droplet size of filtrate was larger than the original emulsions. Furthermore, OC-BS adapted to various emulsions, can be recycled 30 cycles (separation efficiency ≥ 99.5%), and possessed high stability. The optimally-performing OC-BS was due to the repeated action of synergistic superhydrophilic positive and negative electrodes on the charged oil of emulsion, which were demulsified to the maximum degree inside the superhydrophilic sponge. It was electrostatic action, hydrophobic effect, capillary force, and oil viscosity played the main role in oil–water separation. SC-BS only demulsified the oil droplets twice. PPP was affected by emulsion type. OPPP performance was stable but unremarkable. Meanwhile, positively and negatively charged BS could also attract cationic/anionic dyes in wastewater. The results demonstrated that BS can be effectively utilized as a demulsification material and applied to purify various emulsions.