Metal ion-triggered Pickering emulsions and foams for efficient metal ion extraction

Emulsions and foams were constructed by using surfactant particles as stabilizers. Bis(2-ethylhexyl) phosphate, abbreviated as HDEHP, was used as both an oil in neutral form and an anionic surfactant in deprotonated form, DEHP^-. In the system of HDEHP/H₂O, upon adding NaOH, a portion of HDEHP was deprotonated to form DEHP^- as stabilizers of O/W emulsions. After introducing some certain metal ions, the O/W emulsions were transformed to W/O Pickering emulsions due to the generation of insoluble particles by DEHP^- and metal ions. In addition, DEHP^- could also combine with some metal ions to produce particles absorbed at air/water interface, forming ultrastable foams. Accompanied with the formation of Pickering emulsions and foams, the extraction of metal ions from water could be realized with high removal efficiency. The extractant, HDEHP, could be effectively recycled through convenient demulsification of Pickering emulsions or destruction of foams. This work provides new ideas for the construction of particle-stabilized dispersion systems and proposes methods with potential applications in industrial wastewater treatments.