Demulsifying ionic surfactant-stabilized emulsions remains an emerging issue due to the stringent electrostatic barriers. In this work, a phosphate-mediated anion exchange strategy was proposed to fabricate a metal−organic framework, MIL-100(Fe), with adjustable surface charge for effective demulsification toward a cationic surfactant-stabilized emulsion. By adjusting the pH of the phosphate precursor solution, the surface charge of MIL-100(Fe) can be fine-tuned. At pH 3.0, the phosphate-exchanged MIL-100(Fe) with the zeta potential decreasing from 21.4 to 6.1 mV exhibited a significant enhancement of the demulsification efficiency (DE) from 35 to 91%. Further elevating the pH to 9.0 results in the zeta potential of the phosphate-exchanged MIL-100(Fe) to be reversed to −2.0 mV, and the DE can be optimized to 96% within 5 min. The demulsification mechanism was systematically explored based on the zeta potential, distribution of the surfactant, viscoelastic modulus evaluation, and morphological characterization of the emulsion in combination with monitoring of the dynamics process of demulsification. It was found that the phosphate-exchanged MIL-100(Fe) captured by the emulsion can lead to the release of the surfactant and heterogenization of the interfacial film, causing the elasticity of the emulsion to decrease and the irreversible deformation of emulsion droplets. Consequently, the destabilized emulsion could be subjected to the effective demulsification either by the fusion pathway mediated by the phosphate-exchanged MIL-100(Fe) or direct rupture. This work emphasized a facile and promising approach to deal with the cationic surfactant-emulsified oily wastewater and disclosed the fundamental demulsification process.
The development of microfiltration (MF) membranes with a high separation flux and antifouling ability is an ultimate goal for the purification of emulsified oily wastewater by MF techniques. Herein, a dual‐functional MIL‐100(Fe)@graphene oxide (MIL‐100(Fe)@GO) membrane is fabricated by in situ growth of MIL‐100(Fe) with a graphene oxide (GO) nanosheet. The porous MIL‐100(Fe)@GO composite endows the MF membrane with superhydrophilicity and demulsification functions. The MIL‐100(Fe)@GO membrane achieves an outstanding water permeation flux of 12 457 L m−2 h−1 bar−1, as encouraged by the versatility of MIL‐100(Fe) in enabling superhydrophilicity of the MIL‐100(Fe)@GO membrane and providing rich water permeation channels. An impressive emulsion separation efficiency of >99% and maximum emulsion separation flux of 5529 L m−2 h−1 bar−1 are thus delivered by the MIL‐100(Fe)@GO membrane. Moreover, a 131% recovery ratio of emulsion separation flux (7135 L m−2 h−1 bar−1) is attained with this system after regeneration by ethanol. In virtue of the demulsification of MIL‐100(Fe), the rational design of the MIL‐100(Fe) hybrid membrane exhibits a self‐cleaning effect on the emulsion separation. The demonstrated high performance of the membrane system opens up new avenues for the development of membranes with antifouling abilities and ultrahigh flux for emulsion separation.
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