Amino-Functionalized Zirconium-Based Metal–Organic Framework Membrane on Stainless Steel Mesh for Oil-in-Water Emulsion Separation

Abstract

Oil-in-water emulsions in oilfield wastewater are highly stable and difficult to separate, posing a major challenge for efficient wastewater treatment. Herein, an amino-functionalized zirconium-based metal–organic framework-coated stainless steel mesh membrane, denoted as ZMN-S, was fabricated through dopamine-assisted surface modification, in situ growth of zirconium-based metal–organic framework crystals, and postsynthetic ligand exchange. The structure, morphology, surface chemistry, and thermal stability of the membrane were systematically characterized by XRD, FTIR, SEM, EDS, XPS, laser confocal microscopy, and thermogravimetric analysis. The results confirmed that Zr-MOF crystals were uniformly anchored on the stainless steel mesh surface. After amino functionalization, the membrane retained good crystallinity and surface integrity, forming a micro/nanostructured interface enriched with hydrophilic functional groups. Separation experiments demonstrated that the ZMN-S membrane exhibited high separation performance toward various oil-in-water emulsions. The separation efficiency for a dichloromethane-in-water emulsion reached 99.51 ± 0.41%, with a water flux of 101.7 ± 1.9 L·m–2·h–1. Even for crude oil-in-water emulsion, the membrane maintained a separation efficiency of 97.81 ± 0.61%. After exposure to high-salinity and high-temperature conditions, the separation efficiencies remained as high as 96.51 ± 0.58% and 97.92 ± 0.63%, respectively, indicating favorable environmental adaptability. Based on calculations using the oil droplet deformation breakthrough pressure model, the critical breakthrough pressure of the ZMN-S membrane for an n-hexane oil-in-water emulsion is approximately 14.09 kPa, indicating that the membrane’s pore structure and the hydrated underwater superoleophobic interface effectively prevent oil droplets from penetrating the membrane pores after deforming under pressure. Mechanistic analysis suggests that the hydrated membrane surface, oil droplet coalescence and demulsification, and pore-size sieving effect synergistically promote water permeation and oil droplet retention. This work provides a feasible strategy for constructing robust MOF-based membranes for efficient separation of stable oil-in-water emulsions.