Electrospinning Machine| Doped-Mn electrospun nanofiber membrane with targeted ozone activation: High-efficiency complete pollutant removal via photocatalytic ozonation during oil-water separation

Membrane treatment systems with oil-water separation properties are one of the most widely used and efficient processes in the advanced treatment and reuse of petrochemical wastewater. However, oily wastewater contains not only emulsified oil but also a large amount of refractory organic pollutants, especially emerging contaminants (ECs), which are difficult to remove through secondary treatment processes, severely affecting water reuse efficiency and the service life of membrane modules. During membrane treatment, the dual challenges of dissolved organic pollutants in oily wastewater and membrane fouling coexist and have become increasingly prominent in membrane technology applications.To address this, developing membrane separation processes with self-cleaning activity to effectively treat ECs and mitigate membrane fouling has emerged as a viable solution.

Recently, Prof. Liu Fang and Assoc. Prof. Wang Ming from China University of Petroleum (East China) published their latest research, "Doped-Mn electrospun nanofiber membrane with targeted ozone activation: High-efficiency complete pollutant removal via photocatalytic ozonation during oil-water separation" in Journal of Membrane Science. The researchers incorporated Mn-doped carbon nitride nanotubes (CNT_Mn) into polyacrylonitrile (PAN) nanofiber membranes (M_PAN-CNTMn), endowing them with dual functionalities: efficient oil-water separation and photocatalytic ozonation (COP) self-cleaning activity.

Compared to conventional nanofiber membranes, M_PAN-CNTMn exhibits superior oil-water separation, pollutant degradation, and reusability. Additionally, the hydrophilic nano-interface of M_PAN-CNTMn facilitates effective mass transfer between pollutants and reactive species. Mn doping enhances the conversion of free O₃ into *O and O₃•⁻, enabling pollutant degradation via a synergistic pathway of *O non-radical surface activation and O₃•⁻ radical-mediated liquid-phase chain reactions. Overall, the proposed COP self-cleaning oil-water separation nanofiber membrane system offers a novel approach to synergistically address membrane fouling control and pollutant removal in oily wastewater treatment.

Fig. 1: Characterization of CNT_Mn nanopowder.

CNT_Mn nanopowder primarily consists of aligned nanotubes and nanosheets coated on their periphery. Mn atoms induce irregular bending at nanotube tips without disrupting the crystalline structure. CNT_Mn is composed of C, N, O, and Mn elements. Deconvolution of N1s spectra reveals significant differences between CNT and CNT_Mn. While both exhibit C=N-C, NC₃, C-N heterocycles, C-N-H bonds, and π-π* stacking structures, CNT_Mn shows Mn-N_x bonds alongside reduced C-N=C and C-N-H content, suggesting Mn doping sites at N in these bonds. Mn2p fine spectra deconvolution yields 2p_3/2 (624.5 eV) and 2p_1/2 (652.2 eV) peaks.

Fig. 2: Characterization of M_PAN-CNTMn nanofiber membrane.

As shown in Figure 2a, the MPAN-CNTMn nanofiber membrane primarily consists of MPAN and CNTMn. Hydrophilicity/hydrophobicity tests demonstrate that the MPAN-CNTMn nanofiber membrane exhibits excellent hydrophilic properties, with water droplets completely penetrating the membrane within 5.79 seconds. When an oil droplet is pressed onto the surface of the MPAN-CNTMn nanofiber membrane to ensure full contact until droplet deformation occurs, the oil droplet spontaneously recovers its spherical shape and detaches from the membrane surface without external force as the oil tube is slowly lifted. Furthermore, the underwater oil contact angles (UWOCA) of MPAN-CNTMn for dodecane, hexadecane, and n-hexane all exceed 150°, further confirming the membrane's exceptional underwater oleophobicity.

Fig. 3: Oil-water separation and COP catalytic performance.

Fig. 4: Self-cleaning performance.

CNT_Mn enhances M_PAN’s oil-water separation efficiency to >98% for emulsions of these oils.For COP self-cleaning, M_PAN-CNTMn degrades 95.2% of TC and 84.4% of PNP in 60 minutes, outperforming CNT_Mn powder in resisting water-quality interference. The membrane shows stable separation performance, with flux recovery rates improving by 22.7% (dodecane-TC), 24.9% (dodecane-PNP), and 19.4% (crude oil) compared to M_PAN.

Fig. 5: Mechanism of oil-water separation and COP catalysis.

The hydrophilic-superoleophobic properties enable nanoscale mass exchange with aqueous pollutants, confining COP reactions to the surface liquid film. During COP, doped Mn promotes free O₃ conversion to O1-type and O1-3-type *O₃. Stretched O1-type *O₃ bonds break to yield *O, while O1-3-type *O₃ forms a charge depletion layer at the CNT_Mn interface, generating O₃•⁻. *O and O₃•⁻ are key primary activation products, ensuring efficient *O non-radical surface reactions and O₃•⁻-mediated liquid-phase ROS chain transfer. Thus, M_PAN-CNTMn achieves high-efficiency COP self-cleaning during oil-water separation.

Paper link: https://doi.org/10.1016/j.memsci.2025.124413