Electrospinning Equipment: A Novel Janus Membrane for Oil-Water Separation & Cr(VI) Removal

The problem of water pollution is becoming increasingly serious, with oily wastewater and heavy metal - contaminated wastewater being particularly prominent. Oily wastewater mainly comes from oil spills and industrial discharges, containing oil - water mixtures and emulsions. Heavy metal - contaminated wastewater originates from industries such as battery and nuclear industries. Hexavalent chromium (Cr (VI)) has attracted much attention due to its high toxicity. Therefore, it is urgent to develop efficient technologies for oil - water separation and Cr (VI) removal. Membrane separation technology shows great potential in wastewater treatment due to its advantages of high efficiency, low energy consumption, and easy scale - up. Janus membranes have significant advantages in oil - water separation and heavy metal removal because of their unique asymmetric wettability (one side is hydrophobic/lipophilic, and the other side is hydrophilic/oleophobic). However, methods for achieving ultra - high contrast wetting selectivity still need to be further explored. Recently, a research team led by Hualin Wang from the School of Chemistry and Chemical Engineering, Hefei University of Technology, published a study titled "A novel multifunctional Janus membrane for on - demand oil - water separation and Cr (VI) removal highly efficient" in Separation and Purification Technology. In this study, a novel multifunctional Janus membrane was prepared via electrospinning machine for oil - water separation and heavy metal removal, providing a new strategy for wastewater treatment.

I. Abstract

The research team developed a novel Janus membrane (PCP@PAN - PEI) via electrospinning for the separation of oil - water (mixtures and emulsions) and the removal of Cr (VI). The asymmetric wettability of this membrane is derived from the superhydrophobic Janus - PCP side (with a water contact angle of 154.4 ± 0.6°) and the superhydrophilic Janus - PAN - PEI side (with a water contact angle of 0°). This superhydrophobicity is attributed to the hydrophobic groups (such as −CF₂ and –CH₃) in PCP and its high surface roughness. The membrane exhibits excellent separation performance for oil - water mixtures (flux: 7428.5 L·m⁻²·h⁻¹, separation efficiency > 99.5%) and emulsions (flux: 1797.7 L·m⁻²·h⁻¹, separation efficiency > 98.1%), and has good cycling stability. In addition, the relevant separation mechanisms were explored. Moreover, the membrane can effectively remove 78.7% of Cr (VI) at pH = 3, and the adsorption behavior can be well described by the pseudo - second - order kinetics and Langmuir models. The Janus membrane PCP@PAN - PEI shows great potential in oil - water separation and heavy metal removal, and this study also provides a practical strategy for the design of multifunctional Janus membranes.

II. Main Findings

Membrane Preparation and Asymmetric Wettability: The PCP@PAN - PEI Janus membrane was successfully prepared by the layer - by - layer electrospinning device method, and there is a significant difference in wettability on its two sides. The Janus - PCP side has a water contact angle (WCA) of 154.4 ± 0.6° and exhibits superhydrophobicity due to hydrophobic groups such as - CF₂ and - CH₃ and high roughness. The Janus - PAN - PEI side has a WCA of 0° and is extremely hydrophilic. As can be seen from Figure 1, water droplets on the PCL membrane are difficult to separate, while water droplets on the Janus - PCP side can be easily squeezed and lifted without residue, intuitively reflecting the difference in wettability between the two sides and confirming the unique asymmetric wettability of the Janus membrane.

Fig 1：Anti-water adhesion behavior of (a) PCL membrane and (b) Janus-PCP side. (c) WCA of PCP layer with different modification time of PFDTS. (d) The behavior of different droplets on Janus PCP side. (e) Spreading behavior of Janus-PAN-PEI side. Influence of AN/PEI ratios on (f) time required for water droplets to completely wet on Janus-PAN-PEI side and (g) UOCA of Janus-PAN-PEI side. (h) Photographs of the underwater oil droplet adhesion test on Janus-PAN-PEI side. For graphs (c and f-g), different letters indicate a significant difference (p<0.05)

Oil - Water Separation Performance: The membrane performs outstandingly in the field of oil - water separation. The separation flux for oil - water mixtures is as high as 7428.5 L·m⁻²·h⁻¹, and the separation efficiency exceeds 99.5%. The separation flux for emulsions is 1797.7 L·m⁻²·h⁻¹, and the efficiency is greater than 98.1%. Multiple cyclic experiments show good cycling stability. As can be seen from Figure 2, after 20 cycles of separating the light oil (n - hexane)/water mixture, the flux remains stable at 6371.1 L·m⁻²·h⁻¹, and the efficiency reaches 97.7%. After 10 cycles of separating water - in - oil and oil - in - water emulsions, although the flux changes, the separation efficiency still remains at about 95%, highlighting its high efficiency and stability in separating complex oil - water systems.

Figure 2: Flux and separation efficiencies of PCP@PAN-PEI for (a) oil/water mixtures and representative mixtures of (b) n-hexane/water and (c) CS2​/water for 20 cycle. Flux and separation efficiencies of PCP@PAN-PEI for (d) water-in-oil and (e) oil-in-water emulsions, and representative emulsions of (f) water-in-n-hexane and (g) n-hexane-in-water for 10-cycle. For graphs (a-g), different letters indicate a significant difference (p<0.05)

Cr (VI) Removal Performance: The PCP@PAN - PEI membrane has a remarkable effect on removing Cr (VI). At pH = 3, the removal rate can reach 78.7%. The adsorption process follows the pseudo - second - order kinetics (PSO) and Langmuir models, indicating that intermolecular interactions dominate the adsorption. As shown in Figure 3, the membrane has a high removal rate of Cr (VI) under acidic conditions, and the rate decreases with the increase of pH. The fitting curves of adsorption kinetics and isotherms also confirm that the PSO and Langmuir models can effectively describe this adsorption process, revealing the interaction mechanism between the membrane and Cr (VI).

Figure 3: (a) Schematic diagram of Cr (VI) distribution in aqueous solution at different pH (based on the ionisation equilibrium equation). (b) Removal efficiency of the Janus membrane towards Cr (VI) at various pH values. Adsorption kinetics of Cr (VI): (c) PFO and (d) PSO. Adsorption isotherm models of Cr (VI): (e) Langmuir and (f) Freundlich. (g) Schematic illustration of Cr (VI) removal mechanism. (h) SEM images of PCP@PAN-PEI after Cr (VI) adsorption. For graph (b), different letters indicate a significant difference (p<0.05).  

Article Source: https://doi.org/10.1016/j.seppur.2025.132588