Preparation of Electrospun PVDF Membranes Modified by Blending and Their Application in the Process of Lithium-Enriched Solution Concentration

Using the blending electrospinning modification process, PVDF nanofiber layers modified with PVP, PVA, and GO were constructed on the surface of dense PVDF base membranes to investigate the mechanisms by which different modification elements influence fiber morphology, membrane surface physical structure, chemical properties, and membrane distillation performance. The study explored the structure-activity relationship of such membrane materials during the concentration of lithium-rich solutions via membrane distillation. Furthermore, through membrane structure characterization and long-term continuous membrane distillation experiments, the efficiency and stability of the modified PVDF membranes in concentrating lithium-rich solutions were investigated.The electrospinning device used in this study was the Qingzi Nano E04 electrospinning equipment manufactured by Foshan Qingzi Precision Measurement and Control Technology Co., Ltd., primarily consisting of a high-voltage power supply, syringe pump, metal needle, environmental control system, and receiving device. The membrane distillation device used was the DCMD flat-sheet membrane testing equipment from Tongxin Environmental Protection Technology Co., Ltd., mainly including temperature control systems for the hot and cold sides (accuracy ±0.2°C), intelligent flow control system, online conductivity monitoring system, membrane mass data acquisition system, insulation system, integrated electronic control system, and temperature monitoring system before and after the membrane.(1) Preparation of Unmodified Composite Structure MembranesPVDF (7%) was dissolved in a mixed solvent system of DMF and acetone (DMF:acetone = 4:6) to prepare the spinning precursor solution. The precursor solution was delivered at a constant flow rate via a syringe pump to the high-voltage electrospinning system for spinning, with a commercial hydrophobic PVDF membrane used as the substrate for collection, thereby preparing unmodified composite structure PVDF membranes. The spinning process was conducted at room temperature (25 ± 2°C) and a relative humidity of 30-50%. The electrospinning process parameters were as follows: voltage 12-25 kV, receiving distance 15 cm, solution flow rate 0.5 mL/h, and flat receiver size 400 × 250 mm.(2) Preparation of Hydrophilically Modified Composite Structure MembranesPolyvinylpyrrolidone (PVP, 2%), polyvinyl alcohol (PVA, 2%), and graphene oxide (GO, 0.013%) were respectively incorporated into the aforementioned spinning precursor solution to prepare modified spinning precursor solutions. The modified spinning precursor solutions were continuously stirred under constant temperature oscillation conditions at 60°C for 12 hours to ensure complete dissolution of polymers and additives and the formation of a uniform solution system. The solutions were then allowed to stand for cooling and degassing for 12 hours. Using the same electrospinning process and steps as for preparing unmodified composite structure membranes, hydrophilically modified composite structure membranes modified with different modifiers were prepared.(3) Post-treatment of MembranesTo remove residual solvents and enhance membrane mechanical stability, the prepared composite structure membranes were placed between two glass plates and heat-treated in a vacuum oven at 60°C for 12 hours.Membrane surface morphology was observed using a scanning electron microscope (FE-SEM, SU8010, Hitachi/Oxford Instruments, Japan); membrane samples were sputter-coated with gold before observation. Membrane surface chemical composition was analyzed using Fourier transform infrared spectroscopy (iS50, Thermo Fisher Scientific, USA), with a scanning wavenumber range of 4000–500 cm⁻¹. X-ray diffraction (D8 Discover, Bruker, Germany) was used to analyze the composition of crystals formed during membrane distillation. Membrane hydrophilicity/hydrophobicity was determined using a static water contact angle meter (Krüss DSA25S, Germany). The concentrations of Li⁺ and Na⁺ in samples were determined using an inductively coupled plasma optical emission spectrometer (Avio 200, PerkinElmer, USA). All samples were measured in parallel, and the results were averaged.