Abstract:

Objective Water pollution is a worldwide challenge, and it has attracted much research attention. Poly (L-lactic acid) (PLLA) is a biodegradable polymer displaying excellent prospects in water purification, and it is mostly designed to purify single pollutants displaying limited purification effect on others. Therefore, it is necessary to develop novel PLLA materials with more purifying functions to enhance the applicability of PLLA in water purification. Herein, solvent-induced crystallization and surface modification methods were applied to functionalize PLLA nanofiber membranes (NFMs) to enable oil/water separation and copper ions (Cu²⁺) adsorption performances.

Method PLLA NFMs were prepared by electrospinning, and were then treated via solvent-induced crystallization to prepare porous PLLA NFMs, followed by N-(3-aminopropyl)-imidazole (API) modification to obtain imidazole-modified PLLA NFMs. The microstructures and chemical compositions of PLLA NFMs were respectively characterized by SEM and FT-IR, and their wettability and mechanical properties were respectively measured on contact angle goniometer and universal material testing machine. Oil/water separation performances of PLLA NFMs were characterized by testing their separation flux and efficiency. Qualitative and quantitative methods were used to evaluate the Cu²⁺ adsorption performances of imidazole-modified PLLA NFMs.

Results Smooth and straight PLLA fibers with the average diameter of 885 nm were successfully prepared by electrospinning. After solvent-induced crystallization treatment based on the orthogonal experiments, the porous fibers in sample 6 displayed a better morphology than others, and the optimal condition was that the volume ratio of acetone and water was 10∶1, the applied dosage of acetone and water mixed solvent was 0.4 mL/mg, and the treatment time was 150 s. Compared to porous PLLA NFMs, the porous morphology of the imidazole-modified PLLA fibers did not change obviously, and their average diameter about 1 041 nm was close to that of porous fibers, probably owing to the occurrence of imidazole-modification only on the fiber surface without damage to the bulk structure of originally porous fibers. The changes of chemical groups of different PLLA NFMs were illustrated in FT-IR spectra, where the imidazole characteristic peak appeared at 922 cm^-1. The new stretching vibration peaks at 1 295 cm^-1 and 693 cm^-1 were assigned to the amide groups, indicating the success of API modification. The mechanical property results of different PLLA NFMs indicated that the breaking strength of the PLLA NFMs was not affected by modification treatment, although the brittleness of the as-prepared PLLA NFMs increased slightly. The water contact angle of different PLLA NFMs were also scrutinized, and the wettability of the imidazole-modified PLLA NFMs were improved owing to the imidazole and amide groups, which was conducive to the adsorption of ions. The oil/water separation process was successfully achieved using different PLLA NFMs. Compared with pristine PLLA NFMs, the separation flux of the imidazole-modified PLLA NFMs was slightly decreased, while the separation efficiency was improved, which may be caused by the reduction of the effective pore size in the modified PLLA NFMs. The measured separation flux and efficiency of the imidazole-modified PLLA NFMs were 1 044.9 L/(m²·h) and 99.1%, demonstrating their excellent oil/water separation performance. Moreover, the colorimetric method confirmed the Cu²⁺ adsorption ability of the imidazole-modified PLLA NFMs. The effect of API volume fraction on Cu²⁺ adsorption performance by imidazole-modified PLLA NFMs was discussed. With the volume fraction of API increased to 10%, the Cu²⁺ adsorption capacity of imidazole-modified PLLA NFMs gradually increased to 39.27 mg/g. 10% of API volume fraction was chosen because the API volume fractions higher than 10% induced a very limited improvement of Cu²⁺ adsorption capacity, but was expected to affect the mechanical properties of the NFMs. The results of quantitative determinations indicated that the best solution pH for Cu²⁺ adsorption was 6. The adsorption equilibrium could be achieved after 12 h, and the maximum adsorption capacity was 41.46 mg/g after adsorption for 24 h at pH 6, indicating the good Cu²⁺ adsorption property of the imidazole-modified PLLA NFMs.

Conclusion In this paper, porous PLLA NFMs with rich imidazole groups were prepared by the combination of electrospinning, solvent-induced crystallization and API modification. The various characterizations had demonstrated the successful preparation of imidazole-modified PLLA NFMs that exhibited well mechanical property and an improved wettability. The separation flux of the imidazole-modified PLLA NFMs reached 1 044.9 L/(m²·h) and the separation efficiency was up to 99.1%, which indicated their excellent oil/water separation performances. The qualitative and quantitative tests proved the good Cu²⁺ adsorption performance of the imidazole-modified PLLA NFMs, and the maximum of Cu²⁺ adsorption capacity reached up to 41.46 mg/g. In summary, the imidazole-modified PLLA NFMs possessed both oil/water separation and Cu²⁺ adsorption performance, and displayed promising application prospects in water purification.

Key words: poly(L-lactic acid), porous fiber, imidazole-modification, oil/water separation, adsorption of heavy metal ion, electrospinning, sewage treatment