Abstract:

Objective With people gradually raising the awareness of environmental protection and increasingly strict environmental policy, the pollution of dye wastewater has been attracting extensive attention. The adsorption method is one of the most cost-effective methods and environmentally friendly absorbent materials are widely explored. Polyasparthydrazide (PAHy) is a polyaspartic acid derivative with good biodegradability and abundant amino groups, associated with low cost. It is an ideal absorbent material for wastewater treatment. However, in practical applications, challenges such as poor structural stability due to the complexity of post-adsorption separation caused by water-soluble polymers, along with limited adsorption capacity, are often encountered. To address these issues, this study aims to develop a novel insoluble PAHy nanofibrous membrane with regenerative performance through electrospinning and chemical cross-linking techniques. In this study, PAHy nanofibrous membrane with high porosity was prepared and used for Congo Red (CR) adsorption.

Method Polysuccinimide (PSI) solution was prepared by dissolving PSI into N,N-dimethylformamide solvent and nanofibrous membrane was prepared utilizing electrospinning technology. The prepared membrane was soaked in 0.25 mol/L ethanediamine (ED) solution to form PSI crosslinking. Then, different concentration of hydrazine hydrate solution was added to transform PSI into PAHy. After that, 5% acetic acid solution was employed for the neutral reaction, and the samples were washed with deionized water and immersed in ethanol for deswelling. Finally, PAHy nanofibrous membrane was characterized and the Congo Red adsorption properties were evaluated.

Results The result confirmed the successful modification of PSI to PAHy, with characteristic peak imide groups shifting upon crosslinking and hydrazine treatment. The PSI nanofibrous membrane exhibited adjacent bands at 1 794 cm^-1 and 1 709 cm^-1 due to the coupled effect of two neighboring carbonyl groups. After crosslinking and hydrazine treatment, the residue succinimide rings in PSI were opened and the PAHy was generated. PAHy membrane was able to maintain the nanofibrous morphology with partly fused together, and the fiber diameter of nanofibrous membranes increased initially after modification. PSI nanofibrous membrane exhibited an excellent swelling property after ED crosslinking. PAHy nanofibrous membranes prepared by different concentration of hydrazine hydrate had a relatively small impact on its swelling properties. PAHy nanofibrous membrane with high porosity and abundant amino groups exhibited excellent adsorption capacity for CR dye. PAHy nanofibrous membrane modified by 15% hydrazine hydrate and at pH value of 5 displayed optimal adsorption capacity. And the maximum adsorption capacity was 1 679.3 mg/g. The adsorption process followed a pseudo-second-order kinetic model, as evidenced by the high correlation coefficient, indicating a chemisorption-dominated mechanism with possible physical adsorption contributions. The adsorption isotherm analysis revealed that the Langmuir model provided a better fit than the Freundlich model, suggesting a monolayer adsorption process. The regenerative capacity of PAHy nanofibrous membranes was confirmed through repeated adsorption-desorption cycles. The desorption was carried out by alkaline elution and acid neutralization. After 4 adsorption-desorption cycles, the PAHy nanofibrous membranes retained 83.4% of its initial adsorption capacity, demonstrating the potential for reuse in practical applications.

Conclusion In this study, PAHy nanofibrous membrane was successfully prepared through PSI nanofibrous membrane ethylenediamine crosslinking and hydrazine hydrate ammonolysis. The PAHy nanofibrous membrane with a large number of cationic amino functional groups exhibited an excellent CR adsorption capacity. PSI formed a structure after opening the ring with ethylenediamine. Cross-linked PSI nanofibrous membrane had good water absorption performance. Then PAHy nanofiber membrane was prepared by ammonolysis with hydrazine hydrate of cross-linked PSI nanofibrous membrane. The optimal adsorption conditions of PAHy nanofibrous membrane for CR were as follows: the concentration of hydrazine hydrate was 15%, the adsorption pH value was 5, and the adsorption time was 6 h. Moreover, PAHy nanofibrous membrane was reusable for CR adsorption after alkaline elution and acid neutralization. It suggests that PAHy nanofibrous membrane has a broad range of applications in dye wastewater treatment.

Key words: polysuccinimide, nanofiber, electrospinning, adsorption capacity, reproducibility, water treatment, adsorption material