Design and preparation of non-porous amorphous PEI-based polymers and their adsorption properties for anionic dyes

In this study, we report the design and synthesis of a series of non-porous amorphous polyethyleneimine-based adsorbents, specifically PEI-PD, PEI-TC, and PEI-CC, for selective adsorption of anionic dyes from aqueous solutions. Utilizing pyromellitic dianhydride (PD), terephthaloyl chloride (TC), and cyanuric chloride (CC) as cross-linking agents, we produced adsorbents with distinct morphological and adsorption characteristics. Scanning electron microscopy (SEM) revealed that PEI-PD and PEI-TC exhibit rough, folded surfaces, with total pore volumes of 0.0001 cm³ /g, 0.0002 cm³ /g, respectively, whereas PEI-CC displays particulate features with total pore volumes of 0.0040 cm³ /g, confirming non-porous nature. XRD and SAED analyses confirmed the amorphous state. Zeta potential increased for the PEI based adsorbents, correlating with enhanced electrostatic adsorption. IGMH analysis revealed non-covalent interactions, including hydrogen bonding, also contribute to adsorption. Further adsorption studies indicated pseudo-second-order and Langmuir model fits, suggesting chemisorption and monolayer adsorption. The adsorption performance was evaluated using various dyes, demonstrating that cross linker structure significantly influences adsorption efficiency. PEI-TC demonstrates the highest adsorption capacity for Congo red, reaching 3089.49 mg/g, due to its para-substituted linkage enhancing molecular flexibility, while the adsorption capacity of PEI-CC was 793.85 mg/g, as the meta-substituted CC linkage causes the molecular chains stack closely together and reduces the available free space, leading to a decreased specific surface area. Under simulated real conditions, the removal rate remained above 90 %. This study offers insights into the critical role of building blocks in the development of efficient adsorbents, highlighting the importance of polymer structure on adsorption performance.