Influence of Glutaraldehyde Cross-Linking on the Structure and Metal-Binding Properties of PEI-Modified Pineapple Leaf Microfiber

Pineapple leaf microfiber (PALMF) was functionalized with polyethylenimine (PEI) and cross-linked with glutaraldehyde (GA) at controlled GA-to-PEI ratios to examine how cross-linking density influences nitrogen incorporation and amine accessibility. Elemental analysis, FTIR, XPS, and TGA were used to characterize structural changes associated with GA cross-linking. Increasing GA content to a moderate level enhanced nitrogen incorporation, whereas excessive cross-linking reduced measurable nitrogen due to partial amine consumption and increased carbon contribution from GA. Cu(II) ions were employed as a molecular probe to evaluate the accessibility and effectiveness of PEI-derived amine groups, and the highest Cu(II) uptake (35.21 mg g^–1) was observed at a GA:PEI ratio of 1:25. Further increases in GA dosage led to a disproportionate decrease in Cu(II) uptake relative to the nitrogen content, indicating reduced availability of effective binding sites despite the presence of nitrogen functionalities. This behavior is interpreted as arising from restricted conformational flexibility and diminished cooperative multidentate coordination at higher cross-link densities, demonstrating that GA cross-linking governs not only nitrogen incorporation but also the functional accessibility of amine groups in surface-immobilized PEI systems.