Tannic acid-assisted construction of amino-functionalized cellulose nanofiber composite aerogel for high-performance adsorption of Cr(VI), Cu(II) and Congo red: Experimental and DFT studies

With industrialization, heavy metals and dyes pose potential environmental and human health risks. Developing adsorbent materials with three-dimensional structures and abundant active sites is essential for removing these pollutants. In this study, we developed an environmentally friendly, high-adsorption composite aerogel (CNF-TA-PMMT-PEI) comprising cellulose nanofiber (CNF), polydopamine-modified montmorillonite (PMMT), tannic acid (TA), and polyethyleneimine (PEI). The introduction of TA and PEI increased the number of adsorbed active sites and the interaction between CNF and PMMT through strong hydrogen and chemical bonding. CNF-TA-PMMT-PEI has a honeycomb-like pore structure, high porosity (98.29 %), abundant functional groups, and showed rapid and excellent adsorption performance for Cr(VI), Cu(II), and Congo red (CR), with the Q_m of 456.62, 289.86, and 3429.23 mg/g, respectively. The adsorption process followed the pseudo-second-order kinetic model and Langmuir model, indicating that the chemisorption of the monomolecular layer controlled the adsorption, and the thermodynamic adsorption indicated a spontaneous heat absorption reaction. The aerogel had good anti-interference properties and excellent adsorption performance in natural water. After 7 cycles, the removal of Cr(VI), Cu(II) and CR was maintained at 74.81 %, 68.23 % and 81.54 % of the initial performance, respectively. FTIR, XPS and DFT calculations revealed the amine group of the PEI and the phenolic hydroxyl group on TA are the main adsorption sites. The CR had the lowest adsorption energy (−4.23 Ha) compared to Cu(II) and Cr(VI), suggesting stronger interactions and greater stability on CNF-TA-PMMT-PEI. The adsorption mechanism for Cr(VI) involved electrostatic interactions, reduction, and chelation. For Cu(II), including chelation. For CR, adsorption through electrostatic attraction, π-π interactions, and Schiff base formation. In summary, CNF-TA-PMMT-PEI is an eco-friendly adsorbent with outstanding adsorption capacity and effective recovery performance for simulated wastewater.