Floatable chitosan-based cryogels embedded with copper nanoparticles as efficient and reusable heterogeneous...

In this work, chitosan-based cryogels, either native (CS) or functionalized with thiourea (CSTU), were first evaluated as sorbents for Cu(II) ions, and then transformed into heterogeneous catalysts for hydrogenation of 4-nitrophenol (4NP). In batch experiments, the maximum Cu(II) uptake by the CS and CSTU cryogels reached 1.56 mmol Cu(II)/g and 1.74 mmol Cu(II)/g, respectively. XPS and EPR investigations showed that amino and hydroxyl groups on the CS backbone are the main contributors to Cu(II) sorption. The sorbed Cu(II) ions were subsequently reduced with NaBH₄ to generate copper nanoparticles (CuNPs) uniformly dispersed in the pore walls of the cryogels. The prepared CS/CuNPs and CSTU/CuNPs composites were characterized using FTIR, SEM, EDX, TEM, TGA, and uniaxial compression measurements. The kinetics of the 4NP hydrogenation reaction were influenced by the chemical structure of the cryogels and by the NaBH₄ concentration. The apparent rate constants (k_app) increased proportionally with the concentration of NaBH₄, up to 1.24 ± 0.053 min^-1 and 1.01 ± 0.064 min^-1 for the CS/CuNPs and CSTU/CuNPs composites, respectively. The catalytic activity was maintained over four consecutive cycles, while XPS analyses demonstrated that the chemical structure of the composite catalysts was preserved after use. Furthermore, the composites achieved 4NP conversion efficiencies above 91% in both tap and lake waters, demonstrating their adaptability across diverse aqueous environments. This study presents a straightforward strategy to convert Cu(II)-loaded cryogels into reusable, high-performance catalysts, thereby contributing to sustainable wastewater remediation practices.