Fabrication of polyvinyl alcohol/sodium alginate in-situ growth ZIF-67 hydrogel and its application to enhance the adsorption performance of Cu(II)

In this study, an effort was made to enhance the mechanical properties of natural polymer gels and address the limitations of powdered MOF materials in the adsorption of Cu(II) in water. To this end, a MOF and polymer composite hydrogel material was designed and prepared through the use of metal-organic framework materials (ZIF-67) and polyvinyl alcohol/sodium alginate (PVA/SA) hydrogels via a situ grow method. The synthesized hydrogel spheres were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), confirming the in-situ growth of metal-organic framework (MOF) material on polyvinyl alcohol/sodium alginate (PVA/SA) hydrogel spheres. The specific surface area and pore size of the composite hydrogel were measured using nitrogen adsorption-desorption isotherm to reach 613.7 mg·g⁻¹ and 2.44 nm, respectively. The adsorption characteristics, thermodynamic and kinetic modelling and adsorption mechanism of Cu(II) in water by composite hydrogels were investigated. The experimental results demonstrated that the optimal adsorption pH of Cu(II) was 5, with a maximum adsorption capacity of 128.4 mg·g⁻¹, which was 3.3 and 1.8 times that of ordinary SA and PVA/SA hydrogels, respectively. The adsorption process was found to be more consistent with the Freundlich isotherm model (R² = 0.96-0.99), while the adsorption kinetics was more consistent with the pseudo-second-order model (R² = 0.973-0.998). Thermodynamic analysis revealed that the adsorption process was endothermic and spontaneous, and that increasing the temperature is more conducive to the adsorption process. The main adsorption mechanisms of composite materials are complexation, cation exchange and physical adsorption. This is due to the participation of —OH and —COOH groups and the ion exchange of Ca(II) and Cu(II) in the adsorption process. PVA/SA@ZIF-67 can still maintain more than 90% of its original adsorption capacity after 5 adsorption cycles, as demonstrated by cyclic adsorption analysis experiments. The selective removal of Cu(II) was tested in different water sample environments. The adsorption performance of PVA/SA@ZIF-67 for Pb(II) and Cu(II) is much better than Cd(II), Zn(II), Ni(II) and Mn(II).