3.3 The mechanism of absorption of curcumin on NiFe2O4@SiO2)@HKUST-1 x (x=30%wt)
Based on the analysis of experimental data, the mechanism of adsorption of curcumin on the metal-organic framework nanocomposite adsorbent are attributed to (π-π) bonds, hydrogen bonds and electrostatic interactions between the curcumin functional groups and the metal-organic framework. These high adsorption capacities were obtained at pH = 7, where curcumin molecules are natural or neutral. In addition, coordinate bonds have been established between Cu2+ open sites within the HKUST-1 metal-organic framework and CO- and O-groups in curcumin molecules. Hydrogen bonds are also formed between Cu-O-Cu molecules in HKUST-1 and OH in curcumin. On the one hand, curcumin molecules can form covalent bonds with the Si-O group in nanocomposites through silica-oxygen bonds in D-ketones [29, 52, 53]. On the other hand, curcumin binds strongly to most metal ions, because curcumin has acted as a ligand and are attached to the central metals that act as the core of the metal-organic framework and create stable complexes [54-57].