Preparation of a covalent organic framework-modified silica-gel composite for the effective adsorption of Pd(II), Zr(IV) and Mo(VI) from nitric acid solution.

In this study, a novel covalent organic framework-modified silica-gel composite (Si-COF) was synthesized for the adsorption of palladium [Pd(II)], zirconium [Zr(IV)], and molybdenum [Mo(VI)] from nitric acid solutions and its adsorption behaviors were systemically investigated under the effects of contact time, nitric acid concentration, solution temperature and others. The pseudo-second-order kinetic model governed the adsorption of these metal ions onto the Si-COF composite, and the Langmuir isotherm model well-matched with the experimental data, with maximum adsorption capacities of 0.588, 0.221, and 0.417 mmol/g for Pd(II), Zr(IV) and Mo(VI), respectively. The adsorption of these metal ions was clarified to originate from the interaction with the abundant nitrogenous groups on the Si-COF composite by the X-ray photoelectron spectroscopy (XPS) method.