Structure Design of Cellulose-Based Solid Amine Adsorbents and Evaluation of Their Cr(Vi) to Cr(Iii) Reduction Mechanism in Situ Based on Nuclear Magnetic Resonance Spectroscopy

The branched structure of adsorbents employed for the efficient removal of heavy Cr(VI) metals from aqueous solutions has been demonstrated to be the key factor affecting their Cr(VI) adsorption rate. Moreover, stable chemical bonding between functional groups and the absorbent matrix further supports the possibility of recycling and low energy consumption for adsorption methods. However, the effects of the different structures of these materials on the adsorption performances remain poorly understood owing to the difficulty in analyzing the Cr(VI) adsorption and the Cr(VI) to Cr(III) reduction in situ. The present work addresses this issue by in situ analyzing the Cr(VI) adsorption and Cr(VI) to Cr(III) reduction mechanisms of three cellulose-based solid amine adsorbents based on an operando nuclear magnetic resonance (NMR) spectroscopy. The adsorbents are synthesized by a one-step rapid cross-linking method using microcrystalline cellulose as the substrate and three different functional reagents, including tetraethylene pentamine with a small molecule structure, polyethyleneimine with a linear structure, and hyperbranched polyamines with terminal amino groups. The resulting adsorbents, which have essentially equivalent functional amino group densities, represent ideal materials for evaluating the impacts of molecular structures and functional groups on the Cr(VI) adsorption. Operando NMR spectroscopy enables the adsorbed Cr(VI) and reduced Cr(III) to be differentiated and their concentrations quantified with good precision during the adsorption process. The results reveal the different adsorption mechanisms of the specific structures such as the small volume molecule and the hyperbranched structure and provide the possible way to enhance the adsorption by optimizing the structural design