Probing the Adsorption Behavior of the Amine Environment in As(III) Direct Removal via Postsynthetic Functionalization of MOF-808 with Amino Acids

The direct removal of trivalent arsenic (As(III)) from water remains a significant environmental challenge due to its prevalent neutral speciation and low affinity for conventional adsorbents. This study addresses this challenge by systematically engineering the active-site microenvironment of a robust Zr-based metal-organic framework (MOF-808). Through a postsynthetic solvent-assisted ligand exchange (SALE) approach, three distinct amino acids─glycine, asparagine, and histidine─were anchored into the framework to introduce nitrogen-donor sites with varying chemical complexity, creating MOF-808/Gly, MOF-808/Asp, and MOF-808/His. Comprehensive characterization (PXRD, FTIR, XPS, NMR, and BET) confirmed the preservation of the MOF structure and successful functionalization. The adsorption performance revealed a dramatic enhancement in As(III) uptake, with the histidine-functionalized variant (MOF-808/His) achieving a remarkable capacity of 181 mg/g, far surpassing that of the pristine framework. Advanced XPS analysis and adsorption isotherm modeling elucidated the critical role of the amine environment, demonstrating that the imidazole side chain of histidine facilitates superior As(III) binding via synergistic coordination and electrostatic interactions. Our findings establish that side-chain donor strength and pH-dependent protonation are key factors controlling both the thermodynamics and the kinetics of adsorption. This work provides a rational design strategy for creating highly efficient MOF-based adsorbents tailored for the removal of recalcitrant oxyanions like As(III).