Competing-ion-promoted U(VI) adsorption in in situ-formed hydrogen-bonded organic frameworks

Abstract

Hydrogen-bonded organic frameworks (HOFs) offer a versatile platform for efficient capture of U(VI), in which carboxylic acid groups as coordination sites can be densely incorporated and spatially organized within porous architectures. However, U(VI) adsorption is often constrained by the inherent coupling between carboxylic acid deprotonation, which favors uranyl coordination, and framework dissolution in aqueous media. Herein, two isomeric naphthalenediimide-based HOFs (m-HOF and p-HOF) with identical carboxylate densities but distinct inter-carboxylate geometries and pore structures reveal geometry-dependent adsorption behavior: the shorter inter-carboxylate spacing in m-HOF facilitates more efficient U(VI) coordination at low concentrations, whereas the larger pore architecture of p-HOF allows higher uptake at elevated concentrations. Notably, the presence of competing ions such as Gd3+ induces a pronounced salting-out effect that suppresses framework dissolution, leading to markedly enhanced maximum adsorption capacities of 1130 and 1500 mg g- 1 at pH 5 for m-HOF and p-HOF, respectively, without compromising rapid uptake kinetics. The resulting robustness enables efficient U (VI) removal from simulated and natural seawater with facile acid-triggered desorption, highlighting a competing-ion-promoted adsorption strategy for HOF sorbents operating in complex ionic environments.