Optimizing Th(IV) Capture via Side-Chain Carboxyl Configuration in MIL-101(Cr) Adsorbents

Efficient and selective removal of thorium from radioactive effluents remains important for environmental protection and sustainable nuclear technologies. In this work, two amino-acid-modified MIL-101(Cr) adsorbents, denoted MIL-101-1COOH and MIL-101-2COOH, were prepared to clarify how the side-chain carboxyl configuration regulates Th(IV) uptake. Structural characterizations show that both modified materials preserve the MIL-101 framework, allowing the adsorption behavior to be compared mainly from the viewpoint of ligand chemistry. Compared with MIL-101-1COOH, MIL-101-2COOH delivers a substantially higher maximum capacity of 224.16 mg/g, whereas MIL-101-1COOH reaches 110.03 mg/g. The adsorption data are better described by the pseudo-second-order kinetic and Langmuir models. Selectivity, regeneration, XPS, and theoretical calculations further indicate that Th(IV) is immobilized principally through inner-sphere coordination with carboxyl oxygen atoms. The improved performance of MIL-101-2COOH arises from the combined effects of increased oxygen-donor site density and a more favorable local coordination environment introduced by the Asp side chain. These findings provide a structure-oriented strategy for designing MOF adsorbents for radioactive wastewater treatment.