Ligand selectivity and competitive adsorption of heavy metals using beta cyclodextrin-based metal organic framework and crosslinked polymers

Considering the significant threats posed by heavy metals (HMs) to ecosystems and public health, the aim of the current study was the synthesis and characterization of β-cyclodextrin-based adsorbents, including citric acid crosslinked βCD (CA-βCD), adipic acid crosslinked βCD (AA-βCD), and βCD-based metal-organic framework (βCD-MOF), and evaluation of their efficacy in the removal of Pb 2+ , Cd 2+ , and Cu 2+ ions. The synthesized polymers were analyzed by FTIR, XRD, SEM, BET, and zeta potential methods to assess their structural, morphological, and surface characteristics. In the multicomponent system (adsorbent dosage 1 g/L, initial metal concentration 100 mg/L, pH 6.0 at 25℃), βCD-MOF exhibited the highest removal efficiency, particularly for Cu 2+ (~ 80%), followed by Pb 2+ (~ 40%) and Cd 2+ (~ 20%), outperforming AA-βCD and CA-βCD. The preferential adsorption of copper is attributed to its smaller ionic radius and higher charge density, which enhance electrostatic interactions with polymer ligands. Single-component experiments using βCD-MOF (as optimal adsorbent, 3 g/L) revealed excellent removal efficiencies, Cd 2+ : 99.93%, Pb 2+ : 97.02%, Cu 2+ : 96.30%. Interestingly, while Cd 2+ was most efficiently removed alone, despite its larger hydrated radius and higher pKa 1 , competitive adsorption favored Cu 2+ , indicating ligand selectivity shifts under mixed conditions. Equilibrium data were best fitted by the Langmuir model, with maximum single-state adsorption capacities of 142.85, 136.99, and 120.48 mg/g for Cd 2+ , Pb 2+ , and Cu 2+ , respectively. The results of real water treatments demonstrated positive relationship between water hardness and Cd 2+ removal efficiency using βCD-MOF adsorbents. These findings highlighted the role of βCD-MOF as a promising candidate for scalable water purification technologies.