Defective SO₃H-MIL-101(Cr) for capturing different cationic metal ions: Performances and mechanisms

For broad-spectrum adsorption and capture toward cationic metal ions, a facile strategy was adopted to fabricate defective SO₃H-MIL-101(Cr) (SS-SO₃H-MIL-101(Cr)-X, X = 2, 3, 4) with enhanced vacancies using seignette salt (SS) as the modulating agent. The boosted adsorption performances of SS-SO₃H-MIL-101(Cr)-X toward eight different ions, including Ag⁺, Cs⁺, Pb²⁺, Cd²⁺, Ba²⁺, Sr²⁺, Eu³⁺ and La³⁺ in both individual component and mixed component systems, could be ascribed to the effective mass transfer resulting from the exposure of defective sites. Especially, the optimal SS-SO₃H-MIL-101(Cr)-3 could remove all the selected metal cations to below the permissible limits required by the World Health Organization (WHO) in the continuous-flow water treatment system. Furthermore, SS-SO₃H-MIL-101(Cr)-3 exhibited good adsorption capacity (189.6 mg·g^-1) toward Pb²⁺ under neutral condition and excellent desorption recirculation performance (removal efficiency > 95% after 5 cycles). Moreover, the adsorption mechanism involved the electrostatic adsorption and coordinative interactions resulting from complexation between the adsorption active sites and targeted cations (like Cr-O-M and S-O-M), which were explored systematically via both X-ray photoelectron spectroscopy (XPS) determination and density functional theory (DFT) calculations. Overall, this work provided guidance for modulating SS-SO₃H-MIL-101(Cr)-X to promote its potential application in widespread metal cations removal from wastewater.