A new MIL-101-type chromium-based metal–organic framework with densely packed sulfonic groups: an ultra-high uptake of toxic Pb ²⁺ and Cu ²⁺ ions from an aqueous medium

Metal-organic frameworks have been demonstrated to be effective adsorbents of heavy metal ions in recent decades. Nevertheless, their practical applications remain limited because of their slow uptake rates and a lack of functionalization techniques. To overcome these drawbacks, a new sulfonic-functionalized chromium-based metal-organic framework with a MIL-101-type structure was successfully fabricated, termed MIL-101-SO₃H(N), <i>via</i> a solvothermal procedure, and it demonstrated a unique uptake ability for highly toxic Pb²⁺ and Cu²⁺ cations from solution. Accordingly, MIL-101-SO₃H(N) demonstrated the highest adsorption capacity of 1449.7 mg g^-1 and 1328.4 mg g^-1 for Pb²⁺ and Cu²⁺ ions, respectively, which are much higher than those of previously reported adsorbents. It is noteworthy that the obtained data of heavy metal ion adsorption over MIL-101-SO₃H(N) best fit with the pseudo-second-order kinetic and Langmuir isothermal models, indicating that chemisorption occurred during the uptake process. In particular, the effective uptake of Pb²⁺ and Cu²⁺ ions is depicted by the strong electrostatic interaction between the positively charged metal ions and negatively charged sulfonate groups inside the MOF backbone as well as the large and suitable pore sizes of the material, leading to a considerable enhancement of metal ion uptake from an aqueous medium. These findings illustrate that the new SO₃H-modified Cr-based MOF is a potential candidate for use as an efficient absorbent in eliminating highly toxic heavy metal ions under practical conditions.