A new MIL-101-type chromium-based metal-organic framework with densely packed sulfonic groups: an ultra-high uptake of toxic Pb2+ and Cu2+ ions from an aqueous medium

Abstract

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), via 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.

Fig. 1. The backbone of MIL-101-SO₃H(N) is generated from the {Cr₃(OH)(H₂O)₂(μ₃-O)} SBUs with H₃SNAA. Atom colors: Cr polyhedra, green; C, black; O, red; S, yellow. All H atoms are omitted for clarity.

Fig. 2. Powder X-ray diffraction patterns of as-synthesized MIL-101-SO₃H(N) (red), activated MIL-101-SO₃H(N) (blue), and resolvated MIL-101-SO₃H(N) immersed in H₂O (green) in comparison with the simulated MIL-101(Cr) (black) (a); Raman spectra of MIL-101-SO₃H(N) (red) in comparison with the H₃SNAA linker (black) (b); Fourier transform infrared spectra of the H₃SNAA linker (black) and activated MIL-101-SO₃H(N) (red) (c); TGA curve (red) and DSC diagram (blue) of activated MIL-101-SO₃H(N) (d); SEM image of MIL-101-SO₃H(N) at a scale bar of 500 nm (e); TEM image of activated MIL-101-SO₃H(N) at a scale bar of 50 nm (f).

Fig. 3. The XPS analysis of MIL-101-SO₃H(N): the XPS survey of MIL-101-SO₃H(N) (red) (a); high-resolution spectrum of C 1s in MIL-101-SO₃H(N) (b); high-resolution spectrum of O 1s in MIL-101-SO₃H(N) (c); high-resolution spectrum of S 2p in MIL-101-SO₃H(N) (d); high-resolution spectrum of Cr 2p in MIL-101-SO₃H(N) (e).

Fig. 4. Effect of the amount of SO₃H groups within the structures of MIL(Cr)-Z1 and MIL-101-SO₃H(N) on the Pb²⁺ and Cu²⁺ uptake capacity (a) [m = 10 mg, V = 100 mL, C₀ = 50–300 mg L⁻¹, t = 24 h, pH = 5]; the dependence of the original pH on the final pH for calculating pH_pzc (b); influence of solution pH on the adsorption capacity of Pb²⁺ and Cu²⁺ over MIL-101-SO₃H(N) at different concentrations of Pb²⁺ and Cu²⁺ (c) [m = 10 mg, V = 100 mL, C₀ = 250 mg L⁻¹, t = 24 h, pH = 1–5.5 for the Pb²⁺ uptake and pH = 1–5 for the Cu²⁺ uptake]; influence of MIL-101-SO₃H(N) content on the Pb²⁺ and Cu²⁺ uptake (d) [m = 5–30 mg, V = 100 mL, C₀ = 50 mg L⁻¹, t = 24 h, pH = 5.5 for the Pb²⁺ uptake and pH = 5 for the Cu²⁺ uptake].

Fig. 5. Effect of initial concentration on the Pb²⁺ adsorption capacity (a), and the Cu²⁺ uptake capacity (b) over MIL-101-SO₃H(N) [m = 15 mg, V = 100 mL, C₀ = 100–1000 mg L⁻¹, t = 24 h, pH = 5.5 for the Pb²⁺ uptake, and pH = 5 for the Cu²⁺ uptake]. Data fitting with the adsorption isotherm models: Langmuir for Pb²⁺ (c), Langmuir for Cu²⁺ (d), Freundlich for Pb²⁺ (e), and Freundlich for Cu²⁺ (f).

Fig. 6. The kinetic results for the Pb²⁺ (a) and Cu²⁺ (b) uptake by MIL-101-SO₃H(N) [m = 5 mg, V = 50 mL, C₀ = 40 mg L⁻¹, t = 1–20 min, pH = 5]; fitting results with the adsorption kinetic models: pseudo-first-order for Pb²⁺ (c) and for Cu²⁺ (d), pseudo-second-order for Pb²⁺ (e) and for Cu²⁺ (f).

Fig. 7. The reusability of MIL-101-SO₃H(N) in Pb²⁺ uptake (a), and in Cu²⁺ uptake (b); the PXRD analysis of MIL-101-SO₃H(N) before uptake (black) in comparison with the PXRD pattern of MIL-101-SO₃H(N) after desorption of Pb²⁺ (red), and Cu²⁺ (blue) (c); the FT-IR spectra of MIL-101-SO₃H(N) before uptake (red) and after desorption of Pb²⁺ (blue) and Cu²⁺ (green) (d).

Fig. 8. The XPS measurement of MIL-101-SO₃H(N) before and after Pb²⁺ and Cu²⁺ adsorption: (a) the XPS survey; (b) high-resolution curve of O 1s in MIL-101-SO₃H(N); (c) high-resolution spectrum of O 1s in Pb⊂MIL-101-SO₃H(N); (d) high-resolution spectrum of O 1s in Cu⊂MIL-101-SO₃H(N); (e) the Pb 4f XPS spectrum in Pb⊂MIL-101-SO₃H(N); (f) the Cu 2p XPS spectrum in Cu⊂MIL-101-SO₃H(N).