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MOF Membranes for Heavy Metal Removal
Summary:
The authors from the Industrial Membrane Research Institute, Department of Chemical and Biochemical Engineering, University of Ottawa (Canada) developed Zr-based MOF-808 and PAN/MOF-808 nanofibrous composite membranes with high water stability and large specific surface area, achieving excellent adsorption performance in heavy metal ion (Cd²⁺, Zn²⁺) removal from aqueous solutions.
Background:
1. To address the limitation that metal-organic frameworks (MOFs) for heavy metal removal are only used in laboratory scale (due to large particle demand for practical use) and conventional MOF activation (e.g., thermal/vacuum drying) causes pore shrinkage, previous researchers immobilized MOFs on substrates like ceramic membranes or used them as fillers, but these had low MOF accessibility or structural distortion.
2. The authors proposed co-electrospinning to fabricate PAN/MOF-808 composite membranes and a "hydractivation" method, realizing high MOF accessibility and pore expansion, thus enhancing adsorption capacity.
Research Content:
1.Synthesis
- MOF-808: 0.699 g ZrCl₄ and 0.210 g H₃BTC dissolved in 45/45 mL DMF/formic acid, 400 W microwave irradiation for 30 min, centrifuged, DMF-washed, 70°C dried (sample 1); some samples underwent solvent exchange (acetone/water) then 100°C vacuum drying (1A/1B/1C).
- Composite membranes: 20 wt% MOF-808 mixed with PAN/DMF, co-electrospun (PAN: 15 kV, 0.15 mm/min; MOF/PAN: 0.11 mm/min), and layered with PVDF membrane.
2.Characterizations
1) BET: Sample 1 had 939 m²/g surface area, 0.162 cm³/g pore volume, 18.5 Å average pore width; 1C (hydractivated) had 1118 m²/g, 0.202 cm³/g, 19.2 Å.
2) SEM/TEM: MOF-808 was octahedral (200 nm diameter); PAN/MOF-808 fibers had larger diameter and higher roughness than pure PAN.
3) Other tests: PXRD showed 1C’s (111) peak shifted from 4.36° to 3.91° (lattice expansion); DSC found PAN/MOF-808’s Tg (82°C) 3°C higher than pure PAN; tensile tests showed PAN/MOF-808’s Young’s modulus (146.5 ± 6.2 MPa) higher than pure PAN (127.3 ± 4.1 MPa).
3.Application
- Batch adsorption: MOF-808 (1) had max adsorption capacities of 225.05 mg/g (Cd²⁺) and 287.06 mg/g (Zn²⁺); 1C (hydractivated) reached 247.51 mg/g (Cd²⁺) and 312.68 mg/g (Zn²⁺); equilibrium in 10 min.
- Filtration: 1c (hydractivated membrane) treated 580 mL Cd²⁺ solution (30 ppb → 3 ppb) at 348 ± 55 L m⁻² h⁻¹ flux, better than 1a (464 mL); reusable for 4 cycles with no MOF leakage.
4.Mechanism
- Adsorption kinetics fitted pseudo-second-order model (R² > 0.99), indicating physical sorption; isotherms fitted Langmuir model (monolayer adsorption).
- Hydractivation caused water to expand MOF lattice (PXRD peak shift), maintaining expanded pores after vacuum drying, increasing adsorption sites; PAN’s hydrophilicity enhanced MOF accessibility.
Outlook:
This research realizes efficient preparation of MOF-based composite membranes and innovates "hydractivation" technology, providing a new candidate for practical heavy metal-contaminated water treatment, with significance for promoting MOF application in environmental engineering.
Insight Studies on Metal-Organic Framework Nanofibrous Membrane Adsorption and Activation for Heavy Metal Ions Removal from Aqueous Solution
Authors: Johnson E. Efome, Dipak Rana, Takeshi Matsuura, Christopher Q. Lan
DOI: 10.1021/acsami.8b01454
Link: https://pubs.acs.org/doi/10.1021/acsami.8b01454
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