Evaluation of the influence of various metals in iron-rich acid mine drainage on crystallinity, morphology and textural properties of iron-based MIL-101

Synthesis of a series of iron-based MIL-101 (MIL = materials institute lavoisier) using iron (Fe) extracted from acid mine drainage waters (AMD) and terephthalic acid extracted from waste polyethylene terephthalate is reported here. Specifically, various metal concentrations that make up AMD are evaluated for their effects on crystallinity, morphological and textural properties of the MOFs. Each metal (Al, Ca, Mg, Mn and Zn) is paired with Fe (dominant metal in AMD) to make bimetallic combinations. Two metals are also paired with Fe to prepare the trimetallic combination series. Generally, MIL-101 was formed with different degrees of crystallinity. Only trace concentrations (<1.25 ppm) of Mg, Mn and Zn were detected, while Al and Ca were undetected in the MOFs, indicating that Fe is the main metal node in all prepared MOFs. The evaluated metals resulted, to some extent in octahedral morphology and/or mixed octahedral and hexagonal bipyramidal of the MOFs. The obtained BET surface areas ranged from 26–2117 m² g⁻¹ with the highest pore volume being 0.91 cm³ g⁻¹. At 77 K and 1 bar, a hydrogen uptake of 1.44 wt% was obtained for MIL-101 from FeMn combination, which also exhibited the highest surface area. This MOF further displayed the highest CO₂ uptake of 6.17 mmol g⁻¹ at 298 K and 20 bar. The study proves that at low concentrations, metals within AMD result in MOFs with comparable properties to those derived from pristine precursors. This offers the possibility to use raw AMD directly as precursor for the synthesis of Fe-MOFs, without the need to first extract Fe from AMD.