Synthetic modifications of metal organic framework adsorbents for environmental remediation

Abstract

The widespread usage of organic chemicals has led to an unprecedented level of pollution and associated health risk concerns. Although activated carbon (AC) based adsorption is commonly used in wastewater treatment and air purification processes, now, new metal-organic frameworks (MOFs) adsorbents with superior surface areas, chemically tuneable structures, and excellent reusability are available. This thesis evaluates the application of selected MOFs and their modified variants for removing organic pollutants from aqueous and humid air streams, reporting adsorption capacity and kinetics. For the removal of aqueous phase pollutant 2-chlorophenol, the higher surface area of MIL-101 (Cr) even with improved surface amination, gave inferior adsorption capacity compared to the hydrophobic AC, indicating the importance of MOF’s hydrophobicity. A hydrophobic MIL-101 (Cr) was synthesized using a PDMS vapour coating protocol, creating a new material with the same surface area and pore volume as pristine MIL-101 (Cr). For 0.5% toluene P/P0 vapour co-adsorption at 40% RH, this composite showed a 60% higher uptake capacity and a 34% higher aggregate adsorption rate compared to pristine MIL-101 (Cr), and 360% faster kinetics relative to AC. A solution-based treatment for MIL-100 (Fe) was developed using calixarene, producing super hydrophobic surfaces, which at 40% RH and 0.5% toluene P/P0, exhibited a 68% higher aggregate uptake rate, despite having lower pore volumes and surface areas. Finally, MIL-96 (Al) was modified using hydrolysed polyacrylamide polymer, which enlarged the 3.2-µm particles by 225% and transformed their crystal morphology. The polymer also contains amide with NH2 moieties which improved the modified MIL-96 (Al)’s uptake capacity of perfluorooctanoic acid. Overall, this thesis highlights the complexity of co-adsorption when hydrophobic and hydrophilic adsorbates are both present. It also recalls the importance of adsorption kinetics, in contrast with current MOF research emphases, which are on surface area and adsorption capacity. Faster adsorption kinetics may be preferred over a slow-diffusing adsorbate, even if the final uptake capacity is superior, for some industrial applications.