Metal–Organic Framework-Based Aerogel: A Novel Adsorbent for the Efficient Removal of Heavy Metal Ions and Selective Removal of a Cationic Dye from Aqueous Solution

Abstract

Ameliorating the existing purification methods with more affordable, efficient, and industrially adoptable methods that remove heavy metal ions from contaminated drinking water constitutes a persistent technological challenge. Metal–organic framework (MOF)-based materials with their excellent adsorption performance are recently explored as an alternative method to the traditional methods. Here, we report a MOF/biopolymer composite-based aerogel that is a cost-effective, highly efficient, and environmentally sustainable material for the removal of large concentrations of heavy metal ions (1277 mg of Pb2+, 466 mg of Cd2+, and 706 mg of Cr6+ per gram of the aerogel) and the selective removal of the cationic dye crystal violet (310 mg/g) from its mixture with (anionic) methyl orange from contaminated water samples. The MOF used herein is 2-methylimidazole zinc complex (ZIF-8), whereas the biopolymer is the nontoxic and biodegradable Iota-Carrageenan (iCG), a sulfated polysaccharide extracted from red seaweeds. We have transformed the ZIF-8 powder into a moldable, load-bearable, flexible, and eco-friendly ZIF-8@iCG aerogel. SEM images confirmed that ZIF-8 is physically embedded in Iota-Carrageenan, forming a honeycomb structure. The ZIF-8 is located onto the fibers of iCG, forming a thermally stable, flame-retardant ZIF-8@iCG hydrogel, with high mechanical strength and intrinsic porosity. The aerogel removes 92.03% of Pb2+ ions from an intentionally spiked Pb2+ solution (1 ppm), making this water sample drinkable upon three successive filtrations. The thermodynamics of metal ion adsorption suggest that the adsorption kinetics follow the Langmuir adsorption isotherm model and the adsorption process is endothermic. Ion leaching experiments suggest that leaching of zinc ions from the aerogel is negligible. Thus, the newly designed ZIF-8@iCG aerogel is a potentially effective adsorbent for the removal of heavy metal ions and organic dyes from drinking water and is intended for industrial use.