Crosslinked α-Cyclodextrin-based metal-organic frameworks: Water-stable adsorbents for enhanced removal of direct textile dyes from aqueous media

Water-stable crosslinked Metal Organic Frameworks α-Cyclodextrin-based are presented in this work for environmental applications as highly performant adsorbents. Specifically, hexamethylene diisocyanate was used as crosslinker, enhancing the high-water stability of the supramolecular assembly, otherwise not observed. The chemical and physical features of the adsorbent were thus investigated through the use of ATR-FTIR, DSC, TG, SEM, Z-potential and XRPD analyses. If α-Cyclodextrin and the corresponding Metal Organic Frameworks (assembled via K⁺ ion coordination), were highly soluble, the crosslinked structures occurred stable, exhibiting a Z-potential of −15 mV, a reduced crystalline order and thermal stability. The high performance of the adsorbent was demonstrated by testing Direct Blue-78, and its mixtures with Direct Red 83:1 and Direct Yellow 86. When Direct Blue-78 was in use, the role of different parameters affecting the adsorption process, such as pH, ionic strength, and temperature was assessed, demonstrating the key role of electrostatic attractions between the pollutant and adsorbent. The adsorption mechanism was multiple, involving also the salting out of the dye, much more evident in presence of salt-based solutions. The dye removal was endothermic and spontaneous, occurring with an increase of entropy. Langmuir, Freundlich, Temkin, and Dubinin-Raduschkevich isotherms described the process, demonstrating that a pollutants’ multilayer should be considered, and the energy distribution of the adsorption sites was not uniform, changing during the process. Accordingly, pseudo-first and second order kinetic models described the dye removal, confirming the complex mechanism of adsorption, favoring the observed high maximum adsorption capacity that occurred 75 ± 5 mg/g.