Preparation of Ice-Templated MOF-Polymer Composite Monoliths and Their Application for Wastewater Treatment with High Capacity and Easy Recycling.

An ice-templating process was used to fabricate polymer/MOF monoliths, specifically chitosan/UiO-66, as adsorbents for water treatment. The ice-templated macropores enhanced mass transport, while the monoliths could be easily recovered from solution. This was demonstrated by the adsorption of methylchlorophenoxypropionic acid (MCPP, a herbicide compound) from dilute aqueous solution. To enhance the stability, the freeze-dried monoliths were treated with NaOH solution, solvent exchanged, and dried. The treated chitosan/UiO-66 monolith achieved an adsorption capacity of 34.33 mg g-1 (a maximum theoretic value of 334 mg g-1 by the Langmuir model), closer to the capacity (36.00 mg g-1) of the freshly prepared UiO-66 nanoparticles and much higher than that of the NaOH-washed UiO-66 nanoparticles (18.55 mg g-1), by performing the tests in 60 ppm MCPP solution. The composite monolith could be easily picked up using tweezers and used for recycling tests. Over 80% of the adsorption capacity was retained after three more cycles. The powder X-ray diffraction and N2 sorption studies suggested the crystalline structure of UiO-66 was destroyed during NaOH washing procedure. This, however, provides the potential to improve the adsorption capacity by developing methods to fabricate true polymer/MOF composites.