Effect of the Synergetic Interplay between the Electrostatic Interactions, Size of the Dye Molecules, and Adsorption Sites of MIL-101(Cr) on the Adsorption of Organic Dyes from Aqueous Solutions

Herein, we report that the surface charge status and size of the metal–organic framework (MOF) MIL-101(Cr) can be tailored by varying the concentration of tetramethylammonium hydroxide. The adsorption capacity of MIL-101(Cr)-2 toward the anionic dye Congo red (CR) was found to be significantly higher than that of other MIL-101(Cr) adsorbents, although MIL-101(Cr)-3 microcrystals exhibited the highest adsorption capacity toward the anionic dye methyl orange (MO) due to the different adsorption mechanisms involved. This is the first demonstration of the synergetic interplay between the electrostatic and π–π interactions, adsorption spaces and sites, and size of the dye molecules, in terms of the uptake capacity of an adsorbent. In addition, the obtained results shed new light on the adsorption mechanism toward organic dyes over MOFs. Moreover, the MIL-101(Cr) microparticles exhibited excellent adsorption capabilities toward CR, achieving a maximum capacity of 1367.1 mg g –1, which is higher than the majority of reported values for other materials. Therefore, these results indicate that the MIL-101(Cr) nanostructure exhibits potential for application in the treatment of anionic dye-containing wastewater.