Preparation of MIL-88A micro/nanocrystals with different morphologies in different solvents for efficient removal of Congo red from water: Synthesis, characterization, and adsorption mechanisms

In this report, the morphology, size, and surface charge of MIL-88A can be tuned by adjusting the DMF-H2O mixed solvent system in the reaction system. The solvothermal method successfully prepared rod-like, spindle-like, and diamond-like crystals. Surprisingly, these crystals formed in different solvents showed considerable differences in yield and adsorption performance of anionic dyes. The yield of MIL-88A-1 synthesized in H2O/DMF biphasic solvent is the most impressive, about 3.7 times that of MIL-88A-2 synthesized in H2O as the solvent and about 7.5 times that of MIL-88A-3 synthesized in DMF as solvent. MIL-88A has a high pore volume, high BET surface area, and many open metal sites, among which MIL-88A-1 has the best structural performance and adsorption performance. Its maximum adsorption capacity is 555.5 mg∙g−1. The materials were characterized by SEM, FTIR, XRD, TGA, BET, and Zeta. Subsequently, the effects of contact time, PH value, concentration, temperature, adsorbent dosage, and other factors on the adsorption performance were systematically discussed. The adsorption removal rate of CR by MIL-88A-1 can reach 92% within 60 min. The research results show that the adsorption process fits the Langmuir isotherm and pseudo-second-order models to a high degree, and the adsorption process of the whole system can be attributed to the chemisorption of a monolayer on a uniform surface. The adsorption mechanism of MLL-88A is mainly the synergistic effect between electrostatic interaction, hydrogen bonding, and the physicochemical properties (specific surface area, pore volume, and pore size) of the adsorbent.