Synthesis of hierarchical hollow MIL-53(Al)-NH2 as an adsorbent for removing fluoride: experimental and theoretical perspective

The MIL-53(Al)-NH2 was designed to remove fluoride with hierarchical hollow morphology. It was used as an adsorbent for fluoride removal at a wide pH range (1-12) due to the positive zeta potential of MIL-53(Al)-NH2. The pH did not significantly influence the fluoride adsorption into MIL-53(Al)-NH2. However, the adsorbent indicated good adsorption capacity with maximum adsorption of 1070.6 mg g(-1). Different adsorption kinetic and thermodynamic models were investigated for MIL-53(Al)NH2. The adsorption of fluoride into MIL-53(Al)-NH2 followed the pseudo-second-order model and a well-fitted Langmuir model indicating chemical and monolayer adsorption process. When mass transfer model was used at initial concentrations of 100 ppm and 1000 ppm, the rates of conversion were 8.4 x 10(-8) and 4.7 x 10(-8) m s(-1). Moreover, anions such as SO42-, PO43-, NO3-, Cl-, and Br- also had less effect on the adsorption of fluoride. Also, experimental and theoretical calculations on adsorption mechanism of MIL-53(Al)-NH2 revealed that the material had good stability and regenerative capacity using alum as regenerant. In a nutshell, the dominant crystal face (1 0 1) and adsorption sites Al, O, and N combined well with F-, HF, and HF2- through density functional theory. It opens a good way of designing hollow MOFs for adsorbing contaminants in wastewater.