A theoretical chemistry framework to unravel the monolayer adsorption of geogenic anions on UiO-66 and ZIF-8 organometallic structures

The arsenic and fluoride adsorption equilibria of UiO-66 and ZIF-8 organometallic structures were analyzed and interpreted. Experimental isotherms were quantified at 20-40 degrees C and pH 7 for both geogenic pollutants. Density functional theory and statistical physics simulations were carried out to explain and characterize the interfacial interaction forces for the adsorption of these toxic geogenic anions. The adsorption properties of UiO-66 were superior to those of ZIF-8 for both geogenic anions. The maximum fluoride and arsenic adsorption capacities were 4.3 and 2.8 mmol/g, respectively. The best adsorption of fluoride and arsenic was obtained using UiO-66 at 20 degrees C and pH 7. These MOFs showed adsorption capacities up to double those reported for several adsorbents used in the depollution of water containing these geogenic pollutants. The adsorption of arsenic and fluoride on ZIF-8 structure was endothermic (16-31 kJ/mol), while the removal of both pollutants using UiO-66 MOF was exothermic (-26 --41 kJ/mol). This adsorption behavior was associated with the presence of a multi-anionic mechanism for both arsenic and fluoride on ZIF-8 surface. On the other hand, a double monodentate interaction was found for the arsenic adsorption on the external UiO-66 surface due to steric restrictions for mass transfer, while the fluoride adsorption corresponded mainly to a mono-anionic interaction where the internal pore diffusion played a relevant role. The calculated energies for fluoride adsorption were higher than those obtained for arsenic removal indicating stronger interfacial interactions. These results demonstrate the promising potential of these MOF structures for developing cost-effective water treatment processes to remove arsenic and fluoride from groundwater.