Ultrahigh Chemically Stable Silica-Embedded UiO-66-SO3H MOF for the Efficient Recovery of Pd(II) from an Acidic Medium: Experimental and DFT Study

A highly stable acid-resistant Si@-UiO–SO3H MOF was synthesized and utilized for the rapid and efficient extraction of Pd from an acidic medium (nitric acid). FTIR, powder XRD, TGA, NMR, UV–vis absorption spectroscopy, SEM, and EDX were employed for the characterization of synthesized UiO-66-SO3H and Si@-UiO-66-SO3H MOFs. Chemical stability studies were performed to both the MOFs in nitric acid and aqueous medium (pH 1–9). Chemical studies demonstrated that Si@-UiO-66-SO3H is stable up to 8 M acid (HNO3), while UiO-66-SO3H is stable only up to 0.01 M acid (HNO3). However, two MOFs exhibited good stability in the whole range of pH from 2 to 9. Both UiO-66-SO3H and Si@-UiO-66-SO3H MOFs had been investigated for Pd(II) sorption from aqueous (pH) as well as acidic medium via the batch method. Sorption studies revealed that Si@-UiO-66-SO3H showed rapid (∼5 min) and high sorption efficiency (∼84%) toward Pd(II) with a sorption capacity (Cs; mg/g) of ∼248 mg/g (Co;1000 mg/L) from an acidic medium at 1 M, whereas in the pH medium, it exhibited a saturation sorption of only about 35% at pH = 5 within 15 min. The bonding between Pd(II) species and the MOF sample along with the chemical environment before and after the adsorption of Pd(II) onto MOF was confirmed by FTIR and X-ray photoelectron spectroscopy. In addition, the inherent bonding mechanism between Pd(II) and the functional moiety (O═S═O) of MOF was studied by applying DFT calculations. The bonding mechanism between Pd(II) and Si@-UiO-66-SO3H is facilitated by complex formation via the monodentate coordination of the S = O group with Pd(II); in addition, hydrogen bonding also plays an important role. To get back the sorbed Pd(II) onto the surface of MOFs, it was analyzed by desorption studies. Desorption studies revealed that 1 M Na2CO3 has high recovery efficiency (∼70%) toward Pd(II) sorbed onto MOFs. This work provided the facile method for the extraction and removal of Pd(II) by a highly stable MOF for practical applications as well as environmental concerns.