A series of derivatives of the Zr-based MOF UiO-66 were
synthesized,
fully characterized, and tested for their efficiency in the adsorption
and removal of the highly toxic elements lead (Pb(II)), cadmium (Cd(II)),
arsenate As(V), and selenite Se(IV). Different structural engineering
techniques were employed to alter the properties of the Zr-MOFs, such
as linker functionalization and defect creation to increase the density
of adsorption sites. The functionalization of the UiO-66-based structures
with carboxylate groups increased the removal efficiency of Pb(II)
cations by more than 70% and that of Cd(II) cations by more than 60%
to reach 125.2 and 69.1 mg/g, respectively. Moreover, the induced
cluster defects were found to be the preferred adsorption sites of
the anion pollutants. The adsorption of As(V) and Se(IV) by the high-defect
UiO-66-200FA was 132.5 and 30.8 mg/g, respectively, which was higher
than that of the less defect UiO-66, while almost no adsorption was
achieved by UiO-66(COOH)2. Furthermore, a multivariate
(MTV-MOF) approach was employed to investigate the effect of the functional
groups on the diffusion of the adsorbates within the porous networks.
Interestingly, adsorption equilibrium was reached within minutes for
the best-performing MOFs, highlighting their potential to be used
in practical water treatment applications. Finally, chemical adsorption
seemed to be dominant among binding mechanisms for all the pollutants,
which were analyzed in detail to determine the characteristics that
govern the adsorption efficiency of UiO-66-based adsorbents. The conclusions
provided by this comprehensive study help serve as a guideline for
future researchers aiming to employ UiO-66-based adsorbents by providing
them with a deep understanding of the design strategies required to
maximize the performance of this robust MOF.