Abstract
The presence of hexavalent chromium water pollution is a growing global concern. Among the currently applied technologies to remove CrVI, its adsorption and photocatalytic reduction to CrIII less mobile and toxic forms are the most appealing because of their simplicity, reusability, and low energy consumption. However, little attention has been paid to bifunctional catalysts, that is, materials that can reduce CrVI to CrIII and retain both hexavalent and trivalent chromium species at the same time. In this work, the dual CrVI adsorption–reduction capacity of two iconic photoactive water-stable zirconium and titanium-based metal–organic frameworks (MOFs) has been investigated: UiO-66-NH2 and MIL-125. The bifunctionality of photoactive MOFs depends on different parameters, such as the particle size in MIL-125 or organic linker functionalization/defective positions in UiO-66 type sorbents. For instance, the presence of organic linker defects in UiO-66 has shown to be detrimental for the chromium photoreduction but beneficial for the retention of the CrIII phototransformed species. Both compounds are able to retain from 90 to 98% of the initial chromium present at acidic solutions as well as immobilize the reduced CrIII species, demonstrating the suitability of the materials for CrVI environmental remediation. In addition, it has been demonstrated that adsorption can be carried out also in a continuous flux mode through a diluted photoactive MOF/sand chromatographic column. The obtained results open the perspective to assess the bifunctional sorption and photoreduction ability of a plethora of MOF materials that have been applied for chromium capture and photoreduction purposes. In parallel, this work opens the perspective to develop specific chemical encoding strategies within MOFs to transfer this bifunctionality to other related water remediation applications.
Highlights
Photocatalytic reduction of hexavalent chromium is becoming an environmentally friendly and applicable technology to remove highly toxic and mobile chromate oxyanions from polluted water sources [1,2,3,4,5]
Among the varied metal–organic frameworks (MOFs) that could potentially exhibit bifunctional photocatalytic and adsorption activities, zirconium amino-terephthalate (UiO-66-NH2 ) and titanium terephthalate (MIL-125) compounds have been selected. Both MOFs exhibit a cubic structure with an “fcu” topology arising from the connectivity between the inorganic hexanuclear clusters through twelve terephthalate type organic linkers [38,39]
X-ray diffraction (XRD) patterns show a significant increase of the peak width in the specific case of the samples with smaller particle size, which is ascribed to the reduction of the crystalline domains as a consequence of the synthesis conditions
Summary
Photocatalytic reduction of hexavalent chromium is becoming an environmentally friendly and applicable technology to remove highly toxic and mobile chromate oxyanions from polluted water sources [1,2,3,4,5]. Diluted hexavalent chromium solutions require high-cost chemicals for its reduction to CrIII until the legal maximum concentration limits for CrVI (ground water = 0.1 ppm, drinking water = 0.05 ppms) are accomplished This scenario is where bifunctional photocatalyst sorbents could play a key role to face at the same time the water treatment coupled with the chromium recovery. Our findings confirmed that CrVI to CrIII chemical and photoreduction processes are driven by the intermediate generation of pentavalent chromium transient species inside UiO-66-based materials [37] this research does not unravel the fate of trivalent chromium phototransformed species in solution. The photoreduction and adsorptive capacities of two iconic photoactive water-stable zirconium and titanium-based MOFs, UiO-66 and MIL-125, have been addressed For both materials, the experimental monitoring of CrVI and CrIII in solution during and after the operation has been determined. Our conclusions clearly point out that the studied bifunctional MOFs are able to completely photoreduce CrVI to CrIII and at the same time immobilize reduce CrIII species
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