Chromium Speciation in Zirconium-Based Metal-Organic Frameworks for Environmental Remediation

Acute Cr^VI water pollution due to anthropogenic activities is an increasing worldwide concern. The high toxicity and mobility of Cr^VI makes it necessary to develop dual adsorbent/ion-reductive materials that are able to capture Cr^VI and transform it efficiently into the less hazardous Cr^III . An accurate description of chromium speciation at the adsorbent/ion-reductive matrix is key to assessing whether Cr^VI is completely reduced to Cr^III , or if its incomplete transformation has led to the stabilization of highly reactive, transient Cr^V species within the material. With this goal in mind, a dual ultraviolet-visible and electron paramagnetic spectroscopy approach has been applied to determine the chromium speciation within zirconium-based metal-organic frameworks (MOFs). Our findings point out that the generation of defects at Zr-MOFs boosts Cr^VI adsorption, whilst the presence of reductive groups on the organic linkers play a key role in stabilizing it as isolated and/or clustered Cr^III ions.