Efficient extraction of inorganic selenium from water by a Zr metal–organic framework: investigation of volumetric uptake capacity and binding motifs

Abstract

Strict monitoring and control of selenium concentrations in freshwater supplies is critical to safeguarding human health and aquatic life. A handful of previously investigated sorbents exhibit noteworthy gravimetric (mg g⁻¹) Se uptake capacities; however, often display insufficient volumetric (mg cm⁻³) capacities, thereby requiring large volumes of material for commercial implementation. In pursuit of mitigating this material inefficiency, we investigated the selenite (SeO₃²⁻) and selenate (SeO₄²⁻) affinity of MOF-808, a Zr-based metal–organic framework with a high density of potential Se oxyanion binding sites. MOF-808 recorded exceptional volumetric and gravimetric Se oxyanion capacities of 133 mg g⁻¹ (127 mg cm⁻³) and 118 mg g⁻¹ (112 mg cm⁻³) for aqueous selenite and selenate, respectively. Single-crystal X-ray diffraction studies revealed that selenite and selenate can bind at the MOF node via two distinct binding motifs, an η₂μ₂ motif in which the oxyanion coordinates to two different metal atoms in a single node, and a μ₂ motif in which the oxyanion interacts with only a single metal atom. Furthermore, powder X-ray diffraction (PXRD) patterns and N₂ adsorption/desorption isotherms confirm the retention of bulk crystallinity and porosity after the uptake of Se oxyanions.