Open-Framework Vanadate as Efficient Ion Exchanger for Uranyl Removal

Abstract

The elimination of uranium from radioactive wastewater is crucial for the safe management and operation of environmental remediation. Here, we present a layered vanadate with high acid/base stability, [Me₂NH₂]V₃O₇, as an excellent ion exchanger capturing uranyl from highly complex aqueous solutions. The material possesses an indirect band gap, ferromagnetic characteristic and a flower-like morphology comprising parallel nanosheets. The layered structure of [Me₂NH₂]V₃O₇ is predominantly upheld by the H-bond interaction between anionic framework [V₃O₇]_n^n- and intercalated [Me₂NH₂]⁺. The [Me₂NH₂]⁺ within [Me₂NH₂]V₃O₇ can be readily exchanged with UO₂²⁺. [Me₂NH₂]V₃O₇ exhibits high exchange capacity (q_m = 176.19 mg/g), fast kinetics (within 15 min), high removal efficiencies (>99%), and good selectivity against an excess of interfering ions. It also displays activity for UO₂²⁺ ion exchange over a wide pH range (2.00-7.12). More importantly, [Me₂NH₂]V₃O₇ has the capability to effectively remove low-concentration uranium, yielding a residual U concentration of 13 ppb, which falls below the EPA-defined acceptable limit of 30 ppb in typical drinking water. [Me₂NH₂]V₃O₇ can also efficiently separate UO₂²⁺ from Cs⁺ or Sr²⁺ achieving the highest separation factors (SF_U/Cs of 589 and SF_U/Sr of 227) to date. The BOMD and DFT calculations reveal that the driving force of ion exchange is dominated by the interaction between UO₂²⁺ and [V₃O₇]_n^n-, whereas the ion exchange rate is influenced by the mobility of UO₂²⁺ and [Me₂NH₂]⁺. Our experimental findings indicate that [Me₂NH₂]V₃O₇ can be considered as a promising uranium scavenger for environmental remediation. Additionally, the simulation results provide valuable mechanistic interpretations for ion exchange and serve as a reference for designing novel ion exchangers.

Keywords: Born−Oppenheimer molecular dynamics; density functional theory; ion exchange; open-framework compound; uranyl.