Covalent binding of amines to hierarchically porous MOFs for enhanced arsenic removal

Arsenite (As(III)) threatens to environmental sustainability and human health due to its high mobility and toxic nature. Metal-organic frameworks (MOFs) have gained attention as effective adsorbents for removing As from wastewater systems. Nevertheless, their practical implementation is frequently hindered by several challenges, including inadequate functional stability and insufficient availability of binding sites. In this study, four innovative functionalized adsorbents were developed by covalently grafting ethylenediamine (ED), diethylenetriamine (DETA), tris(2-aminoethyl)amine (TAEA), and polyethyleneimine (PEI) onto the hierarchically structured UiO-66-NH₂ (referred to as HP-UiO-66-NH₂) through a bromoacetyl bromide-assisted reaction. The creation of hierarchical pores in the MOFs not only enabled the accommodation of amine groups but also enhanced the mass transfer of As(III). Notably, the DETA/HP-UiO-66-NH₂ material exhibited an exceptional adsorption capacity for As(III), reaching up to 406.5 mg g⁻¹, which exceeds the performance of the majority of adsorbents reported thus far. In practical applications, a mere 1 mg of DETA/HP-UiO-66-NH₂ is sufficient to effectively remove trace amounts of As(III), meeting the World Health Organization’s recommended limit for drinking water (< 10 µg L⁻¹). Fixed-bed column experiments revealed that this adsorbent could treat approximately 1530 bed volumes of arsenic-laden groundwater, achieving a five-fold improvement over commercially available activated carbon. Additionally, the material possessed considerable anti-interference capacity (85 %-96 % effectiveness relative to the blank control) and high regenerability, retaining 91 % of its adsorption capacity after five cycles. Investigations utilizing SEM, N₂ adsorption, XPS, FT-IR, and DFT calculations substantiated that both physical and chemical adsorption were the principal mechanisms driving the adsorption of As(III). This research provides a versatile methodology for the development of ultrahigh-performance MOF adsorbents aimed at addressing arsenic contamination.