Solid Brønsted acidity boosts adsorption reactivity of nano-adsorbent for water decontamination

Abstract

Despite the development of various Lewis acidic nano-adsorbents for fluoride removal through inner-sphere coordination, strong competition for hydroxyl ions still hinders efficient water defluoridation. In addition, the critical issue of polysilicate scaling that results from the ubiquitous silicates must be addressed. To tackle these issues, an alternative approach to enhancing adsorption reactivity by modifying nano-adsorbents with dual Lewis and Brønsted acidity is proposed. The feasibility of this approach is demonstrated by growing zirconium phosphate (ZrP) inside a gel-type anion exchanger, N201, to produce nanocomposite ZrP@N201, in which the confined ZrP contained an otherwise metastable amorphous phase with Lewis acidic Zr⁴⁺ sites and Brønsted acidic monohydrogen phosphate groups (–O₃POH). Compared with the Lewis acidic nano-zirconium oxide analog (HZO@N201), ZrP@N201 exhibited a greatly improved adsorption capacity (117.9 vs. 52.3 mg/g-Zr) and mass transfer rate (3.56 × 10⁻⁶vs. 4.55 × 10⁻⁷ cm/s), while bulk ZrP produced a thermodynamically stable α-phase with Brønsted acidity that exhibited negligible adsorption capability toward fluoride. The enhanced defluoridation activity of ZrP@N201 is attributed to Brønsted acidity and the increased outer electron density of Zr⁴⁺ sites, as corroborated using XPS and solid-state NMR analysis. Moreover, Brønsted acidity strengthens the resistance of ZrP@N201 to silicate, allowing its full regeneration during cyclic defluoridation. Column tests demonstrated 3–10 times the amount of clean water from (waste) for ZrP@N201 as compared to both HZO@N201 and the widely used activated aluminum oxide. This study highlights the potential of developing nano-adsorbents with dual acidities for various environmental remediation applications.