Symmetry-reduction enhancement of nitrate removal on record-breaking layered yttrium hydroxide adsorbents

A new class of layered metal hydroxide materials, namely layered yttrium hydroxides (LYH-X, X^- are anions such as Cl^- and Br^-), are reported to be excellent adsorbents to capture nitrate-nitrogen from neutral water. More importantly, the adsorption properties are correlated with the crystal symmetry of adsorbents: both the adsorption capacity and rate constant of LYH-Cl (orthorhombic <i>P</i>2₁2₁2 space group) are almost twice of those of LYH-Br (monoclinic <i>P</i>2₁ space group). A comprehensive study combining multinuclear solid-state NMR spectroscopy and other multiscale characterization techniques was then performed to understand the origin of the symmetry-related adsorption behaviors. The results reveal an increasing trend of change in local environments of Y(OH)₇·H₂O, Y(OH)₈·H₂O, and Y(OH)₈, implying that the adsorbed nitrate anions are located within the pocket constructed by alternating Y(OH)₈·H₂O and Y(OH)₈. The splitting of the ⁸⁹Y NMR peak of Y(OH)₈·H₂O of LYH-Cl after adsorption is consistent with the reduction of crystal symmetry from <i>P</i>2₁2₁2 to its translationengleiche subgroup <i>P</i>2, providing an additional driving force for nitrate capture. This work thus not only discovers a nitrate adsorbent with a superior capacity of 44.56 ± 0.17 mg g^-1, but also demonstrates a new concept of symmetry-driven adsorption enhancement.