Origin of enhanced boric acid adsorption in light-burned magnesium oxide

Light-burned magnesium oxide (MgO) possesses a high surface area and has attracted interest as a promising candidate for boron adsorption materials; however, the detailed molecular structures decisive for enhancing the adsorption performance have not yet been elucidated. Here, the origin of enhanced boric acid adsorption for the light-burned MgO is studied by multiple probes, including positronium (Ps) annihilation spectroscopy, Fourier transform infrared spectroscopy, and sorption experiments coupled with molecular simulations. The state-of-the-art technique of open space analysis using Ps revealed the detailed structure of the interfaces between MgO nanograins: ∼10 Å and ∼30 Å open spaces, participating in the chemisorption of B(OH)₄ ^- and BO₃ ^3- simultaneously with the physisorption of neutral B(OH)₃ molecules. Furthermore, in addition to the fraction of open spaces, a proton quasi-layer formed on the interior surfaces of the above-mentioned angstrom-scale open spaces was identified to be attributable for enhancing both the chemisorption and physisorption.