Tracking Internal Electron Shuttle Using X-ray Spectroscopies in La/Zr Hydroxide for Reconciliation of Charge-Transfer Interaction and Coordination toward Phosphate.

Metallic hydroxides have been applied as the adsorbents of oxyanion contaminants, with the oxygen-bonded metal (M-O) considered as the core site for adsorption. For enhanced adsorption toward oxyanions, multimetallic modification of M-O is a promising approach for high removal performance. Here, bimetallic La/Zr hydroxides were prepared via a solvothermal route with varying La/Zr dosages. Bimetallic La/Zr hydroxides exhibited higher oxyanion adsorption capacity than La or Zr hydroxide. A maximum phosphate adsorption capacity of ∼160 mg g-1 was achieved under the La/Zr atomic ratio of 1:1, representing a new record among comparable adsorbents. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy showed that the incorporation of [LaO6] and [ZrO6] can induce an internal charge shuttle owing to the electronegativity difference of La and Zr. The charge transfers from La to Zr through the surrounding O 2p ligand, where the electrons in the highest occupied molecular orbitals of the [LaO6] octahedron filled unoccupied π orbitals of [ZrO6]. The as-induced internal electron shuttle in the bimetallic hydroxides primarily strengthened the formation of [MO6]···PO4 species (backdonation interaction). Further, the σP-O donation interaction between [LaO6] and [PO4] was clearly increased via intensification of the covalent coordination with O, as confirmed by the P K-edge XANES spectra for the as-used samples.