Competitive arsenate and phosphate adsorption on α-FeOOH, LaOOH, and nano-TiO₂: Two-dimensional correlation...

Competitive adsorption of arsenate (AsO₄^3-) and phosphate (PO₄^3-) on α-FeOOH, LaOOH, and nano-TiO₂ was studied using batch adsorption experiments and in-situ flow cell ATR-FTIR coupled with two-dimensional correlation spectroscopy (2D-COS) for the first time. With a higher temporal resolution, our results found a highly dynamic adsorption sequence for AsO₄^3- and PO₄^3-. When AsO₄^3- and PO₄^3- were simultaneously exposed to the adsorbents at the same concentrations, AsO₄^3- was preferentially adsorbed by α-FeOOH and TiO₂, but PO₄^3- adsorption was dominant on LaOOH. The results implied that the PO₄^3- adsorbed on LaOOH had to be remobilized to allow for AsO₄^3- adsorption, but that PO₄^3- adsorption on α-FeOOH and TiO₂ was hindered by faster AsO₄^3- adsorption. Crystal orbital Hamilton population (COHP) analysis revealed that AsO₄^3- complexes bonded more strongly on α-FeOOH and TiO₂, whereas PO₄^3- complexes were more stable on LaOOH. Different adsorption sequences and the stability of the complexes were attributed to the diverse geometric configurations of AsO₄^3- and PO₄^3- on metal oxides surfaces with specific bonding chemistry. The presence of Ca²⁺ did not affect AsO₄^3- and PO₄^3- adsorption sequence on α-FeOOH or LaOOH, but it reversed the adsorption sequence on TiO₂ due to the formation of ternary surface complexes on TiO₂ surfaces.