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

Competitive adsorption of arsenate (AsO43−) and phosphate (PO43−) 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 AsO43− and PO43−. When AsO43− and PO43− were simultaneously exposed to the adsorbents at the same concentrations, AsO43− was preferentially adsorbed by α-FeOOH and TiO2, but PO43− adsorption was dominant on LaOOH. The results implied that the PO43− adsorbed on LaOOH had to be remobilized to allow for AsO43− adsorption, but that PO43− adsorption on α-FeOOH and TiO2 was hindered by faster AsO43− adsorption. Crystal orbital Hamilton population (COHP) analysis revealed that AsO43− complexes bonded more strongly on α-FeOOH and TiO2, whereas PO43− complexes were more stable on LaOOH. Different adsorption sequences and the stability of the complexes were attributed to the diverse geometric configurations of AsO43− and PO43− on metal oxides surfaces with specific bonding chemistry. The presence of Ca2+ did not affect AsO43− and PO43− 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.