Arsenate adsorption on an Fe–Ce bimetal oxide adsorbent: EXAFS study and surface complexation modeling

The mechanism of arsenate (As(V)) adsorption on an Fe–Ce bimetal (hydrous) oxide (Fe–Ce) was investigated using complementary analysis techniques including extended X-ray absorption fine structure (EXAFS) and surface complexation modeling. The As K-edge EXAFS spectra showed that the second peak of Fe–Ce after As(V) adsorption was the As–Fe shell, which supported the finding that As(V) adsorption occurred mainly at the Fe surface active sites. Two As–Fe distances of 3.30 Å and 3.55 Å were observed from the EXAFS spectra of As(V) adsorbed samples at three pH levels (5.0, 7.6, and 9.0) and two initial surface loadings of 70 and 130 mg/g, which indicated that monodentate mononuclear and bidentate binuclear As surface complexes coexisted. When compared with the reported dominant species of bidentate binuclear complex for As existing on iron (hydro)oxides, the existence of Ce atoms in the bimetal oxide and the high surface loading were the likely reasons for the existence of the monodentate complex. A Charge Distribution-Multi-site Sites Complexation (CD-MUSIC) model showed that protonated monodentate (MH) and deprotonated bidentate (B) complexes preferred to exist on the Fe–Ce surface in a high surface loading range (Γ = 5.11–14.4 μmol/m2). The MH complex was shown to be dominant at pH < 8. Based on the results from EXAFS analysis and the CD-MUSIC model, the adsorptive behavior of As(V) on Fe–Ce with high surface loadings was satisfactorily interpreted and understood.