Abstract:

Adsorption of arsenate on TiO2 surfaces was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) calculation. The adsorption process was independent of ion strength, indicating the formation of inner-sphere complexes of arsenate on TiO2 surfaces. EXAFS analysis showed that the —AsO4 tetrahedral geometry remained relatively rigid before and after adsorption with an As-O distance (R) of (0.169±0.001) nm. When —AsO4 approached TiO2 surfaces, the As—O—Ti bond formed through the O atoms in —AsO4 clusters. Furthermore, based on the combined results from EXAFS and DFT, As(V) in the “equilibrium”adsorption samples was in at least two metastable equilibrium adsorption (MEA) states: double corner (DC) linkage mode (R1=(0.321±0.002) nm, stronger adsorption) and single corner (SC) linkage mode (R2=(0.360±0.002) nm, weak adsorption). The proportion of coordination number between DC and SC MEA states (CN1/CN2) decreased from 3.3 to 1.6 as the surface coverage increased from 9.79 to 28.0 mg·g-1. This result indicated that at low surface loading arsenate was adsorbed in more stable DC MEA state, but As(V) tended to be adsorbed in SC MEA state as surface coverage increased.

Key words: TiO2, As(V), Adsorption, Extended X-ray absorption fine structure, Density functional theory, Microscopic adsorption mode