The effect of pH on the adsorption of arsenic(III) and arsenic(V) at the TiO2 anatase [101] surface.

Octahedral TiO2 nanocrystals (OTNs) have been prepared by a hydrothermal method with the main surface of (101). Then the arsenic adsorption behavior on OTNs is investigated in a broad experimental pH range from 1.0 to 13.5. The maximum adsorptions of arsenite (As(III)) and arsenate (As(V)) appear at pH values 8 and 4, respectively. It is interesting to see that the minimum adsorptions of As(III) and As(V) are both at pH 12 and then their adsorptions increase again at higher pH values such as 13.0 and 13.5. To our best knowledge, it is quite new to report the arsenic adsorption on the controlled TiO2 surface especially at very high pH values. These results might be helpful to understand the adsorption mechanism. On the other hand, periodic slab models of TiO2 anatase (101) surface with some H(+) cations, some water molecules or some OH(-) ions are suggested to simulate the pH effect. Using these models, the adsorptions of As(III) and As(V) are simulated by the density functional theory (DFT) method. Qualitatively, the adsorption abilities of arsenic species, water and OH(-) follow the order of AsO3(3-)>OH(-)>HAsO3(2-)>H2AsO3(-)>H2O>H3AsO3 for As(III) and AsO4(3-)>OH(-)>HAsO4(2-)>H2AsO4(-)>H3AsO4>H2O for As(V). It implies that H2AsO3(-) should be the major As(III) species at pH 8 and H2AsO4(-) should be the major As(V) species at pH 4, and the most negative charged ions AsO3(3-) and AsO4(3-) should correspond to the adsorptions at the high pH values 13 and 13.5.