Surface complexation of organic arsenic on nanocrystalline titanium oxide

The adsorption mechanisms of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) on nanocrystalline titanium oxide (TiO₂) were investigated with X-ray absorption spectroscopy (XAS), surface charge and zeta potential measurements, adsorption edge, and surface complexation modeling. XAS data demonstrated that MMA and DMA formed bidentate and monodentate inner sphere complexes with the TiO₂ surface, respectively. The charge and zeta potential behaviors of TiO₂ as a function of ionic strength suggested that the point of zero charge (PZC) and isoelectric point (IEP) of TiO₂ were identical at pH 5.8. Adsorption of MMA and DMA on TiO ₂ shifted the IEP to pH 4.1 and 4.8, respectively, indicating the formation of negatively charged surface complexes. A satisfactory interpretation of the experimental data was provided by the charge distribution (CD) multi-site complexation (MUSIC) model with the triple plane option under the constraint of the XAS evidence.