Insights from molecular-level mechanisms
of arsenite [As(III)]
and cadmium (Cd) co-adsorption on TiO<sub>2</sub> can further our
understanding of their synergistic removal in industrial wastewaters.
The motivation for our study is to explore the interfacial interactions
of neutrally charged As(III) and cationic Cd<sup>2+</sup> on nanocrystalline
TiO<sub>2</sub> using multiple complementary techniques. The results
of adsorption edge, ζ potential, and surface complexation modeling
suggest that coexistence of As(III) and Cd<sup>2+</sup> enhanced their
synergistic adsorption on TiO<sub>2</sub> and, consequently, resulted
in the formation of a ternary surface complex. This ternary surface
complex, in turn, inhibited the metal release into the aqueous phase
and, therefore, facilitated the immobilization of the heavy metals.
Our <i>in situ</i> flow-cell attentuated total reflectance
Fourier transform infrared (ATR–FTIR) spectroscopy and extended
X-ray absorption fine structure (EXAFS) spectroscopy evidence showed
that, regardless of the order of contact, As(III) was preferentially
adsorbed on TiO<sub>2</sub> rather than Cd. In agreement with our
spectroscopic analysis, quantum chemistry calculations also illustrated
that the Cd–As(III)–TiO<sub>2</sub> ternary surface
complex should be formed with the adsorbed As(III) as the bridging
molecule. At high As(III) concentrations, the formation of the Cd–As(III)–TiO<sub>2</sub> complex is responsible for the Cd removal. The simultaneous
removal mechanisms will further our understanding of the removal of
multiple pollutants in industrial wastewaters
