How Doping Regulates As(III) Adsorption at TiO Surfaces: A DFT + U Study.

The efficient adsorption and removal of As(III), which is highly toxic, remains difficult. TiO shows promise in this field, though the process needs improvement. Herein, how doping regulates As(OH) adsorption over TiO surfaces is comprehensively investigated by means of the DFT + D3 approach. Doping creates the bidentate mononuclear (Ce doping at the Ti site), tridentate (N, S doping at the O site), and other new adsorption structures. The extent of structural perturbation correlates with the atomic radius when doping the Ti site (Ce >> Fe, Mn, V >> B), while it correlates with the likelihood of forming more bonds when doping the O site (N > S > F). Doping the O, O rather than the Ti site is more effective in enhancing As(OH) adsorption and also causes more structural perturbation and diversity. Similar to the scenario of pristine surfaces, the bidentate binuclear complexes with two Ti-O bonds are often the most preferred, except for B doping at the Ti site, S doping at the O site, and B doping at the O site of rutile (110) and Ce, B doping at the Ti site, N, S doping at the O site, and N, S, B doping at the O site of anatase (101). Doping significantly regulates the As(OH) adsorption efficacy, and the adsorption energies reach -4.17, -4.13, and -4.67 eV for Mn doping at the Ti site and N doping at the O and O sites of rutile (110) and -1.99, -2.29, and -2.24 eV for Ce doping at the Ti site and N doping at the O and O sites of anatase (101), respectively. As(OH) adsorption and removal are crystal-dependent and become apparently more efficient for rutile vs. anatase, whether doped at the Ti, O, or O site. The auto-oxidation of As(III) occurs when the As centers interact directly with the TiO surface, and this occurs more frequently for rutile rather than anatase. The multidentate adsorption of As(OH) causes electron back-donation and As(V) re-reduction to As(IV). The regulatory effects of doping during As(III) adsorption and the critical roles played by crystal control are further unraveled at the molecular level. Significant insights are provided for As(III) pollution management via the adsorption and rational design of efficient scavengers.