Molecular-level insights into surface complexation of arsenite, selenium and cadmium on {2 0 1} TiO2

Abstract

Arsenic (As) and Selenium (Se) contaminations present an urgent environmental concern, whereas its removal remains a significant challenge due to the lack of fundamental knowledge of the adsorption mechanism and effective adsorbents. Herein, {201} TiO2 exhibited the application potential for As(III) and Se(IV) removal, and the antagonistic effect was examined at the molecular level using X-ray absorption spectroscopy and density functional theory calculations. The Langmuir adsorption capacity of As(III) and Se(IV) on {201} TiO2 was 0.32 mmol/g and 0.25 mmol/g, respectively. The rate constant k for Se(IV) adsorption was 5.46 g/(mmol/h), which was 10.2 times higher than that of As(III) (0.48 g/(mmol/h)). EXAFS and CD-MUSIC results demonstrated that both As(III) and Se(IV) formed bidentate binuclear inner-sphere complexes at the bridge-Ti4C of {201} TiO2. The pd hybrid orbital energy of Ti-O bonds in Ti2O2SeO complex (−2.42 eV) was lower than that in Ti2O2AsO- complex (−2.26 eV), indicating the better adsorption stability and stronger affinity of Se(IV) adsorbed on {201} TiO2. The coexisting Cd(II) formed ternary complexes with As(III)/Se(IV) on {201} TiO2, enhancing the Freundlich adsorption of Se(IV) to 0.69 mmol/g and As(III) to 0.43 mmol/g. The introduction of Cd(II) altered the orbital hybridization interaction, and the charge transfer from Ti-3d to Cd-5s orbitals improved the adsorption energies of As(III)/Se(IV) on {201} TiO2. The ICOHP value of the Se-O-Cd bond was higher than that of the As-O-Cd bond, indicating the stronger affinity of Cd(II) and Se-O dangling bond. Gaining insights into the surface complexation modeling at the molecular and electronic level reveals the nature of coexisting ions adsorption behaviors.