Studies on the reaction pathway of arsenate adsorption at water–TiO2 interfaces using density functional theory

Abstract

a b s t r a c tReaction pathway information of transition states and intermediate species is crucial for understanding the adsorption mechanism of pollutants at mineral-water interfaces. However, it has been difficult to obtain such information using existing experiments. Here, the activation barriers, transition states, intermediate species and surface complexes of arsenate adsorption on TiO 2 surfaces were studied using DFTbased reaction pathway calculations. The results indicated that the bidentate binuclear (BB) adsorption structure was formed through a monodentate mononuclear (MM) metastable-equilibrium adsorption (MEA) state. A two-step adsorption mechanism was proposed on the basis of the detailed picture of bond breaking and bond formation during each reaction step. When the adjacent surface sites were occupied, the transform from MM mode to BB mode was greatly inhibited so that both MM and BB coexisted in the equilibrium adsorption sample. The BB complex was energetically more stable than the MM complex, and so, the adsorption irreversibility was fundamentally related to the ratio BB:MM in the final equilibrium state. This mechanism may also explain the initial concentration effect, where, for the given adsorption experiment of arsenate on TiO 2 under the same thermodynamic conditions, both equilibrium constants and the BB:MM ratio in equilibrium adsorption samples changed with the reaction kinetics.

How TiO2facets determine arsenic adsorption and photooxidation: spectroscopic and DFT studies

“…First, BB complexes are more stable than MM on both facets as evidenced by their more negative adsorption energies (Table S4 †), in agreement with previous reports. 36,37 Second, both BB and MM complexes are more energetically favorable on {001} than on the {101} facets. In fact, the adsorption energy for the BB configuration on the {001} facets was 3.36 and 2.89 eV more negative than on the {101} facets for AsIJIII) and AsIJV), respectively.…”

Tridentate arsenate complexation with ferric hydroxide and its effect on the kinetics of arsenate adsorption and desorption

“…When examined on the time scale of laboratory experiments, arsenate adsorption does not exhibit reversible equilibrium or kinetic behavior (Yean et al, 2005;Yang et al, 2012;He et al, 2011). The rate of arsenate adsorption to ferric hydroxide has been shown to display biphasic kinetics, with the majority of arsenate uptake occurring on a time scale of minutes, while the remaining uptake occurs on a time scale of weeks to months (Zhang and Stanforth, 2005;Zhang and Selim, 2005;Grossl et al, 1997;Yang et al, 2007).…”

Binding mechanism of arsenate on rutile (1 1 0) and (0 0 1) planes studied using grazing-incidence EXAFS measurement and DFT calculation

“…These advantages may trigger wide applications of GI-EXAFS in geosciences, chemistry and environmental sciences as synchrotron facilities are improving (Roscioni et al, 2013). Density functional theory (DFT) calculation can provide a detailed description of adsorbed molecules at solid surfaces, including aspects of structure, bonding, and energy (He et al, 2011;Li et al, 2012). Therefore, the combination of GI-EXAFS and DFT calculation is able to identify the structural properties and relative stability of arsenic on specific crystal planes.…”