Combined DFT and IR evidence on metastable-equilibrium adsorption of arsenate on TiO2 surfaces

Journal of Colloid and Interface Science

2011

Self Cite

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.

Section: Methods and Theorymentioning

confidence: 99%

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

Journal of Colloid and Interface Science

2011

Self Cite

“…Based on our previous research (He et al, 2009a,b;Zhang et al, 2009), adsorption of arsenate on SMNP surfaces may proceed through a two-step reaction process. The initially formed MM adsorption complexes could either keep stable temporarily or transfer to the thermodynamically more stable BB adsorption complexes in the second step depending on the surface coverage.…”

Section: Xafs Analysis Of Arsenate-loaded Smnpmentioning

confidence: 99%

XAFS study of starch-stabilized magnetite nanoparticles and surface speciation of arsenate

Environmental Pollution

Zhao

et al. 2011

Self Cite

a b s t r a c tIt has been shown that starch can effectively stabilize nanoscale magnetite particles, and starch-stabilized magnetite nanoparticles (SMNP) are promising for in situ remediation of arsenic-contaminated soils. However, a molecular level understanding has been lacking. Here, we carried out XAFS studies to bridge this knowledge gap. Fe K-edge XAFS spectra indicated that the FeeO and FeeFe coordination numbers of SMNP were lower than those for bare magnetite particles, and these coordination numbers decreased with increasing starch concentration. The decrease in the average coordination number at elevated stabilizer concentration was attributed to the increase in the surface-to-volume ratio. Arsenic K-edge XAFS spectra indicated that adsorbed arsenate on SMNP consisted primarily of binuclear bidentate (BB) complexes and monodentate mononuclear (MM) complexes. More BB complexes (energetically more favorable) were observed at higher starch concentrations, indicating that SMNP not only offered greater adsorption surface area, but also stronger adsorption affinity toward arsenate.

“…[5][6][7] However, previous studies using these experimental and theoretical methods are largely restricted to identifying the geometry of the final adsorption states. [8][9][10][11] There is little investigation on the density of states (DOS) of adsorbate before, after, and during the adsorption processes.…”

Section: Introductionmentioning

confidence: 99%

Quantitative XANES Studies on Metastable Equilibrium Adsorption of Arsenate on TiO₂ Surfaces

et al. 2009

J. Phys. Chem. C

Self Cite

Quantitative information on the electronic and geometric structures of adsorption complexes of arsenate on TiO 2 surfaces was obtained on the basis of XANES analysis combined with DFT calculation. The density of states (DOS) showed that the As(V) K-absorption edge corresponded to the dipole transition from the As 1s electron into the As(4p)-O(2p) antibonding molecular orbital, suggesting that the bonding between arsenate and TiO 2 was primarily due to the interaction of As 4p and O 2p orbitals. Previous EXAFS studies indicated that both bidentate binuclear (BB) and monodentate mononuclear (MM) surface complexes coexisted in the As(V)-TiO 2 adsorption system. DFT calculation showed that the XANES transition energy of the BB complex was significantly higher than that of the MM complex. Thus, quantitative information on adsorption microstructures can be obtained by measuring the blue-shift of the XANES absorption edge. XANES results of adsorption samples at different kinetic stages indicated that there was a structural evolution from the MM complex to the BB complex as the reaction processed to adsorption equilibrium. Therefore, the final proportion between BB and MM complexes in the real equilibrium state was fundamentally affected by surface coverage and kinetic pathways. This fact implied that the real equilibrium constant, when defined by macroscopic parameters of concentration and adsorption density (mol/m 2 ), are generally inconstant in nature and hence cannot describe the real equilibrium properties of adsorption, because different microscopic metastable equilibrium adsorption structures that construct the real adsorption equilibrium cannot be counted for both mass and energy by the macroscopic parameter of surface concentrations (mol/m 2 ).