Theoretical study on the electronic and catalytic properties of Fe-doped TiO2 and its use in the removal of arsenic

Abstract

Arsenic (As) is atoxic element that can be fatal to humans. In the environment, it exists indifferent oxidation states (-III, 0, + III and + V). However, in natural water,the As is mainly found in inorganic form forming as oxyanions of trivalentarsenite As (III) or pentavalent arsenate As (V) depending on the redoxpotential and the pH of the water. The contamination of water with arsenic is aglobal problem related mainly to its extensive presence in water resources usedfor human consumption1. Arsenate is more stable, less mobile andtoxic than As (III) and more efficiently removed from water. Therefore, commonmethods for the removal of arsenic from water involve the oxidation of the As(III) to As (V) species. As an alternative to conventional procedures,different proposals have attracted considerable attention in recent years,including heterogeneous photocatalysis with titanium dioxide (TiO2)and the use of zero valent iron (Fe0) known as ZVI. At present, two experimental studies can be mentioned,a recent one by López-Muñoz2 and another study by Nguyen et al. informing the use ofphotocatalysis with TiO2and ZVI nanoparticles (NP-ZVI) for the removal of As (III) from aqueous systems3.The results obtained demonstrated an increase in the arsenic removal efficiencyof the aqueous solution when ZVI particles were added to the reactor. As photocatalysis with both systems pure TiO2and in combination with iron offer attractive advantages for the treatment ofarsenic in aqueous systems, it is interesting to compare their performance andexplore the potential synergism between them. That is why in this work bothsystems were modeled with the objective of determining the mainphysical-chemical parameters that control the activity and selectivity towardsthe oxidation of arsenic (As). The calculations were made using the VASP4code within the formalism of the Functional Density theory, with the inclusionof the Hubbard coefficient (DFT + U). The chosen surfaces of the titania are thecatalytically most active, anatase TiO2(101) and rutile TiO2(110). The As/TiO2 interaction was modeled by depositing 1, 2 or 4As0 atoms. Different Fe-doping options (cationic and interstitial)were studied. The As adsorption energies were calculated on both surfaces. The study is completed with the analysis of the adsorption of different arsenious species.