Experimental and theoretical study of Cr(VI) photoreduction and adsorption onto SO42−-doped TiO2 obtained by plasma electrolytic oxidation

TiO2/SO42− surfaces obtained by plasma electrolytic oxidation were used for the reduction of Cr(VI) to Cr(III) present in wastewater, in order to study the behavior of TiO2/SO42− in Cr(VI) and Cr(III) ion adsorption by X-ray photoelectron spectroscopy and density functional theory calculations. High-resolution X-ray photoelectron spectra showed the presence of Cr(III) and Cr(VI) in the form of Cr(OH)3 and CrO42−, respectively. In the case of CrO42−, it was experimentally observed that once the surface is washed, they do not remain on it. In the case of Cr(OH)3, it does remain on the TiO2/SO42− surfaces. In order to validate the previous results, the interactions of Cr(VI) in the forms CrO42− and HCrO4− and Cr(III) in the forms Cr(OH)2+ and Cr(OH)3 were modeled by density functional theory +U on surfaces of: TiO2 without OH− groups (dry surfaces) and with OH− groups (wet surfaces) in addition to modeling on TiO2/SO42− surfaces without OH− groups (dry S-surfaces) and with OH− groups (wet S-surfaces). The results of the simulations showed that the presence of OH− on the surfaces with and without S atoms weakens the interaction of the surface with the Cr(VI). In the same way, the presence of superficial OH− energetically stabilizes Cr(III), compensating with the presence of OH− the formation of Cr(OH)3 and Cr(OH)2+ on the TiO2 surface with and without TiO2 atoms present. This results shed new light on the understanding of the reduction of Cr(VI) to Cr(III) on TiO2 surfaces and show how the surface plays an active role in the reduction of this contaminant.