Impact of the duty cycle on the morphology and photocatalytic properties of S-TiO2 obtained by plasma electrolytic oxidation to treat real electroplating wastewater contaminated with Cr6+

Abstract

This work reports S-TiO2 doped coatings to reduce Cr6+ to Cr3+ obtained from a Ti electrode through the Plasma Electrolytic Oxidation (PEO) process. The Ti sheets (20 × 20 × 1 mm) were submerged on 0.1 M H2SO4, and values of the duty cycle from 2% to 50% were applied to obtain various materials. SEM, XRD, AFM, XPS, and DRS techniques were used to characterize the resultant surfaces. It was observed that the duty cycle strongly influences the crystalline/amorphous ratio, anatase/rutile ratio, porosity density, pores size distribution, and surface roughness. Besides, it is explained that the introduction of SO42- into the TiO2 structure can take place either in the Ti or O places in the crystalline lattice. All materials showed photocatalytic properties to reduce Cr6+ to Cr3+ under UVC light (254 nm), decreasing the efficiency with the increase of the duty cycle. Additionally, the introduction of EDTA showed a positive synergy with the heterogeneous photocatalytic process when the material was obtained with the highest duty cycle. This last result was attributed to the relatively low bandgap and the high recombination rate; furthermore, EDTA acts as holes and hydroxyl radical scavenger. Thus, the photoelectrochemical system for the treatment of wastewater was evaluated, and again, the same materials showed the highest performance. In the same way, the re-use of the material obtained with the 2% duty cycle was tested, getting satisfactory results, obtaining 96.14 ± 2.77% Cr6+ reduction even after seven cycles of reuse.