Application of TiO2–WO3 Composite for Continuous Reduction of Chromium(VI) in Light-limited Condition
Authors: Elshorafa Rand , Han Dong Suk , Park Hyunwoong , Abdel-Wahab Ahmed 
Published: July 2019

Hexavalent chromium (Cr(VI)) is toxic and hazard chemical to human health and it is categorized as a carcinogen chemical. Cr(VI) is released from many industrial processes such as electroplating, metal finishing, and textile/leather dyeing. Although there are many treatment technologies for removal of Cr(VI), the photocatalytic treatment using metal oxide semiconductor (SC) materials has been shown to be a promising technology because of the fact that toxic Cr(VI) can be effectively reduced to less toxic Cr(III) by cheap, nontoxic, and stable photocatalyst such as titanium dioxide (TiO2). In this study, two different types of TiO2 particles (nano-sized particle, nanotube particle) were used as photocatalysts for Cr(VI) reduction. These two types have different kinetics of transferring the charge carriers. Based on results of this research, each TiO2 particle type was used for fabrication of multijuntion SC with WO3 particles for continuous reduction of Cr(VI) in water.

For the continuous reduction of Cr(VI), two different methods were applied: Photoelectrochemical(PEC) system and Nanofiltration system. Regardless of the method, WO3 was in junction with TiO2 in order to reduce Cr(VI) even in the dark because WO3 has an ability to storing solar energy through charging/discharging route. Upon light-on, the electrons that was photogenerated from TiO2 transfer to WO3, forming HxW(6-x)O3 of the photocharged surface specie. Upon light-off, the trapped electrons in WO3 are released gradually to the environment that requires electrons for certain chemical reactions to take place thermodynamically. The fabricated heterojunction TiO2/WO3 will have a good example of the photoelectrode to perform the multi-functionalities: solar light conversion and simultaneous solar energy storage, and electron transfer for application of environmental remediation.

In case of the PEC system, TiO2 was doped with WO3 on the fluorine doped tin oxide (FTO) glass by a layer-by-layer deposition technique. The TiO2/WO3 composites that was charged at AM 1.5 G irradiation (100 mW/cm2) shows a gradual reduction of Cr(VI) in the dark until 12 hours. Also, the Cr(VI) results were dependent on TiO2 type, doping ratio of TiO2 and WO3, and charging time.

For the nanofiltration system, the suspended TiO2/WO3 particles were filtered by anodic membrane filtration (20 nm of pore size) and subsequently the deposited TiO2/WO3 particles were charged by illumination of sunlight with light intensity higher than 200 mW/cm2. The charged TiO2/WO3 retained on the membrane was inserted to dead-end filtration apparatus and the membrane system was used for continuous reduction of Cr(VI) over filtration time. For this, various experimental conditions were applied, including loading amount of TiO2/WO3 with different ratio of TiO2/WO3, initial concentration of Cr(VI), charging time, reduction capacity over number of recycle of charging/discharging, etc. This system will have a benefit of no application of UV or light source in the filtration system, which is a major challenge when photocatalysis and filtration are combined for a specific treatment process. Therefore, we believe that two approaches will provide scientific and technical information for continuous removal of Cr(VI) in the light-limited condition.